There are several bridge types which feature elements which require special inspection procedures. The most notable bridge types are:
Figure 16.1.1 Golden Gate Bridge
This topic is limited to the cable and its elements. All other members of a cable-supported bridge have been described in earlier topics and are to be referred to for the appropriate information. For each of the above bridge types, this topic provides:
Figure 16.1.2 Maysville Cable-Stay Bridge
A cable-supported bridge is a bridge that is supported by or “suspended from” cables.
A suspension bridge has a deck, which is supported by vertical suspender cables that are in turn supported by main suspension cables. The suspension cables can be supported by saddles atop towers and are anchored at their ends or self-anchored to the bridge superstructure. Suspension bridges are normally constructed when intermediate piers are not feasible because of long span requirements (see Figure 16.1.3). Modern suspension bridge spans are generally longer than 1400 feet.
A cable-stayed bridge is another long span cable-supported bridge where the superstructure is supported by cables, or stays, passing over or anchored to towers located at the main piers. Cable-stayed bridges are the more modern version of cable-supported bridges. Spans generally range from 700 to 1400 feet (see Figure 16.1.4). Evolving for approximately 400 years, the first vehicular cable-stayed bridge in the United States was constructed in Alaska in 1972 (John O'Connell Memorial Bridge at Sitka, Alaska).
In suspension bridges, vertical suspender cables attach the deck and floor system to the main suspension cables. Cable-stayed bridges are much stiffer than suspension bridges. In cable-stayed bridges, the deck and floor system is supported directly from the tower with fairly taut stay cables.
Figure 16.1.4 Sunshine Skyway Cable-Stayed Bridge in Tampa Bay, Florida
A cable may be composed of one or more structural wire ropes, structural wire strands, locked coil strands, parallel wire strands, or parallel wires.
Parallel wire cable consists of a number of parallel wires (see Figure 16.1.5 and Figure 16.1.10). The diameter varies depending on the span length and design loads. Parallel wire cables used in cable-stayed bridges conforms to ASTM A421, Type BA, low relaxation. It is basically stress-relieved wire used for prestressed concrete.
Structural wire strand is an assembly of wires formed helically around a center wire in one or more symmetrical layers. Sizes normally range from 2 to 4 inches in total diameter (see Figure 16.1.6).
Structural wire rope is an assembly of strands formed helically around a center strand (see Figure 16.1.7).
Parallel strand cable is a parallel group of strands (see Figure 16.1.8). Seven-wire strand commonly used for cable-stayed bridges conforms to ASTM A416, low relaxation steel (see Figure 16.1.11). It is basically seven-wire stress-relieved strand for prestressed concrete.
Locked coil strand is a helical type strand composed of a number of round wires, and then several layers of wedge or keystone shaped wires and finally several layers of Z- or S-shaped wires (see Figure 16.1.9). Locked coil strand has not been used for cable-stayed bridges in this country, but it is commonly used for cable-stayed bridges in Europe.
Several types of cables have been used for cable-stayed bridges. The three most common are locked-coil strand, parallel wire, and parallel seven-wire strand. The majority of existing cable-stayed bridges in the world, other than the United States, use preformed prestretched galvanized locked-coil strand. The cable-stayed bridges in the United States incorporate parallel wire or seven-wire prestressing strand in the cables.
Figure 16.1.6 Structural Wire Strand
Figure 16.1.7 Structural Wire Rope
Figure 16.1.8 Parallel Strand Cable
Figure 16.1.9 Locked Coil Strand Cross-Section
Figure 16.1.11 Parallel Strand
Methods used for corrosion protection include:
Figure 16.1.12 Cable Wrapping on the Wheeling Suspension Bridge
Figure 16.1.13 Shapes of Towers Used for Cable-Stay Bridges
Towers are constructed of reinforced concrete or steel or a combination of the two materials (see Figures 16.1.14 and 16.1.15).
Figure 16.1.14 Tower Types: Concrete “Portal Tower” and “A-Frame Tower”
Figure 16.1.15 Tower Types: Steel “Portal Tower” and Concrete “Single Column Tower”
The deck structures are also constructed of concrete or steel.
In this subtopic, only those bridge elements that are unique to suspension bridges are presented. Refer to the appropriate topic for other bridge elements that are common to similar bridge types (i.e. floor systems, open web girders, box sections, etc.).
Main suspension cables are generally supported on saddles at the towers and are anchored at each end. Sometimes, main suspension cables are referred to as catenary cables. Suspender cables are vertical cables that connect the deck and floor system to the main suspension cables (see Figure 16.1.16).
Figure 16.1.16 Three-Span Suspension Bridge Schematic
If a suspension bridge has only two main suspension cables, the cables are considered to be fracture critical members since there is no load path redundancy. Refer to Topic 6.4 for a detailed description of fracture criticality and redundancy.
Another type of suspension bridge uses a self-anchoring system where the main suspension cables are anchored into the edge girders that span continuously from end to end in the suspension spans. The force from the main suspension cables puts the edge girders into compression. The edge girders support the floor system and the suspender cables support the edge girders in this arrangement.
This type suspension bridge may be used to create long clear spans for navigation and not have to continue the suspension spans to the shorelines for anchorage. The alignment for the approach spans can be different than the suspension spans. These benefits are seen in the new Oakland Bay View Bridge in California, where the approach alignment is on a curve, the suspension span creates the wide navigation channel and the anchoring is self contained within the superstructure of the suspension spans.
In bridges with common earth anchored cable systems, either above or below ground, the total force of the main suspension cable has to be transferred into the anchor block (see Figure 16.1.17). The void area inside the anchor block is referred to as the Chain Gallery. The force from the main cable is distributed through the splay saddle, bridge wires, strand shoes and anchor bars. The anchor bars are embedded and secured in the concrete of the anchor block. The anchor bars may consist of steel bars, rods, pipes, or prestressed bars / strands.
Figure 16.1.17 Anchor Block Schematic
The connection between main cable and tower is usually made through saddles. The saddle supports the main cable as it crosses over the tower (see Figure 16.1.18). Saddles are commonly made from fabricated steel or castings.
Figure 16.1.18 Cable Saddles for Manhattan Bridge, NYC (Main Span 1,480 ft)
The connection between the main suspension cable and suspender cable is made by means of a cable band. The cable band consists of two semi-cylindrical halves connected by high-tensile steel bolts to develop the necessary friction.
Grooved cable bands have been used in the majority of suspension bridges (see Figure 16.1.19). The top surfaces of the bands are grooved to receive the suspender cables, which are looped over the band.
Instead of looping the hanger cables around the main suspension cable, the hanger might be socketed at the upper end and pin connected to the cable band. This connection is called an open socket (see Figure 16.1.20). Connection to the deck and floor system can also be a similar open socket arrangement or it can be connected directly to a floorbeam - similar to the tied arch bridge.
Figure 16.1.19 Grooved Cable Bands
Figure 16.1.20 Open Socket Suspender Cable Connection
The flexibility of cable-supported structures, associated with high stress levels in the main load carrying members, makes these structures especially sensitive to dynamic forces caused by earthquake, wind, or vehicular loads. The term local vibration is used when dealing with the vibration in an individual member (see Figure 16.1.21). When the vibration of the entire structure as a whole is analyzed, it is known as global vibration (see Figure 16.1.22). Due to the amount of vibration in cable-supported structures, it may be common to see various types of damping systems attached to cables. Damping systems may be a tie between two cables, neoprene cushions, shock absorbers mounted directly to the cables, or other systems that act to dampen the cable vibrations (see Figures 16.1.23 and 16.1.24).
Figure 16.1.21 Cable Vibrations Local System Schematic
Figure 16.1.22 Cable Vibrations Global System Schematic
Vibrations can affect suspension cables in several ways. Vibration opens cable wires allowing entry of corrosive chemicals and accelerates corrosion. Vibrations create fretting, cracks in the protective coating and cement grout, and accelerate corrosion and possibly fatigue.
Figure 16.1.23 Cable Damping System - Wheeling Suspension Bridge (Photo Courtesy of Geoffrey H. Goldberg, 1999)
Figure 16.1.24 Cable Tie Damper System
Only the cable and its elements are described in this subtopic. Refer to the appropriate topic for other bridge elements that are common to similar bridge types (i.e. floor systems, open web girders, box sections, etc.).
Due to the complexity of the various cable arrangements and systems, fracture criticality for individual cable-stayed structures can only be determined through a detailed structural analysis.
Cable-stayed bridges may be categorized according to the various longitudinal cable arrangements. These cable arrangements are categorized into the following four basic systems:
In this system, all cables are leading to the top of the tower at a common point. Structurally, this arrangement is the most effective. By anchoring all the cables to the tower top, the maximum inclination to the horizontal is achieved (see Figure 16.1.25).
Figure 16.1.25 Radial or Converging Cable System Schematic
The harp system, as the name implies, resembles harp strings. In this system, the cables are parallel and equidistant from each other. The cables are also spaced uniformly along the tower height and connect to the deck floor system or superstructure at the same spacing (see Figure 16.1.26).
Figure 16.1.26 Harp or Parallel Cable System Schematic
The fan system is a combination of the radial and the harp systems. The cables emanate from the top of the tower at equal spaces and connect to the superstructure at larger equal spaces (see Figure 16.1.27).
Figure 16.1.27 Fan or Intermediate Cable System Schematic
In the star system, the cables intersect the tower at different heights and then converge on each side of the tower to intersect the deck structure at a common point. The common intersection in the anchor span is usually located over the abutment or end pier. The star system is uncommon compared to the three systems previously presented. The star system requires a much stiffer superstructure since the cables are not distributed longitudinally along the deck and superstructure (see Figure 16.1.28).
Figure 16.1.28 Star Cable System Schematic
The cables may lie in either a single or a double plane, may be symmetrical or asymmetrical, and may lie in oblique or vertical planes.
The single-plane cable arrangement is used with a divided deck structure with the cables passing through the median area and anchored below the deck. A single-plane cable system generally utilizes single column or A-frame towers (see Figure 16.1.29).
Figure 16.1.29 Single Vertical Plane Cable System
The double vertical plane system incorporates two vertical cable planes connecting the tower to the edge girders along the deck structure. The structure may utilize twin towers or a portal frame tower (see Figure 16.1.30). The portal frame tower is a twin tower with a connecting strut at the top. Wider bridges may utilize a triple plane system that is basically a combination of the single and double plane systems.
Figure 16.1.30 Double Vertical Plane Cable System
In this two plane system the cable planes are oblique, sloping toward each other from the edges of the deck and intersecting at the tower along the longitudinal centerline of the deck (see Figure 16.1.31). Generally the tower is an A-frame type, receiving the sloping cables that intersect close to the centerline on the tower.
Figure 16.1.31 Double Inclined Plane Cable System
The cables may be continuous and pass through or over the tower or be terminated at the tower. If continuous across the tower, a saddle is incorporated.
The cable saddles are constructed from fabricated plates or steel castings with grooves through which the cables pass (see Figure 16.1.32). Between the end and center spans differential forces will occur at the cable saddles unless they are supported by rollers or rocker bearings. When the saddles are fixed, the rigidity of the system is at the maximum.
If terminated at the tower, an end fitting or anchorage is incorporated. A similar end fitting is utilized at the edge girder (see Figure 16.1.33).
Figure 16.1.33 Cable Deck Anchorage
A socket widely used for the anchoring of parallel-wire strands is a poured zinc socket. The wires are led through holes in a locking plate at the end of the socket and have the bottom heads providing the resistance against slippage of wires. The cavity inside the socket is filled with hot zinc alloys. To improve the fatigue resistance of the anchor, a cold casing material is used. The zinc cools and locks the wire strands into the socket.
Figure 16.1.34 Anchor Inspection on Veterans Bridge
The problems encountered with low fatigue strength of zinc-poured sockets lead to the development of HiAm sockets in 1968 for use with parallel wire stays.
This anchorage incorporates a flat plate with countersunk radial holes to accommodate the geometry of flared wires that transition from the compact wire bundle into the anchorage. The anchorage socket is filled with zinc dust and with an epoxy binder. This method of anchoring the stays increases the magnitude of fatigue resistance to almost twice that for the zinc-poured sockets.
A common anchorage type for strands is the Freyssinet type anchor.
In the Freyssinet socket the seven wire strand is anchored to an anchor plate using wedges similar to prestressing wedges. This wedge anchor is used during erection. After application of the permanent dead load, the anchor tube is filled with an epoxy resin, zinc dust, and steel ball composition. Under transient live load, the additional cable force will be transformed by shear from the cable strand to the tube.
Several of the primary causes of vibration in stay cables consist of rain-wind induced vibrations, sympathetic vibration of cables with other bridges elements excited by wind, inclined cable galloping, and vortex excitation of single cable or groups of cables. Due to the amount of vibration in cable-supported structures, it may be common to see various types of damping systems attached to cables. Damping systems may be a tie between two cables, neoprene cushions, shock absorbers mounted directly to the cables, or other systems that act to dampen the cable vibrations (see Figure 16.1.35).
Vibrations can affect stay cables in several ways. Vibration opens cable wires allowing entry of corrosive chemicals and accelerates corrosion. Vibrations create fretting, cracks in the protective coating and cement grout, and accelerate corrosion and possibly fatigue.
Figure 16.1.35 Damper on Cable-Stayed Bridge
Common deficiencies that can occur on steel cable members:
Refer to Topic 6.3.5 for a more detailed presentation of the properties of steel, types and causes of steel deficiencies, and the examination of steel.
The inspection and maintenance methods presented in this Topic are not exhaustive, but are unique to the particular bridge type. Therefore, include both the procedures presented in this Topic as well as the general procedures previously presented in this manual during the inspection of special bridges.
These bridges are considered to be complex according to the NBIS regulations. The NBIS requires identification of specialized inspection procedures, and additional inspector training and experience required to inspect these complex bridges. The bridges are then to be inspected according to these procedures.
Due to the specialized nature of these bridges and because no two cable-supported bridges are identical, the inspection should be led by someone very familiar with the particular bridge. Many major bridges, such as cable-supported bridges, will have individual inspection and maintenance manuals developed specifically for that bridge, like an "owner's" manual. If available, use this valuable tool throughout the inspection process and verify that specified routine maintenance has been performed. Use customized, preprinted inspection forms wherever possible to enable the inspector to report the findings in a rigorous and systematic manner.
The anchorage system, at the ends of the main cables, consists of a number of elements that require inspection (see Figure 16.1.36).
Figure 16.1.36 Anchor Block Schematic
Inspect the splay saddles for missing or loose bolts and the presence of cracks in the casting itself. There is a possibility of movement up the cable away from the splay. Signs of this movement may be the appearance of unpainted strands on the lower side or “bunched up” wrapping on the upper side
In parallel wire type suspension bridges, inspect the unwrapped wires between the strand shoes and the splay saddle. Carefully insert a large screwdriver between the wires and apply leverage. This will help reveal broken wires. Inspect the wires for abrasion damage, corrosion, and movement.
At the anchorages of parallel wire type suspension bridges, inspect the strand shoes for signs of displaced shims, along with movement, corrosion, misalignment, and cracks in the shoes.
At the anchorages of prefabricated strand type suspension bridges, inspect the strand sockets for signs of movement, slack or sag, corrosion, and broken sockets. Unpainted or rusty threads at the face of the sockets may indicate possible “backing off” of the nuts.
Inspect the anchor bars or rods for corrosion (section loss), deficiencies, or movement at the face of their concrete embedment. Check for corrosion or other signs of distress over the entire visible (unencased) portion.
Inspect the interior of the anchorage for corrosion and deficiencies of any steel hardware, and cracks and spalls in the concrete anchor. Note if there is protection against water entering or collecting where it may cause corrosion, and also if there is proper ventilation (see Figure 16.1.37).
Figure 16.1.37 Anchorage Interior of Ben Franklin Bridge, Philadelphia, PA
Inspect the main suspension cables as follows:
Inspect the main suspension cables for indications of corroded wires. Inspect the condition of the protective covering or coating, especially at low points of cables, areas adjacent to the cable bands, saddles over towers, and at anchorages.
Inspect the wrapping wire for cracks, staining, and dark spots. Check for loose wrapping wires. If there are cracks in the caulking where water can enter, this can cause corrosion of the main suspension cable. Check for evidence of water seepage at the cable bands, saddles, and splay castings (see Figure 16.1.38).
Figure 16.1.38 Tape and Rubber Seal Torn Around Cable Allowing Water Penetration into Top of Sheath
Inspect the hand ropes and connections along the main cables for loose connections of stanchion (hand rope supports) to cable bands or loose connections at anchorages or towers. Check also for corroded or deteriorated ropes or stanchions, bent or twisted stanchions, and too much slack in rope.
Note and record all excessive vibrations.
Inspect the saddles for missing or loose bolts, and corrosion or cracks in the casting. Check for proper connection to top of tower or supporting member and possible slippage of the main cable.
Inspect the suspender cables for corrosion or deficiencies, broken wires, and kinks or slack. Check for abrasion or wear at sockets, saddles, clamps, and spreaders. Be sure to note excessive vibrations.
Inspect the suspender rope sockets for:
Inspect the cable bands for missing or loose bolts, or broken suspender saddles. Signs of possible slippage are caulking that has pulled away from the casting or “bunching up” of the soft wire wrapping adjacent to the band. Check for the presence of cracks in the band itself, corrosion or deficiencies of the band, and loose wrapping wires at the band.
Prepare a set of customized, preprinted forms for documenting all deficiencies encountered in the cable system of a suspension bridge. A suggested sample form is presented in Figure 16.1.39. Separate forms are to be used for each main suspension cable. Designations used to identify the suspender ropes and the panels provide a methodology for locating the problems in the structure. Note and describe vibrations whether local or global, while performing inspections of cable-supported structures.
Figure 16.1.39 Form for Recording Deficiencies in the Cable System of a Suspension Bridge
A cable-stayed bridge is a bridge in which the superstructure is supported by cables, or stays, passing over or attached directly to towers located at the main piers (see Figures 16.1.40 and 16.1.41). There are several special elements that are unique to cable-stayed bridges.
See the National Cooperative Highway Research Program (NCHRP) Synthesis 353 “Inspection and Maintenance of Bridge Cable Systems”, 2005 for a detailed description of inspection locations and procedures for cable-stayed bridge cable element systems.
These bridges are considered to be complex according to the NBIS regulations. The NBIS requires identification of specialized inspection procedures, and additional inspector training and experience required to inspect these complex bridges. The bridges are then to be inspected according to these procedures.
Figure 16.1.40 Cable-Stayed Bridge
Figure 16.1.41 Cable-Stayed Bridge Cables
Cable element inspection includes:
Common wrapping methods for corrosion protection of finished cables include spirally wound soft galvanized wire, neoprene, or plastic wrap type tape (see Figure 16.1.42). Inspect the wrappings for corrosion and cracking of soft galvanized wire, staining and dark spots indicating possible corrosion of the cables, and loose wrapping wires or tape. Bulging or deforming of wrapping material may indicate possible corrosion or broken wires (see Figure 16.1.43). Check for evidence of water seepage at the cable bands, saddles, and castings.
Figure 16.1.42 Cable Wrapping Placement
Figure 16.1.43 Deformed Cable Wrapping
The most common types of cable sheathing assemblies are steel sheathing and polyethylene sheathing.
If steel sheathing is used, inspect the system for corrosion (see Figure 16.1.44), condition of protective coatings, and weld fusion. Bulging may indicate corrosion or broken wires (see Figure 16.1.45). Splitting may be caused by water infiltration and corrosive action. Cracking is sometimes caused by fatigue (see Figure 16.1.46).
If polyethylene sheathing is used, inspect the system for nicks, cuts, and abrasions. Check for cracks and separations in caulking and in fusion welds. Bulging may indicate broken wires (see Figure 16.1.45). Splitting is sometimes caused by temperature fluctuations (see Figure 16.1.47). Coefficient of the thermal expansion for polyethylene is three times higher than the value for steel or concrete. Cracking is sometimes caused by fatigue.
Figure 16.1.44 Corrosion of Steel Sheathing
Figure 16.1.45 Bulging of Cable Sheathing
Figure 16.1.46 Cracking of Cable Sheathing
Figure 16.1.47 Splitting of Cable Sheathing
A variety of damper types may have been installed (see Figure 16.1.48 and 16.1.49). If shock absorber type dampers are used, inspect the system for corrosion, oil leakage in the shock absorbers, and deformations in the bushings. Check for tightness in the connection to the cable pipe, and torque in the bolts.
Figure 16.1.48 Shock Absorber Damper System
Figure 16.1.49 Shock Absorber Damper System
Inspect the tie type dampers (see Figure 16.1.50) for corrosion, and deformations in the bushings. Check for tightness in the connection to the cable pipe, and torque in the bolts.
Figure 16.1.50 Cable Tie Type Damper System
Inspect the tuned mass dampers (see Figure 16.1.51) for corrosion, and deformations in the bushings. Check for tightness in the connection to the cable pipe, and torque in the bolts.
Figure 16.1.51 Tuned Mass Damper System
Inspect the transition area between the steel anchor pipe and cable for water tightness of neoprene boots at the upper ends of the steel guide pipes (see Figure 16.1.52). Check for drainage between the guide pipe and transition pipe, and deteriorations, such as splits and tears, in the neoprene boots (see Figure 16.1.53). Check for sufficient clearance between the anchor pipe and cable, noting rub marks and kinks.
Figure 16.1.52 Neoprene Boot at Steel Anchor Pipe Near Anchor
Figure 16.1.53 Split Neoprene Boot
Inspect the cable anchorages for corrosion of the anchor system (see Figure 16.1.54). Check for cracks and nut rotation at the socket and bearing plate, and seepage of grease from the protective hood.
Figure 16.1.54 Corrosion of the Anchor System
Include anchor pipe clearances, flange joints, and polyethylene expansion joints within the inspection of the cable system. Read the load cells and record the forces in the cables. Note and record all excessive vibrations including amplitude and type of vibration along with wind speed and direction, or other forces including vibrations such as traffic. Also evaluate cable and tower lighting systems.
Prepare a set of customized, preprinted forms for documenting all deficiencies encountered in the cable system of a cable-stayed bridge. A suggested sample form is presented in Figure 16.1.55. Use a separate form for each plane or set of cables. Designations used to identify the cables and the panels provide a methodology for locating the deficiencies in the structure. Note and describe vibrations whether local or global, while performing inspections of cable-supported structures.
Figure 16.1.55 Form for Recording Deficiencies in Cable System of a Cable-Stayed Bridge
In bridge cables, whether a suspension bridge or cable-stayed bridge, the greatest problems generally occur due to corrosion and fracture of individual wires. Visual inspection of unwrapped cables is limited to the outer wires, while visual inspection of wrapped cables is limited to the protective sheathing. Therefore, advanced inspection methods are used to achieve a more rigorous and thorough inspection of steel bridges, including:
See Topic 15.3 for Advanced Inspection Procedures for steel.
Other methods specific to cables include:
State and Federal rating guideline systems have been developed to aid in the inspection of steel superstructures. The two major rating guideline systems currently in use are the FHWA's Recording and Coding Guide for the Structural Inventory and Appraisal of the Nation’s Bridges used for the National Bridge Inventory (NBI) component condition rating method and the AASHTO Manual for Bridge Element Inspection for element level condition state assessment.
Using the NBIS component condition rating guidelines, a one-digit code on the Federal Structure Inventory and Appraisal (SI&A) sheet indicates the condition of the superstructure. Component condition rating codes range from 9 to 0 where 9 is the best rating possible. See Topic 4.2 (Item 59) for additional details about NBIS component condition rating guidelines.
Consider previous inspection data along with current inspection findings to determine the correct component condition rating.
In an element level condition state assessment of a cable-supported bridge, possible AASHTO National Bridge Elements (NBEs) and Bridge Management Elements (BMEs) are:
NBE No. |
Description |
|---|---|
Superstructure |
|
147 |
Steel Primary Cable |
148 |
Steel Secondary Cable |
149 |
Other Secondary Cable |
BME No. |
Description |
|---|---|
Wearing Surfaces and Protection Systems |
|
515 |
Steel Protective Coating |
The unit quantity for steel primary cables is feet and the unit quantity for secondary cables is each. The cables are distributed among the four available condition states depending on the extent and severity of the deficiency. The unit quantity for protective coating is square feet, and the total area is distributed among the four available condition states depending on the extent and severity of the deficiency. The sum of all condition states equals the total quantity of the National Bridge Element or Bridge Management Element. Condition State 1 is the best possible rating. See the AASHTO Manual for Bridge Element Inspection for condition state descriptions.
See Chapters 9 and 10 for the inspection and evaluation of concrete and steel girders, floorbeams and stringers.
See Chapter 7 for the inspection and evaluation of decks.
See Chapter 12 for the inspection and evaluation of abutments, piers and bents.
This topic serves as an introduction to the highly specialized area of movable bridge inspection. It focuses on the types of movable bridges and special elements associated with the various bridge types.
Each of these specialized bridge types has their unique features and unique mechanisms for movement. The following sections cover common types of movable bridges, including information on the operation, maintenance and inspection of the structure and the movement mechanisms. The inspection of these specialized bridges requires a diverse team collectively capable of inspecting the structure as well as the mechanical, electrical, pneumatic or other movement mechanisms. The duties of the bridge inspector are defined by the bridge owner for the inspection of these type structures and should be complemented, as necessary, by duties of other inspectors for the inspection of the movement mechanisms and by the duties for maintenance and operation personnel. The bridge inspector should confirm with the owner their role in the inspection of these specialized structures (see Figure 16.2.1).
Movable bridges are normally constructed only when fixed bridges are either too expensive or impractical. Movable bridges are constructed across designated “Navigable Waters of the United States”, in accordance with “Permit Drawings” approved by the U.S. Coast Guard. When a movable bridge is fully open, it must provide the channel width and the underclearance shown on the Permit Drawings (see Figure 16.2.2). If the bridge cannot be opened to provide these clearances, notify the U.S. Coast Guard immediately and take action to restore the clearances. If that is impossible, an application must be submitted to revise the Permit Drawings.
Figure 16.2.2 Typical “Permit Drawing” Showing Channel Width and Underclearance in Closed and Open Position
If any work is to be done in the channel or on the movable span to reduce the clearances from those shown on the Permit Drawing, obtain an additional permit, from the U.S. Coast Guard District covering the scheduled time for the work.
The U.S. Coast Guard publishes Local Notices to Mariners to keep waterway users informed of work in progress that may affect navigation. The permitee keeps the U.S. Coast Guard informed of all stages of construction.
Verify that the bridge conforms to the Permit Drawing and that the operator is instructed to open the bridge to the fully open position every time the bridge is operated. Failure to do this would establish a precedent that a vessel is expected to proceed before the green navigation lights have turned “on”. Any accident caused as a result of this practice could be ruled the fault of the bridge owner.
Early America's engineering literature did not establish where the first iron drawbridge was built. The first all-iron movable bridge in the Midwest was completed in 1859 carrying Rush Street over the Chicago River (see Figure 16.2.3). The bridge was a rim bearing swing span and was probably operated by steam. It was destroyed on November 3, 1863 when it was opened while a drove of cattle was on one end. It was rebuilt but destroyed by the great Chicago fire of 1871.
Figure 16.2.3 The First All-Iron Movable Bridge in the Midwest was Completed in 1859 (Photo on File at the Chicago Historical Society)
All categories of movable bridges are powered by electric-mechanical or hydraulic-mechanical drives with power driven pinions operating against racks, or by hydraulic cylinders. A small number are hand powered for normal operation. A few bridges use hand power for standby operation. Three categories of movable bridges comprise over 95 percent of the total number of movable bridges within the United States. These categories include:
Swing bridges consist of two-span trusses or continuous girders, which rotate horizontally about the center (pivot) pier (see Figure 16.2.4). The spans are usually, but not necessarily, equal. When open, the swing spans are cantilevered from the pivot (center) pier and must be balanced longitudinally and transversely about the center. When closed, the spans are supported at the pivot pier and at two rest (outer) piers or abutments. In the closed condition, wedges are usually driven under the outer ends of the bridge to lift them, thereby providing a positive reaction sufficient to offset any possible negative reaction from live load and impact in the other span. This design feature prevents uplift and hammering of the bridge ends under transient live load conditions.
Figure 16.2.4 Center-Bearing Swing Bridge
Swing spans are subdivided into two types:
Center-bearing swing spans carry the entire load of the bridge on a central pivot (usually metal discs). Balance wheels are placed on a circular track around the outer edges of the pivot pier to prevent tipping (see Figures 16.2.5 and 16.2.6). When the span is closed, wedges similar to those at the rest piers are driven under each truss or girder at the center pier. This relieves the center bearing from carrying any live load. However, these wedges do not raise the span at the pivot pier, but are merely driven tight.
Figure 16.2.5 Center-Bearing Swing Span in Closed Position
Figure 16.2.6 Layout of Center-Bearing Type Swing Span with Machinery on the Span
Rim-bearing swing spans transmit all loads, both dead and live, to the pivot pier through a circular girder or drum to beveled rollers. The rollers move on a circular track situated inside the periphery of the pier. The rollers are aligned and spaced on the track by concentric spacer rings. This type of swing span bridge also has a central pivot bearing which carries part of the load. This pivot bearing is connected to the rollers by radial roller shafts and keeps the span centered on the circular track.
On both types of swing bridges, the motive power is usually supplied by electric motor(s), hydraulic motor(s), or hydraulic cylinder(s), although gasoline engines or manual power may also be used. The bridge is rotated horizontally by a circular rack and pinion arrangement, or cylinders.
Bascule bridges open by rotating a leaf or leaves (movable portion of the span) from the normal horizontal position to a point that is nearly vertical, providing an open channel of unlimited height for marine traffic (see Figure 16.2.7).
Figure 16.2.7 Bascule Bridge in the Open Position
If the channel is narrow, a single span may be sufficient. This is called a single-leaf bascule bridge. For wider channels, two leaves are used, one on each side of the channel. When the leaves are in the lowered position, they meet at the center of the channel. This is known as a double-leaf bascule bridge.
A counterweight is necessary to hold the raised leaf in position. In older bridges, the counterweight is usually overhead, while in more modern bascule bridges, the counterweight is placed below the deck and lowers into a pit as the bridge is opened.
The leaf lifts up by rotating vertically about a horizontal axis. The weight of the counterweight is adjusted by removing or adding balance blocks in pockets to position the center of gravity of the moving leaf at the center of rotation. When the bridge is closed, a forward bearing support located in front of the axis is engaged and takes the live load reaction. On double-leaf bascule bridges, a tail-lock behind the axis and a shear lock at the junction of the two leaves are also engaged to stiffen the deck.
There are many types of bascule bridges, but the most common are the following three types:
The first rolling lift bridge was completed in 1895 in Chicago, and was designed by William Scherzer. The entire moving leaf, including the front arm with the roadway over the channel and the rear arm with the counterweight, rolls away from the channel while the moving leaf rotates open (see Figures 16.2.8 and 16.2.9). On this type of bridge, curved tracks are attached to each side of the tail end of the leaf. The curved tracks roll on flat, horizontal tracks mounted on the pier. Square or oblong holes are machined into the curved tracks. The horizontal tracks have lugs (or teeth) to mesh with the holes preventing slippage as the leaf rolls back on circular castings whose centerline of roll is also the center of gravity of the moving leaf.
Figure 16.2.8 Rolling Lift Bascule Bridge Schematic
Figure 16.2.9 Double-Leaf Rolling Lift Bascule
The simple principal of this type of bridge can be seen easiest with a railroad bridge. The dead load of the bridge is balanced about the centerline of the drive pinion (center of roll). The pinion teeth are engaged with the teeth on the rack casting. When the pinion turns it moves along on the fixed rack and causes the span to rotate on the circular tread casting as it rolls back on the track casting.
The weight of the leaf, including the superstructure and counterweight, is supported by the curved tracks resting on the horizontal tracks. The counterweight is positioned to balance the weight of the leaf.
On one variation of this type, the trusses on the two leaves acted as three-hinged arches when closed. Locks are engaged in the closed position, allowing the bridge to function as a simple span. In the open position, the leaves operate as a cantilever span. There is a 310 feet span between the centerline of bearings. This bridge was built across the Tennessee River at Chattanooga in 1915, and it is believed to be the third longest double-leaf bascule in the world. It provides an 295 foot channel, which is the widest channel spanned by a bascule bridge.
The Chicago Bridge Department staff of Engineers built the first Chicago type simple trunnion bascule bridge in 1902. This type of bascule bridge consists of a forward cantilever arm out over the channel and a rear counterweight arm (see Figure 16.2.10). The leaf rotates about the trunnions. Each trunnion is supported on two bearings, which in turn, are supported on the fixed portion of the bridge such as trunnion cross-girder, steel columns, or on the pier itself (see Figures 16.2.11, 16.2.12 and 16.2.13). Forward bearing supports located in front of the trunnions are engaged when the leaf reaches the fully closed position. They are intended to support only live load reaction. Uplift supports are located behind the trunnions to take uplift until the forward supports are in contact (if misadjusted) and to take the live load uplift that exceeds the dead load reaction at the trunnions. If no forward live load supports are provided or if they are grossly misadjusted, the live load and the reaction at the uplift supports are added to the load on the trunnions. A double-leaf bascule bridge of this type in Lorain, Ohio has 333 feet between trunnions. Of the three types of movable bridges, the simple trunnion is by far the most popular.
Figure 16.2.10 Trunnion Bascule Bridge Schematic
Figure 16.2.11 Double-Leaf Trunnion Bascule Bridge
Figure 16.2.12 Each Trunnion is Supported on Two Bearings
Figure 16.2.13 Trunnion Bascule Bridge Machinery (One Quarter Shown) is Located Outside of the Bascule Trusses on the Pier
The first multi-trunnion (Strauss) bascule bridge was designed by J.B. Strauss and completed during 1905 in Cleveland, Ohio. There are many variations of multi-trunnion bascule bridges, but basically one trunnion supports the moving span, one trunnion supports the counterweight, and two link pins are used to form the four corners of a parallelogram-shaped frame that changes angles as the bridge is operated. The counterweight link keeps the counterweight hanging vertically from the counterweight trunnions while the moving leaf rotates about the main trunnions (see Figure 16.2.14). One variation of this parallelogram layout is the heel trunnion. A double-leaf bascule bridge of this type in Sault St. Marie, Michigan has 336 feet between the span trunnions. It was built across the approach to a lock in 1914, and it is believed to be the longest double-leaf bascule in the world.
Figure 16.2.14 Multi-Trunnion, Strauss Type Bascule Bridge
Vertical lift movable bridges have a movable span with a fixed tower at each end. The span is supported by steel wire ropes at its four corners. The ropes pass over sheaves (pulleys) atop the towers and connect to counterweights on the other side. The counterweights descend as the span ascends (see Figure 16.2.15).
Figure 16.2.15 Vertical Lift Bridge Schematic
There are two basic types of vertical lift bridges:
The first vertical lift bridge completed during 1894 in Chicago was designed by J.A.L. Waddell. This bridge type locates the power on top of the lift truss span. The actual lifting is accomplished using “up-haul and down-haul ropes” where turning drums wind the up-haul (lifting) ropes as they simultaneously unwind the down-haul ropes. Vertical lift bridge machinery is located on top of the lift truss span, and the operating drums rotate to wind the up-haul (lifting) ropes as they simultaneously unwind the down-haul ropes (see Figure 16.2.16). A variation of this type provides drive pinions at both ends of the lift span which engage racks on the towers.
Figure 16.2.16 Vertical Lift Bridge Machinery is Located on Top of the Lift Truss Span, and the Operating Drums Rotate to Wind the Up-Haul (Lifting) Ropes as They Simultaneously Unwind the Down-Haul Ropes
The other basic type of vertical lift bridge locates the power on top of both towers, where drive pinions operate against circular racks on the sheaves. The lifting speed at both towers must be synchronized to keep the span horizontal as it is lifted (see Figures 16.2.17 and 16.2.18).
Figure 16.2.17 Vertical Lift Bridge Machinery is Located on the Towers, and the Rim Gears (and Operating Sheaves) are Rotated to Raise and Lower the Bridge
Figure 16.2.18 Vertical Lift Bridge with Power and Drive System on Towers
Give particular attention to the special elements found in swing bridges, bascule bridges, and vertical lift bridges during inspection. These elements are commonly found on all types of movable bridges:
Open gearing is used to transmit power from one shaft to another and to alter the speed and torque output of the machinery. Beveled gears are also used to change direction (see Figure 16.2.19).
Speed reducers including differentials serve the same function as open gearing (see Figure 16.2.20). However, they may contain several gear sets, bearings, and shafts to provide a compact packaged unit, which protects its own mechanical elements and lubrication system with an enclosed housing. Differential speed reducers also function to equalize torque and speed from one side of the mechanical operating system to the other.
Shafts transmit mechanical power from one part of the machinery system to another. Couplings transmit power between the ends of shafts in line with one another, and several types can be used to compensate for slight imperfections in alignment between the shafts (see Figure 16.2.21).
Bearings provide support and prevent misalignment of rotating shafts, trunnions, and pins (see Figure 16.2.22).
Brakes can be of either the shoe type or disc type, and can be released manually, electrically, or hydraulically (see Figures 16.2.23 and 16.2.24). They are generally spring applied for fail safe operation. Motor brakes are located close to the drive to provide dynamic braking capacity, except that some types of drives can provide their own braking capability, thereby eliminating the need for separate motor brakes. Machinery brakes are located closer to the operating interface between movable and fixed parts of the bridge and are used to hold the span statically, in addition to serving as emergency brakes in many cases. Supplemental emergency brakes are sometimes also provided.
Figure 16.2.23 Shoe Type Break
Figure 16.2.24 Spring Set Hydraulically Released Disc Break
Drives can consist of electric motors, hydraulic equipment, or auxiliary drives.
For electric motors, either AC or DC power may be used. AC power is often used to power wound rotor motors with torque controllers on older bridges, while new bridges may utilize squirrel cage induction motors with adjustable frequency speed control. DC motors can also provide speed control.
For hydraulic equipment, prime movers may include either large actuating cylinders or hydraulic motors (see Figure 16.2.25). Either type of drive must be supplied with pressure to provide force and fluid flow to provide speed to the operating system. Electrically operated hydraulic power units consisting of a reservoir and pump, with controls, provide power to the operating systems.
For auxiliary drives, emergency generators are provided to serve in the event of power failure. Auxiliary motors and hand operators, with their clutches and other mechanical power transmission components, are provided to serve in the event the main drive fails (see Figure 16.2.26). In some cases, to prevent the need for larger auxiliary generators, the auxiliary motors are required for use any time the auxiliary generators are used, requiring increased time of operation.
Figure 16.2.25 Low Speed High Torque Hydraulic Motor
Figure 16.2.26 AC Emergency Motor
Air buffers and shock absorbers are located between the span and the pier at points where impact may occur between the two (see Figures 16.2.27 and 16.2.28). A cross section of the buffer shows the air chamber and seals on the piston. As the span lowers, the rod is pushed in, causing the air inside to be compressed (see Figure 16.2.29). A pressure relief valve allows the air to escape beyond the pressure setting. Forces are required to build-up and keep the pressure of the air at the movement of the span for a “soft” touchdown on the bearings. Shock absorbers provide the same purpose as the air buffers. However, they are completely self-contained and, therefore, require very little maintenance.
Figure 16.2.29 Typical Air Buffer Schematic
Span lock bars at the end of the span are driven when the span is fully closed to prevent movement under live load. Span locks may also be provided at other locations on the span to hold the span in an open position against strong winds or to prevent movement from an intermediate position. They can be driven either mechanically or hydraulically (see Figures 16.2.30 and 16.2.31).
Figure 16.2.30 Typical Mechanically Operated Span Lock
Figure 16.2.31 Hydraulic Cylinder that Drives Lock Bars
Adjustable quantities of counterweight blocks are provided in addition to the permanent counterweight, which is part of the structure so that adjustments may be made from time to time due to changes in conditions (see Figures 16.2.32 and 16.2.33). A movable span is designed to function in a balanced condition, and serious unbalanced conditions will cause overstress or even failure of the mechanical or structural elements.
Figure 16.2.32 Concrete Counterweight on a Single-Leaf Bascule Bridge
Figure 16.2.33 Concrete Counterweight on a Vertical Lift Bridge
Live load shoes and strike plates between the movable and fixed portions of the bridge are designed to bear most or all of the live load when the bridge is carrying traffic (see Figure 16.2.34).
Figure 16.2.34 Closed Span Resting on Live Load Shoes
Traffic barriers are heavy-duty movable gates or posts that are designed to prevent a vehicle from plunging from the roadway into the draw or into the pit below the bridge (see Figure 16.2.35). Their operation is important for public safety. They are used mainly in situations where a large opening exists between the approach span and the movable span when it is open.
Figure 16.2.35 Traffic Barrier
Swing bridges are designed utilizing the following special elements:
In center-bearing types (with balance wheels), the axially loaded thrust bearing is usually composed of spherical discs, attached to top and bottom bases, enclosed in an oil box to provide lubrication and prevent contamination (see Figure 16.2.36). In rim-bearing types, the pivot bearing is also enclosed but will be radial loaded, maintaining the position of the pivot shaft or king pin.
Figure 16.2.36 Center Pivot Bearing
On center-bearing types only, non-tapered balance wheels bear on the circular rail concentric to the pivot bearing only when the span is subjected to unbalanced loading conditions (see Figure 16.2.37). At other times, when the span is not subjected to unbalanced loads, a gap will be present between each wheel and the rail.
Figure 16.2.37 Balance Wheel in-place over Circular Rack
Usually tapered to allow for the differential rolling distance between the inside and outside circumferences of the rail circle, rim-bearing rollers usually bear at all times.
End wedges are used to raise the ends of the span and support live load under traffic (see Figure 16.2.38). The end wedge bearings are under all four corners of the span. Center wedges are used to stabilize the center of the span and to prevent the center bearing from supporting live load. Wedges may be actuated by machinery and linkage, which connects wedges to actuate together or each wedge may have its own actuator (see Figure 16.2.39).
Figure 16.2.39 Hydraulic Cylinder Actuator
Located at the center of one or both rest piers, end latches generally consist of a guided tongue with roller mounted on the movable span that occupies a pocket mounted on the rest pier when the span is in the closed position. To open the span, the tongue is lifted until it clears the pocket at the time the wedges are withdrawn (see Figure 16.2.40). As the span is swung open, the latch tongue is allowed to lower or fall into a position in which the roller may follow along a rail or track mounted on the pier. When closing, the tongue rolls along the rail or track and up a ramp which leads to the end latch pocket where the tongue is allowed to drop to center the span.
Figure 16.2.40 End Wedges Withdrawn and End Latch Lifted
Bascule bridges utilize the following elements specific to their design:
Rolling lift tread and track castings are rolling surfaces which support the bascule leaves as they roll open or closed (see Figure 16.2.41). Tread sockets and track teeth prevent transverse and lateral movement of the span due to unbalanced conditions, such as wind, during operation and especially when held in the open position.
Figure 16.2.41 Circular Lift Tread and Track Castings
In the rolling lift rack and pinion, the driving pinion engages the rack teeth at the centerline of the roll (see Figure 16.2.42).
In the trunnion rack and pinion, the circular rack castings are attached in the plane of the truss (or girder) in front of the counterweight (see Figures 16.2.43 and 16.2.44).
The drive pinions are overhung in order to engage the rack teeth. A cover is placed over the pinions for safety and to keep debris from falling on it.
Figure 16.2.42 Rack Casting and Pinion
Figure 16.2.43 Rack Casting Ready for Fabrication
Trunnions and trunnion bearings (see Figure 16.2.45) are large pivot pins or shafts. Their bearings support the leaf as it rotates during operation as well as supporting dead load when the bridge is closed. Some designs require the trunnions to carry live load in addition to dead load (see Figure 16.2.46).
Figure 16.2.45 Trunnion Bearing
Figure 16.2.46 Trunnion Design Drawing
A Hopkins frame machinery arrangement is provided on some trunnion bascule bridges. The main drive pinion locations are established in relationship to their circular racks by a pivot point on the pier and pinned links attached to the trunnions.
Located at the rear of the bascule girder on the pier, tail locks prevent inadvertent opening of the span under traffic or under a counterweight-heavy condition if the brakes fail or are released (see Figure 16.2.47).
Figure 16.2.47 Rear Lock Assembly
Center locks are provided to transfer shear load from one leaf to the other when the bridge is under traffic. Center locks may consist of a driven bar or jaw from one leaf engaging a socket on the other leaf, or may be a meshing fixed jaw and diaphragm arrangement with no moving parts (see Figure 16.2.48).
The superstructure acts as a cantilever when opening and closing the bridge with the maximum negative moment near the supporting piers and zero moment at the ends of the cantilever. Once the bridge is lowered into position, center locks are engaged. These locking mechanisms are designed to transmit shear necessary to produce equal deflections at mid point under unbalanced transient loads. These center locks are not normally designed to carry superstructure moment.
Figure 16.2.48 Center Lock Jaws
In twin bascule bridges that are split longitudinally to allow flexibility during construction, repair, or rehabilitation; transverse locks between the inside girders are used to keep the pairs together during operation (see Figure 16.2.49). These are usually operated manually, as they are not normally released for long periods of time.
Figure 16.2.49 Transverse Locks on Underside can be Disengaged
Vertical lift bridges may utilize the following elements peculiar to their design:
Wire ropes and sockets include up-haul and down-haul operating ropes and counterweight ropes (see Figures 16.2.50 and 16.2.51). Ropes consist of individual wires twisted into several strands that are wound about a steel core. Fittings secure the ends of the rope and allow adjustments to be made.
Figure 16.2.51 Wire Rope Sockets and Fittings
Drums are used to wind a rope several times around to extend or retract portions of the bridge (see Figure 16.2.52). Pulleys and sheaves change the direction of the rope or guide it at intermediate points between ends of the rope.
Figure 16.2.52 Drums Wind Up the Up-Haul (Lifting) Ropes as they Simultaneously Unwind the Down-Haul Ropes
Span and counterweight guides are located between tower and span or counterweight to prevent misalignment.
Balance chains are provided to compensate for the weight of counterweight rope that travels from the span side to the counterweight side of the sheaves at the top of the tower as the span is raised. Weight of chain is removed from the counterweight and is supported by the tower as rope weight is increased on the counterweight side of the sheaves on the tower.
Mechanical or electrical, span leveling devices compensate and adjust the movement of the two ends of the span during operation to prevent unsynchronized movement.
Common deficiencies that can occur to steel members of movable bridges include:
See to Topics 6.3.4 – 6.3.7 for a detailed presentation of the properties of steel, types and causes of steel deficiencies, and the examination of steel. Refer to Topic 6.4 for Fatigue and Fracture in Steel Bridges.
Common deficiencies that occur to concrete members of movable bridges include:
Refer to Topics 6.2.3 – 6.3.8 for a detailed explanation of the properties of concrete, types and causes of concrete deficiencies, and the examination of concrete.
It is imperative that a movable bridge inspectors coordinate their work with the Bridge Operator and emphasize the need for advance warning of a bridge opening. The Bridge Operator cannot operate the bridge until being notified by all inspectors that they are ready for an opening. There are many ways that this can be accomplished, such as placing a warning note on the control console or opening the circuit breakers and locking the compartment to the equipment that they will be inspecting.
Important considerations for a movable bridge inspector include observing and making comments in the inspection report on the following safety considerations.
Public safety considerations include good visibility of roadway and sidewalk for the Bridge Operator (see Figure 16.2.53), adequate time delay on traffic signals for driver reaction and before lowering gates, all “gates down” before raising bridge (bypass available if traffic signals are on), the bridge is closed before gates can be raised (bypass available if locks are driven), and traffic signals do not turn off until all gates are fully raised (bypass available).
Observe the location of the bridge opening in relation to the gates, traffic lights and bells, and determine whether approaching motorists can easily see them. Check their operation and physical condition to determine if they are functioning and well maintained. Recommend replacement when conditions warrant.
Unprotected approaches, such as both ends of a swing bridge and vertical lift bridge and the open end of a single-leaf bascule bridge, preferably have positive resistance barriers across the roadway, with flashing red lights as provided on the gate arms (see Figure 16.2.54). High-speed roadways and curved approaches to a movable bridge preferably have advanced warning lights (flashing yellow).
Figure 16.2.53 Operator’s House with Clear View of Traffic Signals and Lane Gates
Figure 16.2.54 Traffic Control Gate
Navigational safety considerations include compliance with minimum channel width with any restriction on vertical clearance when span is open for navigation. Minimum underclearance designated on the permit drawing are to be provided. Inspect underclearance gauges for closed bridges for accuracy, visibility, and legibility.
See that all navigation lights have a relay for backup light, and red span lights do not change to green until both leaves are fully open (see Figure 16.2.55). Check navigation lights for broken lenses, deteriorated insulation of wiring and cable, and dry and clean interior, as these lights are very important to navigational safety.
Check that the marine radio communication equipment is functional (see Figure 16.2.56). Verify that the Operator can automatically sound the emergency signal to navigation vessels if bridge cannot be opened.
Figure 16.2.55 Navigational Light
Figure 16.2.56 Marine Two-Way Radio Console
Structure safety considerations include the structural ability to carry the anticipated loads. Pressure relief valves on hydraulic power units are used to limit hydraulic forces applied to machinery and structure. Horsepower applied to machinery and structure are to be kept within design limits by limiting speed.
Operate the movable bridge in both normal and emergency modes to check all interrelated interlocks and to verify every component is operating correctly.
Movable bridges are considered to be complex according to the NBIS regulations. The NBIS requires identification of specialized inspection methods, and additional inspector training and experience required to inspect these complex bridges. The bridges are then to be inspected according to these methods.
During normal operation, verify that each interlock functions properly and can be bypassed (when provided). Verify the controls for the traffic signals, traffic gates, center or rear locks, emergency brakes, and the bridge operation are interlocked so that they can only be operated in the following sequences.
The bridge opening sequence:
The bridge closing sequence:
Inspection of the control house is necessary to assure the safety of a movable bridge. The operator is responsible for public and navigational safety during operation and, together with maintenance personnel, is usually the most familiar with any known structural or operational issues. Operational and maintenance log books are to be kept in the control house for reference. The resources within the control house can therefore provide a great deal of general information, through the knowledge of its personnel and the records stored there. The position of the control house provides the best general view of the bridge itself.
Consult with the bridge operators to ascertain whether there are any changes from the normal operation of the bridge. Note whether all Coast Guard, Corps of Engineers, and local instructional bulletins are posted. Check for obvious hazardous operating conditions involving the safety of the operator and maintenance personnel.
Note where the control panel is located in relation to roadway and waterway, and also whether the bridge operator has a good view of approaching boats, vehicles, and pedestrians (see Figure 16.2.57). Check operation of all closed circuit TV equipment, and evaluate its position for safe operation. If controls are in more than one location, note description of the other locations and include their condition as well as the information about the control house. Note whether alternate warning devices such as bullhorns, lanterns, flasher lights, or flags are available.
Note whether the structure shows cracks, and determine whether it is windproof and insulated. Check for any accumulations of debris, which may be readily combustible. Check controllers while bridge is opening and closing. Look for excess play and for sparking during operation. Note whether the submarine cables are kinked, hooked, or deteriorated, especially at the exposed area above or below the water. In tidal areas, check for marine and plant growth. Note if the ends of the cable have been protected from moisture.
During the inspection of any type of movable structures, be sure to note any deficiencies that are detrimental to all steel and concrete structures. Most of the bridge structure deficiencies are listed in Chapter 6: Materials, as potential problems apply to movable spans also.
Fatigue can be a problem with movable bridges due to the reversal or the fluctuation of stresses as the spans open and close (see Figure 16.2.58). Carefully inspect any member or connection subject to such stress variations for signs of fatigue.
Figure 16.2.58 Stress Reversals in Members
Inspect the counterweights to determine if they are sound and are properly affixed to the structure. Also check temporary supports for the counterweights that are to be used during bridge repair and determine their availability in the event such an occasion arises. Determine whether the counterweight pockets are properly drained. On vertical lift bridges, be sure that the sheaves and their supports are well drained. Examine every portion of the bridge where water can collect. All pockets that are exposed to rain and snow are to have a removable cover. Check for debris, birds, animals, and insect nests in the counterweight pockets.
Where steel members pass through or are embedded in the concrete, check for any corrosion of the steel member and for rust stains on the concrete. Look for cracks and spalls in the concrete.
Where lift span counterweight ropes are balanced by chains (or other means), make sure the links hang freely, and check these devices along with slides, housings, and storage devices for deficiencies and for adequacy of lubrication, where applicable.
Determine whether the bridge is balanced and whether extra balance blocks are available. A variation in the power demands on the motor, according to the span's position, is an indication of an unbalanced leaf or span. If the controls provide a “drift” position, use this to test the balance. Several coats of paint can increase the structure dead load. Otherwise, the counterweights will eventually be inadequate due to excess paint dead load.
Take notice of any rocking of the piers when the leaf is lifted. This is an indicator of a serious deficiency or critical finding and is to be reported at once. Survey the spans including towers to check both horizontal and vertical displacements. This will help to identify any foundation movements that have occurred.
Check the braces, bearings, and all housings for cracks, especially where stress risers would tend to occur. Inspect the concrete for cracks in areas where machinery bearing plates or braces are attached (see Figure 16.2.59). Note the tightness of bolts and the tightness of other fastening devices used.
Check the pier protection system (see Figure 16.2.60).
Figure 16.2.59 Concrete Bearing Areas
Figure 16.2.60 Pier Protection Systems – Dolphins and Fenders
Verify that structural welds are sound and the grid decks have adequate skid resistance. Check the roadway surface for evenness of grade and for adequate clearance at the joints where the movable span meets the fixed span. For more information on steel grid decks, see Topic 7.4.
A solid concrete deck is used over the pier areas (pivot or bascule pier) to keep water and debris from falling through onto the piers and mechanical devices. Since the machinery room is usually under the concrete deck, check the ceiling for leaks or areas that allow debris and rust to fall on the machinery. For more information of concrete decks, see Topic 7.2.
Other structural considerations include:
Mechanical, electrical, and hydraulic equipment includes specialized areas, which are beyond the scope of this reference manual. Since operating equipment is the heart of the movable bridge, it is recommended that expert assistance be obtained when conducting an inspection of movable spans. In many cases, the owners of these movable bridges follow excellent programs of inspection, maintenance, and repair. However, there is always the possibility that some important feature may have been overlooked. Any problems noted during the inspection are reported to the owner.
Conduct trial openings as necessary to insure proper operational functioning and that the movable span is properly balanced. Trial openings are specifically for inspection. During the trial openings, the safety of the inspection personnel, traveling public and boat operators is a primary concern.
On all movable structures, the machinery is so important that considerable time is to be devoted to its inspection. The items covered and termed as machinery include all motors, brakes, gears, tracks, shafts, couplings, bearings, locks, linkages, over-speed controls, and any other integral part that transmits the necessary mechanical power to operate the movable portion of the bridge. Inspect machinery not only for its current condition, but also for operational and maintenance methods and analysis of the characteristics of operation. The items listed below and items similar to them are to be inspected and analyzed by a machinery or movable bridge specialist. Refer to FHWA-IP-77-10, Bridge Inspector’s Manual for Movable Bridges and the AASHTO Movable Bridge Inspection, Evaluation and Maintenance Manual, Manual for Bridge Evaluation for further information on inspecting these items. The FHWA-IP-77-10 manual is published by the Federal Highway Administration (FHWA), but is currently out of print.
Observe the general condition of the machinery as a whole, and its performance during operation. Check for smoothness of operation, and note any abnormal performance of components. Note any noise or vibration and the source determined. Document any unsafe or detrimental methods followed by the operator to prevent injury to the public or to personnel, or deficiencies to the equipment. Also note the condition of the paint system.
Perform an evaluation of maintenance methods in light of design details for the equipment. Check application methods and frequency of lubrication in the maintenance logbook, if available. Note general appearance of existing applied lubricant.
Check open gearing for tooth condition and alignment including over- and under-engagement. Verify that the pitch lines match. Note excessive or abnormal wear. Inspect the teeth, spokes, and hub for cracks. Observe and note the general appearance of the applied lubricants on open gearing. If the lubricant has been contaminated, especially with sand or other gritty material, remove the old lubricant and have new lubricant applied. If there is a way to prevent future contamination, recommend this appropriate procedure as part of the inspector’s comments in the report. Check the teeth of all gears for wear, cleanliness, corrosion, and for proper alignment.
Examine the exterior of the housing and mountings for cracks and deficiencies (see Figure 16.2.61 and 16.2.62). Check bolts for tightness and note any corrosion. Inspect the interior of the housing for condensation and corrosion. Check the condition of gears. Watch for abnormal shaft movement during operation, indicating bearing and seal wear. Periodically check oil levels and condition of lubricant. Check that circulating pumps and lubricating lines are properly operating. Any abnormal noise is to be documented. Leaking oil may indicate the presence of a crack.
Figure 16.2.61 Cracked Speed Reducer Housing
Figure 16.2.62 Leaking Speed Reducer
Examine shafts damage, twisting, and strain. Cracks, if suspected, may be detected using non-destructive evaluations (NDE) such as magnetic particle or dye penetrant (see Figure 16.2.63). Various advanced inspection methods for steel members are presented in Topic 15.3. Cracks in mechanical components may be determined to be a critical finding. Note misalignment with other parts of the machinery system. Document cracks in shafts and record the exact location. Examine other shafts in the same locations as they were probably made from the same material and fabricated to the same details. They have also been exposed to the same magnitude and frequency of loading. Check coupling hubs, housings, and bolts for condition. Inspect seals and gaskets for leaks. Internal inspection of couplings is warranted if problems are suspected and can be used to determine tooth wear in gear couplings.
Figure 16.2.63 Hairline Crack Revealed on Shaft from Dye Penetrant Test
Examine bearing housings, pedestals, and supports for external condition, noting any cracks. Check bolts in housings and those used for anchors for tightness, damage, and corrosion, noting apparent lubrication characteristics. Grinding noises can be caused the lack of lubricant (see Figure 16.2.64). In sleeve bearings, inspect the bushings for damage and excessive wear. Note evidence of seal damage in anti-friction bearings. Investigate any unusual noise. Check the trunnion bearings for excessive wear, lateral slip, and loose bolts.
Figure 16.2.64 Leaking Bearing
Inspect all braking devices for proper setting of braking torque and for complete release of the brakes when actuated. On shoe brakes, check drums and shoes for wear, damage, and corrosion, for misalignment of shoes with drums, and for clearance when released. Determine if worn linings need replaced. Check for proper actuation without leakage by actuators. Verify that linkages and hand releases are free but not sloppy. On enclosed hydraulic disc brakes, make certain there is proper actuation without leakage at connections or seals. Check the brakes, limit switches, and stops (cylinders and others) for excessive wear and slip movement. Note whether the cushion cylinder ram sticks or inserts too easily. Inspect the brake limit switches for proper setting. Observe the surface of the brake drum for indications of contact with the brake shoes. Check the pressure developed by each disc brake power unit to be sure the brakes are releasing. Also check the manual release on all of the brakes.
Check the housing and mountings for damage, corrosion, and fastener condition. Inspect bearings for lubrication and note indications of wear (movement) and seal leakage at shaft extensions.
Look for any leakage at connections and seals. Note any corrosion on the cylinder rods. Listen to motors and pumps, and note any unusual noise. Check power units to make sure all components are functioning and that pressures are properly adjusted. Sample fluid periodically and examine for contamination and wear metal. Check all main hydraulic power units for charge pressure setting and maximum pressure that can be developed by the unit. Check all filters routinely and replace as needed. Also check the level of fluid in the vertical reservoir.
Check emergency generators for operation and readiness, verifying that there are no oil leaks or abnormal noises. Mechanical service specialists and electrical inspectors are required for more thorough inspections. Auxiliary motors and hand operators, with their clutches and other transmission components, are to be checked for adjustment and readiness to perform when called upon.
Detailed inspections of internal combustion engines are made by mechanical engine specialists. The inspection may include but is not limited to checking of the following conditions:
Examine the center locks and tail locks (if used) on double-leafed bascule spans, and the end locks on single-leaf bascule bridges, swing bridges, and vertical lift bridges. Note whether there is excessive deflection at these joints or vibration on the bridge. Inspect the locks for fit and for movement of the span or leaf (or leaves). Check lubrication and for loose bolts. Verify that the lock housing and its braces have no noticeable movement or misalignment. The paint adjacent to the locks will have signs of paint loss or wear if there is movement. Check lock bars, movable posts, linkages, sockets, bushings, and supports for damage, cracks, wear, and corrosion.
Check all rear locks in the withdrawn position for clearance from the path of the moving leaf as it opens and for full engagement when the leaf is closed. Measure the gap, if any, between the lock plate and the moving leaf bearing plate. Check each rear lock hydraulic drive unit for leakage of oil and operation for correct length of movement of the lock.
On bascule bridges, see if the front live load bearings fit snugly. Also observe the fit of tail locks at the rear arm and of supports at the outer end of single-leaf bridges.
Examine actuators for operational characteristics, including leakage if hydraulic. Note both the quantity and quality of the lubricant. Check for alignment, and analyze the type of wear that is occurring. Note condition of movable operators.
Inspect the fasteners and structure for deficiencies and corrosion. Note contact surface conditions. Check for alignment and movement under load.
Note indications of lack of pressure or stickiness during operation. Check piston rod alignment with strike plate. Note the condition of the rod and housing, and verify if hydraulic leakage is present. Check the air filter and function of any pressure reading or adjusting devices and the operating pressure, if possible. Verify that the air buffers have freedom of movement and development of pressure when closing. Inspect the fully open bumper blocks and the attaching bolts for cracks in the concrete bases.
Check that there is no cracking in the steel or concrete. Note corrosion and damage. Check for deflection and movement under load. Ensure that the linkages and pin connections have the proper adjustment and are in functional condition. Check motor mounting brackets to ensure secure mounting.
Inspect the fasteners for corrosion, loss of section, and tightness.
Check the wedges and the outer bearings at the rest piers for alignment and amount of lift. This can be recognized by excessive vibration of span or uplift when load comes upon the other span.
Examine the live load bearings and wedges located under the trusses or girders at the pivot pier for proper fit alignment and amount of lift.
Check center bearings for proper and adequate lubrication, oil leaks, and noise. Examine the housing for cracking, pitting, fit of joints, and note indications of span translation (irregular rotation) at racks and track. Measure for proper clearance of balance wheels above track. Verify that the tracks and balance wheels are free of wear, pitting, and cracking. Check for proper and adequate lubrication at all lubrication points.
Note balance characteristics as indicated by loads taken by balance wheels, and by drag on the rest pier rail.
Check the rim bearing for wear on tracks and rollers, particularly at rest positions where the bridge is carrying traffic. Examine the center pivots and guide rings for proper fit, and for wear, pitting, and cracking. Check for proper and adequate lubrication at all lubrication points.
Examine the center (live load) wedges located under the trusses or girders at the pivot pier for proper fit (no lifting) and alignment. Check end wedges and bearings at the rest piers for alignment and amount of lift. This can be recognized by excessive vibration of the span or uplift when live load crosses the other span. Inspect the end lift jacks, shoes, and all linkages for wear, proper bearing under load, and proper adjustment.
Note the condition of end latches, including any modification that adversely affects their functional design.
On rolling lift bascule bridges, check the segmental and track castings and their respective supporting track girders (if used) for wear on the sides of track teeth due to movement of sockets on segmental castings. Inspect the trunnion assemblies for deflection, buckling, lateral slip, and loose bolts. Examine the trunnions for any signs of corrosion, pitting, or cracking, particularly at stress risers. Laser leveling may be used during the inspection of trunnions. Check the balance of each leaf. Compare all wear patterns for indications of movement of the leaves. Check for cracking at the fillet of the angles forming the flanges of the segmental and track girders, cracking in the flanges opposite joints in the castings, and cracking of the concrete under the track. Inspect rack support for lateral movement when bridge is in motion.
Check trunnion bearings for lubrication of the full width of the bearing. Verify that extreme pressure (EP) lubrication oil of the proper grade is used.
The condition of wire ropes and sockets, including wire rope lubrication, is important. Look for flattening or fraying of the strands and deficiencies between them. This is reason for replacement. Similarly, check the up-haul and down-haul ropes to see if they are winding and unwinding properly on the drums. Note any need for tension adjustments in up-haul and down-haul ropes. Determine whether ropes have freedom of movement and are running properly in sheave grooves. Look for any obstructions to prevent movement of the ropes through the pulley system, and check the supports on span drive type bridges. Check rope guides for alignment, proper fit, free movement, wear, and structural integrity of the longitudinal and transverse grooved guide castings. Inspect the grooved guide castings closely for wear in the grooves. Examine the cable hold-downs, turnbuckles, cleats, guides, clamps, splay castings, and the travel rollers and their guides.
Check that balance chains hang freely, that span leveling devices are functioning, and that span and counterweight balance closely. Observe if span becomes "out of level" during lifting operation. Inspect spring tension, brackets, braces, and connectors of power cable reels.
Check for damage, including cracking, at drums and sheaves. Note the condition and alignment of span guides.
An available electrical specialist is required for the inspection of the electrical equipment. For this inspection, use current AASHTO guidance on inspection of movable bridges. Observe the functional operation of the bridge and look for abnormal performance of the equipment. Check the operational methods and safety features provided. Evaluate the maintenance methods being followed and check the frequency of services performed.
Examine the normal power supply, standby power supply, and standby generator set (for emergency operation of bridge and service lighting) and note the following:
If the power cable has been repaired with a splice, note the condition of the splice box seal.
If no standby power supply has been provided, determine whether a portable generator could be used. A manual transfer switch would be a convenient way of connecting it.
Examine span drive motors, lock motors, brake thrustor motors, and brake solenoids for the same items as given for power supplies.
Check dry transformer coil housings, terminals, and insulators, including their temperature under load. Observe the frames and supports for rigidity to prevent vibration. Check the liquid filled transformer in the same way, along with checking the oil level while looking for leakage. Examine oil insulation test records.
Check circuit breakers (e.g., air, molded case, and oil) and fuses, including the arc chute, contact surfaces, overload trip settings, insulation, and terminal connections. Examine oil insulation test records, and observe the closing and tripping operation. Record all fuse types and sizes being used.
Examine the wiring and cables for both power and control. Note whether the submarine cables are kinked, hooked, or deteriorated, especially at the exposed area above and below the water. In tidal areas, look for marine and plant growth. Note if the ends of the cable have been protected from moisture. Record the insulation value of each wire as measured by megger. Look for cracking, overheating, and deterioration of the insulation. Check for wear against surfaces and especially sharp edges. Check the adequacy of supports and that dirt and debris do not accumulate against the conduit and supports. Check terminal connections, clamps, and securing clips for tightness, corrosion, and verify that there are wire numbers on the end of each wire. The weight of the wires or cables will be carried by the clamps and not by the wire connections at the terminal strips.
Examine the programmable logic controller (PLC) cabinets, control consoles and stations, switchboards (see Figure 16.2.65), relay cabinets, motor control centers (MCC), and all enclosures for deficiencies, debris inside, drainage, operations of heater to prevent condensation, and their ability to protect the equipment inside. Check the operation of all traffic signals, traffic gates, traffic barriers, and navigation lights. Verify that the bridge is open to provide the clearance shown on the permit drawing before the green span light turns on. Check the traffic warning equipment and control circuits, including the advanced warning signals (if used), traffic lights/signals, gates, barriers, and the public address and communication equipment.
Figure 16.2.65 Open Switchboard
See if conduit is far enough away from all surfaces to avoid debris from collecting against it. Note if it is adequately supported and pitched to drain away from junction boxes and pull boxes, so that water is not trapped within. Also, note if all conduits have covers with seals. Report deteriorated conduit so that it can be replaced with new conduit. Seal and re-coat the connectors at the ends of all PVC coated conduit after all fittings are installed.
Examine the covers on all junction boxes (JBs) for an effective seal, dry interior, functioning breather-drains, heaters having enough power to prevent condensation inside, and terminal strips all secured to the bottom of horizontal JBs or to the back of vertical JBs.
Observe if all voltmeters, ammeters, and watt meters are freely fluctuating with a change in load. Check that all switches and meters are operable.
Check the operation of this equipment under load, and watch for arcing between contacts, snap action of contacts, deterioration of any surfaces, and drainage of any moisture. Look for signs of corrosion and overheating.
Set all limit switches so they do not operate until they are intended to stop the equipment or complete an interlock. Verify that the interior is clean and dry, with all springs active.
Check for power to the field and signal being sent from the transmitter to the receiver. Observe the receiver tracking the rotation of the bridge as it operates. Observe the mechanical coupling between the driving shaft and the transmitter, checking for damage and misalignment.
Check to see if power is going to each light and outlet. Note if there is a shield or bar for protecting each bulb and socket. It is desirable to have service lights available when power is removed from all movable bridge controls and equipment.
A hydraulic power specialist is required for the inspection of the hydraulic equipment (see Figure 16.2.66). Observe the functional operation of the bridge and look for abnormal performance of the equipment. Check the safety features provided and evaluate the maintenance methods being followed, checking the frequency of services performed. Due to the inter-related function of components, the requirements for fluid cleanliness, and the need for personnel safety, do not open the reservoir or hydraulic lines. In addition, do not shut off or adjust any component or part of the power circuit without complete understanding of their function and knowledge of the effect such action will have upon the system. Items which are checked during a hydraulic inspection include the following:
Figure 16.2.66 Hydraulic Power Specialists
The owner of a movable bridge keeps a complete file available for the engineer who is responsible for the operation and maintenance of the bridge. See Topic 4.4 for general record keeping and documentation. The file includes (if applicable), but not be limited to, the following:
Review inspection and maintenance reports with preventative maintenance measures in mind. An example would be the “megger” readings on wiring insulation; especially those taken on damp rainy days when moisture could influence (reduce) the values. An acceptable minimum reading is usually 1 megaohm. If the value on a wire is decreasing on progressive reports, preventative maintenance may save a “short” that could burn out equipment and put the bridge out of operation.
Examples of inspection and maintenance records are shown in Figures 16.2.67 through 16.2.73.
Figure 16.2.67 Example of Notes on Operating Machinery (Gears-General)
Figure 16.2.68 Example of Notes on Operating Machinery (Gears-Teeth)
Figure 16.2.69 Example of Notes on Operating Machinery (Bearings)
Figure 16.2.70 Example of Notes on Operating Machinery (Mechanical Components)
Figure 16.2.71 Example of Notes on Electrical Equipment (Motors)
Figure 16.2.72 Example of Notes on Electrical Equipment (Limit Switch)
Figure 16.2.73 Example of Notes on Electrical Equipment (Megger Insulation Test of the Submarine Cables)
State and Federal rating guideline systems have been developed to aid in the inspection of movable bridges. The two major rating guideline systems currently in use are the FHWA's Recording and Coding Guide for the Structural Inventory and Appraisal of the Nation's Bridges used for the National Bridge Inventory (NBI) component condition rating method and the AASHTO Manual for Bridge Element Inspection for element level condition state assessment.
Using the NBI component condition rating guidelines, a one-digit code on the Federal Structure Inventory and Appraisal (SI&A) sheet indicates the condition of the superstructure. Component condition rating codes range from 9 to 0 where 9 is the best rating possible. See Topic 4.2 (Item 59) for additional details about NBI component condition rating guidelines.
Consider previous inspection data along with current inspection findings to determine the correct component condition rating.
In an element level condition state assessment of a movable bridge, possible AASHTO National Bridge Element (NBEs) and Bridge Management Elements (BMEs) are:
| NBE No. | Description |
|---|---|
Superstructure |
|
Box Girder |
|
102 |
Steel Closed Web/Box Girder |
Floor System |
|
107 |
Steel Open Girder/Beam |
113 |
Steel Stringer (Stringer-Floorbeam System) |
152 |
Steel Floorbeam (Stringer-Floorbeam System) |
Steel Truss |
|
120 |
Steel Truss |
161 |
Steel Pin and Pin-and-Hanger Assembly or both |
162 |
Steel Gusset Plate |
Steel Arch |
|
141 |
Steel Arch |
Cable |
|
147 |
Steel Main Cable |
148 |
Steel Secondary Cable |
| BME No. | Description |
|---|---|
Wearing Surfaces and Protection Systems |
|
515 |
Steel Protective Coating |
The unit quantity for the superstructure elements is feet. The total length is distributed among the four available condition states depending on the extent and severity of the deficiency. The unit quantity for the steel main cable elements is feet. The total length is distributed among the four available condition states depending on the extent and severity of the deficiency. The unit quantity for steel secondary cables is each, with each steel secondary cable element placed in one of the four available condition states depending on the extent and severity of the deficiency. The unit quantity for gusset plates, pins, and pin-and-hanger assemblies is each, with each gusset plate or assembly element placed in one of the four available condition states depending on the extent and severity of the deficiency. The unit quantity for protective coating is square feet, and the total area is distributed among the four available condition states depending on the extent and severity of the deficiency. The sum of all condition states equals the total quantity of the National Bridge Element or Bridge Management Element. Condition State 1 is the best possible rating. See the AASHTO Manual for Bridge Element Inspection for condition state descriptions.
For mechanical, electrical, and hydraulic movable bridge members, individual bridge owners may choose to create their own Agency Developed Elements (ADEs).
Although uncommon, some states have floating bridges that are not supported by a substructure. Instead, they are supported by, or float on the water. The bridge elevation will change as the water level fluctuates (see Figure 16.3.1).
Floating bridges are cost-effective solutions for crossing large bodies of very deep water with a very soft bottom where conventional piers are impractical. For a site with a 100- to 200-foot deep water and a very soft bottom extending another 100 to 200 feet, a floating bridge is estimated to cost three to five time less than a conventional multi-span fixed bridge or a tunnel.
Floating bridges perform well in areas subjected to high winds, moderate currents and moderate waves. They also have low environmental impact and perform well in seismic events.
Washington State is known for its floating bridges with four of the longest and heaviest floating bridges. They are the SR 520 Evergreen Point Bridge, the I-90 Lacey V. Murrow Bridge, the I-90 Homer M. Hadley Bridge, and the SR 104 Hood Canal Bridge.
Figure 16.3.1 Floating Bridge, SR 520 Evergreen Point Bridge, Seattle, WA During Stormy Weather
Floating bridges take advantage of the natural law of buoyancy of water to support the loads. This is achieved through the use of giant pontoons secured into place by an anchoring system. Conventional piers and foundations are not used.
Since a floating bridge "sits" on the water, the bridge itself creates an obstacle to vessels attempting to cross the waterway. For this reason, many floating bridges employ a movable bridge section for vessels to pass through, or an elevated span for vessels to pass under (see Figures 16.3.2 and 16.3.3).
Figure 16.3.2 Movable Bridge Section of Evergreen Point Bridge, Seattle, WA
Figure 16.3.3 Elevated Section of Evergreen Point Bridge, Seattle, WA
Floating bridges may be constructed of wood (see Figure 16.3.4), concrete, steel, or a combination of materials depending on the design requirements although concrete pontoons are generally used in the newer bridges.
The pontoons are large water-tight chambers constructed off site and floated into place (see Figures 16.3.5 and 16.3.6). Despite their heavy concrete composition, the weight of the water displaced by the pontoons is equal to the weight of the structure (including all traffic), which allows the bridge to float. They may be prestressed concrete or reinforced concrete and are classified as either continuous pontoon type or separate pontoon type. The pontoons are held into place by huge steel cables anchored deep in the soil below water.
Figure 16.3.4 Brookfield, Vermont, Floating Bridge Constructed from Timber
Figure 16.3.5 Concrete Pontoons Under Construction
Figure 16.3.6 Concrete Pontoons Transported for Hood Canal Project
To control water leaking into the interior of the pontoons and ultimately sinking the bridges, each pontoon contains several water tight cells. This confines any flooding to a small area of the bridge. Access doors to the interior cells are watertight. Each cell may be equipped with water sensors for early detection of any leaks in the pontoons and a bilge pumping system to pump out water.
Bridge pontoons are designed to safely withstand wind and wave forces, major storms and vessel collisions.
Continuous pontoon bridges are made of individual pontoons, longitudinally connected to each other. The top of the pontoons may be the roadway or a superstructure may be built on top of the pontoons. The size of each pontoon is determined by design requirements as well as constraints imposed by the constructions facilities and the transportation route to the bridge site.
The floating bridges in use today in Washington State are of the continuous type (see Figure 16.3.7).
Figure 16.3.7 Continuous Pontoon-Type Structure
A separate pontoon type of floating bridge consists of individual pontoons. These pontoons are placed transversely to the structure and are spanned by a steel or concrete superstructure (see Figures 16.3.8 and 16.3.9). The superstructure needs to be strong enough and rigid enough to maintain the position of the separated pontoons. A series of cables are attached to each pontoon and are anchored deep in the soil below water.
Figure 16.3.8 Separate Pontoon Type Structure
Figure 16.3.9 Bridge Constructed with Separate Pontoons
Floating bridges are held in place in various ways such as a system of piles, caissons, cables, anchors and fixed guide structures. The most common type of system consists of cables and anchors. Anchor cables are normally two and one half inches in diameter and consist of dozens of individual steel strands (see Figure 16.3.10).
Figure 16.3.10 Cross-Section of Anchor Cable
Anchor cable saddles are used within the pontoon to guide and hold the cable in place (see Figure 16.3.11). Hydraulic jacks inside the pontoon tighten or release the pressure on the cables as the water level fluctuates under the bridge.
Figure 16.3.11 Anchor Cable Saddle
Depending on the depth of the water and the soil conditions, there are four primary types of anchoring systems used on the floating bridges: precast concrete fluke style anchor, pile anchor, open-cell gravity block anchor, and solid gravity slab anchor (stackable).
Precast concrete fluke style anchors are used in deep water with very soft soil conditions. Anchors weighing 60 to 86 tons are lowered to the soil below water. Water jets are turned on allowing the anchors to sink to the proper depth (see Figure 16.3.12).
Figure 16.3.12 Precast Concrete Fluke Style Anchor
Pile anchors are designed for use in water depths less than 88 feet and with hard soil. Piles are driven into the surface to a specified depth and tied together to increase capacity (see Figure 16.3.13).
Open-cell gravity block anchors are a gravity type of anchor. They are reinforced concrete boxes with an open top that are lowered into position and filled with gravel to a predetermined weight. This type or anchor is used in deep water where the soil is hard (see Figure 16.3.14).
Figure 16.3.14 Open-Cell Gravity Block Anchor
Solid gravity slab anchors are a gravity type of anchor. They can be used in either shallow or deep water where the soil is hard. These anchors are solid reinforced concrete slabs weighing up to 270 tons each. The first slab is lowered into position, and then additional slabs are added until the required anchoring capacity has been reached. Solid gravity slab anchors are the preferred anchor type because they are easy to cast and can be placed quickly (see Figure 16.3.15).
Figure 16.3.15 Solid Gravity Slab Anchor
Common deficiencies that occur on floating bridges are:
Floating bridges may be constructed from steel, concrete or timber. Therefore, deficiencies will depend on the material used to construct the bridge. See Topics 6.1 (Timber), 6.2 (Concrete), and 6.3 (Steel) for specific information regarding deficiencies of each material type.
Because of their uniqueness and depending on the material used, floating bridges can prove challenging to an inspector. Floating bridges can be constructed of steel, concrete or timber, therefore a variety of inspection methods are utilized to thoroughly inspect the bridge. Additionally, since many floating bridges include an elevated conventional bridge structure or a moveable bridge section, those inspection methods and locations are to be considered by the inspection team.
See Chapter 6 for detailed description of anticipated modes of deterioration for common bridge materials. See Chapters 8 through 12 for the inspection and evaluation of timber superstructures, concrete superstructures, steel superstructures, bearings and substructures. See Topic 16.2 for detailed information about movable bridges.
Visual inspection of each pontoon cell will reveal any cracks or leaks. Pontoons have access hatches to allow for maintenance and inspection (see Figure 16.3.16).
Visually inspect concrete pontoons for the following deficiencies:
Visually inspect steel pontoons for the following deficiencies:
Figure 16.3.16 Inspector Opening Pontoon Access Hatch
Measure and record the depth of any water found in each cell. The length, location and width of cracks found are to be accurately measured and recorded (see Figure 16.3.17). For steel pontoons and cables, remove corrosion and rust down to bare metal. With calipers or a D-meter, measure and record remaining section thickness. Use a hammer to check for delaminated areas in concrete pontoons.
Figure 16.3.17 Sample Pontoon Inspection Plan
Many of the advanced inspection tools used above water have been adopted for underwater use. See Chapter 15 for the advanced inspection methods of timber, steel and concrete.
Anchors may be embedded 100 or more feet below the water surface. Inspection of the anchors will require underwater divers and equipment with the ability to detect any deficiencies present. See Topic 13.3, Underwater Inspection. Underwater cameras, sonar and other specialized equipment can provide access to cables and anchors.
Examine the floor of each pontoon cell for standing water. Examine pontoon walls and surfaces for cracks. Examine access doors, locks and hatches verifying that they are water tight and in proper working condition. Check the bilge pumping system and verify that it is in working order. Convey any noted problems with the pumping system to specialized maintenance personnel responsible for the system.
Examine the cable ends and anchor cable saddle inside the pontoon. Look at the connections in the pontoons for frayed or broken strands. Verify the presence and functioning of any cathodic protection system on the anchor cables.
When continuous pontoons are used, inspect the joint between the pontoons. Typically a rubber membrane or grout is used between the pontoons. Examine the alignment of the pontoons across the structure looking for signs of differential movement or distortion. This may indicate water leaking into one of the pontoons or some type of ballast balancing problem within the structure.
Examine the cable ends at the pontoon portals and check for cable misalignment and fraying. Check for broken wires that may indicate undue stress on the cable securing the pontoon (see Figure 16.3.18). Also check cables for heavy corrosion or section loss (see Figure 16.3.19).
Figure 16.3.18 Frayed Cables Removed from a Floating Bridge
Figure 16.3.19 Typical View of Heavy Corrosion within Pontoon Port
Floating bridges are subjected to wind, tides and wave forces that are unpredictable and always changing. This exerts high levels of strain and stress on the cables and the anchors. Inspection of the anchors is not easily accomplished. Underwater remote equipment can provide information on each anchor. Look for any indication of anchor movement, misalignment or undermining of the anchor. Check the ballast on open-cell gravity block anchors to verify if there is enough material to keep the anchors in place.
State and Federal rating guideline systems have been developed to aid in the inspection of floating bridges. The two major rating guideline systems currently in use are the FHWA’s Recording and Coding Guide for the Structural Inventory and Appraisal of the Nation’s Bridges used for the National Bridge Inventory (NBI) component condition rating method and the AASHTO Manual for Bridge Element Inspection for element level condition state assessment.
Using the NBI component condition rating guidelines, a one-digit code on the Federal Structure Inventory and Appraisal (SI&A) sheet indicates the condition of the superstructure. Component condition rating codes range from 9 to 0 where 9 is the best rating possible. See Topic 4.2 (Item 59) for additional details about NBI component condition rating guidelines.
Consider previous inspection data along with current inspection findings to determine the correct component condition rating.
In an element level condition state assessment of a floating bridge, possible AASHTO National Bridge Elements (NBEs) and Bridge Management Elements (BMEs) are:
| NBE No. | Description |
|---|---|
Superstructure |
|
102 |
Steel Closed Web/Box Girder |
107 |
Steel Girder/Beam |
113 |
Steel Stringer |
147 |
Steel Primary Cable |
148 |
Steel Secondary Cable |
152 |
Steel Floorbeam |
104 |
Prestressed Concrete Closed Web/Box Girder |
109 |
Prestressed Concrete Girder/Beam |
115 |
Prestressed Concrete Stringer |
154 |
Prestressed Concrete Floorbeam |
105 |
Reinforced Concrete Closed Web/Box Girder |
110 |
Reinforced Concrete Girder/Beam |
116 |
Reinforced Concrete Stringer |
155 |
Reinforced Concrete Floorbeam |
111 |
Timber Girder/Beam |
117 |
Timber Stringer |
156 |
Timber Floorbeam |
106 |
Other Closed Web/Box Girder |
112 |
Other Girder/Beam |
118 |
Other Stringer |
149 |
Other Secondary Cable |
157 |
Other Floorbeam |
| BME No. | Description |
|---|---|
Wearing Surfaces and Protection Systems |
|
510 |
Wearing Surfaces |
515 |
Steel Protective Coating |
520 |
Concrete Reinforcing Steel Protective System |
521 |
Concrete Protective Coating |
The unit quantity for the superstructure elements is feet. The total length is distributed among the four available condition states depending on the extent and severity of the deficiency. The unit of quantity for secondary cablesis each, with each secondary cable element placed in one of the four available condition states depending on the extent and severity of the deficiency. The unit quantity for wearing surfaces, protective coatings and protective systems is area, and the total area is distributed among the four available condition states depending on the extent and severity of the deficiency. The sum of all condition states equals the total quantity of the National Bridge Element or Bridge Management Element. Condition State 1 is the best possible rating. See the AASHTO Manual for Bridge Element Inspection for condition state descriptions.