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Bridge Structures” Structural Engineering Handbook Ed Toma, S.; Duan, L. and Chen, W.F. “Bridge Structures” Structural Engineering Handbook Ed. Chen Wai-Fah Boca Raton: CRC Press LLC, 1999 BridgeStructures 10.1General 10.2SteelBridges 10.3ConcreteBridges 10.4ConcreteSubstructures 10.5FloorSystem 10.6Bearings,ExpansionJoints,andRailings ShoujiToma 10.7GirderBridges DepartmentofCivilEngineering, Hokkai-GakuenUniversity,Sapporo,Japan 10.8TrussBridges 10.9RigidFrameBridges(RahmenBridges) LianDuan 10.10ArchBridges DivisionofStructures,California 10.11Cable-StayedBridges DepartmentofTransportation,Sacramento,10.12SuspensionBridges CA 10.13DefiningTerms Wai-FahChen Acknowledgment SchoolofCivilEngineering, References PurdueUniversity, FurtherReading WestLafayette,IN Appendix:DesignExamples 10.1 General 10.1.1 Introduction Abridgeisastructurethatcrossesoverariver,bay,orotherobstruction,permittingthesmoothand safepassageofvehicles,trains,andpedestrians.Anelevationviewofatypicalbridgeisshownin Figure10.1.Abridgestructureisdividedintoanupperpart(thesuperstructure),whichconsistsof theslab,thefloorsystem,andthemaintrussorgirders,andalowerpart(thesubstructure),whichare columns,piers,towers,footings,piles,andabutments.Thesuperstructureprovideshorizontalspans suchasdeckandgirdersandcarriestrafficloadsdirectly.Thesubstructuresupportsthehorizontal spans,elevatingabovethegroundsurface.Inthischapter,mainstructuralfeaturesofcommon typesofsteelandconcretebridgesarediscussed.Twodesignexamples,atwo-spancontinuous, cast-in-place,prestressedconcreteboxgirderbridgeandathree-spancontinuous,compositeplate girderbridge,aregivenintheAppendix. c 1999byCRCPressLLC FIGURE 10.1: Elevation view of a typical bridge. c 1999 by CRC Press LLC 10.1.2 Classification 1. Classification by Materials Steel bridges: A steel bridge may use a wide variety of structural steel components and systems: girders, frames, trusses, arches, and suspension cables. Concrete bridges: There are two primary types of concrete bridges: reinforced and prestressed. Timber bridges: Wooden bridges are used when the span is relatively short. Metal alloy bridges: Metal alloys such as aluminum alloy and stainless steel are also used in bridge construction. 2. Classification by Objectives Highway bridges: bridges on highways. Railway bridges: bridges on railroads. Combined bridges: bridges carrying vehicles and trains. Pedestrian bridges: bridges carrying pedestrian traffic. Aqueduct bridges: bridges supporting pipes with channeled waterflow. Bridges can alternatively be classified into movable (for ships to pass the river) or fixed and permanent or temporary categories. 3. Classification by Structural System (Superstructures) Plate girder bridges: The main girders consist of a plate assemblage of upper and lower flanges and a web. H- or I-cross-sections effectively resist bending and shear. Box girder bridges: The single (or multiple) main girder consists of a box beam fabricated from steel plates or formed from concrete, which resists not only bending and shear but also torsion effectively. T-beam bridges: A number of reinforced concrete T-beams are placed side by side to support the live load. Composite girder bridges: The concrete deck slab works in conjunction with the steel girders to support loads as a united beam. The steel girder takes mainly tension, while the concrete slab takes the compression component of the bending moment. Grillage girder bridges: The main girders are connected transversely by floor beams to form a grid pattern which shares the loads with the main girders. Truss bridges: Truss bar members are theoretically considered to be connected with pins at their ends to form triangles. Each member resists an axial force, either in compression or tension. Figure 10.1 shows a Warren truss bridge with vertical members, which is a “trough bridge”, i.e., the deck slab passes through the lower part of the bridge. Figure 10.2 shows a comparison of the four design alternatives evaluated for Minato Oh-Hasshi in Osaka, Japan. The truss frame design was selected. Arch bridges: The arch is a structure that resists load mainly in axial compression. In ancient times stone was the most common material used to construct magnif- icent arch bridges. There is a wide variety of arch bridges as will be discussed in Section 10.10 c 1999 by CRC Press LLC FIGURE 10.2: Design comparison for Minato Oh-Hashi, Japan. (From Hanshin Expressway Public Corporation, Construction Records of Minato Oh-Hashi, Japan Society of Civil Engineers, Tokyo [in Japanese], 1975. With permission.) Cable-stayed bridges: The girders are supported by highly strengthened cables (often composed of tightly bound steel strands) which stem directly from the tower. These are most suited to bridge long distances. Suspension bridges: The girders are suspended by hangers tied to the main cables which hang from the towers. The load is transmitted mainly by tension in cable. c 1999 by CRC Press LLC This design is suitable for very long span bridges. Table 10.1 shows the span lengths appropriate to each type of bridge. 4. Classification by Support Condition Figure 10.3 shows three different support conditions for girder bridges. Simply supported bridges: The main girders or trusses are supported by a movable hinge at one end and a fixed hinge at the other (simple support); thus they can be analyzed using only the conditions of equilibrium. Continuously supported bridges: Girders or trusses are supported continuously by more than three supports, resulting in a structurally indeterminate system. These tend to be more economical since fewer expansion joints, which have a common cause of service and maintenance problems, are needed. Sinkage at the supports must be avoided. Gerber bridges (cantilever bridge): A continuous bridge is rendered determinate by placing intermediate hinges between the supports. Minato Oh-Hashi’s bridge, shown in Figure 10.2a, is an example of a Gerber truss bridge. 10.1.3 Plan Before the structural design of a bridge is considered, a bridge project will start with planning the fundamental design conditions. A bridge plan must consider the following factors: 1. Passing Line and Location A bridge, being a continuation of a road, does best to follow the line of the road. A right angle bridge is easy to design and construct but often forces the line to be bent. A skewed bridge or a curved bridge is commonly required for expressways or railroads where the road line must be kept straight or curved, even at the cost of a more difficult design (see Figure 10.4). 2. Width The width of a highway bridge is usually defined as the width of the roadway plus that of the sidewalk, and often the same dimension as that of the approaching road. 3. Type of Structure and Span Length The types of substructures and superstructures are determined by factors such as the surrounding geographical features, the soil foundation, the passing line and its width, the length and span of the bridge, aesthetics, the requirement for clearance below the bridge, transportation of the construction materials and erection procedures, construction cost, period, and so forth. 4. Aesthetics A bridge is required not only to fulfill its function as a thoroughfare, but also to use its structure and form to blend, harmonize, and enhance its surroundings. 10.1.4 Design The bridge design includes selection of a bridge type, structural analysis and member design, and preparation of detailed plans and drawings. The size of members that satisfy the requirements of design codes are chosen [1, 17]. They must sustain prescribed loads. Structural analyses are performed on a model of the bridge to ensure safety as well as to judge the economy of the design. The final design is committed to drawings and given to contractors. c 1999 by CRC Press LLC c 1999 by CRC Press LLC TABLE 10.1 Types of Bridges and Applicable Span Lengths From JASBC, Manual Design Data Book, Japan Association of Steel Bridge Construction, Tokyo (in Japanese), 1981. With permission. FIGURE 10.3: Supporting conditions. FIGURE 10.4: Bridge lines. 10.1.5 Loads Designers should consider the following loads in bridge design: 1. Primary loads exert constantly or continuously on the bridge. Dead load: weight of the bridge. Live load: vehicles, trains, or pedestrians, including the effect of impact. A vehicular load is classified into three parts by AASHTO [1]: the truck axle load, a tandem load, and a uniformly distributed lane load. Other primary loads may be generated by prestressing forces, the creep of concrete, the shrinkage of concrete, soil pressure, water pressure, buoyancy, snow, and centrifugal actionsorwaves. c 1999 by CRC Press LLC 2. Secondary loads occur at infrequent intervals. Wind load: a typhoon or hurricane. Earthquake load: especially critical in its effect on the substructure. Other secondary loads come about with changes in temperature, acceleration, or tempo- rary loads during erection, collision forces, and so forth. 10.1.6 Influence Lines Since the live loads by definition move, the worst case scenario along the bridge must be determined. The maximum live load bending moment and shear envelopes are calculated conveniently using influence lines. The influence line graphically illustrates the maximum forces (bending moment and shear), reactions, and deflections over a section of girder as a load travels along its length. Influence lines for the bending moment and shear force of a simply supported beam are shown in Figure 10.5. For a concentrated load, the bending moment or shear at section A can be calculated by multiplying the load and the influence line scalar.
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