Chapter 34 Bridge Construction

Objectives After reading the chapter and reviewing the materials presented the students will be able to: Identify the major components of bridges Identify and describe types of fixed bridges Identify and describe movable bridges Describe the major steps in designing bridges Describe the bridge building process

Purpose of Bridges Bridges carry roadways, railroads, and walkways over obstacles such as rivers, other roadways, or railroads. Bridges may also be built to carry pipelines and conveyors over obstacles. The two primary components of bridges are the superstructure and the substructure (fig 34-1, page 542). The substructure may include abutments, piers, and footings. Footings rest on bedrock or piles that are driven into the bedrock. Piers extend from the footings to the superstructure. Abutments hold the earth at the approach to the bridge, and support the ends of the superstructure. The superstructure supports the load that the bridge carries.

Fixed Bridges There are seven basic types of fixed bridges: 1. Slab bridges: have superstructures that are reinforced concrete slabs. Slab bridges may be used for short spans. 2. Truss bridges: employ a rigid framework made of steel, wood, or aluminum to make the superstructure. Triangular shapes formed by the components of the truss make the frame rigid. 3. Arch bridges: are used to span deep ravines and to provide clearance for boat traffic. 4. Cantilever bridges: can span relatively long distances. 5. Suspension bridges: provide the longest spans. 6. Segmented concrete box girder bridges: are made with precast concrete segments that are joined with high tensile strength cable. 7. Cable stayed bridges: feature one or more towers with cables supporting the bridge deck.

Movable Bridges Movable bridges are used where boat traffic requires more clearance than can be provided by fixed bridges. There are 3 types of movable bridges: 1. Bascule bridges: rotate upward on a trunnion (pivot) to provide clearance for boats. A counterweight makes it possible to open and close the bridge with a relatively small motor. 2. Lift bridges: include a section that lifts vertically. Counterweights suspended on cables greatly reduce the power needed to raise the movable section. 3. Swing bridges: rotate on center piers so that boat traffic can pass on both sides. Swing bridges pivot horizontally and require a strong structure to support the pivot.

Planning and Design Engineers prepare feasibility studies, preliminary designs, cost estimates, and final designs. Safety and durability are major factors considered during the design process of bridges. Important questions that need to be answered are: What type of bridge is needed? What materials will be used for the superstructure? What will be the length of each span? How many piers will be required? What type of substructure is necessary? How much clearance is required below the bridge superstructure?

Bridge Building Materials Steel reinforced concrete is the major material used in the substructure of bridges. Wood or steel piles are driven into the earth to support the footing where bedrock is too deep to be reached by excavating. Bridge superstructures are made from steel, concrete, masonry, or wood. Combining materials is common. Steel beams may support a concrete roadway.

Building the Substructure Abutments support the ends of the bridge superstructure and serve as the retaining walls for the earth at the approach to the bridge. Bridges that are too long to span from one abutment to another require one or more piers between the abutments. Abutments and piers are typically made from steel reinforced concrete. Ideally footings for piers and abutments should rest on bedrock. When this is not practical, wood or steel piles are driven into the earth to provide needed support. The pile cap becomes the footing for the abutment or pier.

Building the Superstructure Steel superstructures are built in steel fabrication plants. Large sections of steel trusses are assembled at plants and transported to the site. Precast concrete components are also made off site.

Summary Bridges carry roadways, railroads, and walkways over obstacles such as rivers, other roadways, or railroads. Bridges may also be built to carry pipelines and conveyors over obstacles. The two primary components of bridges are the superstructure and the substructure. The substructure may include abutments, piers, and footings. Footings rest on bedrock or piles that are driven into the bedrock. Piers extend from the footings to the superstructure. Abutments hold the earth at the approach to the bridge, and support the ends of the superstructure. The superstructure supports the load that the bridge carries. There are seven basic types of fixed bridges: 1. Slab bridges: have superstructures that are reinforced concrete slabs. 2. Truss bridges: employ a rigid framework made of steel, wood, or aluminum to make the superstructure. 3. Arch bridges: are used to span deep ravines and to provide clearance for boat traffic. 4. Cantilever bridges: can span relatively long distances. 5. Suspension bridges: provide the longest spans. 6. Segmented concrete box girder bridges: are made with precast concrete segments that are joined with high tensile strength cable. 7. Cable stayed bridges: feature one or more towers with cables supporting the bridge deck. Movable bridges are used where boat traffic requires more clearance than can be provided by fixed bridges. There are 3 types of movable bridges: 1. Bascule bridges: rotate upward on a trunnion (pivot) to provide clearance for boats. 2. Lift bridges: include a section that lifts vertically. 3. Swing bridges: rotate on center piers so that boat traffic can pass on both sides. Safety and durability are major factors considered during the design process of bridges. Steel reinforced concrete is the major material used in the substructure of bridges. Bridge superstructures are made from steel, concrete, masonry, or wood. Steel superstructures are built in steel fabrication plants. Precast concrete components are also made off site.

Home Work 1. What are the parts of the substructure of a bridge? 2. What type of fixed bridges provides the longest spans? 3. What are the major factors considered during the design process of bridges?