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Unit 7: Statics and Strength of materials

Bridge Basics. These four factors are used in describing a bridge. By combining these terms one may give a general description of most bridge types. Span (simple, continuous, cantilever), Material (stone, concrete, metal, etc.), Placement of the travel surface in relation to the structure (deck,

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Unit 7: Statics and Strength of materials

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    1. Unit 7: Statics and Strength of materials Bridges

    2. Bridge Basics These four factors are used in describing a bridge. By combining these terms one may give a general description of most bridge types. Span (simple, continuous, cantilever), Material (stone, concrete, metal, etc.), Placement of the travel surface in relation to the structure (deck, pony, through), Form (beam, arch, truss, etc.).

    3. Spans

    4. Placement Examples of the three common travel surface configurations are shown in the Truss type drawings below. A Deck configuration, traffic travels on top of the main structure A Pony configuration, traffic travels between parallel superstructures which are not cross-braced at the top A Through configuration, traffic travels through the superstructure (usually a truss) which is cross-braced above and below the traffic.

    5. Deck Configuration

    6. Pony configuration

    7. Through configuration

    8. Forces in Bridges Compression and tension are forces engineers have to overcome in all bridges. Buckling is the inability of an object to withstand compression. Snapping is the inability of an object to withstand tension. How do engineers deal with the forces? To dissipate force is to spread it out over a greater area, so that no one spot has to bear the brunt of the concentrated force. (Arch bridge) To transfer force is to move it from an area of weakness to an area of strength, an area designed to handle the force. (Suspension bridge)

    9. Types of Bridges There are basically 4 different types of bridges: Arch bridge Suspension bridge Beam bridge Cable-stayed bridge An engineer would choose the correct bridge based on how far it must span from one support to the next. Each bridge deals differently with tension and compression. If part of the bridge cannot stand the compression, it will buckle. If part of the bridge cannot stand the tension, it will snap.

    10. Arch Bridge Arch bridges have great natural strength. Instead of pushing straight down, the weight of an arch bridge is carried outward along the curve of the arch to the supports at each end, which are called abutments. The abutments prevent the ends of the bridge from spreading out.

    11. Arch Types There are several ways to classify arch bridges. The placement of the deck in relation to the superstructure provides the descriptive terms used in all bridges: deck, pony, and through. Also the type of connections used at the supports and the midpoint of the arch may be used - - counting the number of hinges, which allow the structure to respond to varying stresses and loads.

    12. Suspension Bridge The most expensive type of bridge. These are some of the longest bridges in the world bridge and can span from 2,000 to 7,000 feet. True to its name, a suspension bridge suspends the roadway from huge main cables, which extend from one end of the bridge to the other. The deck is hung from suspenders of wire rope, eyebars, or other materials. Materials for the other parts also vary: piers may be steel or masonry; the deck may be made of girders or trussed.

    13. Cable-stayed Bridge Cable-stayed bridges look similar to suspensions bridges – both have roadways that hang from cables and both have towers, but the two bridges support the load of the roadway in very different ways. The difference lies in how the cables are connected to the towers. In suspension bridges, the cables ride freely across the towers, transmitting the load to the anchorages at either end. In cable-stayed bridges, the cables are attached to the towers, which alone bear the load.

    14. Beam Bridge A beam bridge is the most inexpensive kind of bridge, mostly due to it’s size…. they rarely span more than 250 feet. A beam bridge consists of a horizontal beam that is supported at each end by piers. The weight of the beam pushes straight down on the piers. Pre-stressed concrete is an ideal material for beam bridge construction; the concrete withstands the forces of compression well and the steel rods imbedded within resist the forces of tension. To create very tall beams, bridge designers add supporting lattice work, or a truss, to the bridge's beam. This support truss adds rigidity to the existing beam, greatly increasing its ability to dissipate the compression and tension. Once the beam begins to compress, the force is dissipated through the truss.

    15. Types of Beam Bridges Truss design determines the type of beam bridge. In a beam bridge, the very top of the beam experiences the most compression, and the very bottom of the beam experiences the most tension. The middle of the beam experiences very little compression or tension. Adding more material to the top and bottom, enables bridges to handle forces more easily. Trusses allow engineers to add strength in weak areas. In addition to the above-mentioned effect of a truss system, a truss has the ability to dissipate a load through the truss work. The design of a truss, which is usually a variant of a triangles.

    16. Truss Designs

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