An Introduction to Bridge Design

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An Introduction to Bridge Design

• Cambridge University Engineering Department

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Types of Bridge

• Beam Bridges
• Box Girder Bridges
• Arch Bridges
• Truss Bridges
• Suspension Bridges
• Cable Stayed Bridges

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Beam Bridges
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Beam Bridges

• A beam bridge is the simplest kind of bridge
• The farther apart its supports, the weaker a beam
  bridge becomes
• Cheap and cheerful
• Easy to manufacture
• Not very beautiful
• Beam needs careful design

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Box Girder Bridges
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Box Girder Sections
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Box Girder Bridges

• Similar to beam bridges
• With well designed girders will be stronger than
  a simple beam bridge
• Cheap and cheerful
• Easy to manufacture
• Not very beautiful
• Girder needs careful design

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Supporting Beams

• To improve the performance of a simple beam
  bridge the beam has to be supported
• There are three basic ways to do this
• Arches
• Trusses
• Cables

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Arch Bridges
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Arch Bridges

• Arch bridges are one of the oldest types of
  bridge and 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
• These supports (called abutments) carry the load
  and keep the ends of the bridge from spreading out

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Arch Bridges
Over
Under
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Arch Bridges

• Very strong if well designed
• Can be very beautiful
• Tend to be very heavy
• Need very strong abutments

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Truss Bridges
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Truss Bridges

• Are mostly empty space, but very effective
• Solid components (beams, arches etc.) are
  replaced by triangulated assemblies of thin
  (usually metal) members

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Truss Bridges

• Very good strength to weight performance
• Possibility of lots of repeated parts reduces
  manufacturing costs
• Can be incorporated into almost any design
• Can be very beautiful
• Lots of parts to be manufactured

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Suspension Bridges
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Suspension Bridges

• Apart from the towers, all the highly stressed
  parts of a suspension bridge are in tension
• A suspension bridge suspends the roadway from
  huge main cables, which extend from one end of
  the bridge to the other
• These cables rest on top of towers and are
  secured at each end by anchorages

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Suspension Bridges

• Aesthetic, light and strong
• Can span distances far longer than any other kind
  of bridge
• The most expensive bridges to build
• Susceptible to wobble if badly designed

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Suspension Bridge Wobble
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Cable Stayed Bridges
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Cable Stayed Bridges

• 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

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Cable Stayed Bridges
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Cable Stayed Bridges

• Undeniably beautiful
• Compared to suspension bridges
• Require less cable
• Are easier and faster to build
• Need stronger towers