BRIDGES

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BRIDGES
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Bridge Basics

• Bridge is a structure built to span a valley,
  road, river, body of water, or any other physical
  obstacle.
• Designed to carry their own weight, (dead load)
  and to carry people and traffic, (live load) and
  to resist natural forces, such as wind and
  earthquakes.
• Must consider effects of contraction and/or
  Expansion due to temperature changes
• Designs of bridges will vary depending on the
  function of the bridge and the nature of the area
  where the bridge is to be constructed.

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Bridge Basics

• Bridges have been used since the dawn of
  humankind
• Bridges are based on one or more of three basic
  structures that are derived from forms found in
  nature.
• Beam a log fallen across a stream
• Arch natural rock formations
• Suspension from a hanging vine
• Though traditionally constructed from wood,
  stone, concrete, or steel, new bridges made from
  high-performance composite materials such has
  reinforced plastic are emerging

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Bridge Terminology

• Abutment- A substructure element supporting each
  end of a single span or the extreme ends of a
  multi-span superstructure and, in general,
  retaining or supporting the approach embankment.
• Deck- The roadway portion of a bridge that
  directly supports vehicular and pedestrian
  traffic.
• Span- The distance between the main supports of
  a bridge.
• The length normally measured when describing the
  size of the bridge
• Simple Span- a plank across a stream
• Continuous Span- supported along its length by
  piers and abutments
• Cofferdam- A watertight structure allowing
  underwater foundations to be built in the dry.

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Bridge Terminology
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Bridge Forces

• Four types of forces act on a bridge, either
  separately or in combination.
• Tension
• Compression
• Torsion
• Shear
• http//www.pbs.org/wgbh/buildingbig/lab/forces.htm
  l

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Bridges

• Beam Bridge (Girder)
• The simplest form of a bridge
• A rigid horizontal structure that is resting on
  two piers, one at each end.
• The weight of the bridge and any traffic on it is
  directly supported by the piers. The weight is
  traveling directly downward.
• Mostly built from steel and concrete beams, or
  girders.
• The size of the beam, and in particular the
  height of the beam, controls the distance that
  the beam can span.

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Truss Bridges

• Truss bridges are a type of beam bridge made up
  of many small beams attached together in
  triangular configuration to support a large
  amount of weight and span great distances.
• They function by compression and tension forces
  and not by bending forces.
• They are identified according to the way the
  chords are arranged.

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How Truss Bridges Work

• A truss is an interconnected framework of beams
  designed to hold something up.
• Trusses dont bend, they get pulled apart and
  pushed together.
• However once the weight is increased the bridge
  will stag in the middle. This is due to the
  individual beams expanding and contracting due to
  the weight.

• Trusses must be stable, and not able freely in
  any direction in order for them to work.
• The beams have to be placed carefully in the
  right angles and in equal distances away from
  each other so they can distribute the weight
  equally.
• They are usually supported at the ends by
  abutments and sometimes in the middle by piers.

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Arch Bridges

• Arch bridges are one of the oldest types of
  bridges and have great natural strength.
• The load on the roadway of this bridge is carried
  by the arch. The arch is supported at each end by
  a support called an abutment
• The arch is held together by the main piece in
  the middle which is called the keystone
• The roadway can be placed above or below the arch.

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Arch Bridges
Arch Bridges consist of compression and tension.

• Tension
• The greater the degree of curvature (the larger
  the semicircle of the arch), the greater the
  effects of tension on the underside.
• The natural curve of the arch and its ability to
  dissipate the force outward greatly reduces the
  effects of tension on the underside of the arch.
  

• Compression
• Arch bridges are continuously under compression.
  The force of compression is pushed along the
  curve of the arch toward abutments.

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Suspension Bridges

• Suspension Bridge
• Tall towers on both sides of the roadway support
  the main cables.
• The cables run the entire length of the
  suspension bridge and are anchored in the
  concrete abutments at each end.
• Smaller cables are suspended from the main cables
  and these cables support the roadway.
• Almost all suspension bridges have, in addition
  to the cables, a supporting truss system beneath
  the bridge deck (This helps to stiffen the deck
  and reduce the tendency of the roadway to sway
  and ripple.

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Suspension Bridges

• Light, and strong, suspension bridges can span
  distances from 2,000 to 7,000 feet far longer
  than any other kind of bridge.
• They are ideal for covering busy waterways.
• This type of bridge is the only practical type
  suitable for very long spans or when it would be
  hazardous to maritime traffic to add central
  supports.

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Cable-Stayed Bridges

• Similar to the suspension bridge but the
  difference lies in how the cables are connected
  to the towers
• This bridge supports the roadway by cables that
  run from towers to the actual roadway
• Cable-stayed require less cable, can be
  constructed out of identical pre-cast concrete
  sections, and are faster to build.
• Cost-effective bridge that is undeniably
  beautiful.

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Cable-Stayed Bridges

• A cable stayed bridge is a bridge with one or
  more pillars.
• The two types of cable-stayed bridges are
  parallel attachment design and radial attachment
  design.
• In a parallel attachment design the cables are
  attached at different heights along the tower and
  are parallel to one another
• in a radial attachment design the cables are
  attached at a single point at the top of the
  tower and on several places on the road.

Parallel Attachment Design
Radial Attachment Design
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Cantilever Bridges

• A structure or beam that is unsupported at one
  end but supported at the other, like diving
  boards
• A cantilever does not reach all the way across
  the bridge so they are connected in the middle by
  a part called the suspension span
• The suspended span may be built off-site and
  lifted into place, or constructed in place using
  special traveling supports.

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Cantilever BridgeForth Railway Bridge

• The Forth Railway Bridge (or Firth of Forth
  Bridge) is made of a pair of cantilever arms that
  extend out from two main towers.
• The beams are supported by diagonal steel tubes
  projecting from the top and bottom of the towers.
  
• These spans support a central suspended span.
  Some structural members of the bridge are as
  large as 12 feet in diameter.

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Cantilever Bridges
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Movable Bridges

• This bridge is usually used to span canals and
  rivers that carry heavy boat traffic
• Usually powered by electric motors
• In the past they were powered by steam engines
• original movable bridges were the drawbridges of
  medieval times, raised by ropes and pulleys
• There are three main types
• 1.Bascule
• 2.Vertical lift
• 3. Swing

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Bascule Bridge or Drawbridge

• Used for short distances
• Have two movable spans the rise upward, opening
  in the middle
• When open the weight is supported by the
  stationary section of the bridge

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Vertical-lift Bridge

• Used for longer distances
• Straight bridge, held between two towers
• Lifted by steel ropes, attached to counterweights
• -as the counterweights go down the bridge goes
  up and vise-versa.
• Operate in an elevator like fashion

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Swing Bridges

• Mounted on a central pier
• The central pier allows the bridge to rotate to
  the side
• Uncommonly used because the central pier is
  located in the area where boats like to travel

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Moveable Bridges
Bascule Bridge
Swing Bridge
Lift Bridge
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Understanding Bridges

• Video
• http//videos.howstuffworks.com/hsw/20829-understa
  nding-bridges-video.htm