3. Introduction to Bridges
By Doug Knight
This introduction discusses bridge types and real-world considerations for building bridges
Why have a bridge
To get across a gap - often water-filled
May also carry water - called an aquaduct
How can we get across the gap
Ford or swim
Get wet
Current may be too strong
Cold water
Stepping stones
Cannot use for wheeled traffic
Slippery
Deep water
Ferry
Floating bridge
Not carry heavy traffic
Must be tended
Cut down a tree
Two logs tied together
One log rolls
Limited span
Need a tree
1
So we build a bridge
Rope Bridge
How do you get the rope across
Maintenance
Not too stable
Beam
Solid pillar on both ends
Truss
Uses strength and rigidity of triangles
Cantilever
Anchor one end, and rest the other on beam with a bit hanging out
Cantilever with suspended span
2
Suspension
Need real solid anchor for the cable
May flap in the wind
Portable bridges - pontoon bridges
What materials are used in bridge construction?
Wood
Quick
Deteriorates quickly
Burns
Stone
More durable
But span is limited
Iron
Strong
Expensive
Upkeep is difficult (rust)
Concrete
Usually with steel rods inside it - rebar
Road surface
Asphalt
3
What does an engineer have to consider in building
a bridge
Length of the crossing
Weight the bridge must carry - vehicle weight plus its own
Ice loads
Ship collision loads
Water eroding the support piers - erosion
Seismic effects - earthquake - applies even here
Height of the bridge - do ships have to go underneath? CONFEDERATION
BRIDGE VG
Strength of the land it will sit on - PEACE RIVER BRIDGE VG
Weather conditions – HARTLAND BRIDGE VG
Wind affect - TACOMA NARROWS BRIDGE VG
Environmental effects - what will the bridge do to the environment?
How long must it last?
Funding
4
What if we have to put a support in the middle of the
river?
Supports must be on bedrock
Use a cofferdam and pump out the water - good for shallow water
Large metal cylinder (caisson) and gradually dig it into the river bed pressure becomes a problem, because the pressure inside the
cylinder must be high enough to keep the water out - nitrogen
narcosis - the bends - Brooklyn Bridge
Dredge the silt and place pre-assembled pieces in place from a floating
barge - Confederation Bridge
Expansion Joint
Allows the bridge to expand with differences in temperature
One end fixed - the other slides
Cable stretches - bridge moves up and down
5
Famous Canadian Bridges
Quebec Bridge - longest cantilever RR bridge in the world
Confederation Bridge - longest bridge in the world over an ice-covered
waterway
Sault Ste Marie Bridge - railway swing bridge
Hartland NB - longest covered bridge
Peace River Bridge - suspension bridge - fell down
http://www.abcdpittsburgh.org/webshots.htm
http://www.swishweb.com/Science_and_Technology/Bridges/
www.confederationbridge.com
www. ufacademy.com/webmania/bridges
www.buildingtechnology.com/bcba/bridges/basics.htm
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Simple Beam Bridge
Truss - Uses strength and rigidity of triangles
7
Cantilever Span
Anchor one end, and rest the other on beam with
a bit hanging out
Cantilever with suspended span in the middle
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Suspension Bridge
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Peace River Bridge
Built in 1942, the foundations of the Peace River Bridge at Taylor, BC, were
unknowingly built on unstable soil. Ground water eroded away the banks,
and despite regular maintenance the bridge collapsed in October 1957.
Peace River Bridge after collapse
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Tacoma Narrows Bridge (Washington, USA)
Wind – 1, Bridge – 0
The wind rushing down the gorge started the bridge swaying and twisting.
Eventually the bridge collapsed.
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The Longest Covered Bridge in the World
Hartland, New Brunswick
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Cantilever Bridges
The Longest Cantilever Road Bridge in the World – The Firth Bridge
The Longest Cantilever Railroad Bridge in the World – Quebec Bridge
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The Longest Bridge over an Ice-covered
Waterway
The Confederation Bridge linking New
Brunswick and Prince Edward Island
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The Sioux Narrows Bridge
An Example of a Truss Bridge
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Aquaduct – A Bridge That Carries Water
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Pontoon Bridge
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