Portlandjib.com
suspension bridges
 

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There are 3 basic bridge types
 
                  BEAM    ARCH  SUSPENSION
 
A beam is simply a horizontal structure supported at each end 

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You're in the middle of a forest and you have to cross a stream.
What can you do?     Cut down a tree so that it falls across the stream.

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The original meaning of the word beam was    tree

A couple of #6 blocks form this beam bridge

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Adding some #2 blocks and now we have a deck for our bridge

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Here's another beam bridge. More complex than the others, but still just a beam bridge.

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But look what happens if we push on the end of the bridge.

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Now if we add a pair of #4 blocks diagonally, the bridge will stand.

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We've built a truss bridge.

A truss is a type of beam, made of lots of smaller pieces (called members) arranged

so that some of them form triangles. 

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The Sellwood Bridge was a deck truss. (The deck was on top of the truss).

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The Hawthorne Bridge is a through truss- ( the deck, and the cars, go through the truss).

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