Why Are Triangles the Optimal Shape for Bridge Design Over Arches?

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So triangles are apparently triangles are the best shape to use in bridges... But why?
I'm pretty sure that a circle or even an arch is stronger than a single triangle. So is it possible to combine circles or arches on a bridge like how a truss design combines triangles?
 
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A triangle is simple to construct. Circles and arches are harder to build I suspect.

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thezi said:
So triangles are apparently triangles are the best shape to use in bridges... But why?
I'm pretty sure that a circle or even an arch is stronger than a single triangle. So is it possible to combine circles or arches on a bridge like how a truss design combines triangles?
Triangles are simple to construct and calculate loads on (important prior to the advent of computers). Many bridges combine some of both. And note: many suspension bridge configurations are inverted arches.
 
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Here is a picture of the Jacques Cartier bridge in Montreal.
Would you call that a truss bridge or an arch bridge?
800px-Jacques-Cartier_Canada.jpg

or the Chaotianmen Bridge in China
220px-Chaotianmen_Bridge-1.jpg


Note that what one considers the truss making up the arch is composed of triangles for both bridges.
The arch redistributes the loading to the piers.
 
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Take a look at the construction of a Gothic cathedral. Note the 'buttresses.' Triangles have an intrinsic tension member which make 'buttressing' unnecessary. Improvements in materials (since the heyday of Gothic cathedral construction) make this limitation of arches less obvious and more easily addressed.
 
thezi said:
So is it possible to combine circles or arches on a bridge like how a truss design combines triangles?
To make accurate (accurate enough to be temptingly stronger than triangles built from trivial straight elements) circles and arches is not an easy ( or cheap) task at that size. Big CNC still comes with horrible bills.
 
just my opinion but when you had lack of materials like steel, arc hwas one way to construct a bridge. Note the arch lends itself to compression. Stones do well in compression. Old arch bridges had roadway on top.
Triangles is great to construct and are great at handling weight in tension. Roadway is placed low and is in tension. Look at the load vectors.