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

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    Bridges Truss
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Discussion Overview

The discussion revolves around the structural advantages of triangles in bridge design compared to other shapes like circles and arches. Participants explore the reasons behind the preference for triangular shapes in truss designs, the feasibility of combining different shapes, and the historical context of bridge construction.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants assert that triangles are the best shape for bridges due to their simplicity in construction and load calculations.
  • Others question whether circles or arches might be stronger than triangles, suggesting the possibility of combining these shapes in bridge designs.
  • One participant notes that many bridges incorporate both triangular and arch elements, highlighting that suspension bridges can be seen as inverted arches.
  • A participant mentions the construction of Gothic cathedrals, arguing that triangles eliminate the need for buttressing due to their tension properties.
  • Concerns are raised about the complexity and cost of accurately constructing large circles and arches compared to simpler triangular designs.
  • Another viewpoint emphasizes that historical material limitations influenced the use of arches, which are effective in compression, while triangles are advantageous for tension handling.

Areas of Agreement / Disagreement

Participants express differing opinions on the optimal shape for bridges, with no consensus reached on whether triangles are definitively superior to arches or circles. The discussion remains unresolved regarding the potential for combining these shapes effectively.

Contextual Notes

Participants acknowledge that the effectiveness of different shapes may depend on material properties, construction techniques, and historical context, but these factors are not fully explored or resolved in the discussion.

thezi
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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.

Cheers
 
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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.
 

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