Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Structure question

  1. Dec 18, 2005 #1
    What makes truss bridges so strong? Like if you have something supported by triangles, why is it good? I dont really understand the physics behind it.
  2. jcsd
  3. Dec 18, 2005 #2
    you must look at the entire structure itself to understand. each member is supported by teo other members whose stress is oppisite the one being supported
  4. Dec 19, 2005 #3
    A truss bridge gains it's strength from the numerous triangular forms contained in it's structure. Were the structural members of the span arranged in rectangles instead of triangles, the bridge would be weak and collapse. This is because in a rectangle, a lateral force can change the angles without changing the lengths of any of the sides (the beams), allowing the rectangle to collapse without any compressive or tension forces to resist the collapse.

    But in a triangular object (or I think any other polygon with an odd number of sides - isn't that right?) you cannot change the angles without changing the lengths of the sides. So the beams push back on any force that tries to change their shape. Since the triangle has the least number of angles of any odd sided figure, even a small change in angle would require a large change in side length, resulting in a large resistive force.
  5. Dec 19, 2005 #4


    User Avatar
    Gold Member

    Nope. Stick with triangles.

    Build a pentagon from straws. You can easily distort its angles without any change in the length of its sides.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook