Truss Design Advice?

Question

Hello. I am taking an Engineering Introduction class in college, and our project is to build a truss bridge that can sustain a 2000 lb load. The length is 24 in exactly, and the height and width can not be more than 5 inches. Its a two parallel symmetrical level truss bridge. Do you have any advice before my group starts on the project building? Thanks.

Answer 1

Depending on your constraints (materials, available technology, etc.) this can either be fairly trivial (especially if you can weld) or a major challenge.

Take a look at:
http://www.garrettsbridges.com/design/trussdesign/

It gives you a nice overview of the relative advantages of different trusses.

1 ton is a rather heavy load for a 24in bridge. Is this going to be a point load or distributed?

Either way, you have to make sure that the load gets transferred properly.

Pay careful attention to both the constraints of the design and the criteria by which the results will be judged.

If you can model your design you'll be a lot better off. If your school doesn't already provide it, get some CAD/CAM software that offers Finite Element Modeling support. All the major vendors offer major student discounts. Learning how to use it well enough for this relatively simple structural analysis problem should not be difficult.

(Note - if your bridge doesn't break but bends enough that the load "falls off" you still lose, so you have to model deflection as well as strength. Also, reality says that your load will be off-center by at least a little so your symmetric bridge will give asymmetric results.)

If you are going to build your truss out of pieces (as opposed to machining it out of a solid block of material, etc.) pay extra attention to your joints. Not only are they the most vulnerable parts, but a joint can easily fail catastrophically, hurling pieces of the truss around at high speed (i.e. with the energy that comes from a 1 ton load, you can end up with deadly missiles if you are not careful).

If you have a way to test them (check the material science lab at the school - maybe they have X-ray, Magnaflux, ultrasound, or some other examination equipment you can take advantage of.), go for it.

Good luck.

Answer 2

When you decide on the web configuration, make sure all of your critical members will be in tension, not compression. Tension makes more efficient use of the material while compression members may buckle.