Comments on: I Beam Bridge

By: Garrett Boon

Garrett Boon — Wed, 11 Nov 2009 07:09:41 +0000

TD Miller,

Thank you for posting. So one can look at a model bridge as simply an optimized beam. I had not thought of that before.

By: TD Miller

TD Miller — Sun, 08 Nov 2009 21:30:16 +0000

An I-beam (actually called a W-flange) is most useful when the beam is loaded in bending; like the base girder beam of a bridge. In bending, the largest stresses occur at the very top and very bottom of the beam’s cross-section farthest away from the neutral axis. In a bridge, the top flange (horizontal) is in compression (pushed together) and the bottom flange is in tension (pulled apart). The web (vertical) in between the flanges resists the bending and most of the vertical shearing forces. In a sense it IS an optimized vertical rectangular beam. Steel industry handbooks denote them by weight/ft and overall depth with an extra value I for the area-moment of inertia used to compare beams of different geometries. Barring that, structural engineers just go by “vertical strong, flat is wrong”

By: j22

j22 — Mon, 26 Oct 2009 23:04:51 +0000

luv site helped sci-fair a ton

By: Garrett Boon

Garrett Boon — Tue, 30 Dec 2008 18:29:17 +0000

Double A, let me try and explain the I-beam. An I beam is only useful for members in compression, such as the top chord of a truss. The shape of an I is more efficient than a rectangle piece of wood.

The orientation of the pieces of wood in an I beam is what gives it strength. A rectangle piece of wood is much stronger across its longer side than its thinner side. If you have a popsicle stick lying around you can easily test this. An I beam creates a shape that uses only what is necessary for a beam to be strong in both the vertical and horizontal planes without extra mass.

Unfortunately I do not have any more pictures of this bridge. I built it 5 or 6 years ago, and it has long been destroyed.

By: Double A

Double A — Tue, 30 Dec 2008 16:27:00 +0000

I am confused about the I-beam. Could you please tell me a little about beams and how they work. I don’t understand how an I-beam is so useful in bridge building. I was also wondering if you could give photos of the same I-beam bridge, but from different angles.
Thank you so much for the help! It really helped me.

By: Garrett Boon

Garrett Boon — Thu, 18 Sep 2008 15:01:52 +0000

Thank you, Balsa Fan, for your comment. If you have any feedback on how I can make this website better, please let me know.

By: Balsa Fan

Balsa Fan — Wed, 17 Sep 2008 15:20:37 +0000

Great projects. Really informative, well done. I think it is great that you let people disagree with you, and you respond to questions so thoughtfully and respectfully. It is a shame that some people post childish comments that have nothing to do with your site, or balsa engineering projects at all, I can only assume that they choose to have nothing better to do with their time, which is also a shame. Keep up the good work.