I Beam Bridge

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This was one of my earlier bridges. I built it after talking with my uncle, who is a mechanical engineer. He suggested that I try using an I-beam. This was a very good idea, but my construction skills at the time were not the best. In theory an I-beam is excellent for bridges, but I could not make it very well.

I don’t know how much this bridge weighed, but it did not hold very much weight.

I had problems making the two I-beams for this bridge. I had to cut my own strips of wood, and I was not very good at doing that yet. The wood I used was very thin, perhaps too thin. I believe I used 1/32″ x 1/4″ strips.

I could not cut the wood strips very straight, and that was the downfall of this bridge. The pieces to the I-beam were not glued together well at all. I was trying out some unique things with this bridge. And if I had better building skills, it would have turned out much better. In later years I was able to make good I and T beams which performed very well.

7 thoughts on “I Beam Bridge”

  1. 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.

  2. 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.

  3. 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.

    • 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”

      • 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.

      • Sorry TD, an I beam is a I beam. “W-Flange” denotes the I beam is of the wide flange type as apposed to the “S” type otherwise known as “structural beam.”

        You are correct in stating they are typically oriented with the web being vertical. However, there has been a few successful designs (real buildings) that incorporate the opposite orientation.


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