If a barbell bisects a highway overpass midspan between supports, what is the max weight it would carry?

Question

I have researched this - but bridge design is very complicated and I can't come up with an estimate for this load in simple terms. I built a little roman arch model with a width of 4 in and span of 7.5 in. It holds 100lbs! I want to know what this 100lb. load might be in real-life. Is it a million pounds? My bridge is a single lane. I plan to build truss and beam bridges out of the same material to show how the design changes affect the max static load. My Dad says this is no way to design bridges but I'm having fun and this is not for a grade. Thank you.

Answer 1

Darn, this is a sharp kid. You have a bright future ahead of you in engineering. Keep working at it.

As to your question, the loads imparted can be easily calculated using basic static loads equations. Sorry but I can't really draw the diagrams here. As for the strength of your bridge, now that gets VERY complicated. Now you are dealing with load paths, stress/strain relationships, material properties, all the way down to microscopic imperfections in the crystallin structure of your materials. I was never quite a fan of material sciences so I followed my love of fluid dynamics. Considering how bright you seem, I would recommend getting a college Engineering Statics (not statistics) and a Strength of Materials text books. They will teach you what you want to know. Statics uses mostly algebra, but Strength gets into calculus, so some of it may be over your head. Good Luck. Keep working, engineers are in short supply.

Answer 2

Your dad is right, it is a poor way to design bridges, but it surely builds good engineering minds. A good engineer has a sort of intuitive "feel" for what will work and what wont, for how strong it may be, for whether or not it will work, and stuff like that. And the way to get it is to start in 4th grade with toy bridges. But, for real, try linear scaling. That means, if you make every part of your bridge out of double-thickness material, it just may be twice as strong. Making it longer or wider and all bets are off. Try some experiments and keep some notes on your results. Just like the Wright Brothers. And, in engineering, be sure to have a good time.