I have to build a balsa wood bridge for my physics class, I was wondering if anyone has any tips?

Question

the dimensions are
max. 3in width
min 13 in length
max 5 in height

we get 20' of balsa wood and as much glue as we want but our score goes by the mass it holds divided by the bridge's weight

Answer 1

I recommend that you make a 3" circle for where the mass will be placed. Then cut girders that will span the 13" length and make them in the shape of arches so that in the center of the bridge the height is 5" including the thickness of circle to hold the mass. Then taper them to 2-1/2" at the abutments. You will need a few more small pieces to connect the girders together so that they don't buckle when the mass is added.

The design concept is based on the torque the bridge will experience when the mass is placed in the center. That is the maximum stress point for the bridge.

You didn't state where the mass would be placed, I assumed the center. If you could place the 3"disk at one end of the bridge, then you only need girder height there and very thin to the opposite end.

Look at the torque at one end of the bridge:
F*13-(w*7.5+m*g*d)=0
where F is the reaction force at the abutment at the other end of the bridge and d is the distance of the mass from the end you are summing torque:
F=(w*7.5+m*g*d)/13

as d decreases F decreases and as w decreases so does F. If the rules allow you to have the mass supported at the abutment, that is the highest performance you will attain for the bridge. If the mass is in the center, then back to the first solution.

j

Answer 2

Load capability is determined by length x depth x sectional area. You also have to calculate any load moments if you have a load. The triangle is the best shape but it's vertical Strength lessens as the base increases in length'