Designs B/C

[chinesesushi]

So regardless of where the load is placed (generally) . . the top beam is always in compression and the bottom is in tension.

but if the load is placed on the bottom are the vertical supports from the top to the bottom of the bridge in tension? It would seem to make a lighter bridge you'd want as many pieces in tension as possible because those pieces can be lighter.

I suggest you use the simulation program I suggested, may answer some questions. If not, let me know.

[dholdgreve]

Keep in mind that by minimizing the number of pieces in compression within the design, you may be increasing the amount of compression force that the remaining pieces need to carry...

[SOCoach]

Is there an optimal height for a bridge? My students are struggling to get the weight down and it occurs to me making it shorter (not as tall . . . not length) would reduce the weight and help prevent buckling.

[chinesesushi]

The structure isn't so much twisting and breaking as it is deforming downwards. Across the entire span the deformation is actually quite noticeable. But since the break patterns aren't very conclusive, I can't really be 100% sure that's all that is happening to cause failure without high speed footage.

Ahh the lengths I went to trying to get high speed footage of my structures breaking....

From how it sounds you could probably try horizontal braces, if the whole thing deforms try making it taller (I haven't read the rules enough to know if you can...?)

No rules about maximum/minimum heights of bridges, though it is used as a second tiebreaker. One issue I encountered when making my bridges taller while still trying to maintain a low mass was that all my members were longer and less dense so they buckled more. So far my best efficiencies are from my 10cm tall bridges (I've tested bridges from 10cm tall to 17cm tall).

Is there an optimal height for a bridge? My students are struggling to get the weight down and it occurs to me making it shorter (not as tall . . . not length) would reduce the weight and help prevent buckling.

[SOCoach]

So taller helps lessen the effects of downward deformation . . . . but you run into more buckling issues?

[chinesesushi]

So taller helps lessen the effects of downward deformation . . . . but you run into more buckling issues?

If you want to keep the bridge at a mass equal to a shorter bridge, making the bridge taller would mean the compression members would have to be less dense, which leads to more buckling issues.

[bernard]

So taller helps lessen the effects of downward deformation . . . . but you run into more buckling issues?

If you want to keep the bridge at a mass equal to a shorter bridge, making the bridge taller would mean the compression members would have to be less dense, which leads to more buckling issues.

And having compression members of the same density would already lead to more buckling because they are longer. The tallest bridge I've built was 17cm and it buckled way too much and didn't score that well. The best bridges I've built are around 10cm, but I haven't tried anything short than that.

[IvySpear]

So if I were to make a bridge with the load at the top, would Arch form work better, or would beam? By the way, has anyone tried suspension bridges? We tried but it didn't really work out.

[someusername]

So if I were to make a bridge with the load at the top, would Arch form work better, or would beam? By the way, has anyone tried suspension bridges? We tried but it didn't really work out.

I am seeing that a truss like design is going to be the best in this event. Suspension bridges are good if you want to span really long distances mostly.

[dholdgreve]

suspension bridges are only viable if they are either multi-span (i.e. have a center support piling), or have tall buttresses at both ends that cannot be pulled over to the center... The only way to make that happen to to add a high spandrel beam that keeps the proper distance between the 2 end columns... which brings you full circle back to a conventional top and bottom chord bridge...

Also, Suspension bridges function by draping a heavy cable from one high column to the next, then dropping vertical ties from the cable to the road bed. Since we are limited to using only wood, there is no reasonable substitute for the cable portion of the bridge.