Elevated Bridge B/C

[SLM]

...
That wouldn't be my design of choice, as you're supporting up to four times the load in some members and greater than the load in almost all of them. While it's the ratio of loads between members that I find the most helpful, you may find designs that lower the load but do not use substantially more members.

I suppose there are two approaches to this. You can try to "optimize" your design by having the members carrying near the same force, or you can try to "optimize" it by choosing different size members so that the ratio of force to cross-sectional area of the members would be close to each other.

For example, for the bridge that AlphaTauri posted above, a top member carries a compression force of 262, and one of the members at the base of the bridge on either leg carries a force of 85. One approach to "optimization" would be to modify the bridge geometry so that these member forces would be closer together, and then use the same size wood (say 1/4" by 1/4") for both members. Alternatively, you can keep the geometry as is, but use different size wood for each member (say 1/4 x 1/4 for the top member and 1/4 x 1/8 for the leg member).

Here is another important point that you need to keep in mind when designing your bridge. There are two primary reasons why forces are created in structural members. The obvious one is that the structure is subjected to a load, like the bridge being subjected to a load of 15 kg. The less obvious one, the one that probably makes most of SO bridges fail prematurely, is excessive displacement of the joints of the structure. Such displacements could produce rather unanticipated internal stress in members causing them to fail. For example, A few millimeters of lateral displacement at the base of the bridge could result in significant (bending) stress in a member along the top of the bridge. So, when you are designing your bridge take displacement into consideration and try to minimize it as best as you can.

[Bjsong]

This is my second year building elevated bridge and I have some questions about building the bridge. I remembered last year I had a hard time cutting the wood to make the angles of the wood exactly the same. I tried using a miter box but the wood I was using got so small that the blade just splintered the wood. Does anybody have any suggestion to this problem?

[AlphaTauri]

My solution is to draw out a 1:1 scale blueprint of the bridge and simply use an exacto knife to cut all the pieces to fit the blueprint. I cut the pieces to roughly the correct angle and a little long, then carefully trim them to the correct angle and size.

Another solution is to stack all the similar parts on top of each other, hold it in place somehow, and cut through the whole stack at once. If you can keep the blade straight, the angles should all be the same. However, getting exactly the same angle as on the design is not guaranteed.

[andrewwski]

...
That wouldn't be my design of choice, as you're supporting up to four times the load in some members and greater than the load in almost all of them. While it's the ratio of loads between members that I find the most helpful, you may find designs that lower the load but do not use substantially more members.

I suppose there are two approaches to this. You can try to "optimize" your design by having the members carrying near the same force, or you can try to "optimize" it by choosing different size members so that the ratio of force to cross-sectional area of the members would be close to each other.

Correct.

For example, for the bridge that AlphaTauri posted above, a top member carries a compression force of 262, and one of the members at the base of the bridge on either leg carries a force of 85. One approach to "optimization" would be to modify the bridge geometry so that these member forces would be closer together, and then use the same size wood (say 1/4" by 1/4") for both members. Alternatively, you can keep the geometry as is, but use different size wood for each member (say 1/4 x 1/4 for the top member and 1/4 x 1/8 for the leg member).

Agreed as well. Usually you'll end up with a wide range of forces between different members, and you do want to obviously use different sized wood depending on the forces. But I don't think you want almost all of the members to be supporting forces greater than the load. You can definitely design it so that using a similar amount of members you can have lesser forces in general throughout.

Here is another important point that you need to keep in mind when designing your bridge. There are two primary reasons why forces are created in structural members. The obvious one is that the structure is subjected to a load, like the bridge being subjected to a load of 15 kg. The less obvious one, the one that probably makes most of SO bridges fail prematurely, is excessive displacement of the joints of the structure. Such displacements could produce rather unanticipated internal stress in members causing them to fail. For example, A few millimeters of lateral displacement at the base of the bridge could result in significant (bending) stress in a member along the top of the bridge. So, when you are designing your bridge take displacement into consideration and try to minimize it as best as you can.

In my mind this is the hardest thing to account for. You can't exactly simulate it in a model. But it happens in almost all bridges.

[2win]

how do you get a stick of basswood to be more flexible?

[AlphaTauri]

Soak it in water?

We tried to make an arch with balsa wood last year by soaking it in water then bending it and gluing. It didn't work very well- we snapped every piece of balsa we tried to bend. But that may have been because we weren't very patient and didn't let the wood soak for long enough. Definitely not trying that again this year.

[2win]

okay, thanks, i'll try that.

[nejanimb]

I would just recommend not doing it. Arches, from everything I've ever seen, don't work too well.

[2win]

really? okay, i'll talk to my partner about it, thanks.

[AlphaTauri]

Yeah, arches don't work very well because you can't connect the ends to keep the wood in its curved shape. Since the wood wants to spring back to it's original shape of being straight, the legs are going to want to slide outwards and the middle of the bridge is probably going to bend and break. Basically anything curved is a bad idea in a SO bridge.