Elevated Bridge B/C

[blue cobra]

Can you get graph paper big enough to fit an entire bridge on it? Right now I tape two pieces together, but the tape can stretch and slip.

[nejanimb]

It exists - that's what we do! Maybe ask an art teacher?

[andrewwski]

I always just used posterboard with a ruler and protractor - I suppose graph paper may be easier, but you can do without. Make sure your graph paper is in increments of fractions of centimeters, or it's not going to be that useful.

[smartkid222]

the biggest they have is 11 x 17 inches for good clarity (4 boxes an inch)
if you go bigger than that the boxes become like 1 box an inch and the whole thing is huge.

[13bridges]

I have discovered that I use graph paper not for the boxes, but as a reference when I draw my lines. The lines help me build my beams parallel, but otherwise, I have no use for the actual sizes of the squares. Because of this, i usually buy a ghostline tri fold. I use the faint lines they have to draw my bridge design, plus, the foam board allows me to use pins to hold my wood down when gluing.
http://www.craftsetc.com/store/item.asp ... emId=64862

[blue cobra]

Last weekend I drew up a new bridge design, plugged it in to JHU, and the results, were, well, considerably less than desirable.
As seen here.
When the load is moved to the top, the forces are only a bit lower.
Why is this bridge so, terrible? I know it's my first bridge design of the season, but I really wasn't aware you could make the forces that high. I win at that, I suppose

By the way, I don't draw on the foam board so I can use it over. I've never used foam board before, though, so I really don't know if I can use it over.

[AlphaTauri]

Wow. I've never seen it go up that high either. That means with a load of 15kg (probably not going to happen with that bridge, no offense), the force on the center member would be 60kg. I think even the best balsa would snap under that kind of stress.

I'll put up an approximation of my current design soon.

Edit: Um...anyone know how to take a screenshot?

Edit 2: Never mind. I'll post it once my computer starts cooperating.

[SLM]

Last weekend I drew up a new bridge design, plugged it in to JHU, and the results, were, well, considerably less than desirable.
As seen here.
When the load is moved to the top, the forces are only a bit lower.
Why is this bridge so, terrible? I know it's my first bridge design of the season, but I really wasn't aware you could make the forces that high. I win at that, I suppose

By the way, I don't draw on the foam board so I can use it over. I've never used foam board before, though, so I really don't know if I can use it over.

You have placed a load of 150 on your structure. I am assuming that the unit of this force is Newton, since 15 kg is approximately equal to 150 Newton. If my assumption is correct, then you have placed too much load on the truss. You need to place only 75 N on the truss since there will be two such trusses in your bridge. The total load is going to be supported by two trusses, not one. So, each truss should be designed to carry half of the total load.

[andrewwski]

Eh, I say just use 100 as your load, then you can see your forces as a percent. I've never really found use for putting in an actual number, as I really don't think in terms of carrying the whole load (figure the load is going to be dynamically increasing anyway).

The difference between the load on the bottom and top can probably be attributed in this case to dividing it between two separate nodes - the real forces would be the same if loaded from the center. But you also have to consider that while these seem like point-loaded bridges, the loading block actually distributes the load across a fair portion as well.

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.

Keep playing around with it, you'll find there's a lot of different things you can come up with. I've actually drawn up a design this year that I'm extremely pleased with, not that it matters anymore.

[AlphaTauri]

Finally got everything to work. Here's the bridge I designed.
Bridge 1
I put 150 as the load instead of 100, though. Shouldn't have done that.