Elevated Bridge B/C

[sewforlife]

Not sure if this was said or not, but is an arch design for a bridge better or worse than a uhh.. not arch design?

hmm. I saw a lot of arches as well. the only clever design I saw was a small arch and it had a support going from one end to the other, which was just in case the arch wasn't symmetrical. but it looked pretty symmetrical to me.

Do you know if it did well or not?

not too well. maybe an 800 efficiency. but then again, in my state 800 is really good.

[fli754]

The kids I coached (2 different schools) built lamanated bridges which finished first and second at
state. First was over 1400 efficiency. However I believe that particular design would of maxed out
around 1500 plus or minus a 100 or so. Neither team won overall so no trip to the nationals. We laminated wet 3/32 in.strips of 6 to 10 lb.per cu.ft. balsa with thinned carpenters glue (yellow
alaphetic glue). Multiple lamanations tends to reduce the effects of variations in the wood.
In buildings sometimes you can see structural laminations (referred to as Structural Lams) in the form
of pre laminated curved beams that go up the walls and across the roof. They are often seen in churches and gymnasiums.
By the way bending the wood does not weaken it. Wood does not fatigue or strain harden. So long as
you do not exceed it's bend limits (fracture) it is not compromised.
Instead of building a lot of bridges, we repaired our prototype several times, strengthing the areas that failed. One early bridge was repaired 5 times.
This was all B division. Lamination would probably be less effective in C division.
Greg Doe
Smyrna,TN

Would repairing a bridge many times put stress on the pieces? Or is there nothing wrong with it

[Balsa Man]

Would repairing a bridge many times put stress on the pieces? Or is there nothing wrong with it

Repairing doesn't "put stress" on anything; loading puts stress on.
Repairing will add weight, of course - each time you do it.
I believe the process/approach Greg is describing is much like we use (with our saftey tower I described earlier). This is a process for developing a design; sequentially finding out where the weak places/members are - what you need for various members to carry design load. Bridges/structures repaired in such a process aren't something you take to a competitition. You take what you've learned from them, and then build one applying that knowledge - what sized, or density, or construction (as in lamination) piece(s) you need to carry the forces you're dealing with.

[Genny]

about creating an arch-
making arches is pretty easy, just time consuming, and it gets even easier with practice. when i built with arches, my design called for a 3/16x3/16 arch, so i bent 2 pieces of 1/16x3/16 and 2 pieces of 1/8x3/16. I boiled water in a wide saucepan and steamed the wood until it was softened enough to bend to touch the water. I held each end and slid it back and forth through the water in a u shape until it was bent a little past the curve i needed. Then i took the pieces and pinned them to the design to be sure i was getting the correct arch, with the thicker pieces closer to the base of the design. they dried overnight, then i laminated them together with just a little light glue. they arent hard to build, but they arent really needed. in some designs they are important, but like nejanimb said, probably more trouble then its worth

[fli754]

Would repairing a bridge many times put stress on the pieces? Or is there nothing wrong with it

Repairing doesn't "put stress" on anything; loading puts stress on.
Repairing will add weight, of course - each time you do it.
I believe the process/approach Greg is describing is much like we use (with our saftey tower I described earlier). This is a process for developing a design; sequentially finding out where the weak places/members are - what you need for various members to carry design load. Bridges/structures repaired in such a process aren't something you take to a competitition. You take what you've learned from them, and then build one applying that knowledge - what sized, or density, or construction (as in lamination) piece(s) you need to carry the forces you're dealing with.

Oops my bad, i meant by reusing pieces and loading and reloading them, wouldn't that make the overall bridge weaker?

[croman74]

If you reuse pieces, they are weaker the second time. I did that once (due to limited amount of time) and on the 3rd test the bass wood broke.

[cypressfalls Robert]

If i am correct there different grades of wood for the same size peices...right?

[croman74]

Yeah. I put a bit of info in the wiki on this. There are 3 main grains: A, B, and C. A grain is more flexible than the others and is probably better for tension. C grain is good for compression as it is stiffer. And B grain is a mix of the two.

[Balsa Man]

If i am correct there different grades of wood for the same size peices...right?

Yeah. I put a bit of info in the wiki on this. There are 3 main grains: A, B, and C. A grain is more flexible than the others and is probably better for tension. C grain is good for compression as it is stiffer. And B grain is a mix of the two.

Croman is right-on about the different grain types. When you speak of "grades", I'm guessing you may be talking about "weight-grades." Couple things on that.

Balsa, as discussed on this forum and elsewhere, comes in a wide range of density- from around 5 pounds per cubic foot to....mid 20s pounds per cubic foot. It all depends on where out of a log the wood is cut from. So, for a given size - let's use 1/8th square sticks - depending on the density, you can get sticks of widely varying weight. Your local hobby store will have a...bin of sticks; the only way you know what they weigh is to weigh them. Places like Specialized Balsa (check them out on the Web) sell "weight-graded" wood- they weigh sticks, and you can by by weight. Using our 1/8th square exmple, you can buy 36" sticks in 1/10th gr increments - from 0.8gr to 4.4 gr. That doesn't mean each stick weighs exactly 1.0 gr, by the way. If you were to order say six 1.0 gr sticks, you would find they all are close to 1 gr - probably between 0.95 and 1.05.
Hope that answers your question.

[jander14indoor]

Yeah. I put a bit of info in the wiki on this. There are 3 main grains: A, B, and C. A grain is more flexible than the others and is probably better for tension. C grain is good for compression as it is stiffer. And B grain is a mix of the two.

Careful, for a square stick you can't really use A, B, or C to define the stick. It applies to the faces, not the bulk properties. You can have either two A and two C faces (parallel to each other) or all B faces. For straight tension, it just doesn't matter which way the grain is oriented. Strength per mass, not stiffness is most important there. For compression, grain orientation, not grain, can help resist bucklin. If you are equally likely to buckle in all directions, the uniform stiffness of B grain is best. If you are more likely to buckle in one plane than the other, orient the C grain faces parallel to the likely direction of buckling to take advantage of the increase stiffness this way.

Jeff Anderson
Livonia, MI