Elevated Bridge B/C

[SLM]

If you have an 1/8 inch square piece and a 1/8 by 1/16 piece the 1/8 by 1/16 piece of wood would be lighter and about as strong.

Assuming all other factors constant, that is not really true. Please correct me if I'm wrong, but I believe the "theoretical" (ignoring wood inconsistencies) tensile strength would be half, and I think the compressive strength would be less than half, since there is less wood and it would be more prone to buckling.

Assuming wood density is identical in both members, the 1/8 x 1/8 square section is twice as strong as the rectangular 1/8 x 1/16 section with respect to tensile, compressive and buckling strengths. For tension and compression, strength is directly proportional to the cross-sectional area of the member, the larger the area the stronger the member. For buckling, strength is directly proportional to a section property called moment of inertia, which for rectangular sections is (1/12)(b)(h^3) where b is width and h is height of the section. The larger the moment of inertia the more resistance the member offers against buckling.

[nejanimb]

I find it more useful to think in terms of equal weight for the same length of stick, rather than the same density, since that's what matters more in terms of finding an efficient design - finding the lightest and strongest possible pieces to go in the places and connect the distances you want.

Considering pieces of equal weight:

A smaller cross-section piece will have more tensile strength than a larger one. The full reasoning for this was discussed much earlier in the thread. Therefore, since a flat piece will have the same glue area as a square one, the flat piece is superior. That is to say, used as a solo tension member, a piece of 1/8 by 1/16 would be more effective than a 1/16 " piece.

A square piece will have more compressive strength than a flat one. This is because, as Balsa Man went into great detail about on a previous page, the wood will "choose" to buckle across its shortest cross section. By making it flatter, the wood has a very easy decision to make about which way to buckle. This is very easy to test for yourself - try compressing a flat piece with your hands, and it will *always* bend along its short side. With a square piece, it can go either way, so it locks into place better. In general, for the same weight, a larger cross section is better than a smaller one, even if this is at the cost of true density, which is the opposite of what you use for tension. This is why, in the real world, we always see columns being used to hold things up in compression, while skinny cables are use for tension purposes.

However, as I said, I dislike square pieces for both compression and tension. You can either make L-laminations as JimY suggests with your flat pieces to make them more efficient, or use a number of other methods (as we do) to make flat pieces more effective for both compression and tension.

[StampingKid]

Happy Mother's Day to my mom who gave up her diningroom table for Science Olympiad season--Christmas--Easter. Where does everyone else build their bridges?

[blue cobra]

I have a workbench in my basement about 10ft long or so. My dad gave it to me when he built himself a new one, and a piece of plywood over the existing top-which has seen better days-made it suitable for SO.

[sj]

i use my ping pong table which is now completely unsable for its original purpose due to giant globs of CA and deep scratches...

[cypressfalls Robert]

I use a huge piece of plexi-glass .......which was supposed to be for my mission

[AlphaTauri]

I used to use a couple pieces of laminate wood flooring, which I liked because I could move them around the house (i.e. hide when we had company over) and because I didn't have to worry about leaving knife marks in any important/nice-looking piece of furniture. It's also pretty resistant to exacto knives, and quite durable.

[old]

I find it more useful to think in terms of equal weight for the same length of stick, rather than the same density, since that's what matters more in terms of finding an efficient design - finding the lightest and strongest possible pieces to go in the places and connect the distances you want.

Considering pieces of equal weight:

A smaller cross-section piece will have more tensile strength than a larger one. The full reasoning for this was discussed much earlier in the thread. Therefore, since a flat piece will have the same glue area as a square one, the flat piece is superior. That is to say, used as a solo tension member, a piece of 1/8 by 1/16 would be more effective than a 1/16 " piece.

A square piece will have more compressive strength than a flat one. This is because, as Balsa Man went into great detail about on a previous page, the wood will "choose" to buckle across its shortest cross section. By making it flatter, the wood has a very easy decision to make about which way to buckle. This is very easy to test for yourself - try compressing a flat piece with your hands, and it will *always* bend along its short side. With a square piece, it can go either way, so it locks into place better. In general, for the same weight, a larger cross section is better than a smaller one, even if this is at the cost of true density, which is the opposite of what you use for tension. This is why, in the real world, we always see columns being used to hold things up in compression, while skinny cables are use for tension purposes.

However, as I said, I dislike square pieces for both compression and tension. You can either make L-laminations as JimY suggests with your flat pieces to make them more efficient, or use a number of other methods (as we do) to make flat pieces more effective for both compression and tension.

I am afraid that I cannot understand what you are saying. You say "A smaller cross-section piece will have more tensile strength than a larger one." but unless you have left out a significant portion of what you are thinking this just isn't so. A smaller cross section of much densier wood could have a greater tensile strength, but won't necessarilly, but a smaller cross section of similar spec. wood will certainly have a lower tensile strength. THen you say that "A square piece will have more compressive strength than a flat one." If you mean buckling strength then you are likely correct but the compressive strength is unrelated to moment of inertia issues like buckling is.

[T-B]

I know several of the big guns are getting ready for nationals; we got second in our state but our team wasn't anywhere near to moving on to nationals. So it looks like our bridge building career may be over. I'm ready to get started on towers. Has anyone built towers before? How do you connect the sides? What is you want to use three sides instead of four? Bass for the big compressions pieces, laminated balsa? What about beam shapes? L, V, T shapes. If you laminate where the two sides meet, then how do you miter those joints along the long side of the wood. I'm probably way ahead of myself here since they haven't even announced the specs yet, but I want to get started over the summer. Don't I say that every year?

[nejanimb]

I find it more useful to think in terms of equal weight for the same length of stick, rather than the same density, since that's what matters more in terms of finding an efficient design - finding the lightest and strongest possible pieces to go in the places and connect the distances you want.

Considering pieces of equal weight:

A smaller cross-section piece will have more tensile strength than a larger one. The full reasoning for this was discussed much earlier in the thread. Therefore, since a flat piece will have the same glue area as a square one, the flat piece is superior. That is to say, used as a solo tension member, a piece of 1/8 by 1/16 would be more effective than a 1/16 " piece.

A square piece will have more compressive strength than a flat one. This is because, as Balsa Man went into great detail about on a previous page, the wood will "choose" to buckle across its shortest cross section. By making it flatter, the wood has a very easy decision to make about which way to buckle. This is very easy to test for yourself - try compressing a flat piece with your hands, and it will *always* bend along its short side. With a square piece, it can go either way, so it locks into place better. In general, for the same weight, a larger cross section is better than a smaller one, even if this is at the cost of true density, which is the opposite of what you use for tension. This is why, in the real world, we always see columns being used to hold things up in compression, while skinny cables are use for tension purposes.

However, as I said, I dislike square pieces for both compression and tension. You can either make L-laminations as JimY suggests with your flat pieces to make them more efficient, or use a number of other methods (as we do) to make flat pieces more effective for both compression and tension.

I am afraid that I cannot understand what you are saying. You say "A smaller cross-section piece will have more tensile strength than a larger one." but unless you have left out a significant portion of what you are thinking this just isn't so. A smaller cross section of much densier wood could have a greater tensile strength, but won't necessarilly, but a smaller cross section of similar spec. wood will certainly have a lower tensile strength. THen you say that "A square piece will have more compressive strength than a flat one." If you mean buckling strength then you are likely correct but the compressive strength is unrelated to moment of inertia issues like buckling is.

I didn't leave it out. What I said is that, for pieces of equivalent weight, a smaller cross-section piece will have more tensile strength than a larger one. That is, a 20cm, 1 gram piece of 1/16 by 1/16 will be stronger in tension than a 20 cm, 1 gram piece of 1/8 by 1/8. This does, of course, mean that the piece is denser, as you suggested. As I said, since when constructing a bridge the important part is how much mass you spend connecting two nodes, it's therefore more practical to speak of comparing pieces of equivalent mass rather than equivalent density.

Also, practically speaking, buckling strength and compressive strength are synonymous. Perhaps a better way to say what I said would have been "A square piece will be a more effective compression member than a flat one." That is, a 20cm, 1 gram piece of 1/8 by 1/8 will be better as a compression member than a 20 cm, 1 gram piece of 1/8 by 1/16.

Hopefully that makes more sense!