Bridge Designs

[homesciencenerd]

The arc bridge designs are very difficult to build evenly and efficiently, but is not impossible. My partner and i had an arc design that was getting an efficiency of around 1400-1600, which is good but not good enough. We are ending up with a sorta triangle style design.

[Mr_Pep_Band]

I agree with the last few, building and arc or arch with balsa wood is quite difficult and may not perform as well as it could due to load eccentricity, wood imperfections, etc.

I stick with straight members so they perform similar to a "two-force member", and they are easier to build.

For tension members, the best choice is to have a member going from point to point in a line, not curved.
Liken it to a rope or a chain, when it is pulled taught, it forms a line, (in reality a parabola due to gravity, but close enough over short distances)

[dholdgreve]

Something to consider... When a member is put in tension, it can actually be used to brace off of... So if you are having problems with a compression member buckling, brace that spot 90 degrees back to a tension member, and repeat... My guess is that you will end up with a triangle... full of triangles.

[havenguy]

I was wondering what type/size of wood people have been using for the tension piece running across the length of the bridge? I'm currently using a 3/32 by 3/32 piece of basswood, but I would like to slim this down as much as I can.

Thanks.

[Balsa Man]

Well, its been quite a while since I posted anything, but finally got motivated to jump back in. Got out of coaching a couple of years back, just doing a little consulting with a few local teams. Anyway, on tension members in a bridge....

In archives, you'll find my previous input on the subject.
Briefly, the best solution I've found is 1/64th" paired strips. By "best", mean the lightest. By "paired strips", mean two on each of the two trusses (sides of the bridge)- one on each side of the legs, run parallel. Width and density depend on required/design loading.

On a C Division bridge I'm consulting on this year, all four together weigh 0.9 grams, for a span of a bit over 18" 3/16" wide, cut from 3"x36"x1/64" sheet weighing 7.2 grams (this falls into "heavy" density range as that term is used by Specialized Balsa). Tested one- as in a single strip) to a bit over 8kg, with the design load (i.e., the tension load on the members, on one truss at 15kg structure loading of 5.5kg. So, even at this weight, way over-engineered- massive safety factor. For comparison, typical weight for 3/32nds bass is around 1.5gr per 24", so a LOT heavier.

You need to be careful using this technique to make sure the strips are of very equal length (w/in a fraction of a millimeter), otherwise, one of the pair gets all/most of the loading, stressing it, putting a twisting load into the leg, and if lengths on the two sides/trusses aren't equal, then one side gets all/most of the loading. The other thing to take into account is that there will be some stretching/elongation at full load. Testing will get you to the answer for how much in your particular design. In the bridge noted above, we're at a bit less than a millimeter. So, when you shorten, which 'pulls the legs inwards' a bit, as the bridge loads, the legs slide out, and at full load, everything 'straightens out to design configuration.'

Last, re: the post above, on bracing off of a tension member, while it can be.... effective if you're bracing off of something like a 3/32nds bass tension member, won't work well on paired 1/64th strips.

[JonB]

I was wondering what type/size of wood people have been using for the tension piece running across the length of the bridge? I'm currently using a 3/32 by 3/32 piece of basswood, but I would like to slim this down as much as I can.

Thanks.

We use bass tension pieces that are ~1mm thick and relatively wide (I do not know the exact width off the top of my head). We get great efficiencies using these pieces (the exact methods will remain unspoken for now).

[Mr_Pep_Band]

I was wondering what type/size of wood people have been using for the tension piece running across the length of the bridge? I'm currently using a 3/32 by 3/32 piece of basswood, but I would like to slim this down as much as I can.

Thanks.

I use 1/16" x 1/16" Balsa wood. Its the smallest available size i can get and it is also big enough so that there is some give when transporting or handling it.

Using 4 strands on a Division B bridge that stands 9cm tall above the 5cm tall Test Support will get you about 5.3 kg of tension in EACH tension member. The balsa wood can easily handle this load if glued sufficiently at the contact points.

I would point out though that in the vast majority of designs, the tension cords make up a small percentage of the mass of the bridge compared to the compression members. Balsa wood is quite efficient in its tensile strength. I would recommend using 1/16" Balsa wood for ALL tension members. But make sure the wood is of medium strength (density) or better. The really light and flimsy balsa wood is not acceptable IMO.

But if you are looking to reduce weight, I would look at the compression members first.

[bernard]

I was wondering what type/size of wood people have been using for the tension piece running across the length of the bridge? I'm currently using a 3/32 by 3/32 piece of basswood, but I would like to slim this down as much as I can.

Thanks.

I've had successes with and have heard of successes with 1/16" square bass, 1/16" square balsa, or thinner, wider sheets of balsa (like 1/4" x 1/32"). 3/32" square bass sounds a bit overdone given how well 1/16" square bass works, just reinforce joints better as you move to smaller sizes.

[havenguy]

Great, thanks for all the input. I'm going to try 1/16" square bass and see how that goes. I've only built 1 bridge this year that I've used for all competitions, so I better start testing some more...