Bridge Designs

[dholdgreve]

hello, i have been recently trying out a new design, a triangle topped bridge but on the bottom instead of a straight line to connect the two legs of the triangle it is a curve. It has been working like magic, any thoughts?

I would suggest you do some research on the engineering principles involved in bridges... Identify which members are in compression, and which are in compression, then determine if the bottom chord should be arched or not.

[embokim]

I have done my research and I have consulted a structural engineer and according to him, this design is pretty accurate

[dholdgreve]

I have done my research and I have consulted a structural engineer and according to him, this design is pretty accurate

It sounds to me that your confidence in this design is pretty high, so you do not need our buy-in or approval... That being said, generally the top chords are considered to be in compression, as the load is applied to the top of them. A bridge MUST be in equilibrium, so this means that the bottom chord must then be in tension. That being said, if a member is "pulled" on both ends, a curved member would not be the most efficient design.

Now there are ways to reverse this premise, so that the compression members are on the bottom, where a curve could possibly work, ... but most involve some sort of buttress into the existing structure, which is not allowed in this competition. In any case, good luck with your design... Hope it holds the family Volkswagon!

[JonB]

I have done my research and I have consulted a structural engineer and according to him, this design is pretty accurate

It sounds to me that your confidence in this design is pretty high, so you do not need our buy-in or approval... That being said, generally the top chords are considered to be in compression, as the load is applied to the top of them. A bridge MUST be in equilibrium, so this means that the bottom chord must then be in tension. That being said, if a member is "pulled" on both ends, a curved member would not be the most efficient design.

Now there are ways to reverse this premise, so that the compression members are on the bottom, where a curve could possibly work, ... but most involve some sort of buttress into the existing structure, which is not allowed in this competition. In any case, good luck with your design... Hope it holds the family Volkswagon!

I would agree with dholdgreve. Also, I will add that arcs of any sort are almost impossible to build well with balsa (notice: I did not say impossible- it is possible, just exceptionally difficult). I would stray from arcs- you can have very, very successful bridges without any sort of arc.

[Bazinga+]

I have done my research and I have consulted a structural engineer and according to him, this design is pretty accurate

It sounds to me that your confidence in this design is pretty high, so you do not need our buy-in or approval... That being said, generally the top chords are considered to be in compression, as the load is applied to the top of them. A bridge MUST be in equilibrium, so this means that the bottom chord must then be in tension. That being said, if a member is "pulled" on both ends, a curved member would not be the most efficient design.

Now there are ways to reverse this premise, so that the compression members are on the bottom, where a curve could possibly work, ... but most involve some sort of buttress into the existing structure, which is not allowed in this competition. In any case, good luck with your design... Hope it holds the family Volkswagon!

I would agree with dholdgreve. Also, I will add that arcs of any sort are almost impossible to build well with balsa (notice: I did not say impossible- it is possible, just exceptionally difficult). I would stray from arcs- you can have very, very successful bridges without any sort of arc.

I dont think any of the top 5 bridges at nats ever had arcs, but I guess there's a first time for everything.

[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.