Elevated Bridge B/C

[jcms science boy]

At the regionals, my ratio was 630 and the 2nd place was 360 or so. I was surprised there were 14 teams competing and i personally thought that 360 and lower is a horrible score... (no offense). I'm building a new one 4 state, hoping 2 get 1,000, but that's probably out of my reach. Since i'm in division B, i think 800-1000 would be good enough 2 get a medal.

[kidneyfailure52]

so guys, if I put a quarter inch thick piece of balsa, with a quarter inch gusset, and another quarter inch piece of wood there, would the piece be out of specks or ok????







frm beckendorff and im asian!!!!

[computernerd4826]

I overbuilt a bridge by a lot. it weighed 17.5 grams, but held 58 pounds (26kgs)
That would give me an efficiency 1485 if it could go beyond 15kgs...
Do you think reducing wood size would also keep my efficiency that high?

[Pleiades]

You could either reduce the dimensions of the wood, reduce the density, or reduce the amount of members. It will probably depend on your design. I personally like just reducing the density rather than reducing the wood size. Someone could better answer your question than me. The elevated bridge person on my team brought his bridge in the other day to test it. The bridge weighed 58 grams. It was made out of very high density 1/4 x 1/8 balsa. It's obvioulsy very overbuilt but his last bridge couldn't even hold the loading block so i dont really blame him that much. I didnt watch the testing of the bridge but when i came back into the room i saw the bucket that it held and it held a full 5 gallon bucket of sand. My school doesn't have a scale but it was probably way more than 15kg. I was actually pretty surprised. I took the bridge and put it on a table and the legs had 8 contact points [4 on each side] but only 4 of them touched the table. When i pressed down on the bridge it also had some give. I'm hoping he can come up with something lighter.

[smartkid222]

I overbuilt a bridge by a lot. it weighed 17.5 grams, but held 58 pounds (26kgs)
That would give me an efficiency 1485 if it could go beyond 15kgs...
Do you think reducing wood size would also keep my efficiency that high?

It depends. What wood size are you using now?

[gandhiji]

has anyone had any success building a bridge with slanted legs or are straight legs better?

[bob3443]

has anyone had any success building a bridge with slanted legs or are straight legs better?

straight legs are probably stronger, because the force goes straight down, but then they're also heavier.

[Balsa Man]

straight legs are probably stronger, because the force goes straight down, but then they're also heavier.

Actually, your conclusion on overall bridge weight is probably correct, but not on the legs.

On a leg - a member - that is carrying a load, so its carrying a compression force - if it is straight up and down, the compressive force it carries is equal to the load. In a bridge, 4 legs, 15kg max load, = 3.75 kg per leg. If those legs are at an angle, carrying the same vertical load, the compressive force goes up as the angle away from vertical increases. Understanding what the forces are is important - as a number of previous posts have noted, the Bridge Builder app free on the web is an easy way to see. For instance, in our team's design this year, the (slanted) legs are carrying about 6kg ea. They're about 30 degrees from the vertical. So, about 30 degrees of slant, a (roughly) 55% increase in load.

So, slanted legs are longer, and they need to be stronger. So, they - the legs themselves are/would be heavier than straight legs. The other wood you would need to have the bridge span enough distance so the legs could go straight down, however, would add to the overall weight of the bridge. How this trade-off will work out will depend on exact design, but in most cases straight legs = heavier bridge than one with slanted legs, even though the straight legs are lighter than slanted ones would be.

Len Joeris
Fort Collins, CO

[computernerd4826]

I overbuilt a bridge by a lot. it weighed 17.5 grams, but held 58 pounds (26kgs)
That would give me an efficiency 1485 if it could go beyond 15kgs...
Do you think reducing wood size would also keep my efficiency that high?

It depends. What wood size are you using now?

1/8 X 1/8
and for the main leg pieces i am using 1/4 X 1/8
right now i am switching from 1/8 to 3/32

[smartkid222]

I overbuilt a bridge by a lot. it weighed 17.5 grams, but held 58 pounds (26kgs)
That would give me an efficiency 1485 if it could go beyond 15kgs...
Do you think reducing wood size would also keep my efficiency that high?

It depends. What wood size are you using now?

1/8 X 1/8
and for the main leg pieces i am using 1/4 X 1/8
right now i am switching from 1/8 to 3/32

ok, make sure you weigh your wood and choose the pices with the density you want.
good luck.