Building a balsa wood bridge

[Orange Stang 04]

Any civil engineers here? Or build a balsa wood bridge before?

If so, please post up what you found to be the best design. I'm thinking keeping it simple with something like a deck truss would probably be the best idea, instead of being a junkyard civil engineer and making things complicated and making a bad design. Or possibly an arch design?

It has to be made completely from balsa wood, and must be atleast 12" long. The object is to have the bridge that has the best bridge weight-weight supported ratio.

Any insight will be greatly appreciated. :beer:

 

[thomas91169]

this is SVTP.

most of us would take a 12" piece of balsa stock, and reinforce it with our ****. Talk about spanning distances......

 

[Moparkidd]

is it a project to see how much weight it can hold? i've done one of these before a long time ago, i cant remember but mine sucked. lol

 

[Orange Stang 04]

is it a project to see how much weight it can hold? i've done one of these before a long time ago, i cant remember but mine sucked. lol

Yeah, updated my original post.

 

[lloyyd]

is it a project to see how much weight it can hold? i've done one of these before a long time ago, i cant remember but mine sucked. lol

I built one too in high school. Mine actually did really good but it was just luck. I didn't really know what I was doing, just threw something together.

 

[BayouFX4]

Mine in high school held 88 lbs before a tiny joint failed near the top. If I hadn't been lazy and recut the piece it would have held more. Super glue is the best glue to use imo. I don't have any pics of mine here, but I might be able to post one when I get home this weekend. It was a simple A shape design. I was in the top 10% as far as weight to support weight ratio goes.

 

[03vert70charger]

I remember this from physics as well. My teacher said we can use as much glue as we want, so I lathered mine with gorilla glue lol. Unfortunately, doing it the night before with gorilla glue doesn't work out so well. It took 2 days to fully dry. Mine was late but I still managed to get a B.

 

[Orange Stang 04]

Yeah, this one is more of a serious competion. We cant just cover the whole thing with glue, any excess that isn't being used to hold joints needs to be removed.

 

[Stroszek]

I believe an arch design would be your best bet.

 

[James Snover]

An arch with the radius at least equal to the span. For example, if you're going to span 12 inches, the arch should be at least 6 inches high. Higher and steeper up to a point is better, but 6 inches for an absolute minimum. Triangles are your friends.

And if the road surface can have a bit of arch in it, too, that'll give you two arches. Unless they have a rule that the road surface must be flat.

Jim Snover

 

[Moparkidd]

Mine in high school held 88 lbs before a tiny joint failed near the top. If I hadn't been lazy and recut the piece it would have held more. Super glue is the best glue to use imo. I don't have any pics of mine here, but I might be able to post one when I get home this weekend. It was a simple A shape design. I was in the top 10% as far as weight to support weight ratio goes.

we were only allowed hot glue

 

[BayouFX4]

we were only allowed hot glue

Oh we could use any glue we wanted.

 

[99cobradave]

Design is not as important as quality construction. Pick a good simple truss design, 45 degree angles. Keep it simple, be very careful with your joints - make your cuts as careful as possible. Use tightbond wood glue. Trim any dried excess glue with a razor blade, as you want to minimize the weight (I'm assuming you are being judged by capacity to weight ratio). The failure modes of these type of student structures are in the joints, man! Don't get sloppy! Lastly, don't underestimate the design of the reaction supports, you don't want a shear failure at the support - beef it up.

Good luck.

 

[thomas91169]

Design is not as important as quality construction. Pick a good simple truss design, 45 degree angles. Keep it simple, be very careful with your joints - make your cuts as careful as possible. Use tightbond wood glue. Trim any dried excess glue with a razor blade, as you want to minimize the weight (I'm assuming you are being judged by capacity to weight ratio). The failure modes of these type of student structures are in the joints, man! Don't get sloppy! Lastly, don't underestimate the design of the reaction supports, you don't want a shear failure at the support - beef it up.

Good luck.

Structural engineer i take it?

 

[99cobradave]

Structural engineer i take it?

Yes.

 

[Black1999Cobra]

I did this. I designed an awesome one and came in 2nd. The first place team's design wasnt working out, so they just laid layer after layer of balsa wood. It weight the most and held the most as well (they ended up with the best holding force to weight ratio).

I took the balsa wood and made an I-beam. Think about it, how do they make metal beams where you get the most support for the least amount of weight? An I, thats how.

I would have won I think if I would have glued the top and bottom layer to the middle vertical support all at once instead of glueing the top one day and the bottom the next (the bottom didnt ahdere too well since the center support was already dried).

I think ours ended up holding something like 450 pounds (I remember thinking back, man 2 of us could have stood on that some bitch!!). The other teams held like 800 lbs. They just barely beat us from a weight to holding strength ratio.

So, to win...layer balsa wood. I got an A though.

 

[N2DAMYSTIC]

What will they use to test the bridge with? Depending on how they go about the testing, and what they use is how I would design the bridge.

 

[Turkish]

I built one in HS, make sure to notch where the balsa wood pieces connect to one another, kinda like piecing a puzzle together. Makes the structure rigid.

 

[focussvtracer]

did that in 7th grade :lol:

 

[WHITESVTCOBRA]

google usually knows all.