# Bridge Project

## Main Question or Discussion Point

Hello! In my geometry class we are going to be making a bridge.

The materials are:
15 3mm x 3mm x 683mm balsa wood
glue
cutters

Arch bridges are to difficult for our materials and we cant make a suspension bridge because we have no string. What is the strongest bridge I can make?

Related General Engineering News on Phys.org
minger
How is your bridge going to be loaded? If all you need is vertical strength for a single span, then look at what makes I-beams strong.

Here's what I would look at. I would decide on a truss design. For example, two horizontal beams connected by angled webbing. What you're going to need to decide is how high to make the beam. The higher you make it, there is less material to make it wide, which may affect how the loading gets applied. However, making it higher would seemingly increase the area moment of inertia and could make it stronger.

There should be plenty of information online to get you started.

Thanks for the reply The bridge will be graded by how many pennies it can hold so the stronger the better. We start the project tomorrow.

I am in Vorbis' class as well. Basically, what you are saying is make a truss bridge as tall as possible keeping with the minimum road bed requirments? With a truss bridge, it gets stronger if it is made higher? Is this correct?

Hi there haydo ;) welcome to the forums

RonL
Gold Member
Some years back, I recorded a PBS show that covered the building of a bridge, by two engineers. If memory serves correctly, the title was "Rainbow Bridge", it was about an arched bridge with all straight beams that interlock at angles which produce the arch. It might be something that will work for you.

I'll see if I can find a link, and get back.

Ron

Found it

http://www.pbs.org/wgbh/nova/lostempires/china/builds.html

Last edited:
Interesting, but is it supposed to be any stronger that a truss bridge?

RonL
Gold Member
Interesting, but is it supposed to be any stronger that a truss bridge?
I don't have any information at this time, other than the foundation at each end was a key factor in the building of the one mentioned in the project. Any thing else will depend on strength of the materials used.
Others on the forum might throw some information in. Do you have a particular time frame for the project for your class ?

P.S. I did look for the video and it appears that it is only a purchase item, I'll pull it out of my collection and watch it again to see if there is any mention of specific data that might help you.

Last edited:
We have to have it completed in a week, so time may be an issue. I noticed you mentioned that the foundation was key, but the bridge has to be mobile, so a rainbow bridge may not work if it relies heavily on the foundation.

RonL
Gold Member
We have to have it completed in a week, so time may be an issue. I noticed you mentioned that the foundation was key, but the bridge has to be mobile, so a rainbow bridge may not work if it relies heavily on the foundation.
That is a short time. Not knowing what your restrictions might be, anything to keep the ends from pushing outward will make the arch capabile of carrying a very large load for it's size.

Try google for arch bridge information, also Nova rainbow bridge. I'll come back if I come up with anything else.

Ron

That is a short time. Not knowing what your restrictions might be, anything to keep the ends from pushing outward will make the arch capabile of carrying a very large load for it's size.

Try google for arch bridge information, also Nova rainbow bridge. I'll come back if I come up with anything else.

Ron
I don't think we can use the sides of the desk (we're spanning two desks if I'm not mistaken) for support, so I can't really think of anything that would make an arch bridge stay in place. If I did decide to go with a truss bridge, would it be stronger if I made the roadbed on the top or on the bottom?

RonL
Gold Member
I don't think we can use the sides of the desk (we're spanning two desks if I'm not mistaken) for support, so I can't really think of anything that would make an arch bridge stay in place. If I did decide to go with a truss bridge, would it be stronger if I made the roadbed on the top or on the bottom?
With the model, you can do anything, and again not knowing the rules you must follow, I would think in real life considerations the road should stay as low as possible.

The two things that seem inportant to me would be

A. If a ramp has to be constructed, support for the road becomes more expensive.
B. Percentage of grade, poses needs for traffic flow considerations.

Again being as time is such an issue, I would encourage doing a google on "bridge design".

I am not an engineer, my thoughts come from driving large trucks, and some years in excavation related business.

Ron

Thanks for your time, I just have one more question (probably ) Which is better, a truss bridge out of repeating right triangles, or a truss bridge out of repeating equalateral trianges. It seems to me that a bridge out of equalateral (sp) triangles would be easier to construct, but if it's weaker, I may want to make one out of right trianges.

Thanks for the replys, I'll keep guys updated. By the way I think I'm going to make a truss bridge.

BobG
Homework Helper
Bracing the foundation is the hard part. As the weight increases, the bridge bows down, the pillars move apart, and eventually the bridge snaps when the bridge bows too far.

If the bridge hangs from the desks, with the trusses below the road top, the trusses are braced by the desk. All of the force winds up being straight down. The road top literally has to peel off of the trusses for the bridge to break.

Best design depends on which is stronger - the glue or the wood. Or, if you can notch the wood so the road top interlocks with the trusses (like Lincoln logs), you get the best of both.

You obviously have to know the distance between the desks ahead of time for the idea to work.

Bracing the foundation is the hard part. As the weight increases, the bridge bows down, the pillars move apart, and eventually the bridge snaps when the bridge bows too far.

If the bridge hangs from the desks, with the trusses below the road top, the trusses are braced by the desk. All of the force winds up being straight down. The road top literally has to peel off of the trusses for the bridge to break.

Best design depends on which is stronger - the glue or the wood. Or, if you can notch the wood so the road top interlocks with the trusses (like Lincoln logs), you get the best of both.

You obviously have to know the distance between the desks ahead of time for the idea to work.
I'm fairly sure we can't use the desk to brace the bridge on, so that may not work. Would the same thing work with the roadbed on the bottom?

BobG
Homework Helper
Yes. Keep in mind that being limited to wood doesn't mean you can't have a "suspension bridge". It just means your "cables" are rigid. If the two pillars are braced so that they have to remain parallel, then your stress from the weight of pennies still gets distributed over the entire bridge.

For something like the Golden Gate Bridge, the pillars go all the way to the bottom of the bay and the road bed braces the pillars. Since you cant' do that, you need a brace between the top of the pillars to keep them from bowing in toward each other (kind of like some of those railroad bridges with framework above the railroad bed). The top brace has to bow in order for the roadbed to bow, plus a few other parts of your bridge have to do some flexing as well (the length of your wooden "cables" can't change, for example).

In other words, you're distributing force over the entire bridge so no single point experiences enough force to break.

Just figure out which part of the bridge has to move for the roadbed to bow, put in something to keep that from happening, and so on, until you get to a point where every part of the bridge has to flex before the roadbed flexes.

The lincoln log technique with the notches is going to be key (and the toughest since your cuts have to line up). You don't want to have to rely on the strength of the glue to hold your bridge together unless it's at a point that won't experience much stress (if you have many of these points, then you didn't do a good job distributing your stress).

Edit: By the way, using a compass to show the possible motions for your supports will probably help visualize how to keep the motion from happening when you're designing your bridge. And analyze from different points of view; not just the obvious side view.

Last edited:
I have decided on a truss bridge it this kind of shape: /\/\
There are of course conecting beams between the triangles. To add a little more support, I'm quadrupling up the base roadbed boards as well as the very base boards. It seems to me that this shape of triangle should cause the bridge to bow ONLY if the glue (I may try to notch, but it doesn't seem easy) gives or the top middle board breaks. If I can stabalize the middle board enough, and seeing as it's horozontal, it should holb fairly well. Is there anything you would suggest adding?

Also, we are being weighed on a bucket or something of that sort being hung from the bridge and weight being put in there. I don't know it that really matters, but it does mean we don't really have to build a real road bed.

Mech_Engineer