Design and Construction

Design

Ready to start the hands on part of building a bridge, our class looked at the different designs that had been used in the past, and took heavy consideration into what had and hadn't worked. Here are some common designs that we looked into.

Ah. This example might be a bit small. Here is some detail. These might not be all of the types of bridges out there, but they are the most common.

Beam

Pretty much what it sounds like. All of the load is transferred onto beams that touch the bottom of whatever the bridge is spanning. As a result, they cannot hold up as much as others, since the force causes the beams to bend dramatically. Also, our bridges were not allowed to touch the bottom, so they were ruled out.

Truss

This was one of the more common bridges. The used multiple bracing throughout the bridge to push the forces against each other. Compression and tension here were moved to the outside, so the only place for failure resulted in the strength of the joints and materials used. However, this bridged used a lot of parts at varying angles. I didn't want to use it, as liquid glue would make this very tricky.

Arch

One of the most reliable bridges in history has been the arch bridge. These puppies use the arch and the sides of the gap- the compression from the load on the arch equals out the tension on the top part of the bridge, and over all work very well. 

Suspension

The end all be all of bridges. the suspension bridge is renowned for being the best for spanning large gaps and carrying large loads. They work by transferring the load to large cables that are anchored on the side of the gaps, with the cables resting upon the pillars. This allows the bridge to sway as well.

Cable Stayed

Sometimes confused with the suspension bridge, a cable stayed bridge is only similar in that it uses cables. However, instead of large cables running horizontally the bridge with smaller ones holding up the base of the bridge, a cable stayed bridge has the cables attached to the pillars, and not to counter weights on the edges. Again, this design was ruled out due to it needing cables. 

After looking at many many pictures of bridges, and thoroughly understanding the concepts behind them, we quickly jumped to the suspension bridge for our first design. It was almost drilled into us that it was the best design- it cost less than fancy truss bridges And hey, it even looked nicer!

However! We soon had a problem with that- we had nothing to use for cables. While we could have worked around it, maybe by hanging something, but that would have become quite messy. When then looked at arches. They were indeed strong, and much simpler than a suspension bridge.However, due to the materials, all being very straight and having low elasticity, did not suit a arch. In the end, we we almost forced into a truss bridge, the one thing I wanted to avoid. 

After a number of simulations, it was pretty much decided that a truss would be the best. We would uses small triangles- the strongest of all the geometric shapes- for the inside trusses. This would make the compression resulted from the load put tension on the main part of the bridge, thus holding a large amount of weight. After pages of sketches with lots of triangles, we came up with this:

Excuse the poor drawing. Paint is really terrible.

So, pretty simple, but hopefully very effective. Time to build. We also used this simulator to help in our design process. While this may have not gone as I would have thought in the simulator, it still worked while we built it.

Construction

One of the challenges that I tackled was with the materials. Since we decided as a group to stick to toothpicks and skewers, one of the biggest problems was that they were round, and round surfaces make it hard attach angled joints to. Also, as single units, they were tough to arrange together in a way that seemed strong enough.

My solution was to bundle skewers and toothpicks into threes. This would make them stronger and easier to connect to each other, as well as give them three times the strength, hopefully. For example, we tested one of the bamboo skewers against my new tri-skewer. After one kilogram of weight, the skewer bent and bowed inward toward the weight. This is how the tri-skewer preformed:

That is about 6 kilograms of mass on one of our supports. In our design, we used shortened tri skewers (and the cut of ends in the trusses) as the bottom "bed", hoping that these super strong beams would gives us much more support. The rest of the build was rather simple after that. We used toothpick bundles and the mini-skewers in the trusses, and a few going across the width of the bridge to hold everything together. 

One hard part about this style was joining the two bundles of skewers on the end. We used leftover skewer to hold them together on a flat face. Due to the limitation of lamenting  we could use only one, whereas we planned for three.

Everything was held together with Elmer glue, which from a DIY standpoint is a fickle substance. It holds very well and has a high elasticity, but is a liquid and takes a long time to become solid. We waited about 24 hours for the glue to get a good hold. It was still very messy and time consuming. 

One of our processes that was used in the gluing process was clamping. We made one truss at a time, then clamped the top to the main skewers on the bottom. This was along the lines of of pressure treated concrete or pressure treated wood. If we could get the materials to form under such pressures, than maybe they might stand up to higher forces acting on them. 

In the end we got this:

Notice one change to our design- we added a small truss underneath. We had leftovers and didn't really see why not. In hindsight, we really didn't need it, or at least I don't think we did. 

As for prices, this is what we had in the end:

Toothpicks: 66.....$864

Skewers: 12........$1164

For a grand total of: $2028

Which is under $2500, so under budget!

As for mass, we came in at 48.7g