# Homework Help: Help with plastic bridge design

1. Apr 18, 2013

### Tesla42

I am in a high school Precalculus class. We are about to have a project where we build bridges with PVC and twine. The goal is to hold as much weight as possible.

For this project, I believe we will be given 28 feet of PVC, a roll of twine, and 5 or 6 days to design and construct our bridges. Also, I think we are designing arch bridges, where there is a main part and a parabola rising above it on each side. Another constraint to the design is that the main portion has to be a certain, so far unstated length in order to reach two blocks so that it can be loaded with weight.

All that said, the purpose in this thread was to receive input from the experts that frequent this website. My teacher told us to do as much research as possible, so I am asking you as well as searching for online resources regarding bridge building.

2. Apr 18, 2013

### Tesla42

I apologize for my inattention to the categories on this forum and my misplaced entry. This would definitely fall under the category of coursework and should be designated as such.

Also, my ideas regarding this bridge are relatively simple right now. I only know that I am going to place two arches and that there will be several lengths of pipe going across the main structure to better handle the stresses that it will go through.

To put into perspective how much this design needs to hold, the top bridge from last year held 292.5 pounds, and that was with 4 fewer feet of plastic. This is around the amount that I want my bridge to hold so that I can get a good grade.

I will be back with some more specific ideas later after researching more about how bridges are designed.

3. Apr 19, 2013

### CWatters

Never built one from PVC only balsawood. I would experiment with methods of jointing. For example think about how you might make triangles out of PVC pipe. It's a lot easier to lash together rough wooden poles than smooth PVC tubes. Can you drill holes in the pipe or are there other restrictions?

Perhaps also find out how they plan to load the weight onto the bridge (eg point loading in the middle or nice uniform distribution?).

4. Apr 20, 2013

### Tesla42

We have to build it out of PVC, or at least some type of plastic that is similar. Whatever it is, it is very flexible.

We are allowed to drill holes in the pipes, so connecting joints is far more feasible. Also, I am not sure about how the load will be distributed, but am I correct in assuming that a load more distributed along the span of the bridge would be preferable to a huge load in the bridge's center?

5. Apr 23, 2013

### Tesla42

I have formulated a design but I have quite a bit of leftover pipe. Would it be useful or a waste to use the leftover pipe to reinforce the two pipes that form the main span? If it would be a waste, what would be a better suggestion?