Tuesday, July 2, 2013

Building a Civil War Pontoon Bridge

Do you know what a pontoon bridge is? I didn't before this unit, but as soon as I read about them (they are basically temporary floating bridges that can be built quickly---they were used frequently in the Civil War and were especially important in the Battle of Fredericksburg, among others) I knew we had to build one. It's exactly the type of thing the boys are most interested in. Perhaps not surprisingly, when I searched online for things like "Build pontoon bridge for kids," no results came up. :) So we were on our own. 

We learned about Civil War pontoon bridges at this website.
There are some great photographs here.
This article had really interesting information about pontoon bridges today (Washington State has a whole bunch, and is currently constructing the world's largest pontoon bridge).
This video shows a pontoon bridge failing (what's an engineering lesson without a bridge fail video?)

After finding out what components made up a pontoon bridge (this site was most helpful for methods and terminology), we brainstormed options for building materials. Pontoons are basically like big floating boats, so we used water bottles as our pontoons. We used skewers for the long floor timbers (called balks and side rails) that lie across the pontoons. Popsicle sticks worked well as the chess planks, and we just used yarn for lashing everything together. I wanted to do everything with lashing, but it was just too hard to lash the popsicle sticks securely enough, so we did use a layer of duct tape to secure them to each other, and then lashed that to the balks beneath.

We needed water to build our bridge across, so we took our supplies and relocated to a nearby park with a stream in it. I was sitting against a tree feeding Marigold most of the time, so I told the three boys they were on their own to build the thing. "Use square knots!" was the extent of my advice. (There are probably better knots for lashing, but what do I know? We need to do a unit on knot-tying.)
Gathering materials

I was really impressed with their persistence and cooperation. Malachi got distracted and started playing in the stream a few times, but the two older boys worked pretty tirelessly.

Assembling the pontoons

Measuring the width of the stream

Creating long balks from two shorter skewers

Lashing the balks to the pontoons

Adding the chess planks

Because we were working between rocks instead of dirt riverbanks, we couldn't really do the abutment part of the bridge. The boys just secured the yarn beneath two rocks.

I told them the bridge had to support the weight of some soldiers and some artillery without sinking, and that it had to be sturdy enough not to float downstream or break apart.


This was a really fun project and we all gained a new appreciation for the Army Corps of Engineers, who constructed these bridges under heavy fire and in all kinds of miserable conditions. Hooray for pontoon bridges!

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