Saturday, September 3, 2022

Woven "Rainbow" Arch Bridge Project


Got a bit distracted last week. Saw this in the latest issue of my Timber Frame Guild publication and decided to give it a go. How would this design compare to the Town Lattice truss bridges I built a few years ago?

My idea is to experiment with this woven arch type bridge and if successful put together a 1/2 day learning experience for people interested in this unique design. They would build a small foot bridge spanning 20 feet out of 8 foot long landscape timbers at Shake Rag Alley arts center in Mineral Point, WI next summer.

The woven arch "rainbow" design is from China and over 1200 years old. Sometimes called a "chopstick" bridge to get the idea of how it is put together and how it works to span a distance and support a load I would literally start here.  After a trip to Walmart and getting my chopsticks I was ready to begin by following the steps shown in the pictures. This was much more difficult than it looked.

Several try and 20 chopsticks latter I had it. I splayed out the bottom legs to add stability but can now build this in less than 3 minutes! So now it was on the the next step. I would build my small bridge out of 8 foot long landscape timbers.

Just to keep interest I will show you the finished product. Amazing how there are no fasteners and this can hold 500 pounds!

Being a retired high school Tech Ed instructor and not an engineer I relied on common sense and experience as I experimented. First I designed and built a simple load tester. I modified and added a pressure gauge to my bottle jack. Each landscape timber was positioned in the fixture so the bottle jack could jack it up 1 inch from straight at rest. At this point I recorded the PSI reading on the gauge. The readings ranged from a low of 100 to a high of 600. The lower the PSI reading the weaker the timber's bending strength. Those were used for the lateral cross pieces.

The process starts like this. I clamped and tied the pieces together just to make things a bit easier.

Now this is a little tricky and having an assistant will make things go much smoother. My assistant took the form of two 5 gallon buckets to hold the two raised timber up while I slid the next pair of legs in and placed the next lateral across them. The bridge is assembled from the center out. The section on the right in the picture will end up being the center of the bridge and the two lateral cross pieces will then be level. 

This picture of the finished bridge might help you get the sequence of steps in mind better. You are going to be switching ends each time you add a section.

Here I have added a section on the left side. 

As the bridge grows so does the weight and it is time to replace the helper with a pair of step ladders, beam and two come alongs to raise the bridge legs and add another section.

You can see that in this first model I used a single timber for each bridge side. For the final run I doubled them up for more strength.

With the bridge assembled I wanted to see how much weight it could hold. This would be done by placing a swimming pool on top and filling it with 30 gallons of water (240 lbs).

I was quite pleased with the results. It held and with a 20 foot span brought the bridge height down about 4 inches as it tightened up. In this picture you can see two of the tie downs I attached to the bridge sides. This was done to stop the bridge from racking as there were no deck boards and the sides at the bottom not splayed out.

I took down and rebuilt this bridge three times. Making several improvements in strength and stability. I doubled the side legs and load tested it by filling two 30 gallon garbage cans with water (480 lbs) and splayed the bottom legs for stability. Here is the money shot! I still need to work on building a suitable hand rail system and attachment of it with clamps.

As an extra measure of safety because class participants will want to walk over a bridge they build I will be adding an A-frame type safety stop 6 inches below the center of the bridge to catch the bridge should there be a failure of any parts.




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