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.




Friday, July 29, 2022

KidWind Guyed Tower Raising for Real


I hear it I forget. I see it I remember. I do it I understand. This is the mantra that I try to live by in teaching what KidWind is all about to coaches and students. Take the picture above for example. Most people would see this and say, "Oh, that's a windmill." From a teaching standpoint the questions might be, "What does it do? How do you get it up there?..."

A diagram like this might be in order to show the layout for the guy wires and what it looks like before the tower is raised. This whole process would take only a few minutes and make for small talk in passing. What I would call a "snack" in the world of education on the way to teaching a bit about renewable energy to a classroom of students.

In my mind KidWind is more about providing a "feast" for young students minds and hands to learn and understand what is being taught. This was the case for 24 high school seniors taking part in a two week on campus experience a few weeks ago that I worked with for a day at the UW Madison Wisconsin Energy Institute. Thanks to Allison Binder and Scott Williams invitation to help out.

The morning started out with groups of six students getting familiar with the rigging, terminology and process involved in raising a table top model wind turbine layedout in kit form like this. Everything was covered from a call to diggers hotline and wearing safety helmets. Safety was important from start to finish. The model generator was mounted, raised and tested for output lighting a small LED.

At the end of the hour each group had gone through the process twice. Once without the turbine to prove out the cable system and make any adjustments before mounting the generator to the tower. Finally their model turbine was raised by a small hand cranked winch and looked like this. 

OK time to put what was learned in the morning into practice on the real thing. 

After lunch it was time for the real thing. In groups of 12 they rigged the 9 meter tower and gin pole with a 300 watt Air-X generator for raising. Just like the learned with the practice model the tower was first raised without the turbine to prove out the rigging and make any necessary adjustments.

Mission accomplished! Now if you have or know of a group that would like to get this same firsthand learning experience let me know and we can plan it out. Learn about the future of Renewable Energy today!

Imagine the response these students gave on their first day back in school, to the age old question, "So what did you do over the summer?"