Hello Everyone! In this instructable, I will be guiding you through building an arch bridge. Whether you are reading this and looking for a fun project or hope to apply what you learn to a competition I wish you the best of luck. :)
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Step 1: Materials
Step 2: Get Template and Preparations
Before building a design is needed as the template for the truss. You can either print the above image so it fits an 8.5 by 11-inch piece of paper or refer to my prior instructable in order to make your own design.
1. Print 2 templates and then fold or cut the excess paper so all that is left is the template for the trusses
2. Put the templates one above the other on top of a wooden block
3. Cover the wood block and templates with the wax paper.
4. Tape down the templates and the wax paper to the block.
To build the trusses pieces of wood will be cut and placed over the template then glued to each other. The purpose of the wax paper is to prevent glue from sticking the bridge to the templates and the purpose of the wooden slab is to stick pushpins into that will keep the wood in place.
Step 3: The Arch
1. Soak a couple sticks of wood in water. This makes it more flexible and without soaking trying to make an arch will snap the wood. Leave to soak until the sticks are a dark brown. I left mines to soak overnight. (Don't soak as many as I did, a couple will suffice)
2. Put pushpins along the arch in locations specified in the picture. Be sure to pay attention as to whether the pushpins are on the inside or outside lines. The point of the pushpins is to be a guide for the wood so when it dries it will harden in the arch shape.
3. Dry a stick with a paper towel. After drying work fast because if the wood dries than it will no longer be flexible.
4. Put the stick along the pushpins and mark where you should cut so the size of the wood matches that of the template.
5. Cut the wood where you marked and then return it to the pushpins.
Note: While the arch can be accomplished using multiple pieces of wood using one piece of wood lets the arch be stronger and increases the efficiency in which it disperses the force applied to it.
Step 4: Interior of the Arch
Before you start it is important to know that while gluing you are NOT gluing the wood to the wax paper but to the other pieces of wood.
1. Place and glue the bottom of the arch in. Make sure the directions of the cut and how the end of the arch and bottom connect to each other match the picture. This is important because the direction of the cut influences how the force is distributed. (I originally did this incorrectly and had to fix it midway through the project so some of the further pictures might still have the wrong cuts.)
2. Place and glue the vertical members.
3. Place and glue the diagonal members. Make sure that all cuts are flush.
4. Place and glue the center X. This reinforces the center of the bridge as the center is the most likely to break because that is where force is applied during testing
5. Repeat all steps so you have two trusses, one for either side of the platform that will go in the middle.
Step 5: Platform Construction
1. Place the pushpins like in the first picture and make a wooden a wooden frame with the pushpins bracing the frame while the glue you now put in the joints dries.
2. Measure even intervals and place vertical members along the bridge.
3. Put in diagonal members so there is an X shape between each vertical member.Make sure cuts are flush.
4. Run horizontal sticks along the bridge. I used 10 horizontal sticks evenly spaced.
5. Run another layer of vertical members above where you placed the previous vertical members so it's like a sandwich.
Step 6: Assembly
1. Remove all assembled pieces from the wax paper.
2. Mark and cut a piece of wood so it is the size of the width of the bridge. Use this piece for size to cut approximately 26 more pieces.
3. Glue those width sizes pieces together in groups of two. These combined pieces will be referred to as bundled pieces.
4. This next part will be easier with two people. You will run the combined pieces in between the arch and glue them so they make a sort of platform. Then glue the assembled platform onto it.
5. Next cut more pieces of wood the same size of the bundled pieces. You will glue these in intervals along the arch so a sort of roof is formed. One thing I did wrong in this last step is glue the pieces all along the arch. I should have left a gap in the center for easier testing. I ended up having to break off the centerpieces so there was room to test. If you are not testing then feel free to glue all along the arch because it will improve strength even more.
You have now completed the bridge, congratulations!
Step 7: Testing
How this machine works is as you turn the wheel the screw turns downward and exerts pressure on your bridge. The amount of pressure put on the bridge can be read on the connected readout. The more you turn the wheel the more force is exerted. While testing you will hear little crackles, this is the sound of the wood glue breaking and it is normal to hear this. If you hear a big snap that means something broke. [;)] A way to tell if a silent break has occurred is if the pressure reading keeps on dropping even as you are increasing the pressure. Before testing I slid a foam board onto the bridge and put a small block of wood on top for an even testing surface.
Thanks for reading! If you have any questions feel free to put it in the comments below. Also, does anyone know what the machine called that was used for testing is called?