West Point Bridge Testing

In this project, we assembled a bridge using these instructions. This bridge is a cardboard model of a truss bridge. We used CorelDraw to design the pieces of the bridge. We used the laser cutter to cut out these pieces. The workflow for the medium laser is here. The workflow for the big laser is here. We then cut out more pieces to figure out how much our bridges would hold by testing the tension and compression forces of different pieces. We used these testing instructions to figure out how much tension force and compression force our bridges could handle. 

Bridge Pieces

We were given this template to cut out the gusset plates of our bridges. Then, I was tasked with making the bridge tubes. We were given the dimensions and how many of each tube we needed. To make the tubes, I first made a box and made its length the length of the tube, and its width the dimensions of the tube. For example, I needed a 10mmx10mm tube that was 21cm long. For the width I multiplied 10 by 4, there are 4 sides of the tube, and I added 6 to make a tab to glue the tube together. I then added lines 10mm apart and then another line 6mm from the bottom of the box. I copied and pasted that to make the number of tubes I needed. I repeated this process for the remaining tubes of varying lengths and widths. I cut that file out and then assembled my bridge with all these pieces. We were given a layout of the bridge and taped it to a big cardboard sheet. We laid each of our components on the cardboard and glued them together while on the layout. To figure out how much force our bridges could hold, we needed to create tubes and strips to find out how much compression and tension, respectively, our bridges can hold. I designed three different types of tubes and strips with squares on each end. We cut out three of each piece so that there would be three trials of each piece. We assembled all of our testing pieces by using superglue and finished them off by adding support strips around the ends of each testing tube and glueing the squares to each end of the strips. These pieces correlate to the parts of our actual cardboard bridge. One problem our group faced was glueing our fingers to the cardboard pieces. Superglue's name does not disappoint. 

Force Testing

Before actually testing our bridge, we needed to find out how much force our bridges could hold. We used a testing machine to test each piece to figure our how much force our bridge could support. To find the tension it could support, we used the strips. We attached each strip to the testing machine 10.5 inches behind the fulcrum, the beam in the middle of the testing machine, with clamps and we also secured the testing machine to the edge of the table. We placed a bucket on the groove at the end of the testing machine and filled the bucket with sand until the strip broke. We then weighed the bucket to figure out how much mass the strip could hold until its rupture point, the point where the strip broke, and we recorded that number. We repeated that process for each strip. To calculate the force each strip could hold, we multiplied weight, which can be found by multiplying mass by 9.81m/second^2, by the length from the fulcrum to the testing piece and then dividing it by the length from the bucket to the fulcrum. Instead of having to do every problem with a calculator, we used Google Sheets to do the equations for us. We added the lengths, masses, and sand weights to the sheets and entered the equations we wanted multiplied. We then copied and pasted these equations for each strip so we wouldn't have to enter the equations over and over again. We used these same steps to find compressive force, but instead of putting the tube behind the fulcrum like the strips, we put in 10.5 inches in front of the fulcrum. We added all of this information to our Google Sheets. One problem that we faced while testing for our component strength was when the component broke, there was no warning. Sometimes the cup of sand would continue being poured into the bucket, adding more sand than the component actually held. Another time our bucket fell and spilled all over the floor, so we lost some sand. 

Bridge Testing

Once we calculated the force that the bridge could hold, we could then actually test our bridge. We have a stack of 5 books and 6 Lego bricks of the same size. We placed a Lego on the middle six top joints. We separated two tables and placed the bridge in the middle, with only the ends of the bridge on the tables. The first weight that we added to the bridge was 3 books that weighed 3500 grams or 3.5 kilos. Our bridge held the weight for the required time of 5 seconds. When we went to add the next 2 books, adding the weight on the bridge to 5000 grams or 5 kilos, our bridge could not handle the weight for the required 5 seconds. Our bridge broke at the two end strips at the bottom.