A3 - SCHETLEY

In this assignment we used starting calculating the actual amount of force that is on a bridge, beginning with a simple bridge made up of three triangles. My personal assignment was to perform a truss analysis on a bridge spanning 36 inches with a height of 10 inches and a load of 20 pounds. I was able to calculate the force on each member pretty easily, as they are all just simple sum of forces problems. When I calculated the force on each member, I calculated the amount of force in Newtons, because the video that taught me how to do truss analysis (seen here) used Newtons and in physics, when learning sum of forces problems, all weights needed to be first converted to Newtons. The results of my calculations can be seen here:

Figure 1: Truss Analysis - Page 1

Figure 2: Excel Worksheet Calculating Angles

Figure 3: Truss Analysis - Page 2

Figure 4: Truss Analysis - Page 3

The final results for all the forces on the members can be seen below, labeled in both Newtons and pounds of force:

Figure 5: Forces in Individual Truss Members

There is also a program online called Bridge Designer that does a very basic analysis of bridges, shown below. All you need for this program is a few nodes, some connecting members, a load (or more than one load) and the program calculates all the numbers for you. I designed a bridge in the program like the one I just did an analysis on by hand and it produced the following results:

Figure 6: Bridge Designer Analysis of Three Triangle Bridge

As you can see in the image, the results I obtained in my hand-written analysis are the same numbers as the results from Bridge Designer. I think this has to do with the fact that I used a load of 20 (it isn't incredibly visible in the screenshot) and I scaled my bridge to the bridge I was analyzing (1 block on Bridge Designer was equivalent to 2 inches). I think that I got the same results because the force on the bridge does not have as much to do with the size of the bridge but rather the angles of the members and the weight of the bridge. Since I attempted to scale my bridge on Bridge Designer as accurately as possible, I ended up getting the same results.

We also had to do a Bridge Designer Analysis of the K'Nex Bridge from a couple weeks ago that spanned 24 inches and held the load of 39.2 pounds. We had to apply a load of 20 to the bridge, and the results are below:

Figure 7: Bridge Design Analysis of K'Nex Bridge

As you can see from the results from the Bridge Designer, a majority of the force and weight was being applied to the middle members. This was scaled properly to one block for every inch, so the loads on each member are probably very accurate. However, as you can see in the picture, the middle member of our bridge is missing. The program believes, for a bridge to be "stable," the number of members plus three must be equal to the twice the number of nodes. In our actual design, there is a total of 28 members and 15 nodes, so the program can't run this simulation, deeming that the bridge would be unstable, when it clearly is not. This forced us to remove the middle member, as it would give as a "stable" bridge while keeping it symmetric and taking away one of the least essential members (we think).

We can now calculate the forces that are on the bridge given a certain load, and there is a study that was done on the tensile strengths of K'Nex here. Knowing this, we can design our K'Nex bridge so that the pieces do not experience more than the given amount of tensile strength listed on the page. This would be beneficial so we would be able to know exactly what the breaking points of certain places would be, so we could adjust our members to compensate for the loads they will experience.