A3 - Lester

 Figure 1

Figure 2

Figure 3

Figure 4

   I found the tension/compression forces in each member for the sample truss given, as can be seen in the calculations sheet above (Figure 1). The forces were calculated using the “Method of Joints”. Following the video instructions, a free body diagram was made for each joint. The X and Y components are separated and, since there was no movement and therefore no acceleration in the system, the sum of the forces equals zero. Apart from the tension/compression forces, there is a force in the X-direction resulting from the fixed node, and a normal force in the Y-direction from the upward resulting force on the table. Some forces were equal to each other, as shown in the calculations. A drawing from the calculations sheet is enlarged in Figure 2. This drawing labels each member with the corresponding force.

   The drawing and analysis was replicated in the Bridge Designer program. The dimensions given were h=8”, L=24”, and W (weight applied)=15 pounds. However, the Bridge Designer field was not big enough for those dimensions. The measurements were scaled down by a factor of two, for a new h=4” and L=24”.  After initializing a load test, it was shown that the tension/compression forces found in the calculations page were very close (rounded to the nearest whole number) to the calculations I found by hand. This analysis can be seen in Figure 3.

   Finally, our group modeled our Knex Truss on the Bridge Designer program (Figure 4) and applied a load of 20 pounds to the midpoint. The dimensions were scaled down using the same method as Figure 3. The corresponding calculations for tension/compression were in the expected range, and not above 100, which would mean the Truss would break with the applied load. The forces increase in magnitude the closer they are to the midpoint and the applied load. However, none of the forces were above 100, so the truss is effective. This information can be used to fine-tune our design. Now that we know the center has the most force, we can add extra support, increase the members, or even slightly change the design to maximize the load it can carry. We will do this between the Week 8 and Week 9 labs, before writing the lab report that summarizes our final design and other aspects of the project. 
   This tests were very helpful in discovering how forces are distributed in a truss system. Knowing which members will have tension and which will have compression is helpful in deciding which members or joints to reinforce. Also, it can be seen that the center of the truss bears the most load. We must make sure that the center of our Knex bridge is strong enough to support that load.