Bridge Process

During class last week, my group and I tested a three-foot bridge we'd designed over the course of the previous week. This bridge held only 14.2 pounds and its point of failure was caused by weak joints for the cross beams. Under 14.2 pounds, the bridge twisted and its members popped apart. To fix this, we replaced the weak connections with stronger ones. The original connections consisted of a Knex chord laying in the gusset plate joint. These connections were weak because the chords could slide freely through the plates, although being held against them. The new connections were created with two grooved 360-degree gusset plates joining the chords' ends like two puzzle pieces interlocking. This type of connection does not allow for movement and helps minimize the bridge's horizontal displacement. Next week we will test out three-foot bridge and compare our results to the rest of the groups' results to see which group has the best overall bridge.


Now that my group and I have almost completed the bridge design process for the term, I am realizing how much I have learned, specifically about bridge design. In class yesterday, Dr. Mitchell put three rough bridge analyses on the board. All were the same width and comprised of three triangles in the same order, however each was a different height. According to the analyses, a bridge whose height is the taller, or whose triangles are the least obtuse, will have the least amount of compression and tension on its members under a given load. Another important key to designing bridges that I've learned is that joints tend to be the weak spots. Before taking this class, I always thought that chords were the "weakest links" in bridges, but it is truly the "links" which are weaker. In testing the Knex, I have read information about which joints can handle the greatest stress force before failing, but we have never analyzed the force it would take to snap a chord in half. This is because of weak joints.