Bridge Process

Now that the term is almost over, I can take the time to reflect on what I have learned.  Before I had taken this class, I didn’t even know what a truss was, let alone that it was a type of bridge.  I was new to this whole topic and was forced to learn a lot in order to succeed in this class.  I thought it was really interesting that bridge designing was such an intense process.  It really has a lot of little details that need to be perfected in order to have a successful bridge.  I had no idea that so much happened behind the scenes during the planning stages of a bridge.  It really is a complicated process.  You need to consider what type of materials to use, how big/long the bridge will be, what type of gusset plates to use, the angles of the trusses, the thickness of each member.  The list goes on and on.  I am just so amazed that all of this goes on.  It has really opened my eyes up to what engineers have to do when designing a real life project. 

Last week in class my group tested our three foot design.  Unfortunately, we weren’t doing very well.  We changed our design a few times and added a lot of gusset plates and more members.  We knew we were raising the cost of our bridge, but it didn’t hold a lot of weight at all.  Our bridge kept failing due to the twisting of the members at the joints.  We tried putting in cross sections in the middle of the bridge to reduce this twisting, but we didn’t get to test it again so we weren’t sure how it was going to work. We hope that this has made our bridge stronger, especially since it is now more expensive.  Next week we hope our prediction of a low weight is wrong.  

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.