Analysis Desires

Last week, my group mates and I tested our 2' bridge in a competition against the rest of the groups in our class. Our bridge would have cost over $305,000 to build full-scale, but it only supported 17.4 pounds of sand during the test. This was the first time we tested one of our bridges using the apparatus, and we were able to get a good view of how our bridge failed. This week we will be testing our 3' bridge which was designed completely differently. We decided to cut costs by using different gusset plates and longer chords. We believe our bridge will be able to support more weight than our last bridge. And even though this bridge is a foot longer than the last, it's nearly $50,000 less expensive.

In designing the bridge using Knex, something that is challenging is having to run several tests and analyze each one to determine where the weaker points are. In West Point Bridge Designer, a table of information was presented after each simulated test. The info included statistics regarding each individual chord's tension and compression forces, and slenderness. I would like to be able to determine the tension and compression forces across the top and bottom of the Knex bridges we've been designing, but the problem is that WPBD bridges normally fail because of weak chords giving out. The pattern I've noticed with the Knex bridges is that the joints, or "Gusset plates," have been failing before the chords. I am not sure how we could calculate the tension and compression forces.

Analysis Desires

When designing a bridge on West Point Bridge Design, it tells us what pieces fail and how much tension and compression is exerted on each piece.  I feel that this information would be really helpful when designing our K’nex bridges.  It is a completely different experience to design a 3D bridge and also including the top and bottom designs, rather than just the side pieces.  We have no idea of how our bridge is going to work until we actually test it.  This makes us put a little more effort into our design, but it also means we might not create the most effective or efficient bridge because we are not able to participate in trial and error type experiments.  If we knew before hand which pieces were most likely to fail, we could design our bridge differently before actually participating in the class competition.  I’m not sure how this could be done, but I know it would be helpful to have the tension and compression numbers known so we could build our bridge to its max potential. 

Last week we tested our bridge in class.  It did much more poorly than we expected it to.  It only held a total of 17 pounds.  It broke right on the edge of the bridge; the piece that was on the horse holding the weight of the bridge.  We did not expect this.  We thought that the reason it would fail would be because the gusset plates would slip apart because they seemed like the weak pieces of the bridge.  We decided that we should scrap our bridge and create a new design.  In class we decided we should go with a square shaped bridge instead of the trapezoid we had originally built.  We think this will help disperse the weight of the bridge off of the end points.  We hope to test our new design in class this week with better results.