Knex Process

Last week in class, our group discussed the bridge designs we'd submitted on paper and chose a strategy for building our Knex bridge. After modeling one of the paper drafts which used a number of 3.375" chords, we chose to shorten the average length of the chords and increase the number of joints. Doing so raised the cost of our bridge but also increased its strength greatly. We are going to test the bridge in class this week and pay close attention to the way the bridge collapses. I would like to figure out how to disperse the load weight across the bridge so that the bridge doesn't have a weak point in the future.

There are important differences between designing a bridge using Knex and designing a twenty-foot "real" steal bridge. The most obvious, I believe, is the length to weight ratio of members. Knex are very light plastic pieces which seem to be very strong for their weight. Without having yet compared a Knex bridge's strength-to-weight ratio to that of a steel bridge, I am imagining that the steel bridge would have more dead weight in proportion to its strength. On this note, a steel bridge with the same design as a Knex bridge may not be able to support as much load as a Knex bridge due to the different strength-to-weight ratios. The steel bridge may give out sooner because of this, or it may hold together longer due to the use of better Gusset plates.

While experimenting with the Knex bridge designs, I have learned that the Knex grooved Gusset plates tend to be the weak points in the designs. The smooth plastic pieces easily slide apart under tension, causing the bridge to fail. On a "real" bridge, there would be stronger Gusset plates due to a greater amount of friction between materials and the usage of bolts for additional support. Both of these elements would help strengthen the steel bridge, but neither of these options are available to strengthen the Knex bridges, making the steel bridge potential stronger.