K'nex Process

There are many differences between using K’nex pieces to build a bridge and using steel to build a ‘real’ 20’ bridge.  K’nex pieces easily fit together and you have a lot of options when picking ‘gusset plates’.  However, all of the connecting pieces are 45 degree angles, and you are very limited to the types of triangles you build.  In real life, you can make any desired shape out of various materials and connecting pieces.  This is a disadvantage of using K’nex pieces. 

Something our group struggled with was figuring out how best to use the connecting pieces.  We found out early on that the grooved pieces can easily slip and slide apart from each other when enough pressure is exerted on them.  We were just hoping that the designs we had would uphold a significant amount of weight.  The other disadvantage of using K’nex pieces is that there is no way to ‘bolt’ or glue the pieces together as you can do on a real bridge.  The pieces just fit inside of each other and we have to hope that’s enough.  The steel bridge in real life has the potential to be more efficient. 

Last week in class we brought sketches of a bridge design and compared them all to see which one we thought would work best.  In the end, we came up with a new design all together.  We had decided to use as many short members as possible to increase the strength of the bridge.  However, this caused us to use a lot of connecting pieces which really hiked up the price.  We are going to test it in class this week and we predict that it will fail due to the gusset plates sliding apart. 

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.

Research Questions

Currently we are working on understanding the different types of bridges and how they work.  As a class we have all made some designs although I’m sure some are better than others, there are a couple of questions I was wondering if Mr. Bhatt could answer for us.

To begin with, after so many years of bridge producing, is there a certain technique or kind of literary work one can read as an engineering to advance in the proper understanding of bridges and their necessary properties?

Additionally,  what kind of specialists would be needed to bring together the perfect combination of a good quality bridge and a good budget demand for the government and/or company offering the capital to build the bridge?

Lastly,  how can one do research from other types of literary works besides books (lets say one wants to read the mentioned on previous journals or newspapers on a certain subject like bridges)?  Like the commentaries that other successful bridge companies might have of certain historical strategy and designs.

- Ana M. Franco