Staquet Weekly Blog - Week 7

Last week in lab each group was required to test their K'Nex bridge to see how much weight it would be able to suspend.  After testing, each group completed a survey on their bridge regarding how much weight the bridge held, its cost, how many pieces it was made of, a short description of the bridge design as well as why/how it failed.  Our bridge was able to hold 39.1 pounds while only costing $133 thousand, making it the strongest bridge in the class in terms of the cost to weight ratio.  This was a major accomplishment for our group, but I think that constructing the 3' span bridge will be much more difficult for not only our group, but for the class as a whole.  This week in lab we will be learning more about the structural analyzation of trusses and be able to gather some figures in reference to the weight capabilities of a specific design.

Working on the K'Nex bridge design without any numbers hasn't been too much of a problem up to this point.  Knowing the length of the members was obviously important in making sure the bridge would be able to span 24", but as far as weight goes, the exact numbers had not been a serious necessity.  Now that the bridge will have to span 36" I think some figures regarding failure in the K'Nex pieces will be much more useful.  The true challenge will be the fact that the bridge will still be experiencing the same 8" of force, only now it will be 6" farther away from each end.  I think the most important number to know will be the maximum tension force a K'Nex gusset plate can withstand before it lets go of the straight member connected to it.  It could be calculated by doing a number of stress tests on a simple connection and getting an average reading for the force required for the connection to fail.  This test would have to be evaluated for a number of different scenarios based on not just the orientation of the gusset piece, but the kind being used; whether it is a 360,180, 90, or 45 degree connector as well as how many members are being connected to it.  Another interesting number to know would be the length that a gusset piece adds to a straight member when they are connected. This can be easily found with the use of a ruler, and I honestly am unsure as to why our group has yet to calculate it.  The more important number is the failure point of the gusset pieces due to tension, because this is the primary reason that bridges collapsed during the lab.  Compression is not an issue with the K'Nex because they will fail elsewhere from tension forces before compression becomes a factor.