Week 6: Analysis Desires

This past week the group took our 2' KNEX bridge design and made a few modifications before the load test. The group wanted to try to decrease the cost and still provide the strength that the original design had.  Our bridge managed to hold 48.4lbs which was the highest in the class.  However our cost/breaking weight was not the best in the class.  Our bridge was not the highest costing bridge in the class but it was among the top costs.  In the next test we will try to make modifications to the design to lower the cost without forfeiting the strength.  The bridge failed at the gusset plates.  The group predicted that the gusset plates underneath the load plate would break.  This was a close prediction.  The plates at the edge of the load plate on the underside of the bridge truss pulled apart.  The KNEX bridge structures week points are in the gusset plates dealing with tension.  They tend to pull apart or snap the small connection points.

If possible it would be great to be able to calculate the tensions on each individual connection on a gusset plate.  It is obvious that the plates close to the load plate on the underside of the bridge experience the most tension.  Trig functions may be able to help break down the forces along different cross members.  The fact that all connections are made at either 45degrees or 90degrees should allow for easy calculations.  I am not sure how to account for the decrease in forces toward the end of the bridge or how much force the slight bending of each individual beam takes out of the overall force.  The compression forces do not seem to be a problem.  The beams are much stronger than the gusset plates and are less likely to bow.  It is also important to figure out how the cross members from side to side strengthen the bridge.  Bridges will fail easily by racking from side to side.

The rules for next week has changed some.  The group must complete a 3' bridge with a hollow section 3"x2" for traffic to travel through.  This provides a whole new set of challenges and lots of modifications to do from the original design.  The group will get together and try to use what we have learned from the first load test to create a bridge with a better cost/breaking weight ratio.