Week 7 Update

In terms of the Knex project, it would be beneficial to know similar information to that given in WPBD.  The compression force/strength ratio as well as the tension force/strength ratio would be important pieces of information to have before testing the bridge.  Another helpful piece of information would be the overall force on the gusset plates.  This would allow us to tweak the design to ensure that the forces on the gusset plates were evenly distributed and not all concentrated on one junction.

Determining the force on the gusset plates could be fairly simple.  Using free-body diagrams to interpret all of the forces acting on the plate would allow us to measure the overall force acting on it.  This can then be plugged into Newton's second law which tells us that the sum of the forces acting on an object is equal to the mass of the object multiplied by the acceleration of the object; this will give us the "acceleration" of each gusset plate.  In an ideal setting, the acceleration should be zero and to achieve this we would need to balance out all forces acting on the gusset plates so that there would be no acceleration.   The only conceivable issue with using free-body diagrams to compute the force acting on each gusset plate would be finding the tension forces of the upper members.  We know the force of gravity, but it may be more difficult to determine the force acting on the gusset plate from above.

To determine the force/strength ratios would be very challenging.  For the purposes of the Knex bridges, there is no setting where we can exaggerate the deflection of the bridge in order to determine which members are compression and which are tensing. 

Last week in lab, the bridge created in week 5 was tested using a bucket of sand.  Our bridge was able to hold a maximum of 21.4 pounds at a cost of $216,000, giving it a weight/cost ratio of 9.91*10^-5 pounds/dollar.  We were slightly surprised by these results as we did not know what to expect.  We had tested the structure during week 5 and it had failed under the weight of one battery. 

This week we hope to begin working on a bridge which will span 36".  Our previous design was preserved and will be used as a starting block for the larger bridge.  More support members will be added and possibly there will be the addition of a secondary upper truss.