As part of Dartmouth's Solid Mechanics course all students are tasked with forming a small group to design and construct a pedestrian style scale model bridge. This bridge is require to span at least 400mm and support a load of 1kN or 225lbs. Each group puts their bridge to the test at the end of the term in final competition where points are awarded for design aesthetics, construction/craftsmanship, total load, and weight to strength ratio. Our bridge achieved a 1,385lb load while only weighing in at 0.90lbs.
As part of the design and testing process students are encouraged to use finite element analysis (FEA). Depicted to the right is a animation of the deflection of our bridge at ten times true scale
FEA is also used to derive a factor of safety under both loading and buckling conditions. By examining these results, our team was better able to make design iterations in order to maximize strength, while reducing weight.
Our bridge was designed in SolidWorks before being laser cut and hand assembled for real-world testing. All parts are designed to be cut from 3mm plywood in the least wasteful manner possible. We chose to laminate our truss for extra thickness as well as achieve a grain offset between layers for added strength.
When tested to failure our bridge failed under buckling at 1,385lbs. The loading consists of a small trolly at the bridge's center whose wheels are the point of contact for the load. This point load encouraged our team to design and arched deck with a great deal of under deck support structure in order to effectively distribute the load into other sections of the bridge. This along with other anti-stress concentration measures like circular trusses allowed our bridge to exceed the project specifications by a factor of six.
