Plywood Bridge
Objective
The task was to design a truss bridge, which could withstand load from an Instron. We were not allowed to use more than 800 cubic centimeters of 3 mm thick Baltic birch plywood. The bridge had to meet specific dimension requirements to fit the Instron. We were also required to predict:
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The maximum load supported by the bridge before failure
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The location of failure
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The maximum deflection expected at a total load of 1,000 N.
Technical Details
Some of the designs I was responsible for are show above. I modelled them in SolidWorks and ran some simulations to test their rigidity. We opted for a simple Howe truss. The compressive diagonals meant we had more strength per material. It’s construction was also feasible in the constricted time frame. I made some sketches after which, we the modelled the bridge in SolidWorks and assembled it from laser-cut plywood.
The semi-circular bumps at the bottom of the truss provide a little more material for force to be distributed to, reducing stress. We chose to glue together long rectangular pieces of plywood for the deck. This was a way stronger option than cutting out one large rectangular piece, as the wood glue provided more resistance to shear force and bending moment than the wood grain would have provided on its own.
Results
We ran tests on our first version of the bridge, to help us identify the locations of failure and maximum load. The results from the test, coupled with simulations on SolidWorks helped us to deduce that the first failure would occur at about 5kN, at one of the vertical members.
On the final day of Instron testing, the bridge was able to withstand 5.933kN of load (approximately one kilo-Newton more than our 'conservative' prediction).