Structural Engineering Design 2

The stringer

Having seen a few hollow timber surfboards on the internet I was surprised in some cases by how much of the stringer had been cutaway to save weight.

Internal structure

Calculating the stresses in the web under loading showed this would not work, but curious to see if something else was at play I made a test piece to test the theory against practice. The test piece was made the same depth and with the same materials and thicknesses as the proposed board.

Test piece

The test piece was put on supports 1m apart and I gently lowered my weight onto it with my feet at third points along it. This loading scenario is not the same as being in the sea but would do as a simple initial load test. If the test piece could not support this load scenario then I would not have confidence the board would survive in the sea with a wave breaking onto it.

Before I had lowered my full weight onto the board I heard cracking sounds and the test piece failed.

Failure!

The failure occurred near the support point and at a corner where the middle depth of the stringer had been cut away. Near the supports the vertical shear force is highest. The web openings would cause vierendeel bending to occur and the stress would be concentrated at the corners. It was clear that this was an area of high stress and the opening size had to be reduced.

Calculation showed that a safe allowable depth of opening to cut out was approximately one third of the section depth for 6mm thick marine ply. To achieve a safe longitudinal stress and vierendeel bending stresses the length of the openings was reduced to the minimum to allow the ribs to be slotted through. The amount of stringer cut away is not a great deal but there seemed little point in trying to cut away more for the sake of saving a few grams and making the board more fragile.

The final stringer profile with the test piece in view behind.