STRUCTURAL MECHANICS - 9

 Your client wants to install a heavy safe on the second floor of a building. As the structural engineer, you decide to reinforce the existing timber bearer by specifying a parallel flange channel (PFC) screw fixed to the bearer in the configuration shown (Fig. 2 overleaf). The beam shear X6 of the composite beam has already been calculated for you. Refer to tables and the appendix for timber bearer sizes and PFC details. NOTE: The modulus of elasticity of spotted gum is approximately 20GPa compared to 200GPa for steel.

a.  The timber beam consists of multiple laminations of 45mm spotted gum. Determine the longitudinal shear stress t at the interface between the two lowermost laminations of the bearer(.e., 45mm above the bottom of the bearer) if the floor was loaded up before the PFC could be installed? 

b. You specify to the builder that the PFC must be fixed to the timber beam with coach screws at 300mm spacings. Allowing a factor of safety of 2, what is the minimum required shear capacity for the coach screws?









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