Shear Stress on the Pin  
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The figure shows a rectangular plate labeled axial member fixed to a gusset plate that is anchored to a column on the left by a pin. The axial load, P, on the plate is directed to the right.

The pin is used to transmit the load P from the axial member to the gusset plate (which is anchored to the column). In transmitting the load P, shear stress is produced in the pin. The shear stress in the pin acts on the surface at the interface between the axial member and the gusset plate.