Fig. 10 shows a shelf bracket fixed to a wall. A heavy box is placed on the shelf. The product has been designed to be both functional and aesthetically ple...

(a) The type of force acting along the diagonal strut labelled A when the load is applied is compression [1]. When the heavy box pushes down on the horizontal shelf, the shelf tries to rotate downward about the wall fixing. The diagonal strut resists this rotation by being pushed together along its length (shortened), which is compression. The strut acts as a prop, transferring the downward load diagonally into the wall.

(b) The type of force acting on the fixing screws at point B is shear [1]. The weight of the box and shelf pulls the bracket downward, while the wall pushes back. The screws are caught between these two opposing forces acting in slightly different planes, which tends to slice across the screw shank. This is a shearing action, as opposed to tension (pulling apart) or compression (pushing together).

(c) One way the bracket could be strengthened to support a heavier load is to add a gusset plate at the corner joints [1]. A gusset plate is a triangular piece of metal welded or bolted across the joint where two members meet, reinforcing the connection and spreading the forces over a larger area. Other valid suggestions include: using a thicker or wider section of metal for the bracket arms; adding a second diagonal strut for additional support; or using a stronger grade of steel.