A rigid bar of weight W = 800 N hangs from three equally spaced vertical wires (length L = 150 mm, spacing a = 50 mm): two of steel and one of aluminum. The wires also support a load P acting on the bar. The diameter of the steel wires is ds = 2 mm, and the diameter of the aluminum wire is da = 4 mm. Assume Es = 210 GPa and Ea = 70 GPa.
(a) What load Pallow can be supported at the midpoint of the bar (x = a) if the allowable stress in the steel wires is 220 MPa and in the aluminum wire is 80 MPa? (See figure part a.)
(b) What is Pallow if the load is positioned at x = a/2? (See figure part a.)
(c) Repeat (b) above if the second and third wires are switched as shown in figure part b.

 

a -+a- L A Rigid bar of weight W (a) a -- a L S S A Rigid bar of weight W (b)