A structural tension member of 10 ft. long with a square cross-section is subjected to a load of 120,000 lb. There is a choice between an aluminum alloy and structural steel, with the following information given for each.
Aluminum:
Unit weight (lb/ft^3) 165
ultimate strength (psi) 45,000
modulus of elasticity (psi) 10.5*10^6
cost per lb $0.45
Steel:
Unit weight (lb/ft^3) 490
ultimate strength (psi) 90,000
modulus of elasticity (psi) 30*10^6
cost per lb $0.21
A) Which material will be cheaper if you want a factor of safety of 5? (F.S = ultimate strength/applied stress).
B) Which material will result in the lightest member if you want to limit the elongation to 0.20 in.? Which material will be cheaper under these conditions?