Actually, the tested strength of the wire is 160,000 psi. Although Roebling had requested 180,000 psi, he got American suppliers who cheated on the quality. The safety factor allowed the inclusion.And it still stands. Many ,if not most old boilers,failed not because of jacket strength, but because of lack of water. If water is then reintroduced, it touches red-hot iron.There is immediate decomposition to hydrogen and oxygen.The resultant recomposition will blow the tube or the boiler.