Actually if the diameter of a metal rod is reduced by half, the cross-sectional area is reduced by about 60 percent. So in terms of tensile strength a safey factor of 2.5 would be needed. For bending stress the safety factor is more complicated - it depends on the shape (area moment). For instance an I-beam is stronger than a beam with a circular cross-section when one attempts to bend it in one direction. Note that a circular or square section is probably alot cheaper than an I-beam. But I don't feel like doing the calculations right now -- and I'm hungry I really need to buy some chicken sticks.