Soft steel is used for rivets always, and sometimes for the whole superstructure of a bridge, but medium steel more generally for the plates, angle bars, &c., the weight of the bridge being then reduced by about 7% for a given factor of safety.
The committee recommended that a factor of safety of 4 should be taken for wind stresses.
It was pointed out as early as 1869 (Unwin, Wrought Iron Bridges and Roofs) that a rational method of fixing the working stress, so far as knowledge went at that time, would be to make it depend on the ratio of live to dead load, and in such a way that the factor of safety for the live load stresses was double that for the dead load stresses.
For stiffened suspension bridges with wire cables, if the dip is 310th of the span the limiting span is 2700 to 3600 ft., and if the dip is *th of the span, 3250 to 4250 ft., according to the factor of safety allowed.
In the next place, it is necessary to confine the pressure, at every point of the masonry, to an intensity which will give a sufficient factor of safety against crushing.
