In a three-span bridge the theoretical advantage of continuity is about 49% for a dead load and 16% for a live load.
Dead Load.-The dead load consists of the weight of main girders, flooring and wind-bracing.
The four cables support a dead load of 7140 tons and a live load of 4017 tons.
The weight of main girders increases with the span, and there is for any type of bridge a limiting span beyond which the dead load stresses exceed the assigned limit of working stress.
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.
