Live-load sentence example
In a three-span bridge the theoretical advantage of continuity is about 49% for a dead load and 16% for a live load.
The four cables support a dead load of 7140 tons and a live load of 4017 tons.
Live Load on Railway Bridges.-The live load is the weight of the heaviest train which can come on the bridge.
In the earlier girder bridges the live load was taken to be equivalent to a uniform load of 1 ton per foot run for each line of way.
He takes as the live load for a bridge two such engines, followed by a train of wagons covering the span.Advertisement
For all these reasons the stresses due to the live load are greater than those due to the same load resting quietly on the bridge.
These percentages are added to the live load 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.
Working Stress for combined Dead and Live Load.
Generally a girder supports both a dead with a travelling load like a railway train, this is greater for partial and a live load.Advertisement
The cost of the main girders for one span will vary nearly as the square of the span for any given type of girder and intensity of live load.
Rankine gives the approximate rule Working deflection =5= l a /t o,000h, where l is the span and h the depth of the beam, the stresses being those usual in bridgework, due to the total dead and live load.
Here again, since the intensity of the pressure on A becomes greater as it descends owing to the increased head, the apparent increase of weight of the lift-ram as it rises is automatically balanced; water from the high-pressure system is admitted down the hollow ram B and does the work of lifting the live load.
A purely empirical allowance for impact stresses has been proposed, amounting to zo% of the live load stresses for floor stringers; 15 for floor cross girders; and for main girders, io% for 40-ft.