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Loadings for Short Bridges

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LOADINGS FOR SHORT BRIDGES Highway Bridges.—Sidewalks of bridges in towns may be considered as carrying a live load of 100 pounds per square foot of sidewalk area. In the more crowded districts of cities, larger loads are sometimes employed, but in general this is ample for all probable occurrences.

Roadways of highway bridges should be able to carry the heaviest motor trucks which may reasonably be expected to come upon them. In the development of truck transportation there is a tendency to in crease the weights carried by a single truck, and careful attention should be given to this possibility in designing bridges intended to last a long time. A motor truck weighing 20 tons, with 6 tons on one axle and 14 tons on the other, the distance between wheels being 6 feet and between axles 12 feet, may reasonably he assumed as a maximum load for a bridge upon an important country highway or street of a town. This load is a very exceptional one•for ordinary highways and probably in most cases a truck weighing 7 to 10 tons is as large as is likely to be met under present conditions, and possibly a road roller may he a more probable maximum load. The use of maximum loads not likely to he exceeded in the near future is always desirable in such work.

For country bridges under moderate or light traffic, a truck weigh ing 8000 pounds on each of two axles, 10 feet apart, may he used as a probable maximum load under present conditions, or a road roller, 6 tons on the front wheel, which is 4 feet wide, and 4.5 tons on each of the rear wheels, each 20 inches wide.

Street Railway the bridge is to carry a street railway, the load of a car weighing 50 tons on four axles spaced 5, 14, and 5 feet apart may be assumed as a probable maximum load. This load may be considered as distributed over an area of bridge floor about 35 feet in length and 10 feet in width, giving a maximum uni form load of about 300 pounds per square foot.

For light traffic roads, a car weighing 35 tons on the same wheel distribution may be used, giving a uniform loading of about 200 pounds per square foot.

150. Distribution of Concentrated Loads.—Investigations of the distribution of concentrated loads upon slabs have been made by Mr. Goldbeck for the U. S. Office of Public Roads. These tests 1 seemed to indicate that for a slab whose width is greater than its span, the effective width of distribution of a concentrated load might be taken at about eight-tenths of the span.

From a series of tests at the University of Illinois, Mr. Slater concluded 2 that for a slab whose width is greater than twice the span, the effective width (e) might be assumed as e=3x+d, where x is the distance from the concentrated load to the nearest support and d is the width over which the load is applied. As the ratio of

width to span decreases, the effective width becomes less, the coeffi cient in the formula becoming about 1.2 when the span equals the width.

From tests for the Highway Department of the State of Ohio 3 Professor Morris recommends for a concentrated load applied to the concrete floor of a highway bridge that e=0.6S+1.7, where e is the effective width in feet for a slab whose width is greater than its span, and S is the clear span in feet. This agrees well with the results of Mr. Slater if the load be placed at the middle of the span Si 2) .

When the load comes upon the floor of the bridge through a pave ment or fill, it may also be considered as distributed lengthwise over a certain area. For earth fill, the length of distribution may be taken as twice the depth of fill. For gravel or macadam road sur face, three or four times the depth of surface may be used.

In T-beam construction, when a slab is continuous over several girders and a load conies upon the slab immediately over one of the girders, the whole of the load will not be borne by the girder under the load, but a portion of it will be transferred by the slab to adjacent girders. In the Ohio tests mentioned above, this distribution was investigated and the following conclusions reached: (1) The percentage of reinforcement has little or no effect upon the load distribution to the joists, so long as safe loads on the slab are not exceeded.

(2) The aniount of load distributed by the slab to other joists than the one immediately under the load, increases with the thickness of the slab.

(3) The outside joists should be designed for the same live load as the intermediate joists.

(4) The axle load of a truck may be considered as distributed uniformly over 12 feet of roadway.

151. Railway Bridges.—For short spans, railway moving loads may be considered as uniformly distributed by the track and ballast. If the heaviest locomotive load per foot of length be distributed over a width of about 10 feet, the result will be well on the safe side. When the bridge is covered by a fill under the tracks, the width of distribution may be increased by twice the depth of fill.

The weights for maximum locomotive loads may vary from about S000 to 10,000 pounds per linear foot of track, or from 800 to 1000 pounds per square foot when distributed over a width of 10 feet. For bridges longer than about 35 feet, it may be preferable to use actual locomotive wheel loads, or to somewhat reduce the load per square foot.