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In the case of it should be noted that only the wheels 10 to 16 inclusive cause the stress (see Fig. 94), and that the loaded length is the distance from wheel 10 to wheel 16.

Some specifications do not call for impact stresses. The unit stresses in these specifications are made low, and the sections designed are large enough to withstand the additional stresses due to impact. In eases where the impact stresses are required, they must be con sidered in computing the maximum and minimum stresses.

49. Snow-Load Stresses. In some localities the snowfall is considerable, and its weight should be taken into account in com puting stresses. This should be done by considering it as an addi tional dead load of 15 pounds per square foot of floor surface for every foot of snowfall. As it covers the .entire floor surface, the stresses will be proportional to the dead-load stresses. Also it is evident that the snow load should not be taken into account in railroad bridges unless they have solid floors, as most of it falls through the open spaces between ties and stringers. .

As an example, let it be required to determine the snow-load stresses in a member of a highway bridge, the dead-load stress in the member being +84.0, the dead panel load being 12 000 pounds, and the snow being 11 feet deep on the roadway, which is 14 feet wide. The snow panel load is: In the above equation, 14 is the width of roadway; 15 is the weight in pounds of one square foot of snow one foot deep; and 20 is the length of one panel. One-half of the weight of snow must be taken, as half is carried by each truss. The snow-load stress is then: In like manner, all snow-load stresses-can be computed.

Most of the standard specifications which have been published do not specify snow loads; and in fact it is not customary to include the snow load in any designs except those for bridges in extreme northern latitudes. It is hardly probable that the greatest load will come upon a country bridge when it is covered with snow. Also, in cities, the sidewalks are cleaned of snow; and so is the roadway if the city is of large size.

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