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Rh which will produce the maximum stress, but which will not involve overlapping of adjacent lanes, nor place the center of the lane less than 5 feet from the roadway face of the curb.

Each lane loading shall consist of a uniform load per linear foot of traffic lane combined with a single concentrated load so placed on the span as to produce maximum stress. The concentrated load shall be considered as uniformly distributed across the lane on a line normal to the center line of the lane. For computation of moments and shears, different concentrated loads shall be used, as indicated in plate 8. The lighter concentrated loads shall be used when the stresses are primarily bending stresses, and the heavier concentrated loads shall be used when the stresses are primarily shearing stresses.

Either truck or lane loading shall be used, depending upon which gives the larger stress. In computing stresses, each 10-foot traffic lane loading or a single standard truck per lane shall be considered as a unit. The number and position of loaded lanes shall be such as to produce a maximum stress, subject to reductions hereafter specified. Fractional lane widths are not to be considered.

On any series of continuous spans, discontinuous lengths of lane loading shall be used where necessary for maximum stress, but only one concentrated load shall be used.

Where maximum stresses are produced in any member by loading any number of traffic lanes simultaneously, the following percentages of the resultant live-load stresses shall be used in view of improbable coincident maximum loading:

The position and number of loaded lanes used shall be such as to produce maximum stresses in all cases.

The reduction in intensity of floor-beam loads shall be determined as in the case of main trusses or girders, using the width of roadway which must be loaded to produce maximum stresses in the floor beam.

Bridges built by State and Federal highway agencies in recent years on the primary rural roads of the country, outside of metropolitan areas, are generally of H –15 design, and the adoption of higher standards for ordinary rural roads would not be justified by any apparent requirements. For major intercity routes and metropolitan areas the H–20 designs are justified and a number of the States have already adopted this higher standard.

Within the usually assumed tolerance of overloads, bridges of design and condition capable of supporting H–15 loading will safely support the loads of all commonly used commercial vehicles in the frequency with which they are normally applied. They will also similarly support the loads of all military equipment other than heavy and medium tanks, without special control of the movement. Load diagrams of the principal types of military equipment are shown in Plates 9 and 10.

Regarding the support of tanks, special studies have been made from which the following conclusions can be drawn as to the effects