plus its proper impact allowance. If it do not so exceed, the regular uniform live load is to be employed.

IMPACT-ALLOWANCE LOAD.

The impact-allowance load is to be a percentage of the uniform live load, found by the formula

where P is the percentage and L the length in feet of span or portion of span that is covered by the live load, when the member considered is subjected to its maximum stress.

DEAD LOAD.

See Specifications for Railroad Structures.

WIND LOADS.

For highway structures the wind loads per lineal foot of span for both the loaded and the unloaded chords are to be taken from the curves shown on Plate VIII.

This diagram was figured for a clear roadway of twenty (20) feet. For wider structures, the wind loads are to be increased two (2) per cent for each foot of width in excess of twenty (20).

The wind loads given on the diagram have been computed from detailed designs for simple spans up to seven hundred and fifty (750) feet in length, but beyond this limit they have been assumed; consequently, in designing spans of greater length than this, it will be necessary to check the assumed wind-pressure after the sections are proportioned, using an intensity of twenty-five (25) pounds per square foot.

The intensities employed in preparing the curves varied from forty (40) pounds for very short spans to twenty-five (25) pounds for very long ones.

For viaducts, the wind pressure on the structure is to be assumed as three hundred (300) pounds per lineal foot on the spans at the level of the floor, and two hundred and fifty (250) pounds for each vertical foot of each entire tower.