loaded structure, in double-track trestles placing the train of empty cars on the leeward track. All wind loads are to be treated as moving loads

INDIRECT WIND LOAD OR TRANSFERRED LOAD.

For both through and deck spans, even with polygonal top chords, the transferred load is to be assumed to produce a tension in the leeward bottom chord that is constant from end to end of span, and a similar release of tension on the windward bottom chord. For trusses with parallel chords this assumption is correct, provided that all the wind-pressure travels directly to ends of span by the horizontal bracing; while for trusses with polygonal top chords the assumption is a compromise, the travel of wind-pressure being ambiguous. The transferred load at one pedestal is to be found by multiplying one half of the total wind load on the top chord by the average truss depth and dividing the product by the perpendicular distance between central planes of trusses.

TRACTION LOAD.

The total traction load on any portion of a structure is to be taken as twenty (20) per cent of the greatest live load that can be placed on that portion of said structure.

In proportioning the towers and columns of trestles and elevated railroads, the towers and columns between consecutive expansion points are to be assumed to receive no aid from neighboring towers and columns, but must be figured for the greatest possible traction load between said consecutive expansion points.

No percentage of impact is to be added to traction loads.

CENTRIFUGAL LOAD.

The centrifugal load is to be computed for the greatest probable velocity of trains by the formula