frames of angles; longitudinal struts at top of towers, plate girders; and longitudinal girders, plate-girder spans, or occasionally, for very long spans, open-webbed, riveted girders or pin-connected trusses.

The detailing for longitudinal girders of trestles and elevated railroads and the bracing between same shall comply with the specifications governing the designing of plate-girder spans and the floor systems of pin-connected spans.

In general, the transverse and longitudinal bracing of trestle towers shall consist of a double-cancellation system of stiff diagonals without any horizontal struts, except at the bottom between pedestals. The latter struts must be strong enough to move the column feet upon their sliding-bearings when said struts are expanded or contracted by changes of temperature. Provision must be made for holding some feet rigidly, and for sliding some in one horizontal direction only, and others in any horizontal direction, at the same time holding them all down so that they shall not he lifted perceptibly by the wind-pressure. Sliding-plates are always preferable to rollers for pedestals of trestles. They shall be planed extremely smooth, and so as to bear properly at all parts.

Occasionally, in solitary bents, it is permissible to use hinged ends for columns at pedestals; but it is generally better to make then fixed, and to figure the columns for the greatest bending produced in them by transverse loads and extreme changes of temperature.

The tops of trestle columns are to be made vertical by bending them just beneath the longitudinal girders where the latter are riveted to them; and the upper transverse struts must be made as deep as the longitudinal girders, and must be riveted effectively to the columns. Corner brackets of double webs are to be used for connecting the columns to the horizontal struts and bracing-diagonals, and at the same time to strengthen the column at the bend. Additional strengthening is to be given by using a solid web or diaphragm in the column extending from the top thereof to a point about two (2) feet below the bend.