however, be made quite deep. The reinforced slab must be cut back from the rail in order to permit of the latter being removed. This detail requires careful watching.

Sidewalks on through-truss bridges are usually placed outside of the trusses and carried on longitudinal stringers supported by cantilever brackets.

In very-long-span bridges, such as cantilever or suspension structures, the matter of economy from deck and floor-system should be very closely studied, in order to eliminate every unnecessary pound of dead load. Preliminary designs should be made for various types and for different spacings of beams and stringers, in order to determine the most economic arrangement. The question of total weight of deck may be far more important for such long-span structures than the total weight of metal in the floor. Timber decks, of course, will be much cheaper than concrete decks; and the decision between the two types must be made by judgment, considering the nature of the traffic, the money available, the possible revenues, the first cost, the cost of maintenance, and especially the danger from fire. In general it may be said that the concrete deck should be adopted if sufficient money can be secured, and, for a toll bridge, if the estimated revenue justifies the expenditure.

There is a light type of floor recommended for long spans by Edward A. Byrne, Esq., Member American Society Civil Engineers, Chief Engineer of the Department of Plant and Structures of the City of New York. It consists of stiffened buckle plate, having the buckle down, covered with plain concrete, and supporting a thin block pavement. Concerning this type Mr. Byrne on June 8, 1920, wrote the author as follows:

Where buckle plates are used to support the roadway pavement the following detail of construction is suggested. The ends of all buckle plates should rest on supports and be properly spliced. The fillets of the buckle plates should be reinforced by angles— 3" X 3" X 3/8" angles have proven effective in buckle plates supported on stringers five feet on centers. The plates should be laid with the buckle down and filled with Portland cement concrete to a depth of at least three inches above the top of the plate. Longitudinal angles, 3" X 3" X 3/8", should be riveted to the buckle plates along the line of the supporting stringers to restrain the concrete foundation and act as a template for laying the concrete. Each buckle plate should have a drain hole; and a hole in the concrete, extending from the top thereof to the buckle plate, should be provided. On top of the concrete foundation a wood block pavement, three inches in depth, should be laid without any cushion, sand to be spread over the pavement and brushed into the joints. The Portland cement concrete should be one part cement, two parts sand, and four parts broken stone or gravel—the top to be rubbed to a smooth finish. ...

It is highly important that, in using a buckle plate floor, all deflections of plates be eliminated, otherwise the concrete foundation will crack and disintegrate.

Tests made by Mr. Byrne on the floor of the Queensboro Bridge, where the stringers were spaced five feet on centers and where the buckles were about four feet long, showed deflections of buckle plates as great as three- eighths of an inch before the stiffening angles were attached, but no appreciable amount after they were put on.