DESIGN OF A PLATE-GIRDER RAIL`YAY-SPAN 68. The Masonry Plan. In some cases the general dimensions of the masonry are limited; such a ease, for example, would occur in the crossing of a street or narrow waterway. Here the length of the span and the distance above the street or the surface of the water, are the limited dimensions. The span and under-clearance may be unlimited, as in the case of a country stream crossed by a roadway which is a considerable distance above the surface of the water. The term unlimited is not here used in its exact meaning, as the span in this case is really limited by the cost, which rapidly increases with the length of the span.
In some cases, as when the engineer is in a bridge office, the masonry plans are sent in by the railroad. In such eases many of the limited dimensions are fixed. The most usual dimensions to be fixed are the elevation of base of rail, the elevation and size of the bridge seat, and the length of the span under coping. These limit the depth of the girder, or the depth of the floor if it be a through girder, and also limit the length of the bearing plates at the end. Fig. 131, the masonry plan of a road crossing, shows in general what can be expected. All the dimensions usually fixed are given, and those marked x and y may or may not be, but x should never be less than 3 feet.
The case whose design is under con sideration will be taken similar to that of Fig. 131, and the span will therefore be a deck one.
70. Determination of the Span, Cen ter to Center. Fig. 132 shows the various spans—namely, under coping, center to eer ter of end bearings, and over all. The span coping is that span from tinder cop ing to under coping lines of the abutments, and is so chosen as to give the required dis tance between the abutments at their base.
The span center to center is equal to the span under coping plua the length of one bearing plate. The span over all is the extreme length of the girder. The length of the bearing plate is influenced by the width of the bridge seat, and also by the maximum reaction of the girder. The length should seldom be greater than 18 inches and never greater than 2 feet, as the deflection of the girder will cause a great amount of the weight to come on the inner edge of the bearing plate and also on the masonry, in which case the masonry is liable to fail at that point and the bearing plates are over-stressed.
Cast-steel bearings are now almost universally used. They decrease the height of the masonry, and distribute the pressure more evenly and for a greater distance over the bridge seat. When these castings are used, the bearing area between them and the girder may be made quite small, thus doing away to a great extent with the deteriorating effect due to the deflection of the girder as mentioned abo%e. Fig. 133 shows the end of a girder equipped with a steel pedestal. Table XX gives the length of the bearing on the masonry for various spans, Cooper's E loading being used and cast-steel pedestals being employed.
As an example, let it be required to determine the span center to center of a deck plate-girder of 60-foot span under coping, the loading being E 40. From Table XX. it is seen that the length of the masonry bearing will be 21 inches, and therefore the span center to center of bearings will be GO + 2 X (i X 1 ft. 9 in.) = 61 feet 9 inches.
In Articles 71 to 77 the above girder will be designed ; and also such information as is of importance regarding the subject-matter of each article will be treated. The dead- and live-load shears are computed by the methods of Part I, and are given in Fig. 134.