DESIGN OF A THROUGH PRATT RAILWAY-SPAN 70. The Masonry Plan. The same remarks which are made in Article 67 apply here. In this case the length of the masonry plate is usually determined by considerations relative to the number and length of the rollers in the bearing, and not by the bearing per square inch upon the masonry, the size of the plate as determined by the above considerations being usually much larger than if it had been determined by the unit bearing stress. A preliminary design of the masonry plate is usually made in a manner similar to that done in the case of the plate-girder; or the length of the masonry plate may be approximately determined from the following: The above masonry plates are for single-track bridges, with or with out end floor-beams, the length being the same in either case.
80. Determination of the Span. The determination of the span is made in exactly the same manner as described in Article 68. Care should be taken, in case end floor-beams are not used, to allow for the pedestal stones, which are square stones resting directly upon the bridge seat, and upon the top of which rest the masonry plates of the stringers. Their height must, of course, be such as to keep the stringers level. In case these stones are used, their size must be determined; and if it is greater than that of the bearing or masonry plates, then their size determines the width of the bridge seat and the span center to center of bearings.
81. The Ties. In the design of the ties, as well as in all the design which follows, the Specifications of the American Railway Engineering & Maintenance of Way Association will be followed. Whenever reference is made to these specifications, the number of the article will be enclosed in parentheses, as "(5)," which signifies that Article 5 of the Specifications is to be referred to.
The stringers in the bridge in question will be taken 6 ft. 6 in. center to center. The maximum loading (7) is such as to bring S 333 pounds on one tie, and to this must be added 100 per cent for impact, making a total of 16 667 pounds. In order to illustrate the
method of assuming the distance, center to center of rails, as 5 feet, that distance will be used in this case. The maximum moment will then be 9 X 16 667 = say, 150 000 pound-inches. The size of the tie will be determined as in Article 71, the allowable unit-stress being 2 000 pounds per square inch (5). If a 7 by 9-inch tie is used, the unit-stress will be 1 590 pounds. If a 6 by 8-inch tie is used, the unit-stress will be 2 340 pounds. It is evident that a 7 by 9-inch tie must be used. See Fig. 166 for spacing of stringers and rails, and for position of the loads. Note that, although impact is taken into account in this case, the size of the tie is the same as that designed for the plate-girder, although the unit allowable stress also differs.
82. The Stringers. The width, center to center of trusses, will be assumed as 17 feet, since this is sufficient to clear the clearance diagram in cases of single-track bridges of spans less than 250 feet.
The span which is to be designed in the following articles is a through-Pratt with 7 panels of 21 feet each, making a total span, center to center of bearings, of 147 feet 0 inches. See Plate HI (p. 251). Rivets inch in diameter will be used throughout, except in channel flanges.
The length of the stringers end to end will be 21 feet, and accord ing to Cooper's p. 32, the maximum moment for the live load will be 226 000 pound-feet per rail. The coefficient of impact (9) will be ( 21 -I- 300 = 0.935, and therefore the moment due to impact will be 0.935 X 226 000 X 12 = 2 535 000 pound inches, making a total of 5 247 000 pound-inches due to live load.
The section modulus for any particular beam is equal to the bending moment divided by the unit-stress, and this is equal to the moment of inertia divided by one-half the depth of the beam. This latter quantity is constant for any given beam, and for I-beams may be found in column 11, Carnegie Handbook, p. 98.