=54. Detail of Track Construction.=—Many crane failures have been due to the spreading of the track between supports. It is better, therefore, to supply considerable lateral rigidity to the beam supporting the track or traveling crane. Where loads are heavy and plate girders are used for the runway tracks, the flanges of the girder are sufficient for this purpose. Where =I= beams are used, however, for the support of the crane track, it is good practice to place on the top of them and rivet with countersunk rivets, spaced about 18 inches apart on each flange, channel irons as indicated at _a_, Fig. 27. By means of these channel irons, which are drilled with open holes _b b_, the rail _c_ may be readily clamped in place by means of wrought-iron clips and bolts, and the rails nicely aligned and adjusted by wedging between these clips and the track. =55. Maximum Stress on Track Girders.=—The principal calculation for the construction of the runway of cranes exists in determining the maximum bending moment. The maximum bending moment on a runway girder occurs when the wheels of the traveling crane are in the position indicated in Fig. 28. It will be noticed that the center of the girder is midway between the center of the near wheel and the center of the crane trolley, that is, the distance _a_ is one-half the distance _b_. The following formula will give the maximum bending moment on a crane girder when the load is in the position indicated in Fig. 28: _w_(_l_ - _a_)² _M_ = --------------- 2_l_ in which _M_ = bending moment, in inch-pounds; _w_ = load on one wheel of crane, in pounds; _l_ = span of girder from center to center of support, in inches; _a_ = distance, in inches, marked in Fig. 28. [Illustration: FIG. 28] In order to illustrate the application of this formula, assume that the wheel load _w_ equals 10,000 pounds; that the distance from center to center of supports of the runway girder is 15 feet, or 180 inches; and that the distance _a_ is 12 inches. By substitution, 10,000 × (180-12)² _M_ = ------------------ = 784,000 inch-pounds 2 × 180 From this bending moment may be found, by the methods given in _Design of Beams_, the proper size girder to use. THE POWER PLANT BOILER ROOM