A Treatise On The Principles And Practice Of Dock Engineering

428 DOCK ENGINEERING. Similarly, from (107), Rc = 142-4 tons, and, by residue, RB = 314-5 „ Now take the ballast. From formulas (129), (130)— R W-F 2 x 712 - 162 \ 8ab (a + b) \8 x 71 x 104 x 175/ = — 3-7 tons; Ro = wZ ; 1 - = 250 Z (6è2 + 4 ai - Z2)1 8 ôa (a + ô) J 16 (6 x 712 + 4 x 71 x 104 - 162) 8 x 712 x 175 = 216 tons, 1 - and, by residue, RB — 37’7 tons. Hence, the nett reactions for stated are— the whole bridge under the conditions RA= 54-1 - RB = 314-5 + Rc = 142-4 + 3’7 = 50-4 tons. 37-7 = 352-2 „ 216-0 = 358-4 „ 761-0 „ The sum is tlxe total weight of bridge structure, imposed load, and ballast. Having determined the reactions at the points of support by calculation as above, it will be found most convenient to obtain the bending moment and shearing stress throughout the bridge by graphical methods. The diagrams admit of superposition, from which the points of maximum stress may be determined under any variation of loading. At this stage, however, the procedure is common to bridge design generally and need not be further investigated. Distinctive Features of Movable Bridges.—The following essential and distinctive features of swing bridges claim some brief attention :— The Pivot.—There are two main systems, or methods, in which a swing bridge is united with the pivot upon which it revolves—viz., the method of suspension and the method of superposition. In the latter instance, the body of the bridge rests directly upon the pivot in a manner analogous to the ordinary balancing of a bar upon any vertical. In the former system, the bridge structure is suspended from the pivot by means of stout bolts, which pass up from the underside of the pivot girders to the extremities of a crosshead, or saddle-piece, carried by the pivot. The structure of the pivot itself follows an almost numberless variety of individual designs, dependent on one or other of the two principles adopted. We will accord a passing notice to a few typical cases. (a) A long, narrow pivot passing through the bridge, nearly to the surface of the roadway, as at Velsen (fig. 405). Such a pivot requires a firm and unyielding foundation, for any inequality of settlement will