A Treatise On The Principles And Practice Of Dock Engineering

GENERAL NOTES ON DESIGN. 441 extremities of the loaded and unloaded leaves and a sharp recoil of one of them when the pressure had been transferred to the other. This connection may take the form of a projection on one of the meeting faces with a corresponding groove in the other face, engagement being made in the ordinary process of rotation. Or, again, where the leaves tilt slightly after turning, so that a tongue-and-groove joint is not feasible, long bolts have been shot home through the faces of the leaves. The motive power in such cases may be a hand lever, a screw, or a hydraulic ram. In a number of instances horizontal interlocking is omitted entirely, partly on account of the necessary clearance required for expansion and partly to avoid the inconveniences of a complicated adjustment. A simple plug dropped into a vertical dovetailed groove serves to unite the leaves and keep them in position. General Notes on Design. Having regard to the maximum resistance of the material to stress and the minimum thickness consistent with stiffness, one-ninth or one-tenth of the unsupported length will generally be found the most effective ratio for the depth of iron or steel girders at the point of support. Towards the nose end of the bridge a reduction is advisable, both on account of economy and headroom. Except for very short spans, lattice girders are preferable to plate girders. The latter make a heavier bridge and expose a larger surface to wind pressure. In bridges carrying a railway track, the cross girders must be designed to take at least the full concentrated load of a pair of engine driving wheels, say 16 to 19 tons, and in order that this intensity of pressure may not be exceeded, it is necessary that their distances apart should not be greater than the distance between two consecutive pairs of wheels, say 6 or 7 feet, while it cannot economically be much less than that amount. But in ordinary cases, 9 to 12 feet is considered a generally advantageous range, in whicli case the load on the cross girders is 32 tons for each track, exclusive of structural load. It will be found economical to give the cross girders a larger proportion of depth than the main girders, say one-seventh or one-eiglith of their span. AVind bracing should be provided to withstand the authorised (but excessive) estimate of 56 Ibs. per square foot. An alternative to the cross girder system is to carry the rails on the main girders themselves, whicli accordingly must lie below the platform. This method, while diminishing the headroom of the closed passage, increases the effective breadth of the bridge by the flange width of two or more girders, which otherwise would protrude above the roadway level, and at the same time provides a clear deck, flush with the coping, when the bridge is swung back into its recess. On the other hand, a deeper