A Treatise On The Principles And Practice Of Dock Engineering

STRESSES IN MOVABLE BRIDGES. 411 If we consider the support afforded by the further abutment in the single- leaf bridge when at rest, the ratio is, of course, considerably reduced. 2. On the other hand, the length of a single-leaf swing bridge is less than the combined lengths of two leaves for the same opening. Ihe icason for this has already been given—viz., that the pivot has to be placed sufficiently far back from the face of the coping to accommodate the whole width of the bridge upon the quay. When there are two pivots, the excess of length thus involved is doubled. 3. A single-leaf bridge only requires a single set of actuating machinery. For a given opening, the set will have to be at least twice as powerful as the two sets combined, but the cost of construction, of repairs, and of general maintenance will certainly not be doubled for a single set. 4. The control of the machinery of a single-leaf bridge is in the hands of one man. Two sets of machinery necessitate two attendants at least, whose co-operation can only be secured by imperfect signals or by shouted instructions, which, in windy weather, are liable to be unheard or mis- understood. 5. Additional apparatus for interlocking is required in the case of double-leaved bridges. 6. The adjustment of the levels of two leaves at their junetion is a matter of some delicacy. Any irregularity (however slight) in the joints of a locomotive track leads to percussive action and the gradual destruction of the rail. The absolutely necessary clearance between the two sets of rails is sufficient to cause this, and repairs or renewal involve inconvenience and delay. 7. A double-leaf swing bridge necessitates less length of passage than a single-leaf bridge, the whole of whose length has to be accommodated on one side. Stresses in Movable Bridges.—It would manifestly be impracticable, within the limits of a single chapter, to attempt to treat with the least degree of precision and finality the very numerous and important considéra- tions peculiarly involved in the design of movable bridges. Still less would it be possible to investigate, with that thoroughness which the question demands, the nature and amount of the stresses set up in the framework of such bridges, generally, under the varying conditions of load and support to which they are subjected. These latter problems form the basis of distinct treatises, to which the reader is referred for information more complété, more detailed, and more comprehensive than could be included here. At the same time, in view of the identification of movable bridges with dockwork and the unique features which they possess in that connection, it would evidently be equally injudicious and inappropriate to abstain alto- gether from presenting some account of the principles, upon the basis of which such structures are adapted to the particular kind of work which they are called upon to perform. Accordingly, we will endeavour to compromise the matter by investiga-