Movable Bridges

Movable Bridges, by Otis E. Hovey, Assistant Chief Engineer of American Bridge Company, Volume 11, Machinery; pp. XVI and 344. Above 125 Figures and Graphic Tables in the text; 5 Folding Plates and a full page half-tone engraving. John Wiley & Sons, Inc., 1927.

This promised second volume, supplementing the first on Super-structure, is truly a worthy and satisfying sequel. In the other the author's masterful treatment dealt with this class of structures simply as bridges; in this one he discusses all the problems involved in swinging, lifting and lowering them. In other words: first, the forms of structure which support the roadway; second, the machinery,—usually confined in small space,—by which the inertia of the mass of the bridge is overcome, accelerated or retarded. In the language of mechanics : the first volume considers the structures as subject to forces acting statically, or forces whose dynamic action is reduced to statical equivalents, —while the second comprehends the wide range of problems concerning the dynamics of the operating machinery, which in turn involves the most intimate knowledge of experimental

data resulting from the experience and research of the last two or three decades. Obviously it is not possible, in, the space at command, to do more than notice briefly a few of the outstanding features of the book, taken in order; enough to indicate the completeness of the treatment, in the cases cited, and the great amount of data made available for use at very point where the action of the forces must be determined.

In the chapter on "Power for Movable Bridges" the author develops formulas of quite general application, and then applies them to two actual swing spans, otle operated by a steam engine and the other by an electric motor in the same time. In due order are deduced the horse-power required; the force; the times of acceleration, uniform velocity, retardation, and total time of opening; the times as modified by the action of wind; and the gear ratio. Similarly a vertical lift-span is treated.

A chapter on the general arrangements and details of the machinery deals with five actual cases of swing bridges, both of ordinary size and some of great weight; including examples of operation on center-pin, on a wheel .circle (known as rim-bearing), and the center-and- rim-bearing type. The machinery for two classes of bascule bridge and for a lift-bridge is also shown by adequate illustrations. Altogether this chapter clearly presents the subject by twenty full-page drawings, several lesser ones in the text, and five folding plates.

As to the materials which enter into the construction of this class of bridges only the expert has knowledge of their number, variety and characteristics;—such as the qualities and adaptations of various grades of iron and steel, bronze.for bearing discs, and the properties and proportions of antimony, zinc, tin, lead and copper which enter into the several alloys used as "anti-friction" metals, so-called. A twopage table of data on the metals named completes the chapter on this subject. This is introductory to a full discussion of the turning resistance of swing bridges and the calculation of co-efficients of friction in many different combinations of machine elements. There follows, naturally, an interesting discussion of motor torques, giving a clear exposition of the different adaptations and limitations of steam, oil and gasoline engines and the electric motor for operating such bridges.

A chapter on shafts, couplings and keys has many points of interest; as also one on journals, bearings, pivots and cellars. Here the designer will find all needful formulas, tables and graphic charts.

A complete discussion of gearing includes the design of teeth of spur wheels, bevel gearing, the arms of gear wheels, theory of worms and wheels and adaptation of screws and nuts to actuate the ends of draw spans. Here again diagrams and copious tables give all essential data for designing. Levers, cranks and springs, sheaves for counterweight wire-rope, and a discussion of the wire rope for this use receive equally adequate treatment. An appendix gives a full discussion of the problems involved in large sheaves loaded by ropes under tension. Particular attention is given to the elastic distortion of the hub tending to enlarge the hole bored for the trunnion; and caution given that the bearing of the trunnion should be uniform over its entire length. An interesting discussion of block brakes and band brakes (with an allusion to automatic brakes) covers everything essential to the engineer in providing the means for controlling the movement of the heavy structures to be manoeuvered. The final chapter gives a sufficient discussion of the essential details of hydraulic machinery, when adopted for operation of movable bridges or used as auxiliary in course of erection and adjustment.

Finally the author, in thirty-four pages, gives complete specifications for the design of movable bridges in every detail. This is the fruit of many years of experience in designing, supervising erection, and testing the operation of this class of structures. Perhaps the most noteworthy examples are the six great emergency dams on the locks of the Panama Canal. These are provided to swing around over the lock chambers, and, from the long arm, allow the lowering of wickets and gates to form a dam adequate to hold back the lake-water in case the chamber must be emptied. As Engineer for the Contractor, the author was responsible for all the details of construction and erection and testing of these structures, which was accomplished without a mishap.

It is safe to state that these two volumes are unique in the entire range of literature on bridges, and constitute a monument to its author.

Director Emeritus, Thayer School of Civil Engineering, Dartmouth College.