264 W. P. Trowbridge — Design for a Bridge. 



channel being built a slight distance out from the shore line. With 

 the spans first mentioned, however, all the piers may be built without 

 exjjensive cofier-dams and with rock beds for foundations. 



For these spans, taking into consideration also the great altitude of 

 the roadway required (135 feet), it appears evident that single straight 

 girders either of the lattice or tubular form are inapplicable, both on 

 account of the excessive weight required and the difficulty of erecting 

 them. 



The system or design which I suggest is represented in elevation in 

 sketch 1. The design may perhaps be appropriately classed with 

 cantilever constructions, although difiering in essential points from 

 any existing structures of long span. It may be explained in detail 

 by reference to sketch 2, which represents a half -span. 



This sketch represents a half span of one of the channels. P repre- 

 sents the pier 150 feet long, 60 feet broad, and 136 feet high, built of 

 masonry, but not necessarily solid throughout. A B represents the 

 vertical elevation of a tubular chord or strut extending fiom A, the 

 middle of the span, across the pier, and resting upon it, to B, There 

 are three of these chords or struts, one at each side, and one in the 

 middle of the breadth of the pier. These chords being designed to 

 sustain thrusts only, will be about 4 feet square in cross section, of a 

 tubular form, made of iron plates ; and as the thrust towards the pier 

 will increase unifoimly from A, where it is 0, to the ])ier, the section of 

 the material will l)e increased towards the pier by adding plates to 

 the interior of the tubes. 



Upon these three tubes or chords, and forming part of them, will be 

 built three iron towers (T), firmly braced laterally to each other. 

 These towers will be 150 feet high. A T and B T represent iron sus- 

 pension or stay-rods, placed at distances of about 10 feet apart; each 

 rod A T having a corresponding stay-rod B T. The lower ends of 

 each pair of rods, A T and B T, are firmly attached to the tubes or 

 chords and the upper ends to short pendulums, tlie design of which 

 is to insure equality of strain in the corresponding rods A T and B T. 

 At the points where the rods BT are attached to the tubes, anchor- 

 ing-rods attached to the tube pass down into the pier through well- 

 holes, at the bottom of Avhich they are secured, by cross bars, to the 

 masonry. 



The widths of the towers at the base is such as to secure perfect 

 stability, the downward thrusts always striking near the centre of 

 the base of each tower. There are two sets of parallel rods AT, and 

 two sets B T, in pairs, for each tube or chord, making six sets of 1 1 



