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Page:Popular Science Monthly Volume 89.djvu/746

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��Popular Science Monthly

��If the reader will pause a uioinent to look at the acxoiiipan\in^ photograph of the bridge, he will note a series of perfectly good cajjital K's made up of vertical steel posts and diagonal rods. This picture shows also the traveler, which is nothing more or less than a great steel tower carrying gigantic movable cranes on top to handle the heavy pieces of steel, some weigh- ing one hundred tons. By examin- ing the diagram of the bridge, it will be seen that both north and south sections of tlu- bridge, symmetric- al about the main piers, are made u|) of a series of K's. The "K" system has a number of

���Cast-steel rocKer which probably foiled Pirst.

��f- Vtfoter

��How did the disaster occur? Probably the steel rocker casting under the south upstream comer suddenly crumpled. The truss then dropped on the carrying girder, kicking it out or turning it enough to let the corner of the truss slip off

��advantages for a bridge of great size. Chief of these is that during erection of the cantilever arms, each panel or "K" can be completed without temporary supports and the traveler moved out to the end. It will be remembered that conditions at this bridge site made temporary supports out of the question.

The top chords of the arms are in tension; that is, the forces acting upon them tend to stretch them. So, they are made up of great e>-ebars, thirty-two of them on either side of the main posts having a cross-section of eight feet of solid steel. The bottom chords are in compression; that is, the forces acting (»n them tend to shorten them.

The great post over the main ])ier is three hundred and ten feet high and weighs fifteen hundn-d tons. It is compcj.sed of four posts latticed together into a rectangular lower nine feet by ten feet. The shcje under the main post has a bearing on the stone pier of twenty- two feet by tAventy-six feet and is nineteen feet high, and weighs four hundred tons. Like some other parts of the bridge, it was shipped in pieces, each weighing one hun(ir<-(l ions. An- other measure of the ni.igiiiludc- of this modern wonder of the worlcl is the jiin

��connecting the slK)e and tower. It is two and a half feet wide and weighs six tons. Two sets of massive steel temporary \iaduct were built under each anchor arm, one set to carry the floor system, which in turn carried the traveler; the other to support the lower chords. During the winter of two years ago, the traveler, weigh- ing a thousand tons, was built on the north shore. In the spring it was moved to the main pier and the shoes placed. The tra\el- er then nu)\ed back front the main pier, placing the lower ciiorcls t)n the t e m p o r a r y \- i a - ducts. It was luovetl out to the main pieragain and on the way back the trusses erectcxl up to the point where the vertical and diagonal legs of the "K" intersect. Arriving at the anchor pier, it began to erect above the iiitersection of the legs t)f the "K."

Thus the anchor arms were erected. But when the traveler reached the main pier, it naturall>- had to erect the cantilever arm in front of it, panel by panel. A "flving liritlge," projecting forward from the finishe<l work, carrietl the periuaiu'iit work ol the panel until it was rixeti'd u]). The iKing bridge was coniposcxl of pieces of steel with one end fastencKl to the coiui)leted work, the other projecting out into space and held up by susjjension rods.

The suspended span was assemiiled in a sliallnw cove some three miles below the bridge' at the same time the .south cantile\er arm was being erected. Six l)argi-s thirty-two feet wide by one huniheil and lift\- feet long were placed untler as many panel points. When the tide came in the span was afloat and w.is towed by tugs to the bridge, where it was aiu'horeil to the h.mging trusses and coupled to the hanger slabs. The plans wi-re to raise it to its liii.il ])()sition in a few hours by eight oni- tiinus.ind-ton lu<lraulic jacks, two .it e.ich <i)rner.

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