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Page:Popular Science Monthly Volume 89.djvu/743

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Popular Science Monthly

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��turning it enough to let the corner of the truss slip off. Had the accident happened an hour earlier, many promi- nent engineers of the I'nited Slates and Canada who were on the span witnessing the lifting would have been lost. As

lost.

��it was but a The failure of the sunmier of 1907 cannot happen to the new bridge. While the lowei- chord of the old bridge was but four and a half feet square and had seven hundred and eighty-one square inches of solid steel in its cross - sec- tion, the same chord of the

��dozen lives were

���Diagram of the new Quebec Bridge. "X" marks the point where the bottom chord of the old structure crumpled. The expansion of cantilever arm and suspended span, due to temperature changes, is taken up by brake shoes at the connections, each capable of resisting a force of one hundred and twenty-five tons. Even the difference of temperature, due to one side of the bridge being in the sun and the other in shadow, was calculated with accuracy

��new bridge is seven feet two inches deep and ten feet four inches wide and has a cross-section of nineteen hundred and two square inches of steel — two and a half times the amount in the old bridge. The familiar spring board at the swimming hole is a good example illustra- ting the principle of cantilever construction adopted for the Quebec bridge. The load is carried by the project- ing portion, which is supported by a weight at the back end suffi- cient to keep it from raising. The suspend- ed span is like the boy standing on the end of the spring board. The suspended span and cantilever arm tend to raise the anchor arm, which must be heavy- enough to pre\ent that under any circumstances. .Naturally the bridge engineers desired to keep the suspended span and cantilever arm as light as possible. Hence they made all the truss members of the suspended span and the greater part of the trusses

��Suspended Spon

���The method of raising the suspended span in position. When the tide came in, the span was afloat and was towed by tugs to the bridge, where it was an- chored to the hanging trusses and coupled to the hanger slabs. It was raised by hydraulic jacks

��of the cantilever arm of nickel steel, which contains one per cent of nickel and is one third stronger than ordinary- steel. The designer must consider first the natural and artifi( ial limitations of the location and then the traffic to be carried. At the location of the Quebec , i bridge, the channel of the St. Lawrence River is nearly two hundred feet deep. The stream is swift and subject to high tides. The trafficof ocean- going ships must not be interrupted. These con- siderations, together with the kind of foundations available, determined that the span over the channel should be eighteen hundred feet long, ninety feet longer than the famous Firth bridge in England, hereto- fore the longest span ever built. The length of span, together with economy and rapidity of erec- tion, determined the type of bridge, a canti- lever. The bridge will carry a double track railroad and two side- walks.

After the main di- mensions of the steel, "su()erstructure" it is called, had been figured out, plans for the ma- sonry' were made. The north stone masonry pier was carried to fifty feet below the bed of the river, twenty feet aboN'e bed rock, where a satisfactory founda- tion was found in the form of large and small boulders firmly wedged together. The south pier en- countered sand for the whole distance. So it was carried to bed rock, eighty-six feet below the river bed. Most of the sand was removed by blowpipes.

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