Popular Science Monthly 



729 



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. 



