The St. Lawrence River Bridge Disaster.

Progress, 1 October 1907, Page 449

The St. Lawrence River Bridge Disaster.

[By W. T. Johnson C E.I

Recently the world was startled by the news that the huge bridge under construction at the St. Lawrence river, Quebec, had collapsed. A heavily laden train was delivering materials upon one of the outer cantilever arms at the time, and in a few seconds sixty workmen lost their lives, and some thousands of pounds worth of material was converted into scrap-iron. In the midst of the distressing influence of the disaster it is some satisfaction to know that the trouble was immediately traceable to faulty material, and not to the design. In order that we may fully appreciate the magnitude of this Canadian undertaking it is necessary for us to compare it with the largest completed bridge in Kew Zealand, viz ,, — thai of Staircase Gully, on the Midland Railway. The accompam-ing outline drawing shows the elevations of the two bridges, and it will be seen that our own structure, large as it is, forms a very minor part of the illustration. It is also instructive to follow 7 up the finality with which the designers ot the Quebec bridge drew the calcilatiors

Intended for a highway and railroad bridge across the St. Lawrence river, the total length of the structure, from centre to centre of anchorage piers, is 2800 feet It consists of two 500-feet anchor spans, extending from the anchor piers to the main piers of the towers, and two 562-^-feet cantilever arms reaching out over the river, which carry between them an enormous central suspended span of 675 feet. This span is the longest simple pin-connected truss span in the world. The cantilever arms and the central span together form a channel span 1800 feet in length, or 90 feet longer than the cantilever span of the Forth bridge. The underside of the channel span is 150 feet above high water, and the depth of the cantilever trusses over the main piers is 350 feet. The total height from low- water level

to the highest point of the cantilever is 414 feet. The two trusses are in vertical parallel planes 67 feet apart, centre to centre. Bet« een them is supported a floor system capable of accommodating two steam railroad tr?cks, two electric cpr tracks md two highways for vehicles, all of which are placed between the two trusses whilst outside of these are two side walks.

In making calculations for the train loads provision was very wisely allowed for an increase m the future of 50% over present weight. These accumulated live loads, together with the wind loads, estimated at 2 5 lbs on eveiy square foot of surface of the structure have to be added to the dead load of the structure itself, which amounts to a total of 40 000 tons. The maximum stress allowed in the tension members is 17,0001bs to the square inch, and 20,0001bs on the secondary members. In the south anchor arm the trusses are 96 feet 9f inches deep over the anchor pier, and 315 ft. deep over the main pier, and are divided into five main panels of 100 feet span, or ten sub-panels of 50 feet. The vertical posts and top andT bottom chords are pin-connected — the pins ranging from 12 to 24 inches in diameter. The two m? in posts over the piers are hea\ ily

brr.ced transversely, and form in fact a vast transverse truss 67 feet wide, 315 feet high, and weighing 1500 tons. The bottom chord is i\ feet deep by 5J feet wide, built up of four webs having a maximum cross section of 842 square inches. The floor beams at each panel point are lOMeet deep, and weigh 30 tons each. Each top chord is made up of twenty eye bars, and the maximum stress

allowed for' reaches" 8000 tons. The total cross-scetion of the top chord to take this stress is 71 1 square inches. The Phoenix Bridge Compai.y, New York, are responsible for the design and construction of this great, but ill-fated, bridge. The structure had, A\hen it collapsed, been under construction for seven years, and w?s only half through