The St. Lawrence River Bridge Disaster.

Progress, Volume III, Issue I, 1 November 1907, Page 32

The St. Lawrence River Bridge Disaster.

[to the editor] Sir, — On rr-ading through this month's Trocr'-ss I was much interested in the article on the St. Lawrence Rivei Bridge Disaster. lam a young engineer, and have taken a course of evening lecture- at the Canterbury College School of Engineering, one of the subjects being elemental strength of materials. I very naturally read this article through carefully, especially when particulars of this great bridge were given. I was surprised that the engineer should call a design like that safe, at least, according to out standard of safe stresses. The llrst departure from our standard that caught my eye was 25 \h pei square foot allowed for wind pressure I remembei^d being taught to allow 50 1b per square foot and. also allow for the area of the train or other traffic likely to be on the bridge, as well as the area of the structure itself. On looking up my notes I found that my memory had not played me false. The next thing was thf* tension and secondary members ; maximum stresses allowed 17,000 and 20,000tb pe~ square inch. We were taught to allon no more than 13,200 th per square inch for live loads dud 20 000 for dead loads, hi your article they have allowed the dead -load stress to members that are subject to live loids. Then the top chord, which is expected to carry a load of Sooo tons, has a cross section of 711 square inches. This equals a load of 11. t tons per square inch, or 24,8001b, which is almost twice as much as we allow per square inch for bridge design. I knew of course thit there was a little difference between the English and American factors of safety, but on looking it up I found thit the difference is not worth speaking of. It would seem, by the number of bridges and other structures that are continually collapsing over there, as though there was no one in authority to see that the standard is worked to. However. lam getting away from the subject. My object in writing to you is to see if there is any chance of the expressions used being not quite correct, for it seems incredible that an engineer should allow such small margins of safety. — I am, etc., Jas. McArthur, Jpn. Christchurch, 16/10/07.

The following reply to our correspondent's communication has reached us from Mr. \V. T. Johnson, C.E , the author of the article in question :— " In reply to Mr, Me Arthur's queries re stresses in the St Lawrence river bridge, as gi\ en in Octotx r Progress, I beg to say that the maximum stress allowed for in tension members, viz : — i/,ooolb per square inch, is considerably in excess of that allowed by the Board of Trade in the United Kingdom, who limit same to 14,5601b per square inch for stxucttires in mild steel, having a bteaking strain of from 62,7201b to yT.fiSotb per square inch. Regarding the allowance made for wind pressure, vxz., 25ft) per square foot, this appears low, and would not be nearly sufficient to covei the pressures experienced vi this country. But this is a very <>pen question, as much depends upon the situation of the structure ; and dealing with very large structures like the St I^wrence bridge no doubt the engineers made tests or had reliable information upon which to base their calculations. In dealing with American weights it is ueces^aiy to bear in mind the fact that a ton with them is equal to 2ooolb."