From some relatively simple calculations it is possible to estimate the deflections and stresses arising from these assumptions. a) The snow load would have been a more critical load case than the weight of people and goods being transported across the bridge. Like today’s bridges, most of the strength is needed to support the structure rather than to carry the live loads. b) Deflections under its own weight would be about 200 mm at the centre, under snow about 300 mm and under traffic 40 mm assum - ing four people on the bridge at the same time. c) The maximum bending stress in the main cantilevers at the springing point would be about 7 MPa giving it a load factor of about 2 assuming a bending strength of approximately 13 MPa. This is comparable to that we would require for a modern structure. COMMENT The fascination of these bridges lies in the wonderful proportions that result from using nature’s materials in the most simple and straightforward manner. Many of the tallest trees at that time would have been capable of spanning the rivers by themselves, but the difficulty of handling them would have been considerable. Instead the builders used materials of a scale that they could handle with, probably, quite a small crew. In return, nature required the users to cross the bridge not with automobile or even horse but using the most surefooted method. The abutment areas appear to have been chosen at locations where further scour was not likely to be a major concern. The bridges were site specific, with the counterweight stones carried but a short distance. The first tree could be load tested to determine its capacity and then be used as a guide for the rest of the structure. As long as the trees were strong enough, the counterweight of soil overburden could be placed by both trial and experience. A simple, effective and delightful solution to an important function and in the tradition of the great feats of human engineering.
Indian Sus- pension Bridge across Sestoot River, B.C.
National Archives PA-083106
Indian Sus- pension bridge over the Alaw- Kish (Kuldo River), B.C. Floor System and North abut- ment.1899
National Archives PA-083030
Indian Suspen- sion Bridge at Moricetown, B.C.
Tom Martin, M.I.C.E is a structural engineer in Calgary, Alberta, with many bridges in his portfolio.
National Archives PA-082992
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