Tom Martin Wet’suwet’en bridges
t here are several photographs in the National Archives taken of river crossings in the Cassiar district of BC, near Hazelton. I thought it might be of interest to look at the structures of these bridges from a modern design point of view. Working from the photographs, this study implies no first hand knowledge of the actual methods used or of the choice of tree species selected. Three of the bridges are remarkably similar suggesting that they were quite common constructions. The first, described as ‘Indian Suspension Bridge over the Wotsonqua River’, photographed in winter by Charles Horetzky in 1872, is particularly dramatic having been built fairly high over the gorge (see page 50) . The second,‘Indian Suspension Bridge over the Alaw-Kish’, has been recorded in a set of three photographs from 1899 which provide clear details of the connections made between the different members used. The third is ‘Indian Suspension Bridge across Sestoot River’, 1899. These two bridges are so similar that, at first, they could easily be mistaken as the same one. STRUCTURES These are really not suspension bridges as we use the term today. They consist of cantilevered beams from each embankment, and the gaps between the ends have a joining piece hung from the tips of the two cantilevers to make up the distance required to be crossed. In order to build such a structure the two cantilevered portions need to be anchored into the embankments so that they will not topple into the river. This could be done by having the beam length on the embankment longer and heavier than the length projecting from it. This, however, might be too limiting on the span and requires fairly long timbers. It appears from the photographs that the root system of the cantilevered trees are rooted in the embankments with some additional soil or rock overburden to ensure that stability is maintained. Perhaps, at first, the eroding of embankments created partial river crossings and were extended by trees on the opposite bank being lowered by the deliberate undermining of the roots. Generally there seem to be four main cantilevered tree trunks from each embankment, two higher and two lower. One or two of these might have originally grown on the spot and the remainder subsequently hauled in from not too far away. Using the first to support the next, it would have been possible to create the finished cantilever section in phases. Once these were in place the parts of the tree trunks on the land could be topped with earth and rocks to form a durable and balanced abutment. With two cantilevers constructed, the connecting section could similarly have been built from smaller logs tied to the tips of the cantilevers. The upper logs form a handrail while sharing the total loads through ropes or sinews connecting lower to upper. By loading the lower, the upper would be forced to deflect with it, thus sharing the load. Some of the photographs indicate additional support and lateral bracing from shorter posts on one or the other river bank. These, of smaller size than the main timbers, may have been most useful during the construction stage before the counterweights had been completed.
Indian Suspension bridge over the Alaw-Kish (Kuldo River), B.C. 1899. National Archives PA-083029
ANALYSIS I have carried out a simple analysis of a model bridge to determine whether these structures would have been durable and safe enough for a reasonable life. To do this some assumptions about dimensions had to be made: Useful length of trees — 23 m Diameter at base — 54 cm Diameter at top — 18 cm Length of centre connection — 10 m Overall span — 57 m Abutment length of trunk — 7.5 m Snow load — 30 kg per metre of each of the main members. These properties were estimated from scaling one of the Sestoot bridge photographs after making some judgement about the size of the stand- ing trees.
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O n S ite review
S urface
I ssue 9 2003
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