Stephanie White photographed and measured a number of lumber installations, includ- ing this bridge, for Large Things in Wood , a Field Notes small book. Tom Martin is a structural engineer in Calgary. We discussed these bridges with Colin Nicol-Smith of Gower Yeung Engineers in Vancouver, a firm long involved in logging bridges. The decks, generally, are 14 feet wide (the trucks 8’-10” wide). Two layers of timber planking lie over 10 x 12 18”o/c beams. The bottom layer is 4 x 12 random length rough sawn treated planks (Coast Douglas Fir #2 or better), with the top row a wearing surface of 3 x 10 random length untreated planks. More recent bridges do not have an earth or gravel covering as this one does. Untreated timber bridges have to be inspected regularly and occasion- ally load tested. Allowance is made for deterioration of the outer timber layer by excluding the outer 2” for size. Some bridges survive in use for twenty or even thirty years. l og stringer bridges were once very common in the logging roads in BC. This one consists of six 80’ long, 3’-6 diameter logs, most likely old growth spruce and fir, whose market value today can be as high as $40,000 per log. Steel and precast concrete bridges are now used to replace log spans when they do finally deteriorate. This bridge spans Cottonwood Creek, west of Youbou on Vancouver Island, and is one of hundreds of bridges on the maze of gravel logging roads between Lake Cowichan and the West Coast. Driving, lost, on this web of roads, one could be forgiven for thinking that BC has a lot of trees . The road is dark and wet with puddles even in the heat of September, as the forest never dries out. Then you burst into the dazzling sunshine of a clear cut — no mystery, no majesty, no shade and very hot. For this kind of bridge, the river bank is stabilised by heavy timber cribs. On these rest four stringers which support the road bed. The top of the bridge is contained by two more stringers on each side, lashed with cables to two crosswise needle beams beneath the four bottom stringers. The six logs thus share the load equally. Off-highway logging trucks have 5 axles with an overall spacing of 48 feet. The total weight of these vehicles, for which there are 4 classes, is 667 kN to 1,468 kN. (The classes designated L-75, L-100, L-150 and L-165 actually denote the Imperial tons of each vehicle. 75 tons = 667 kN. This bridge would have been designed for L-75 loading.) It is likely near the limiting span for its section. Because logging trucks may not be symmetrically loaded the bridges are designed for an unsymmetrical loading.
Log Stringer Bridge, Youbou, BC Stephanie White and Tom Martin
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I ssue 10 2003
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