Italics indicate cases where decay rather than mechanical destruction may determine the life of the tie under the climatic conditions indicated in columns 1, 2 or 3. This table has been prepared under the assumption that the mechanical life of a tie is proportional to its mechanical strength, as indicated by its specific gravity, and that the relative resistance of various woods to decay is independent of climate. Tie plates are assumed, when required, in accordance with average practice. This assumption of tie plates is intended to care for the situation where a hardwood tie fails by decay and under the same conditions a softwood tie, unplated, would fail mechanically. It is recognized that this assumption would not always hold good. |