Crossing Streams and Rivers (continued)

On hiking trails, foot logs can be used to cross streams where safe fords cannot be located or to provide access during periods of high runoff. Constructed foot logs consist of a log, sills, and bulk–heads. The foot log should be level and well anchored. Notch the sill, not the log. The top surface should be hewn to provide a walking surface at least 250 mm (10 in) wide. Don't let the log or rails touch the ground. Remove all bark from logs and poles (Figure 55).

Figure 55—Log footbridge. The sill can be
notched to accommodate the logs, not vice-versa.

If the foot log is associated with a shallow stream ford, be sure to position the log upstream or well downstream of the ford. Logs immediately below the crossing can trap travelers who lose their footing in the ford. If you have handrails, construct them according to plan. Improperly constructed handrails are a big liability, because they are not strong enough.

Choosing the materials for a bridge is not a simple process. Even the use of native material for a simple foot log has consequences. For example, most untreated logs of a durable wood (like coastal Douglas–fir) have a useful life of less than 20 years. Yet a log that is big enough to support visitor traffic and winter snow loads may be 100 years old. Plus, the typical bridge has three to four stringers. Multiply this replacement–to–growth ratio by several replacement cycles and you see how it's possible to create a slow motion clearcut around the bridge site.

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