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Trail Construction and Maintenance Notebook

Crossing Streams and Rivers (continued)

Bridges

Bridges range from a simple foot log with handrails to multiple span, suspended, and truss structures (Figure 54).

Figure 54—Typical log stringer trail bridge.

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 bulkheads. 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.

Imported materials are often used to extend a bridge’s life. Pressure-treated wood, metal, concrete, wood laminates, and even “space-age” composites are being used in bridges. Many of these materials must be trucked or flown to a bridge site and the old materials hauled out. All this is really expensive. Yet, these costs may be less than the more frequent and more dangerous replacement of structures made from native materials. It’s possible to mix-and-match steel or other “unnatural but hidden” components with wood facing and decking to achieve a natural appearance.

Unless your bridge is preassembled and flown right onto a prepared set of abutments, you’ll end up moving heavy materials around the bridge site. Be careful to avoid trashing the site by allowing winch guylines and dragged logs to scar trees and disturb the ground. The damage done in a moment can last for decades.

Other types of bridges include multiple span, suspended, and truss structures (Figure 56). A two-plank-wide suspended foot bridge with cable handrails is more complex than it looks. Midstream piers for multiple spans need to be professionally designed to support the design loads and withstand expected flood events. It does no one any good to win the National Primitive Skills Award for building a gigantic bridge by hand—only to have it fail a year later due to a design or construction oversight.

Figure 56—Suspension trail bridge typical of
the Northern rockies.

Bridges are expensive, so it makes sense to take good care of them. Check foot logs and bridges annually for problems. Loose decking, planking, curbs, or handrails should be repaired as soon as possible. Clean debris and organic material from all exposed wood surfaces on the bridge or supporting structures. Structural members should be checked for shifting, loose, or missing spikes or bolts. Approaches need to be well drained so water does not run onto the bridge.

Report any of the following problems to a qualified bridge inspector who can determine whether the bridge should remain open to traffic: rot; bent, broken, or disconnected steel members; large checks, splits, crushed areas, or insect damage in wood members; permanent sag or excessive deflection; erosion around abutments; broken concrete; concrete with cracks larger than 3 mm (1/8 in); or exposed rebar.

All bridge structures need to be inspected by a qualified bridge inspector at least every 4 years.

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