Crossing Streams and Rivers (continued)

Imported materials are often used to extend a bridge's life. Pressure–treated wood, metal, concrete, wood laminates, and even "spaceage" 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. Mid–stream 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 Norther 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 abut-ments; 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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