R10 SUPPLEMENT 7709.56b-91-1
EFFECTIVE 5/6/91
2.1-2.3
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2.1 - RECONNAISSANCE. Obtain comments from all resource specialists to ensure
that all concerns listed in NEPA documents, Road Cards, and other planning
documents are properly evaluated.
Consider relative costs of road or trail construction and bridge construction. Cost
data collected over several years generally verifies that 1 to 1-1/2 miles of road can
be built for approximately the same cost as one 70- to 80-foot bridge.
2.2 - SITE SELECTION.
1. Carefully consider sites with well-defined bedrock channels, even if such
sites would require curved or skewed structures.
10. Avoid sites where backwater potential could effect non-Forest Service
landownership.
2.3 - FITTING THE STRUCTURE TO THE SITE.
2. Where curved approaches are needed, consider road design speed, sight
distance, and average daily traffic in selecting minimum curve radii.
3. To aid in preventing standing water on bridge decks, place all bridges on a
slight grade of .3 to .5 percent.
4. Where total project costs are not seriously impacted, limit bridge grades to 5
percent. Where steeper grades cannot be economically avoided, limit bridge grades
to 10 percent, and ensure that bridge grade is prominently identified as a structural
design factor. Do not use R10 standard designs for grades above 5 percent, without
modification of the substructure.
6. Locate the structure out of the stream channel waterway associated with the
average annual high flow. For the maximum design flow, consider the risks of scour
and debris passage in selecting the necessary waterway.
7. Do not constrict the waterway associated with the normal annual flow with
approach fills, abutments, or piers. Consider the effects of substructure elements
within the waterway at the maximum design flow.
10. Where skew alignment is necessary, limit skew to a 40-degree maximum,
and use 10-degree increments to the fullest extent possible.
11. Consider the possible use of intermediate supports where bridge spans will
exceed 80 feet. In most instances, savings in superstructure cost will offset any
added cost for the substructure.