FSH 7709.56b - DRAINAGE STRUCTURES HANDBOOK
R-10 SUPPLEMENT 7709.56b-91-1
EFFECTIVE 5/6/91
05 - DEFINITIONS
5. Highway Bridge. A structure, including supports, erected over a depression
or an obstruction, such as water, highway, or railway, and having a track or
passageway for carrying highway traffic, or other moving loads, and having an
opening measured along the center of the roadway of 20 feet or more between
undercropings of abutments, or springlines of arches, or a height above the stream
channel or natural ground of 5 feet or more. Structures not meeting these
dimensions which, in the opinion of the Forest Engineer or Line Officer, should be
inventoried and inspected regularly, may be included in the bridge definition.
6. Trail Bridge. A structure, including supports, erected over terrain features,
or an obstruction carrying pedestrians, pack animals, or nonhighway vehicles and
having a span measured along the center of the trail of 20 feet or more, or a height
above the stream channel, or natural ground of 5 feet or more. Trail structures not
meeting these dimensions which, in the opinion of the Forest Engineer or Line
Officer, should be inventoried and inspected regularly, may be included in the trail
bridge definition.
7. Substructure. The portion of a bridge structure that serves as the foundation
and related support elements for the superstructure. Retaining walls which
support the bridge foundation or footing are considered part of the substructure.
8. Superstructure. The portions of a bridge structure that include all elements
that span the opening and directly support the traffic loads.
9. Design Life. The length of time a facility or structure is needed to meet the
transportation objectives. Long- and short-term facilities are terms for design life.
10. Long-Term Facility. A facility which is developed and operated for longterm land management and resource utilization needs, and which may be operated
for constant or intermittent service (FSH 7709.16).
11. Short-Term Facility. A facility developed and operated for a limited period
of time which will cease to exist as a transportation facility after the purpose for
which it was constructed is completed, and the occupied land is reclaimed and
managed for natural resource purposes (FSH 7709.16).
12. Service Life. The length of time that a specific type of structure is
expected to provide a desired service (FSH 7709.16). Long- and short-term
structures are terms for service life.
13. Long-Term Structure. A structure comprised mainly of permanent
materials with an average expected service life of 20 years or longer.
14. Short-Term Structure. A structure comprised mainly of untreated timber
materials with an expected service life of 10-15 years.
15. Portable (Modular) Structure. A prefabricated bridge superstructure
designed to be readily dismantled, transported, and erected at another site.
16. Temporary Structure. A structure of either long- or short-service life that
is erected to serve a short design life.
For many years, "temporary structure" has been traditionally used for a log stringer
structure; however, many log structures were erected where long-term facilities are
needed, and many modular structures of permanent materials are now in use as
"temporary structures." The term "short-term facility" is a preferred usage.