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4.1 - PRELIMINARY DESIGN PROCESS. The Regional Structural Engineer will
provide preliminary design options for review and concurrence by the Forests when
preliminary design data is provided with sufficient lead time to allow for such
concurrent preliminary design work.
3. Prepare a general layout drawing for the proposed structure. Use a scale of
1 inch = 10 feet for structures less than 100 feet long; 1 inch = 20 feet for longer
structures. Use a contour interval of 2 feet, except where sites are very flat, and a
1-foot interval is needed to show critical elevations. Prepare the layout sheet so
that the center line of the road is parallel with the top edge of the sheet. If the
proposed structure is curved, align the topographic map to place the long chord
between structure limits parallel to the top of the sheet.
For bridge projects, show a plan view and profile parallel to the roadway. Show the
bridge profile to the same scale as the plan view, and as a direct vertical projection
of the plan view layout.
For culvert projects, show a plan view and two profiles, one parallel to the roadway
centerline and one parallel to the axis of the culvert. Show both profiles as direct
projections from the plan view.
This drawing layout will make the determination of fill catch points and final
grading requirements much easier to determine. Sample general layout drawings
are shown as exhibits 01 and 02 (SD 010 and SD 600).
4.2 - SELECTING ALTERNATIVE STRUCTURE TYPES. As a general rule, select
structure types with a service life equal to design life; long-term structures (treated
timber, concrete, or steel) for long-term facilities; and short-term structures
(untreated timber) for short-term facilities. However, portable modular structures
(long-term structures) are appropriate and may be used for short-term facilities
where the intent to relocate the structure at a later date is documented.
Unless justified by an appropriate economic analysis and approved by the Regional
Office, do not use short-term structures for long-term facilities. Unless used for a
short-term facility and intended for relocation, do not place long-term
superstructures on short-term substructures. However, where re-entry into an area
at a future date is likely, use of short-term superstructures on permanent
substructures may be appropriate to avoid repetitive disturbance of streams.
4.3 - ECONOMIC ANALYSIS OF ALTERNATIVES. Use good judgement in
preparing economic analyses. The Least Total Expected Cost, or Life Cycle Cost
methods, are excellent analysis tools when accurate construction costs, reliable
damage predictions, and damage repair costs are available. When using these
methods, choose simplifying assumptions, with care, to maintain the validity of the
analysis. (Certain assumptions contained in the examples presented in the
handbook are at best questionable.)
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4.4 - ALTERNATIVE TO INDIVIDUAL SITE LEAST TOTAL EXPECTED COST
ANALYSIS. Two basic assumptions inherent in the Least Total Expected Cost
method are: (1) funding is always available to repair damage associated with
various risks, and (2) service interruptions, or other potential damage associated
with various risks, is both acceptable and can be measured economically. For
projects where these assumptions are not valid, design criteria that seeks to prevent
project damage is acceptable. Selection of an appropriate flood-return frequency, as
the basis for bridge or major culvert design, is appropriate.
For bridge structures, a return frequency of 50 to 70 years is recommended.
For culverts, a return frequency of 25 years is recommended.
When alternatives of differing service life are considered, make economic
comparisons on the basis of equivalent risk, or equivalent probability that the
design flow will be exceeded during the service life of the structure. For equal risk,
short-term structures can be designed for lower design flows than long-term
structures. Refer to chapter 5, section 5.2 of the parent text.
4.51 - Documentation. Preliminary reports prepared by the Forest/Areas should be
submitted to the RO for review and comment prior to proceeding with final design.
Preliminary reports prepared by the RO will be submitted to the Forest/Areas for
review comment, and approval, prior to final design effort.
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4.1 - Exhibit 01
Sample General Layout (SD 010)
SEE END OF THIS CHAPTER IN THE PAPER COPY
OF THE MASTER SET
FOR SECTION 4.1 - EXHIBIT 01.
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4.1 - Exhibit 02
Structural Plate Culvert, Sample General Layout (SD 600)
SEE END OF THIS CHAPTER IN THE PAPER COPY
OF THE MASTER SET
FOR SECTION 4.1 - EXHIBIT 02.