R10 SUPPLEMENT 7709.56b-91-1
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7.1 - GENERAL.
1. In-House Design. Use R10 Standard designs and drawings to the fullest
extent possible. Where site conditions or other factors require minor modification of
the standard designs, do not change the standard drawings; create a new drawing
based on applicable portions of the standard. For projects where substantial
modification of the standard designs would be required, contact the Regional
Structural Engineer for recommendations regarding major modification or
independent design.
2. Contract Design.
a. Turnkey Contracts. Use turnkey contracts (contractor design and
construct) only when standard designs are inappropriate, and design
complexities are within the capabilities of Forest Service personnel, but
time constraints preclude in-house design. Do not use turnkey contracts for
multi-span structures, other potentially complex designs, or as a means to
avoid design responsibility.
b. A/E Design. For projects that will require multi-span structures, or other
designs of a complex nature, contact the Regional Structural Engineer for
recommendations, providing sufficient lead time to allow the option of
obtaining design services under a formal A/E procurement.
3. Contract Documents. Prepare project drawings, specifications, and cost
estimates in accordance with FSH 7709.56a - Road Preconstruction Handbook.
Use R10 Standard Special Project Specifications to the fullest extent possible.
7.2 - DESIGN CONSIDERATIONS.
7.23 - Long-Term Versus Short-Term Structures. Match service life to design life as
closely as possible. Unless an appropriate economic analysis, considering future
replacement cost, indicates an economic benefit, do not use short-term structures
for long-term facilities. Long-term structures are appropriate for short-term
facilities, if designed for partial disassembly, transport, and reassembly.
7.24 - Modular Portable Bridges. Do not use modular bridges for long-term facilities
unless an appropriate analysis indicates a total life cost less than or equal to the
expected cost of permanent bridge construction.
R10 SUPPLEMENT 7709.56b-91-1
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7.25 - Used Materials. Do not use railroad flatcars, as either short-term or longterm structures, on roads regularly used for log haul or other heavy commercial
traffic. Flatcars may be considered for use on roads carrying only recreational,
administrative, or other light traffic, only when the cars can be thoroughly
inspected and, if necessary, tested by a qualified engineer prior to acquisition.
Where any uncertainty about structural condition exists, obtain coupons of material
from a location near mid-span and from the web of the main girder at the bearing
area. Perform tests necessary to evaluate the allowable stress and fatigue history of
the material.
7.3 - DESIGN SPECIFICATIONS.
7.31 - Widths. Use of 16-foot roadway width, without curb widening, is approved for
use on roads where the predominant traffic consists of the standard R10 design
loads defined below.
7.32 - Design Loads. Use the AASHTO HS20-44 loading only for structures on
roads not serving logging traffic. For structures on roads required for log haul or
other heavy commercial traffic, use the R10 Standard Loads U80, U102 and L90
described in exhibit 01. Heavier design loading (high lead vehicles and mobile spar
equipment) may be used only where an analysis indicates that the cost of
dismantling the equipment to legal loading exceeds the cost of construction, or
reconstruction, to a higher load standard, of all existing and proposed structures
between the site of equipment use and the closest adequate access.
Use the load that produces the most critical stress in each structural element. Use
the basic allowable stress requirements of the AASHTO specifications for U80
loading. Use the Operating Stress level of the AASHTO Bridge Maintenance
Manual for U102 and L90 loading. If Load and Resistance Factor design methods
are used, the beta factor of AASHTO 3.22.lA shall be 1.67 for U80 loads and 1.33 for
U102 and L90 loads.
For U80 and U102 loads, use the impact requirements of the AASHTO design
specifications; for L90 loads, impact requirements may be reduced 50 percent.
For trail bridges refer to section 7.7.
7.34 - Bridge Railings. Use R10 Standard Bridge Railing (SD 100 Series) whenever
possible. Use exhibit 02 to determine the length of bridge rail and approach rail
pay limits.
7.35 - Approach Railing. When bridge railing is installed, use of approach railing is
mandatory. Use a minimum approach rail length of 25 feet at each corner of the
bridge. Consider approach rail length of 50 feet for double-lane structures.
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7.7 - TRAIL BRIDGES. Use R10 standard designs and drawings for all bridges
within the span lengths indicated on the drawings. For unique bridges or bridges
with longer span lengths, contact the Regional Structural Engineer for
recommendations. Schedule the project to provide sufficient lead time to allow the
option of obtaining design services under a formal A/E procurement.
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7.32 - Exhibit 01
Alaska Region Off-Highway Design Vehicles Axle Loads (SD 080)
SEE END OF THIS CHAPTER IN THE PAPER COPY
OF THE MASTER SET
FOR SECTION 7.32 - EXHIBIT 01.
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7.34 - Exhibit 02
Bridge Rail/Approach Rail Pay Limits (SD 070)
SEE END OF THIS CHAPTER IN THE PAPER COPY
OF THE MASTER SET
FOR SECTION 7.34 - EXHIBIT 02.