7709.56b,4 Page 1 of 5 FSH 7709.56b - TRANSPORTATION STRUCTURES HANDBOOK WO AMENDMENT 7709.56b-94-1 EFFECTIVE 7/27/94 CHAPTER 4 - PRELIMINARY DESIGN 4.02 - Objectives. To develop preliminary structure designs for bridges, trail bridges, and major and minor culverts, that include the following: 1. Development and evaluation of road bridge and major culvert alternatives that meet resource and ecosystem management objectives (FSM 7712.3). 2. Development and evaluation of trail bridge alternatives that meet the intended use of the trail and meet resource and ecosystem management objectives. 4.03 - Policy. (Sec. 03). Develop preliminary designs for each bridge and major culvert, and for trail bridges when the size and scope of the project are appropriate. 4.04 - Responsibility. (FSM 7722.04). 4.05 - Definition. Economic Analysis. A systematic method for assessing and relating social, economic, and engineering data and information. See the Least Total Expected Cost (LTEC) method (EM-7700-9, sec. 4.08) for an appropriate analysis procedure. 4.08 - References. American Association of State Highway and Transportation Officials (AASHTO). Manual for Maintenance Inspection of Bridges (AASHTO Manual). Current Edition. AASHTO, 444 North Capitol Street, N.W., Suite 249, Washington, DC 20001. U.S. Department of Agriculture, Forest Service (USDA-FS). The Least Total Expected Cost--An Economic Analysis Method for Selection of Culverts and Other Transportation Structures, EM- 7700-9. September 1994 (est.). USDA-FS, Engineering Staff, P.O. Box 96090, Washington, DC 20090-6090. U.S. Department of Transportation. Federal Highway Administration (FHWA). Hydraulic Design of Bridges With Risk Analysis, FHWA-TS-80-226. March 1980. Available from the National Technical Information Service, 5285 Port Royal Road, Springfield, VA 22161. U.S. Department of Transportation. Federal Highway Administration (FHWA). Hydrology, Hydraulic Engineering Circular No. 19 (HEC-19), FHWA-IP-84-15. October 1984. Available from the National Technical Information Service, 5285 Port Royal Road, Springfield, VA 22161. U.S. Department of Transportation. Federal Highway Administration (FHWA). The Design of Encroachments on Flood Plains Using Risk Analysis, Hydraulic Engineering Circular No. 17 (HEC-17), FHWA-EPD-86-112. 1981. Available from the National Technical Information Service, 5285 Port Royal Road, Springfield, VA 22161. 4.1 - PRELIMINARY DESIGN PROCESS. Complete the following preliminary design steps. 1. Select structure types for analysis and comparison. 2. Analyze the alternatives. a. Develop information needed for conducting the analysis. b. Conduct an economic analysis (sec. 4.05). 3. Select an alternative for the final design. 4.2 - SELECTING ALTERNATIVE STRUCTURE TYPES. Select tentative alternative structure types for analysis. 1. Each alternative should meet criteria and limitations specified in the following: a. The Project Engineering Report (sec. 1.4). b. The road, resource, and ecosystem management objectives (sec. 1.11). c. Established road design standards (sec. 1.41). d. The foundation investigation (sec. 3.1). 2. Also consider the following factors: a. Stream channel alignment, cross-section, stability, need for protective measures, and channel modifications. b. The availability of structural components. c. Resource protection needs, including fisheries. d. Lowering approach embankments, where possible, so that they are overtopped before the structure itself is overtopped or breached. This may save on construction costs and will reduce the possibility of damage to the structure. Allowing for more damage to the approaches may permit the structure to be made smaller and will normally result in less total cost to repair. 4.3 - DESIGN CONSIDERATIONS. 4.31 - Appearance. Design structures to be compatible with the natural surroundings and to meet visual quality objectives in form, color, and texture. Design railings to minimize obstructing the view from the structure. Locate surface drainage features to avoid causing discoloration of structure components. Use materials compatible with the surrounding environment. Miter large culverts consistent with structural adequacy and safety, and bevel inlets to increase flow efficiency. Utilize the input of architects and landscape architects in planning and design activity when appropriate. 4.32 - Waterways. Maintain natural stream conditions at waterway crossings. Ensure that desirable streambed conditions and fish habitat are protected when selecting structure type. When fish use the streams for habitat, spawning, or migration, consider using arches or bridges rather than culverts. 4.33 - Long-Term Versus Short-Term Structures. Match structure design life (sec. 05) with structure service life (sec. 05). Use long-term structures (sec. 05) on long-term road facilities. Use short-term structures (sec. 05) when appropriate for short-term facilities. Avoid the following: 1. Replacing a component of the structure with materials that would have a design life less than, or significantly longer than, the remaining service life of the structure of which it will become a part. 2. Repeated disturbance of the streambed and streambanks when the structure is replaced. 3. Inadequate hydraulic capability for the structure design life. Do not use short-term structures for long-term facilities unless an economic analysis considering future replacement costs including the costs of environmental consequences, management requirements, and maintenance/repair needs, demonstrates a clear value of doing so. Use long-term structures for short-term facilities only if designed for relocation and reuse at other sites. 4.34 - Modular Portable Bridges. Design modular, portable, or reusable bridge superstructures to meet the same criteria used for short-term and long-term structures. Consider site safety requirements, abutment construction and hydraulic design needs, and total life-cycle costs before using a modular portable bridge. Do not use modular bridges for long-term facilities except where permitted by Regional guidance (sec. 05). 4.35 - Use of Previously Used Materials. 4.35a - General Requirements. Do not incorporate structural elements made from previously used materials, including railroad car units, into structures unless a licensed professional engineer certifies in writing that the capacity and remaining life of the material is adequate for the intended use. Consider previously used materials only when a thorough inspection and structural analysis indicates an economic advantage for their use and a level of risk is acceptable based upon the following: 1. The type, grade, and properties of the material. 2. The present condition of each structural element. 3. The allowable design stresses in effect when the structural element(s) was originally manufactured and any code restrictions in effect at the time. 4. The past use of the structural element, including information on the type, magnitude, duration, and number of cycles of loading to which the structural element has been subjected. This information may not be known for previously used material. 5. The recommended allowable working stresses based upon information and conditions assessed in previous paragraphs 1 through 4. When previously used materials are used for structural elements, perform design, load capacity analysis, and load rating in accordance with the requirements of section 8.32 and the "AASHTO Manual" (sec. 4.08). Do not use previously used materials that are damaged or that have reached the end of their service life (sec. 05). Obtain approval from the Regional staff director for engineering activities prior to incorporating previously used materials in any road or trail bridge (FSM 7720.04a). 4.35b - Railroad Car Units. Do not use railroad car body units for bridges on forest development roads or as trail bridges for long-term installations without the approval of the Regional staff director for engineering activities (FSM 7720.04a). Consider their use as temporary structures only after all provisions of section 4.35a are met. 4.4 - ECONOMIC ANALYSIS OF ALTERNATIVES. Make an economic analysis (sec. 4.05) of alternatives as part of the preliminary design activity. Economic analysis may be performed in accordance with professionally acceptable procedures such as analyses that consider costs incurred during the entire life of the structure (life-cycle method), or the least-total-expected- cost method (LTEC) (sec. 4.08). 4.5 - SELECTING THE ALTERNATIVE FOR FINAL DESIGN. In addition to economic analyses as a basis in selecting an alternative for final design. Also consider: 1. The method by which the structure can be fitted to the site (sec. 2.3). 2. Resource protection and other management concerns, including fish passage. 3. Public safety. 4. Ease and speed of construction. 5. Esthetics. 6. Available construction skills and equipment. 7. Federal Flood Insurance Program requirements (E.O. 11988, FSM 7701.4). If the structure is in an area covered by the Federal Flood Insurance Program, make sure the backwater for the 100-year flood does not exceed the elevation specified in the appropriate insurance program. 4.51 - Documentation. Document the results of the preliminary design in a report. Include sketches or drawings, as appropriate. Recommend the preferred alternative. Place a copy of the report, sketches, and drawings in the project file.