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e . . e , e 5.0 ANALYSIS CONSIDERATIONS Prior to developing procedures for selecting the LTEC design, it is necessary to discuss several limiting assumptions and the roles played by such key variables as the service life of the structure and the discount rate used in the analysis. 5. 1 Limiting Assumptions For bridge design, it is assumed in the analysis procedure that the bridge itself will not fail. In other words, the foundation and other critical components of the bridge are not anticipated to fail even during rare flood events. The construction costs are assumed to include allowance for designing against failure of these components. There may be damage to the bridge deck, to embankments and to scour protection measures all of which are included in the assessment of economic risks. On the other hand, the assumption of failure should be included in the analysis for culverts and longitudinal flood plain encroachments. This will require defining failure criteria for these structures. Such criteria might be linked to degree of damage, overtopping flood magnitude, or structure size. Example B illustrates the analysis procedure and assumed failure criteria for a culvert design. Another basic assumption is that no increase in loss of life occurs. Increased accident potential and damage are accounted for in the procedure. The terminal or salvage value of the facility is also assumed to be zero. 5.2 Useful Life of Structure Generally, highway properties are retired from service due to physical wear and tear or deterioration, inadequacy in load capacity or traffic volume capacity, general obsolescence and demand to make room for other improvements. Because of the uncertainties involved, the service life of highways varies over a considerable range. This variation is found not only between components of the highway such as earthwork, bridges and paving but also within components. Generally, highway bridges and embankment have a service life in excess of 30 years. The service life of a highway pavement may be shorter. The service life for culverts can be estimated by application of procedures such as contained in reference 18. In the LTEC design process, the construction or capital cost component is amortized over the service life of the structure. This is accomplished by multiplying the capital cost by the capital recovery factor, CRF. Since the CRF approaches the discount rate as the service life increases, examining these factors in a set of compound interest rate tables indicates that when the discount rate is 7 percent or greater there is not a significant change in the CRF when the service life is greater than 30 years. 15