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Design results using DLF are rarely compared with results from time history dynamic analysis to evaluate the economic consequences of using one approach vs the other. How are the DLF "rule of thumb" values determined, and can they be justified? Having to beef up the structure and foundation can be costly, resulting in project delays. If such changes are truly needed, that's one thing. But if costly design changes are the result of an overconservative and unrealistic analysis approach, that's a problem.
Dr. Yuqing Liu, Bechtel Global Functional Manager of Pipe Stress and Vibration Engineering, points out in a 2022 youtube video presentation that "At the end of the day, dynamic (loads) cause more damage to your plants than static (loads)." In that same video Dr. Liu shared results of a case study involving waterhammer loads where "We saved the client, we saved the project millions of dollars by using time history analysis (instead of DLFs). We also saved the project's schedule" since it would have taken months longer to redesign and fabricate piping, structure and foundation had they relied on results from commonly used DLF values.
Similarly, in seismic zones, design results using a static seismic load approach are not usually compared with results using time history dynamic analysis. However, unlike the use of DLF, at least the static seismic loads are usually based on codified values which have some justification. Although time Time history analysis would offer a more realistic distribution of seismic loads , an additional concern with the static seismic load approach are the effects of pipe-structure interaction which can be significantbecause mass is typically not evenly distributed in a piperack structure, particularly rack structures supporting heavy equipment and piping.
Most piping stress models have nonlinear boundary conditions (gaps, friction, one-way supports, etc.) for nonlinear static analysis, yet most legacy piping stress programs are incapable of nonlinear time history analysis. That means that in order to run time history cases, engineers have traditionally had to linearize all nonlinear supports for the dynamic analysis, which is a dubious approach. This limitation likely helped push engineers toward the DLF alternative. CSiPlant can easily account for nonlinear boundary conditions in nonlinear time history cases.
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