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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 dynamic 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, which in this case study were wildly overconservative (10X higher support load) as compared to results from time history dynamic analysis (10X higher support load).
Similarly, in seismic zones, design results using a static seismic acceleration load approach are not usually compared with results using response spectra spectrum or time history dynamic analysis. However, unlike the use of DLF in waterhammer and slug flow applications, at least the static seismic loads are usually based on codified values which have some justification. Time history analysis would consider a more realistic distribution of seismic loads because mass is typically not evenly distributed in a piperack structure, particularly rack structures supporting heavy equipment with piping. Seismic ground motion acceleration records are becoming increasingly more available through organizations like PEER. CSiPlant can easily import these ground motion time vs acceleration records for nonlinear time history analysis.
Most piping stress models have nonlinear boundary conditions (including 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 would have to linearize all nonlinear supports for all dynamic analysis cases, which is a dubious modeling approach. This limitation in widely used legacy pipe stress programs likely helps push engineers toward the DLF static analysis alternative. CSiPlant can easily account for nonlinear boundary conditions in nonlinear time history analysis, and nonlinear time history cases . Legacy piping in CSiPlant do not typically take long at all to run. For example, a nonlinear time history dynamic analysis of a 400 node/joint model with multiple nonlinear boundary conditions typically runs in less than 2 minutes using CSiPlant's SAPFire solver.
Legacy pipe stress programs have additional limitations in the number of time history cases and/or the size of time history functions, limitations which also push engineers toward the DLF approach. CSiPlant has no limit to the number of dynamic analysis cases which can be analyzed, or the size of the time history function.
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