Although widely considered to be the most accurate approach for analysis of dynamic loads such as waterhammer, slug flow or earthquakeblast load, time history dynamic analysis is rarely performed by piping stress engineers. Whereas use of dynamic load factors (DLF) is much seems to be more common. While no one knows with absolute certainty why that is, we can reasonably assume several factors:
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Similarly, in seismic zones, design results using a static seismic acceleration load approach are not usually compared with results using response spectra 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 offer 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 (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 likely helped helps push engineers toward the DLF static analysis alternative. CSiPlant can easily account for nonlinear boundary conditions in nonlinear time history cases.
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It's generally perceived that creating time history cases is time consuming with a steep learning curve, another factor which may have nudged engineers to use DLF. CSiPlant's ability to import time history data from text files with automatic consideration of scientific notation is straightforward, and can save a ton of time as compared to manually entering time history load data. CSiPlant can import time history data from most commercial piping fluid transient software programs, and most seismic ground motion records as-is with little additional effort. For cyclic dynamic loads from vibrating machinery, CSiPlant has built-in features to automatically generate sine and cosine time history functions for different machine speeds. Once time history load functions have been imported or auto-generated, a CSiPlant time history load case can be defined in only a few minutes.
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