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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 approach is loads are usually based on codified values which have some justification, although . Although 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 significant.
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 dynamic analysis. That means that in order to run time history cases, users engineers have traditionally had to linearize all nonlinear supports for the dynamic analysis, a dubious approach. This limitation helps push engineers toward the DLF 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 has 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 loading 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, CSiPlant has built-in features to automatically generate sine and cosine time history functions. Once time history load functions have been imported, a CSiPlant time history load case can be defined within in only a few minutes.
In a few scenarios, DLF can be unconservative if the dynamic excitations match the resonant frequency of the piping. Another major disadvantage of DLF is that it ignores the frequency of the dynamic excitations.
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