Although widely considered to be the most accurate approach for analysis of dynamic loads such as waterhammer or earthquake, time history dynamic analysis is rarely performed by piping stress engineers. Whereas use of dynamic load factors (DLF) is much more common. While no one knows with absolute certainty why, we can reasonably assume several factors:
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.
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 history analysis would offer a more realistic distribution of seismic loads, an additional concern with the static seismic load approach is 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, engineers have traditionally had to linearize all nonlinear supports for the dynamic analysis, which is a dubious approach. This limitation helped push engineers toward the DLF alternative. CSiPlant can easily account for nonlinear boundary conditions in nonlinear time history cases.
Legacy piping stress programs have 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.
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 in only a few minutes.
In a few scenarios, DLF can be unconservative if the dynamic excitations match the resonant frequency of the piping. A major disadvantage of DLF is that it ignores the frequency of the dynamic excitations.
Other CSiPlant capabilities which make for even more realistic design results are consideration of second order P-delta effects, nonlinear load sequencing, and the ability to integrate CSiPlant piping models with SAP2000 structural analysis models for combined pipe-structure interaction.