Although widely considered to be the most accurate approach for analysis of dynamic loads such as waterhammer or seismic loadsearthquake, time history dynamic analysis has not traditionally been used very often is rarely performed by piping stress engineers. Whereas use of dynamic load factors (DLF) is much more common. While no one knows for sure 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 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 need to be madeneeded, 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 approach is usually based on codified values which have some justification, although time history would offer a more realistic distribution of seismic loads.
Most piping stress models have nonlinear boundary conditions (gaps, friction, one-way supports, etc.) for nonlinear static analysis, yet most legacy piping stress programs force users are incapable of nonlinear time history dynamic analysis. That means that in order to run time history cases, users have had to linearize all nonlinear pipe supports for the dynamic analysis, which is a questionable dubious approach. This limitation helps to push engineers toward the DLF alternative. CSiPlant can easily account for nonlinear boundary conditions in nonlinear time history cases.
...
It's generally perceived that creating time history cases is time consuming with a steep learning curve, another factor which has pushed 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 time history load case can be defined within only a few minutes.
In a few casesscenarios, 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.
...