Improving the accuracy of your model - Why CSiPlant time history analysis?

Although widely considered to be the most accurate approach for analysis of dynamic loads such as waterhammer, slug flow or blast load, time history dynamic analysis is rarely performed by piping stress engineers. Whereas use of dynamic load factors (DLF) seems to be more common. While no one knows with absolute certainty why that is, we can reasonably assume several factors:

Design results using DLF are rarely compared with results from time history dynamic analysis to evaluate the economic and safety 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.

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.

Similarly, in seismic zones, design results using a static seismic acceleration load approach are not usually compared with results using response 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 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. 

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.

In some 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 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.