CSiPlant offers linear and nonlinear time history dynamic analysis using modal and direct-integration methods. There is no practical limit on the number of time history cases which can be analyzed or the size of the time history functions. Time history cases can be load sequenced to continue from the end state of a nonlinear load case for more realistic analysis results, and nonlinear time history analysis cases can account for gaps, multi-linear support behavior, friction, and P-delta effects in the dynamic analysis. If gaps, friction, one-way supports and other nonlinear boundary conditions are important to consider in nonlinear static analysis, then this nonlinear behavior is also important to consider in dynamic analysis cases.

Applications where time history dynamic analysis can be beneficial are analysis of waterhammer or steamhammer fluid transient loads, analysis of piping pulsations loads from reciprocating equipment (sinusoidal time history loading), seismic ground motion accelerations, blast loads, unbalanced vibrating machinery loads (sinusoidal time history), and relief valve openings. Use of static load factors such as DLF or static G accelerations to approximate dynamic loading behavior may be a questionable approach in some designs with design results compared to results from more realistic nonlinear time history analyses to assess the economic and safety consequences of one approach vs the other.

Modal time history cases typically run faster than direct integration load cases and are not as sensitive to time step size. Although nonlinear modal time history cases can account for nonlinear boundary conditions (gaps, friction, one-way supports, etc.), nonlinear modal time history cases do not account for nonlinear P-delta effects. Modal time history cases can only be load sequenced after another nonlinear modal time history case.

Direct integration time history cases are sensitive to time-step size, which should be decreased until results are not affected. Nonlinear direct integration time history cases can be load sequenced after either a nonlinear time history case or after a nonlinear static case, and can account for nonlinear boundary conditions and nonlinear P-delta effects.

Comparison between FNA and direct-integration time-history analyses

Time history analysis may be initiated using the process which follows:

Create the model and assign support conditions.
Select Define > Functions > Time History to define or import a time-history function which characterizes load variation over time.
Assign load conditions to the model.
Define either a modal or direct-integration time-history load case through Define > Load Cases.
Enter on the Load Case Data form under Loads Applied, the following fields:
- Load Type. Select load pattern or specify an acceleration.
- Load Name. Either load pattern name or acceleration direction
- Function. Select the time history function which characterizes load variation in time.
- Scale Factor If acceleration load type apply scale factor as a percentage of G in current units. For example, if you wanted to apply a .3G acceleration load type while in Kip/ft units, you would type a scale factor of 9.66 (.3 X 32.2). If meter units, type 2.94 (.3 X 9.8) and so forth.

On the left side of the time history load case dialogue under Properties, users will find options to specify load sequencing using the 'Continue from End state of' field, an option to select a mass source, option to specify P-delta if it's a nonlinear direct integration time history case, damping, and time step data. Number of time steps X output step size should total a time frame which is at least as long as the time history function used in the time history case.

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