Time-Step Selection in APDL Verification Manual VM74

  • Last Post 22 May 2020
sma4t posted this 21 May 2020

I am trying to do a similar analysis to the APDL verification manual case VM74 (link).

In the Analysis Assumptions and Modeling Notes of VM74, it is stated that the integration time step (ITS) is chosen as 1/120 of the period of oscillations. the natural frequency of the oscillations (f) is as follows: f = sqrt(k/m) = sqrt(200/0.5) = 20 hertz.

Following that, the ITS can be calculated as: ITS = 1/(120*f) = 4.16 E-4 s

Whereas the VM explicitly states that the ITS is selected as 0.0025 s (25 E-4 s)

I was wondering why the suggested ITS does not match the 1/120 rule.

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ekostson posted this 22 May 2020

Think it is a typing error - typically as a rule of thumb, dt = 1/10 * T or 1/20 * T. So it should be I think 1/20 of the smallest period of interest (or largest freq. of interest) and not 1/120 which is very small.


sma4t posted this 22 May 2020


Thank you for your response. The APDL's Structural Analysis Guide for Transient Analyses has stated here that for stepped loads, ITS values as small as 1/(180f) may be needed to follow stepped loads. The explanation is given under the "Resolve the applied load-versus-time curve(s)" section.


ekostson posted this 22 May 2020

That is OK.


In the VM74 the dt=0.0025 s stated is actually a 1/20 of the period (1/20HZ) so it is not 1/120 as written in the VM. Thanks for bringing thus to our attention.


All the best



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sma4t posted this 22 May 2020

I'm sorry, could you please elaborate more on that? I did not understand why dt = 0.0025 s is OK. The impulse loading in this problem takes place in only 0.0025 s. Why having only 1 time-step throughout the application of the impulsive load won't cause resolving issues?

The VM states that the ITS is chosen as such "to allow the step changes in acceleration to be followed reasonably...". I'm thinking that perhaps the typo is the other way around and the ITS is mistakenly calculated for 1/20f.

Thank you for your guidance. BTW, the same issue exists in VM75, as well.