Integration Time Step in Time History analysis

Question

How the default value of Integration time step of 0.0013888 sec is decided in Staad?. what is the basis for it. Could any one please help out?

Aswathi A · Answer

The rule of thumb is that to capture the contribution of a mode, DT must be smaller than (1/10th) the period of that mode. With high-frequency modes, the period of the modes will be so small that the default DT may become larger than one cycle of that mode&rsquo;s response. It is not possible to calculate the response for that mode accurately with that DT. 
 
 0.001388 is obtained as 1/10th the period of 72 cps. 72 cps was chosen as a number that is high enough that most structures won't have a significant mode with such a high frequency.