Dear Peter

Many thanks for your helpful description...This is part of my sensitivity analysis in determining the length of the model(90m-...-180m-200m-220m-240m,...).

under the century city 90degree earthquake component. 

So, lets see what i have understood...

As the 8th mode shape contribution in x direction is 86%(more or less sufficient) and its frequency is 17(Hz)--> so based on the relation 1/(20*f max) = 1/(20*17Hz)=0.003(sec), so 0.003  is considered as a reasonable initial and maximum time step. Am I right?(it is really important for me to understand this concept as it is the basis for my future analyses)

some more queries:

1. is only cumulative effective mass fraction in x direction important? when and how should we consider the very parameter in y direction?

2.How did you derive the acceleration-frequency graph from the acceleration time-history data?

Best Regards