STAAD extrapolates.
Let's say the period of the first mode is 0.1 seconds, the spectrum data starts from 0.2 seconds, and the first 2 values are :
0.2 4.0 ; 0.3 5.0
Then, if the interpolation method is LINEAR, STAAD finds the acceleration for the first mode as
4.0 + (0.1-0.2)*(5.0-4.0)/(0.3-0.2) = 3.0 units
A table showing the spectral acceleration at each mode's period is reported in the output file. It looks like this
MODE ACCELERATION-G DAMPING
-------- ---------------------------- -------
1 0.75870 0.05000
2 0.50025 0.05000
3 0.43549 0.05000
4 0.43538 0.05000
5 0.43125 0.05000