I am using UBC1985 for generating the seismic base shear. But in the result of structure period, I have seen Ta and Tb as attached picture. What is different?


When other related parameters (like input Zone, K, I and Ts) are defined in Seismic Definition part with the Soil Characteristic (Ts though optional), Ta and Tb will be derived from the output.

There are two methods for calculating the building period Method A and Method B. STAAD calculates these from those two methods see clause 1628.2.2.
Please refer to Ubc 1985 | PDF | Building Code | Wall (scribd.com)

Ta (approximate fundamental natural time period) is period defined by method A, Tb is period defined by method B, Tuser is the user provided value for Tb and T (natural time period) is the actual value of period used in the UBC calculations.