Calculation of structural period for using in UBC 97 seismic force

[dollarbulldog]

Dear,

Does anybody know the way ETABS use to computed the period of structure for UBC 97 seismic force? In auto-generated quake load windows, ETABS provides both method A and program calculated options with edit text box nearby. What does ETABS perform in program calculated option?

Best

 

[WillisV]

Eigen-value based modal analysis.

 

[Murali27]

In static method approach in ETABS as per UBC 1997, it also restricts the actual time period based on modal analysis to 1.4 x Method A time period or 1.3 x Method A time period.