hi,
I running a seismic modal analysis in staad for one of our structure, where I am getting the modal mass participation factor as below
RESPONSE SPECTRUM ANALYSIS -- PAGE NO. 30
CALCULATED FREQUENCIES FOR LOAD CASE 11
MODE FREQUENCY(CYCLES/SEC) PERIOD(SEC) ACCURACY
1 0.945 1.05852 4.033E-16
2 1.920 0.52071 1.952E-16
3 2.065 0.48423 3.376E-16
4 2.169 0.46104 1.530E-16
5 2.881 0.34705 3.469E-16
6 3.114 0.32116 1.485E-16
7 3.736 0.26769 1.444E-15
8 3.848 0.25989 4.862E-15
9 3.923 0.25493 1.724E-12
10 4.115 0.24301 1.037E-13
11 4.188 0.23878 3.284E-16
12 4.219 0.23704 4.854E-16
13 4.778 0.20930 5.453E-11
14 4.870 0.20533 1.383E-09
15 5.165 0.19361 2.217E-10
MASS PARTICIPATION FACTORS IN PERCENT BASE SHEAR IN KN
-------------------------------------- ------------------
MODE X Y Z SUMM-X SUMM-Y SUMM-Z X Y Z
1 0.00 42.89 0.02 0.000 42.894 0.018 0.00 993.36 0.00
2 55.83 0.00 0.00 55.825 42.894 0.018 0.00 0.00 0.00
3 0.01 0.00 0.00 55.836 42.894 0.018 0.00 0.00 0.00
4 0.00 0.01 2.95 55.836 42.899 2.965 0.00 0.27 0.00
5 24.03 0.00 0.00 79.867 42.899 2.965 0.00 0.00 0.00
6 0.00 0.28 0.00 79.867 43.182 2.966 0.00 18.10 0.00
7 0.00 40.95 3.13 79.867 84.132 6.092 0.00 2623.18 0.00
8 0.00 11.01 11.21 79.867 95.139 17.306 0.00 705.07 0.00
9 0.04 0.00 0.00 79.911 95.139 17.306 0.00 0.00 0.00
10 0.15 0.00 0.00 80.064 95.139 17.306 0.00 0.00 0.00
11 0.00 0.02 1.00 80.064 95.160 18.311 0.00 1.34 0.00
For Seismic in Y-Direction I am Getting the First mode as High Mass participation Node with Higher time period, But 7th Mode is having slightly lesser participation Factor but the time period low.
if I use first mode time period as my fundamental mode time period for calculating static base shear ( to Limit Modal BAse Shear at 85% of Static ) it is coming low,
If I use 7th Mode time period as my Fundamental Mode Time period for Static Base Shear it is coming HIgh.
Which Time Period to be USed as Fundamental Time Period for Calculating Static Base Shear.
Thanks
Senthil kumar.B