I am performing Response Spectrum Analysis in Staad Pro 2007. I have some queries regarding obtaining base shear and designing the members for seismic forces.
To start with, I have defined various gravity loads in load case No 1 to 5. In load case 6, I have defined applicable masses (in terms of loads) in all three directions X, Y and Z. The spectra in X direction is defined as follows:
Combination Method: CQC
Spectrum Type: Acceleration
Spectrum Table: Plugged in site specific spectra as defined in our seismic report.
Damping: 0.05 (5%)
Scale: 9.8 (as the values of acceleration provided in table are in terms of %g)
Direction: X: 0.385. Please see below how I defined this factor.
Query No. 1:
I believe the response modification factor for the type of frame (R ) and Importance Factor (I) are not explicit inputs to Staad Spectrum Analysis; but these are to be defined in the form of factors X, Y and Z in the input stream. Please clarify.
In my case, my structure in X direction is Ordinary Steel Concentrically Braced Frames. Therefore, R = 3.25 as per ASCE 7-05, Table 12.2-1. I value for Occupancy category III in my case is 1.25 as per ASCE 7-05, Table 11.5-1.
As per ASCE 7-05, Cl. 12.9.2, the base shear should be divided by R/I, which means the base shear in X direction should be multiplied by 1 / (R/I) = 0.385 as per ASCE 7-05. I, therefore, assume that the factor in direction X for Spectrum Analysis in Staad should be X=0.385. Please confirm that this is correct. If not, please advise what factor for X should be used in Staad.
Similarly my Z direction, being Special Steel Moment Frame, R = 8, as per ASCE 7-05, Table 12.2-1. I value for Occupancy category III is 1.25 as per ASCE 7-05, Table 11.5-1. Hence base shear in Z direction should be multiplied by 1 / (R/I) = 0.156. I, therefore, assume that in Staad Spectrum Analysis the factor Z should be 0.156. Please confirm whether or not this assumption is correct; and if not, please advise the correct value to be used.
Query No. 2:
This relates to the load combination. As suggested in Staad technical manual, the response loads will be absolute load. So, we need to consider two cases, Gravity + Seismic and Gravity - Seismic. Please clarify.
In my model, load case 7 is Spectra in Z Direction.
As per ASCE 7-05, Cl. 12.5.3 (a), we need to combine two orthogonal seismic forces for seismic category D. (100% seismic X +or - 30 % seismic Z and vice versa). How to do this in response load case as I get combined seismic force effect in a single load case only?
Query No. 3:
My third question is regarding distribution of seismic base shear in terms of vertical and horizontal distribution? How does Staad take care of it? In ASCE 7-05, this is calculated based on a function of the Effective Seismic Weight at each floor and the height above base. Please refer to equation 12.8-11 & 12.8-12 in ASCE 7-05.
Query No. 4:
My last question is regarding design of various members in Staad for this response spectra load. How should the load combinations be defined, and how the design parameters should be defined?
Thanks & Regards