My question is regarding carrying out a response spectrum analysis in STAAD Pro. This is what I have done so far.
I have set up the model in kN and METERS units. I have allowed for the joint weights in the form of udl loads on the beams in all three X Y and Z directions(should these be global or local??) for the two orthogonal directions X and Z. I have created a custom response spectrum using period and acceleration calculated from clause 11.4.5 of ASCE 7-05 with a damping ration = 0.05.
The values that I used for generating the response spectrum are as follows:
S1 = 0.3, Ss = 0.7 giving Fa = 1.24 and Fv = 1.8
Sds=0.58, Sd1=0.36, To=0.12sec, Sms = 0.87, Sm1=0.54, Ts = 0.62sec, TL = 12secs
My input spectrum looks like this:
SPECTRUM SRSS X 1 ACC SCALE 1 DAMP 0.05 LIN MIS*RESPONSE SPECTRUM X0 0.23; 0.05 0.37; 0.1 0.51; 0.15 0.58; 0.2 0.58; 0.25 0.58; 0.3 0.58;0.35 0.58; 0.4 0.58; 0.45 0.58; 0.5 0.58; 0.55 0.58; 0.6 0.58; 0.65 0.55;0.7 0.51; 0.75 0.48; 0.8 0.45; 0.85 0.42; 0.9 0.4; 0.95 0.38; 1 0.36;1.05 0.34; 1.1 0.33; 1.15 0.31; 1.2 0.3; 1.25 0.29; 1.3 0.28; 1.35 0.27;1.4 0.26; 1.45 0.25; 1.5 0.24; 1.55 0.23; 1.6 0.23; 1.65 0.22; 1.7 0.21;1.75 0.21; 1.8 0.2; 1.85 0.19; 1.9 0.19; 1.95 0.18; 2 0.18; 2.05 0.18;2.1 0.17; 2.15 0.17; 2.2 0.16; 2.25 0.16; 2.3 0.16; 2.35 0.15; 2.4 0.15;2.45 0.15;
My question are:
Generally speaking, your approach is correct. We verified some (but not all) of the numbers you have mentioned and they look right.
Here are the answers to your 3 questions.
1. The value for SCALE should be 9.806 or the appropriate value of acceleration due to gravity in the current unit system. In Figure 11.4-1 on page 115 of ASCE 7-05, along the vertical axis, the spectral response accelerations are annotated with (g) in parentheses. This indicates that the ordinates as shown are normalized with respect to g.
2. If the members are doubly symmetric, and you have the same weights along global X and Z, the eigensolution will yield double root modes - modes which are at 90 degrees to each other. In some programs including STAAD, you may instead get single modes which could be at 45 degrees to the global X and Z directions. This is normal behavior. The participation factor in the 2 directions will also be identical.
Since you have double root modes which are hence closely spaced modes, you must use the CQC method of combination, not SRSS.
3. We presume you are asking about the numbers which appear in the table called MODAL BASE ACTION in the STAAD output file. For any given mode and direction, the force terms are equal to
A * B * C * D
where
A = spectral acceleration for that mode. You can get this from the STAAD output file.
B = participation factor for that mode. You can get this from the STAAD output file.
C = Total vibrating mass along that direction. You can get this value from the file with the name inputfile_MASS.TXT. It will be located in the same folder as the STAAD input file.
D = Acceleration due to gravity in the appropriate units. Since C is reported in units of Pound-mass(inch) = LBf/386.08858, you may want to use 386.08858 for D. In other words, C and D must be in consistent units.
For the moment terms, the level arm between the location of the individual masses and the (0,0,0) point must be calculated and multiplied by the respective mass and summed.
Kris,
Thanks for the response. I have incorporated the CQC method as suggested. I have also omitted the repeated force(used by staad to calculate the masses) from the Z direction Response Spectrum load case as I had already included them in the 1st dynamic case X-direction. Is this correct or should I be repeating them in the Z diretion load case?
I have checked the base shears for the static case and compared them with the dynamic(response spectrum) case. The dynamic base shears are coming out more than twice as high as the static base shear. Normally we expect that the dynamic base shears come in lower than the static . Is there another scale factor that I am missing? When I check the building weight used in the static check and the weight from the dynamic(sum of support reactions for the dead loads), they are coming out pretty close, so not sure what I am doing wrong. My static base shear using equiv. lateral force procedure is 2500kN where the dynamic base shears are coming out around 6500kN(see below). Any help would be great.
MASS PARTICIPATION FACTORS IN PERCENT BASE SHEAR IN KN
-------------------------------------- ------------------
MODE X Y Z SUMM-X SUMM-Y SUMM-Z X Y Z
1 88.11 0.00 0.00 88.107 0.003 0.001 6381.42 0.00 0.00
2 0.00 0.47 0.00 88.107 0.476 0.002 0.07 0.00 0.00
3 0.00 0.32 0.00 88.107 0.799 0.002 0.03 0.00 0.00
4 8.10 0.01 0.03 96.210 0.810 0.034 924.73 0.00 0.00
5 0.26 0.03 12.78 96.468 0.838 12.818 29.42 0.00 0.00
6 1.13 0.00 3.15 97.596 0.838 15.966 128.75 0.00 0.00
ZPA 2.40 0.00 0.00 100.000 0.000 0.000 151.51 0.00 0.00
---------------------------
TOTAL SRSS SHEAR 6451.20 0.00 0.00
TOTAL 10PCT SHEAR 6451.20 0.00 0.00
TOTAL ABS SHEAR 7615.92 0.00 0.00
TOTAL CQC SHEAR 6453.24 0.00 0.00