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:
Answer to your first question regarding repeating the weight data :
The weight data, which includes all possible weights that can vibrate, ought to be specified in the first dynamics case only. STAAD can generate the mass matrix only once per file. So, providing the weights in subsequent dynamic cases isn't of any use.
Answer to your question on comparing the base shears - static method versus the spectrum method
This is not an easy question to answer. Nevertheless, here are a couple of points you may find useful.
In section 12.9.2 of ASCE 7-05, you will find the following.
The value for each force related design parameter of interest, including story drifts, support forces, and individual member forces for each mode of response shall be computed using the properties of each mode and the response spectra defined in either Section 11.4.5 or 21.2 divided by the quantity R/I.
There are also some restrictions described in section 12.9.4 of ASCE 7-05.
Are you taking these into consideration when you make the comparison?
You could try asking the ASCE code committee members or post this question on other discussion groups like SEAINT.ORG and see what they say.