My question is in two parts.
First of all, how does Staad determine where to start the RSA analysis, what determines what the first frequency it looks at is?
The second part, is I am trying to work out how Staad is working for a more complex model by working on a simple model. The simple model is a 60x30 building with 8" concrete walls and no windows or doors. The walls are 30 ft tall and there are plate diaphragms at 9ft and 27 ft made up of 6" concrete. Steel 12X50 beams support the floor at 12 ft O.C. The first 75 or so participation factors are as follows:
PARTICIPATION FACTORS
MASS PARTICIPATION FACTORS IN PERCENT BASE SHEAR IN KIP
-------------------------------------- ------------------
MODE X Y Z SUMM-X SUMM-Y SUMM-Z X Y Z
1 0.00 3.68 0.00 0.000 3.683 0.000 0.00 0.00 0.00
2 0.00 16.38 0.00 0.000 20.062 0.000 0.00 0.00 0.00
3 0.00 0.00 0.03 0.000 20.062 0.028 0.00 0.00 0.00
4 0.00 0.00 0.00 0.000 20.062 0.028 0.00 0.00 0.00
5 0.00 0.67 0.00 0.000 20.727 0.028 0.00 0.00 0.00
6 0.00 2.43 0.00 0.000 23.155 0.028 0.00 0.00 0.00
7 45.73 0.00 0.00 45.727 23.155 0.028 266.13 0.00 0.00
8 0.00 0.00 3.76 45.727 23.155 3.790 0.00 0.00 0.00
9 0.00 0.01 0.00 45.727 23.163 3.790 0.00 0.00 0.00
10 0.00 0.00 0.20 45.727 23.163 3.987 0.00 0.00 0.00
11 0.00 0.00 0.00 45.727 23.163 3.987 0.00 0.00 0.00
12 0.00 0.01 0.00 45.727 23.176 3.987 0.00 0.00 0.00
13 0.00 0.00 5.55 45.727 23.176 9.533 0.00 0.00 0.00
14 23.90 0.00 0.00 69.623 23.176 9.533 139.07 0.00 0.00
15 0.00 0.00 1.76 69.623 23.176 11.293 0.00 0.00 0.00
16 0.00 1.50 0.00 69.623 24.681 11.293 0.00 0.00 0.00
17 0.73 0.00 0.00 70.353 24.681 11.293 4.25 0.00 0.00
18 0.00 0.02 0.00 70.353 24.705 11.293 0.00 0.00 0.00
19 1.15 0.00 0.00 71.500 24.705 11.293 6.67 0.00 0.00
20 0.00 0.00 0.00 71.500 24.709 11.293 0.00 0.00 0.00
21 0.00 0.00 0.00 71.500 24.709 11.293 0.00 0.00 0.00
22 0.00 0.00 0.00 71.500 24.709 11.293 0.00 0.00 0.00
23 0.01 0.00 0.00 71.512 24.709 11.293 0.07 0.00 0.00
24 0.06 0.00 0.00 71.568 24.709 11.293 0.33 0.00 0.00
25 0.00 0.00 0.00 71.568 24.709 11.293 0.00 0.00 0.00
26 0.02 0.00 0.00 71.585 24.709 11.293 0.10 0.00 0.00
27 0.00 0.00 0.00 71.585 24.709 11.293 0.00 0.00 0.00
28 0.00 0.00 3.79 71.585 24.709 15.084 0.00 0.00 0.00
29 0.00 0.00 0.23 71.585 24.709 15.317 0.00 0.00 0.00
30 0.36 0.00 0.00 71.950 24.709 15.317 2.12 0.00 0.00
31 0.00 0.00 8.42 71.950 24.709 23.740 0.00 0.00 0.00
32 0.00 0.00 0.00 71.950 24.709 23.740 0.00 0.00 0.00
33 9.06 0.00 0.00 81.011 24.709 23.740 52.73 0.00 0.00
34 9.48 0.00 0.00 90.494 24.709 23.740 55.19 0.00 0.00
35 0.00 0.00 55.43 90.494 24.709 79.167 0.00 0.00 0.00
36 0.01 0.00 0.00 90.501 24.709 79.167 0.04 0.00 0.00
37 0.00 0.00 0.00 90.501 24.709 79.167 0.00 0.00 0.00
38 0.00 0.00 0.00 90.501 24.710 79.167 0.00 0.00 0.00
39 0.00 0.49 0.00 90.501 25.204 79.167 0.00 0.00 0.00
40 0.05 0.00 0.00 90.555 25.204 79.167 0.32 0.00 0.00
41 0.00 0.00 0.00 90.555 25.204 79.167 0.00 0.00 0.00
42 0.00 0.04 0.00 90.555 25.245 79.167 0.00 0.00 0.00
43 0.00 1.34 0.00 90.555 26.582 79.167 0.00 0.00 0.00
44 0.00 0.00 0.13 90.555 26.582 79.301 0.00 0.00 0.00
45 0.00 0.00 0.04 90.555 26.582 79.342 0.00 0.00 0.00
46 0.00 0.00 0.00 90.555 26.582 79.342 0.00 0.00 0.00
47 0.00 0.27 0.00 90.555 26.849 79.342 0.00 0.00 0.00
STAAD SPACE -- PAGE NO. 37
MASS PARTICIPATION FACTORS IN PERCENT BASE SHEAR IN KIP
-------------------------------------- ------------------
MODE X Y Z SUMM-X SUMM-Y SUMM-Z X Y Z
48 0.00 0.00 0.00 90.555 26.852 79.342 0.00 0.00 0.00
49 0.00 0.00 0.00 90.555 26.852 79.342 0.00 0.00 0.00
50 0.00 0.00 0.09 90.555 26.852 79.428 0.00 0.00 0.00
51 0.00 0.00 0.02 90.555 26.852 79.444 0.00 0.00 0.00
52 0.00 0.00 0.00 90.555 26.853 79.444 0.00 0.00 0.00
53 0.33 0.00 0.00 90.889 26.853 79.444 1.94 0.00 0.00
54 0.00 0.16 0.00 90.889 27.016 79.444 0.00 0.00 0.00
55 0.00 0.10 0.00 90.889 27.121 79.444 0.00 0.00 0.00
56 0.00 0.00 0.12 90.889 27.121 79.560 0.00 0.00 0.00
57 0.00 0.27 0.00 90.889 27.394 79.560 0.00 0.00 0.00
58 0.00 0.00 0.00 90.889 27.394 79.560 0.00 0.00 0.00
59 0.00 0.00 0.00 90.889 27.394 79.560 0.00 0.00 0.00
60 0.00 0.00 0.00 90.889 27.394 79.563 0.00 0.00 0.00
61 0.00 0.00 0.06 90.889 27.394 79.624 0.00 0.00 0.00
62 0.00 0.00 0.00 90.894 27.394 79.624 0.03 0.00 0.00
63 0.00 0.00 0.00 90.894 27.399 79.624 0.00 0.00 0.00
64 0.24 0.00 0.00 91.137 27.399 79.624 1.42 0.00 0.00
65 0.00 4.04 0.00 91.137 31.443 79.624 0.00 0.00 0.00
66 0.00 0.00 0.00 91.137 31.443 79.626 0.00 0.00 0.00
67 0.00 0.00 0.08 91.137 31.443 79.711 0.00 0.00 0.00
68 0.00 0.01 0.00 91.137 31.450 79.711 0.00 0.00 0.00
69 0.00 0.00 0.00 91.137 31.450 79.711 0.00 0.00 0.00
70 0.00 0.00 0.19 91.137 31.450 79.899 0.00 0.00 0.00
71 0.07 0.00 0.00 91.206 31.450 79.899 0.40 0.00 0.00
72 0.00 0.00 4.15 91.206 31.450 84.044 0.00 0.00 0.00
73 0.00 0.00 0.00 91.206 31.450 84.044 0.00 0.00 0.00
I expected that this building would present more regular behavior and would have most of the participation in the first 20 or so Modes but that is not the case for the Z direction. Is this building irregular in some way or is it a result of how the program determines the frequency for analysis? All the plates in the model are 3ft square. I have attached the Staad Input file if that may help.