Response Spectrum Method for the Earthquake Check According to
SIA 261
For any given beam or shell structure, the soil acceleration
and the calculation coefficients are specified according to SIA
261 for the specified earthquake zone, foundation and structure
class.
For the check the reactions of the structure corresponding to
the resonant frequencies are calculated and statistically
superposed according to the response spectrum method.
The resulting internal forces are available for further
superposition with the static load cases and subsequent design. |
 |
To allow for the evaluation of the relevant modes of vibration,
the program outputs the applied as well as the effective modal
masses:
Sum of the applied masses (global)
Mx [t] My [t]
Mz [t]
-----------------------------------------------
Dead
weights: 0.000
0.000 0.000
Point masses:
1500.000 1500.000
0.000
-----------------------------------------------
Sum:
1500.000 1500.000
0.000
Effective modal masses
(global)
Eigenmode
Mx eff [t] My eff [t] Mz
eff [t]
------------------------------------------------
1 935.135
0.000
0.000
2
0.000
935.135
0.000
3
0.000
287.137
0.000
4 287.137
0.000
0.000
5
98.544
0.168
0.000
6
0.168
98.548 0.000
7
47.342
3.197
0.000
8
3.201
47.258
0.000
------------------------------------------------
Sum:
1371.526 1371.443
0.000
According to section 16.5.3.5 of the
standard, the sum of the effective modal masses should be at least
90% of the total weight of the structure.
Example: 3D reinforced concrete frame
 1st
eigenmode |
Corresponding internal forces My from the response spectrum
|
| 
Statistically superposed internal forces My from all
eigenmodes |
|
|
