The seismic loads are taken into account according to the Response Spectrum Analysis method. This method requires a previously calculated number of undamped free vibration frequencies and the corresponding mode shapes.
|Hungarian code||MSZ||EN 1998-1|
These are the steps of creating seismic loads and setting response spectrum parameters:
Calculate the first n vibration mode shapes and frequencies
Check the table of seismic equivalence coefficients in X, Y, Z directions in the Table Browser. Vibration results will appear only if you the Vibration tab is selected. In Eurocode 8 the requirement is ε ≥ 0.9 (the sum of the coefficients must represent at least 90% in each direction) and every mode shape having a coefficient larger that 5% in any direction must be included.
Create a new seismic load case
The program will create multiple load cases:
Setting seismic parameters
Clicking this button you can set the response spectrum and other parameters.
The parameters required depend on the actual design code.
Closing this dialog futher load cases will be created:
Load cases with endings 01X, 02X, ....nX, 01Y, 02Y, ....nY, 01Z, 02Z, ....nZ.
These are the seismic forces in X, Y or Z direction coming from individual mode shapes.
Load cases with endings 01tX, 02tX, ....ntX, 01tY, 02tY, ....ntY.
These are the extra torsional forces due to the seismic effects in X or Y direction.
In certain design methods Eurocode allows to increase seismic forces by an fse factor when finding critical combinations.
The purpose of this is to control failure modes and avoid dangerous collapse mechanisms.