Defining a seismic load case

A general procedure for the definition of a dynamic load case is given in chapter Defining a new dynamic load case. This page describes only the additional parameters that are required for a seismic load case.

Seismic action

Response spectrum	Selection of the seismic response spectrum. The selection contains all the defined spectra stored in the spectrum database (see Defining the seismic spectrum ). Button next to the combo box opens the Spectrum manager and the user may modify the existing spectrum or add a new one.
Direction	Selection of the base direction of the seismic action. Options are: X, Y, Z and General. For directions X and Y, a rotation angle about the Z axis may be defined. For direction General, a direction vector (3 components) is defined manually.
Rotation about Z axis	If the selected base direction is X or Y, this rotate the direction of the seismic action about the vertical axis.
Factor X	X-component of the direction vector of the seismic action. If the selected base direction is X, Y or Z, Factor X is computed. If the selected base direction is General, Factor X must be defined manually.
Factor Y	ditto for Y-direction
Factor Z	ditto for Z-direction
Acceleration factor	All the acceleration values in the spectrum table are multiplied by the given value of acceleration coefficient.
Overturning reference level	This field specifies the height of a point around which the structure may overturn. The height is measured from origin of the global co-ordinate system. The final turning moment is related to this point. This value is also used as a base reference level for the distribution of ELF storey forces and Accidental eccentricity moment when using a linear distribution of accelerations.

Equivalent lateral forces

This sub-section includes settings that are used only for equivalent static analysis of seismic actions.

Those settings are detailed separately in the chapter dedicated to ELF calculation of multi-storey buildings.

Accidental eccentricity

This sub-section includes settings that are used for the analysis of effects of accidental eccentricity.

Those settings are detailed separately in the chapter dedicated to the calculation of accidental eccentricity on multi-storey buildings.

Modal superposition

Type of superposition	Selection of the method for seismic modal superposition SRSS: modal results are combined using the Square Root of Sum of Squares. This method is suitable when all modes can considered as decoupled. CQC: modal results are combined using the Complete Quadratic Combination. This method is suitable in the case of closely-spaces modes. It is identical to SRSS when the modes are fully decoupled. It is more general than SRSS, but also more computationally intensive. MAX: similar to SRSS, but gives more importance to the mode with the highest participation. For more details, see the theoretical background chapter about seismic loading.
Unify eigenshapes	This feature can be used with SRSS modal superposition in the case of closely-spaced modes. It is faster, but less robust then CQC modal superposition. See the theoretical background for more details. If ON, the eigenshapes meeting the precision condition are considered multiple. It is supported only with SRSS and MAX superposition types.
Precision	The precision in the condition for multiple eigenshapes. Typical value: 10%
Damping	Damping value used for CQC modal superposition.

Mode filtering

Mode filtering allows to take modes into account selectively for seismic modal superposition.

Mode filtering can lead to an important speed up of computation time for modal superposition of seismic load cases, without significant loss of accuracy.

Mode filtering

Select the method for mode filtering:

Disabled: no mode filtering is applied. All computed modes are used for modal superposition

Total mass: only modes with the highest modal mass ratio are taken into account for modal superposition. Modes are sorted in decreasing order of their modal mass ratio and superposed until the specified cumulated mass ratio is reached.

Mass threshold: only modes with a modal mass ratio higher than the specified value are taken into account for modal superposition.

Required total mass

Required value of the cumulated modal mass ratio for Total mass filtering method. Typical value: 90%

Required minimal modal mass

Required modal mass ratio for Mass threshold filtering method. Modes with a modal mass ratio greater or equal to specified value are taken into account. Typical value: 5%

Residual mode

The residual mode method is intended for bigger models, where it is difficult to compute the minimal required modes. Most seismic design codes require, that 90% of the modal mass ratio is taken into account in the computed modes.

That compensating technique may be used in case the modal mass ratio cannot be met. However, it is based on the assumption, that the missing modal mass is due to so-called rigid modes, which typical have a frequency that is too high to be set in motion by seismic action. If flexible modes are missing and therefore compensated by that technique, the results can become very inaccurate.

Modes that are within the typical frequency range of seismic action (i.e. 0-33 Hz) should be taken into account. Some codes specify, that at least 70% of the modal mass ratio must be covered by computed modes. The rest may be covered, for instance, by a residual mode analysis.

DO NOT compute e.g. only two modes and take the rest in missing mass or residual mode. It is not the purpose of such a method and would most probably lead to wrong results.

Use residual mode

If OFF, only the modal mass included in the computed eigenmodes is taken into account.

If ON, a residual pseudo-mode is computed, applying the ground acceleration to the missing mass as a static loading. The obtained result is included in the modal superposition as an additional (pseudo-)mode.

Remarks about Residual mode method

The direction of the static equivalent loads is the same as the direction selected by user in the spectrum direction.
If the seismic load is defined in 2 or 3 global directions, then the static equivalent mass in the residual mode is computed in the global 2 or 3 defined directions with respect to the directions defined in the input.
The value of acceleration for the cut off frequency is taken as the last value of the selected response spectrum (i.e. highest frequency). The program doesn’t check the level of the cut off frequency. It is of the responsibility of the user to define the spectrum accordingly.
In case of CQC superposition, it is assumed that there is no correlation between the residual mode and the other modes (i.e. SRSS is applied for the residual mode).

For more detailed information about Residual mode, see the theoretical background chapter on seismic loading.

Signed results

Predominant mode

If ON, the eigenshape selected in the combo box below is used for the definition of signs of result values.

This affects the results on 1D and 2D members.

When the load case is used in a combination, then it is combined once with coefficient 1 and once with coefficient -1.

Mode shape

If the option above is ON, the user may specify the predominant mode = the mode shape which determines the sign of the results.

It is possible to select option "Automatic" or a number from 1 to the total number of selected eigenshapes.

"Automatic" uses the mode shape with the biggest overall mass ratio, calculated as follows:

That approach, however, does not take into account the actual direction of the seismic action and can lead to inconsistent results signature on 3D models. It is therefore recommended to select manually the predominant mode for signed results.

Note : Prior to the definition of the first load case, at least one mass group combination must have been already defined. In addition, Dynamics must have been selected in the Functionality list of the Project setup dialogue.