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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.
| Direction X | |
| Direction X | If this option is ON, the user can select required spectrum for X-direction |
| Response spectrum X | Selection of the seismic response spectrum for direction X. 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. |
| Factor X | Substitute seismic forces are calculated from masses defined on the structure and from the acceleration. The values in this and two adjacent fields (for the two other axis-directions) specify the final direction in which the earthquake acts. Value 1 means full effect along the axis. 0 (zero) stands for no effect along the axis. |
| Direction Y | |
| Direction Y | ditto for Y-direction |
| Response spectrum Y | ditto for Y-direction |
| Factor Y | ditto for Y-direction |
| Direction Z | |
| Direction Z | ditto for Z-direction |
| Response spectrum Z | ditto for Z-direction |
| Factor Z | ditto for Z-direction |
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Acceleration factor |
All the acceleration values in the spectrum table are multiplied by the given value of acceleration coefficient. |
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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 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.
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.
| 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. |
| Damping spectrum |
Selection of the damping spectrum (only for CQC modal superposition). Button [...] opens the Damping database manager (which is a standards SCIA Engineer database manager). |
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).
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.
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Unify shapes |
If ON, the eigenshapes meeting the precision condition are considered multiple. |
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Precision |
The precision in the condition for multiple eigenshapes. Typical value: 10% |
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 lower than the specified value are ignored. Typical value: 5% |
The two methods (Missing mass in modes and Residual mode) are 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.
Those compensating techniques may be used in case the modal mass ratio cannot be met. However, they are 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 those techniques, 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.
| Mass in analysis |
Participating mass only: only the modal mass included in the computed eigenmodes is taken into account. Missing mass in modes: the missing modal mass is computed and distributed in the modes that have been already computed (e.g. the number of modes selected by the user). Residual mode: "Residual mode" method install the missing mass as "weight" (e.g. standard load case). The result of this load case is handled as an "extra mode". |
Remarks about Residual mode method
For more detailed information about Missing mass in modes and Residual mode, see the theoretical background chapter on seismic loading.
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.
