scia.a.spectral - Seismic spectral analysis

Response Spectra calculation

• Type of modal superposition: SRSS or CQC method can be selected
• Mode filtering based on modal mass
• Accidental eccentricity can be taken into account for buildings
• Automatic generation of Newmark seismic combinations
• Results from modal analysis are available: eigen frequencies, mass participation factors and eigenmode shapes
• The same postprocessing menus as for static load cases are available to view numerical or graphical results: displacements, accelerations, internal forces, and reactions.
• Results per storey are also available.
• Spectra of the EC8 and national codes: Austria, Czech Republic, France, Germany, India, Italy, Slovakia, Netherlands, Romania, Switzerland, and U.S. are available by default
• Additional spectrum can be defined using table input (copy/paste from Excel is also possible).

Equivalent Lateral Forces

The seismic Equivalent Lateral Forces (ELF) analysis is the most well-known method for the seismic analysis of structures. Although it is quite conservative, its simplicity makes it a very popular method for seismic design.

• The ELF method is a static analysis method.
• The calculation is based on the distribution of masses in the structure.
• The calculation of storey forces is based on the definition of storeys as well as on the reduced system, which must therefore be defined.

Calculation of the Equivalent Lateral Forces

• The user can select the method for calculation of equivalent lateral forces. The available methods follow the European and American standards - EN 1998 and ASCE 7-10.
• ELF are calculated in the background after a modal analysis.
• Calculated equivalent lateral forces are applied as one concentrated force at the mass centre of each storey.

Possibility to select how the acceleration is distributed in the building:

• Linear distribution of accelerations (EN 1998-1 clause 4.3.3.2.3 eq. (4.11) )
• Polynomial distribution of accelerations (ASCE 7-10 12.8.3)
• Distribution of accelerations from eigenshape (EN 1998-1 clause 4.3.3.2.3 eq. (4.10) )
   

Application of the storey forces to the model

• The calculated storey forces are applied to the structure using the reduced system.
• The transformation matrices of the IRS method make it possible to "smear" the concentrated storey forces in such a way that the resultant of each storey force is applied at the mass centre of the corresponding storey. It is therefore not necessary to define diaphragms to apply storey forces.
• The loads are, however, applied in a distributed way to the entire storey, hence avoiding any numerical singularity, as would be the case if point loads would be applied in a conventional way.

As results, all standard result output can be used in SCIA Engineer, without any restriction. Also, because it is a static load case, none of the issues related to the loss of sign due to the modal superposition apply here.
 

Required modules:

• scia.a.modal