Seismicity

 

2013

Introduction

As a general 3D structural analysis software, SCIA Engineer is able to provide a detailed analysis of civil engineering structures. However, the particular field of seismic design of buildings requires specific tools in order to improve the efficiency of that process. The tools available in SCIA Engineer 2013 are primarily intended for effective modelling and analysis of buildings under seismic actions.

One such tool is “Storeys” which is now also used in seismic analysis. The overall seismic behaviour of a building can very often be analyzed accurately by considering each floor as a mass linked to adjacent floors by walls and columns. The synthesized storey results give a summarised view of results for each storey. Displacements, accelerations, internal forces and resultant forces may be displayed per storey, either as bulk values for each storey, or as detailed results for each wall and column.

In case of a seismic analysis, the IRS (Improved Reduced System) analysis is the recommended method for the computation of eigenmodes of the structure. This technique of condensing the model allows for the number of degrees of freedom to be reduced drastically in comparison to the large number of nodes in the finite element mesh. The computation time is reduced accordingly. Moreover, most of the local vibration modes, which are irrelevant for the overall seismic response of the structure, will not appear in the results – unlike in standard FE eigen buckling analysis.

Highlights
The seismic design functionality includes tools for an effective modelling and analysis of buildings under seismic actions according to design code principles
Powerful IRS (Improved Reduced System) method computes eigenmodes of the structure. Because of the significant reduction in the number of degrees of freedom of a large finite element mesh, the calculation time is reduced notably.
Output of summary results per storey such as location of mass centre, overall mass, displacements and accelerations per storey
Output of detailed results per storey level such as total forces per storey and (resultant) forces per wall/column
Storey results may be used for fast and comprehensive load path output
Automatic creation and update of the mass based on the corresponding loads

Detailed information

The modified IRS method

The current tendency in structural analysis is to use the full 3D finite element model of the analysed structure when performing linear buckling analysis. SCIA Engineer facilitates such analysis and structures, like buildings, scaffolding or installations, are most often modelled in 3D using beam and shell elements.

Once an explicit, detailed 3D model of the structure has already been defined and prepared for the purpose of static analysis, it is common for the same model to be used for dynamic analysis and for seismic design as well. A typical issue of using the full 3D model for obtaining the dynamic response is that seismic design focuses on the overall behaviour of the structure while analysing the full mesh of the model often results in ample information about local vibrations. More specifically in the case of modal analysis, the numerous local vibration modes are irrelevant for the overall seismic structural response. Hence, it seems practical to use a different, reduced mesh when performing dynamic analysis.

To enable this, an Improved Reduced System (IRS) method has been developed, which takes into account not only the stiffness matrix of the system, but also the mass matrix, during the reduction process. This method has proven to give excellent results in dynamic analysis while calculation time is reduced tremendously.

Picture: Option Reduced model in the solver setup to take into account the IRS method

 

Picture: From full 3D model to condensed model

 

Picture: Comparision of the full mesh vs the IRS analysis for a real building (Building: Engineering group GIBES, ACPC Building, Fribourg, Switzerland)

Storey results

As mentioned before, the overall seismic behaviour of a building can very often be analyzed accurately by considering each floor as a mass linked to neighbouring floors by walls and columns. Therefore the concept of storeys is widely used in this context. Several aspects of data input, as well as analysis model, result output and design functionality will be directly linked to the storeys of a building.

After the dynamic modal analysis by means of the condensed model, storey results appear, either as bulk values for each storey, or as detailed results for each wall and column.

Picture: Mapping of each FE of the mesh to the closest R-node

Displacements, accelerations, internal forces and resultant forces may be displayed per storey both in a numerical as in a graphical way. Also masses and mass centre per storey are at the user's disposal.

This feature may also be used for fast and comprehensive load descending output.

Picture: Summary storey results

For more about Storey results read separate article.

Automatic update of masses after changing load

In dynamic analysis, masses need to be introduced in the model in order to represent each load type, relevant for the seismic load combination. The mass approximations simulate the actual load effects on the structure, and can be added in the model manually. SCIA Engineer goes a step further and allows for these mass load-replacements to be generated based on static load cases.

To ensure that no discrepancies occur in the case of updated load values, the mass and load values are linked and continuously controlled. This means that even after the mass has been generated, it stays in function to the load defined in the static load case.

Related documents:

Presentation of Stéphane Rossier, Product Development Engineer of the seismicity project.

Tomislav Matijevic from Institut gradevinarstva Hrvatske d. d. PC Rijeka:
I think that the good thing about IRS is that the designer gets global view on simplest model of real structure. Like Einstein said: 'Analysis should be as simple as possible, but no simpler'. And, of course there are advantages which were mentioned on webinar: faster calculation, easier achievement of 90% mass criterion, and full mass matrix.Good things are also storey results.

Stéphane Rossier, Astrid Bastiaens, Jaroslav Broz

15/05/2013