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Keywords: columns carried by beams, vertical seismic component, vertical acceleration, cut-off columns, cut-off elements, seismic acceleration.

Summary

In building construction there are architectural design solutions, in which several columns of the building are not supported by the foundation of the building. The anchorages of these columns take place by beams, located in intermediate floors of the building. In many situations the analysis of the seismic vertical component is not carried out, because of the computational cost. In this article a modal-spectral analysis has been performed from typical building design situations in Spain, where columns supported by beams exist. The aim is to assess those situations where the vertical component of the earthquake requires a detailed study in the design of these buildings.

The program employed has been ANSYS [1]. Two-dimensional frames have been analyzed, belonging to typical residential buildings in Spain, with cast-in-place structures. The buildings are from the area of Granada, which is the place in Spain with the highest design seismic acceleration.

The aim of this study is to investigate the influence of the length of the beams supporting cut-off columns. Beams of 5m and 8m length have been analyzed. To observe the influence of the number of floors resting on the same beam, two cases have been studied: beams supporting three floors of the building, and beams supporting six floors of the building.

The main conclusions of this study are:

In the two-dimensional analysis performed, several modes of vibration must be extracted to obtain the first vertical mode of the beams supporting the cut-off columns. In a three-dimensional analysis, many horizontal modes in the two main directions of the building and several torsional modes appear before the vertical modes. Therefore when the size of the building is greater (more numbers of floors and more numbers of spans) the vertical modes will appear much later in the modal analysis.
In the cases analyzed the design forces for the beams supporting cut-off columns (flexural moments and shears) matched with the gravitational load combination or the horizontal-seismic load combination, obtained according to Eurocode 2 [2] and Eurocode 8 [3]. The vertical-seismic load combination has never been the most unfavourable for design.
Although it has not been explicitly studied, for beams confined in underground floors, where the horizontal seismic component does not affect the structure, the gravitational load combination is always the most unfavourable one.
In this paper buildings have beenn analyzed that have been designed with ductility class "L", which is the most common design situation in Spain. In cases where the ductility class is "M" or "H", the seismic horizontal component is less strong than the vertical one, because the behaviour factor is higher for the horizontal component. The analyses of these buildings with ductility class "M" or "H" are not included in this article, but in the research carried out the results obtained have been quite similar.
The main conclusion of this paper is that, for the area with the most seismic risk in Spain (with design accelerations as far as a_g = 0.33g), and for typical residential buildings, in half of the cases analyzed the gravitational load combination was the unfavourable one for the design of the beams supporting cut-off columns, in terms of design forces. In the rest of the cases the most unfavourable was the horizontal-seismic load combination, which includes 30% of the vertical-seismic component. Nevertheless, the addition of this last term is not very significant, in terms of flexural moments and shears at the beams supporting the cut-off elements.

References

1: Ansys, Revision 5.7. Universitary Version, Swanson Analysis Systems, Inc., 1998.
2: Eurocode 2. Concrete Design of Structures (ENV 1992-1-1), CEN, Berlin, 1992.
3: Eurocode 8. Earthquake Resistant Design of Structures (ENV 1998-1-1/2/3), CEN, Berlin, 1998.

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Front Page Browse CCP CSETS CTR IJRT Other Authors Search Purchase Guide FAQ Contact us	Civil-Comp Proceedings ISSN 1759-3433 CCP: 81 PROCEEDINGS OF THE TENTH INTERNATIONAL CONFERENCE ON CIVIL, STRUCTURAL AND ENVIRONMENTAL ENGINEERING COMPUTING Edited by: B.H.V. Topping Paper 219 The Effects of Vertical Seismic Acceleration in Columns Supported by Beams J. Lavado School of Civil Engineers, University of Granada, Spain doi:10.4203/ccp.81.219 purchase the full-text of this paper Full Bibliographic Reference for this paper J. Lavado, "The Effects of Vertical Seismic Acceleration in Columns Supported by Beams", in B.H.V. Topping, (Editor), "Proceedings of the Tenth International Conference on Civil, Structural and Environmental Engineering Computing", Civil-Comp Press, Stirlingshire, UK, Paper 219, 2005. doi:10.4203/ccp.81.219 Keywords: columns carried by beams, vertical seismic component, vertical acceleration, cut-off columns, cut-off elements, seismic acceleration. Summary In building construction there are architectural design solutions, in which several columns of the building are not supported by the foundation of the building. The anchorages of these columns take place by beams, located in intermediate floors of the building. In many situations the analysis of the seismic vertical component is not carried out, because of the computational cost. In this article a modal-spectral analysis has been performed from typical building design situations in Spain, where columns supported by beams exist. The aim is to assess those situations where the vertical component of the earthquake requires a detailed study in the design of these buildings. The program employed has been ANSYS [1]. Two-dimensional frames have been analyzed, belonging to typical residential buildings in Spain, with cast-in-place structures. The buildings are from the area of Granada, which is the place in Spain with the highest design seismic acceleration. The aim of this study is to investigate the influence of the length of the beams supporting cut-off columns. Beams of 5m and 8m length have been analyzed. To observe the influence of the number of floors resting on the same beam, two cases have been studied: beams supporting three floors of the building, and beams supporting six floors of the building. The main conclusions of this study are: In the two-dimensional analysis performed, several modes of vibration must be extracted to obtain the first vertical mode of the beams supporting the cut-off columns. In a three-dimensional analysis, many horizontal modes in the two main directions of the building and several torsional modes appear before the vertical modes. Therefore when the size of the building is greater (more numbers of floors and more numbers of spans) the vertical modes will appear much later in the modal analysis. In the cases analyzed the design forces for the beams supporting cut-off columns (flexural moments and shears) matched with the gravitational load combination or the horizontal-seismic load combination, obtained according to Eurocode 2 [2] and Eurocode 8 [3]. The vertical-seismic load combination has never been the most unfavourable for design. Although it has not been explicitly studied, for beams confined in underground floors, where the horizontal seismic component does not affect the structure, the gravitational load combination is always the most unfavourable one. In this paper buildings have beenn analyzed that have been designed with ductility class "L", which is the most common design situation in Spain. In cases where the ductility class is "M" or "H", the seismic horizontal component is less strong than the vertical one, because the behaviour factor is higher for the horizontal component. The analyses of these buildings with ductility class "M" or "H" are not included in this article, but in the research carried out the results obtained have been quite similar. The main conclusion of this paper is that, for the area with the most seismic risk in Spain (with design accelerations as far as a_g = 0.33g), and for typical residential buildings, in half of the cases analyzed the gravitational load combination was the unfavourable one for the design of the beams supporting cut-off columns, in terms of design forces. In the rest of the cases the most unfavourable was the horizontal-seismic load combination, which includes 30% of the vertical-seismic component. Nevertheless, the addition of this last term is not very significant, in terms of flexural moments and shears at the beams supporting the cut-off elements. References 1 Ansys, Revision 5.7. Universitary Version, Swanson Analysis Systems, Inc., 1998. 2 Eurocode 2. Concrete Design of Structures (ENV 1992-1-1), CEN, Berlin, 1992. 3 Eurocode 8. Earthquake Resistant Design of Structures (ENV 1998-1-1/2/3), CEN, Berlin, 1998. purchase the full-text of this paper (price £20) go to the previous paper go to the next paper return to the table of contents return to the book description purchase this book (price £135 +P&P)
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