Research On The Scope Of Structures Considering Seismic Action In Two Directions Simultaneously

It is indicated by theoretical analysis and seismic damage of structures that the essence of earthquake ground motion is multi-dimensional and it should be considered in seismic design of buildings. It is first be stipulated by Code for Seismic Design for Buildings (GB50011-2001) that the torsion effect of structures subjected to two horizontal components of earthquake ground motion should be considered in design when the structure has sharp irregularity of mass and rigidity layout. Since the concept of sharp irregularity of mass and rigidity layout is fuzzy to execute, the scope of structures considering seismic action in two directions simultaneously is researched in this paper. The main factors influencing torsion effect are analyzed and differences between code of china and other country are investigated and some problems in code have been discovered. Referring to three kinds of plan irregularity, structures individually with four kinds of irregularity as eccentricity in one direction, eccentricities in two directions, re-entrant corner irregularity and diaphragm discontinuity have been analyzed to investigate the relationship between torsion effects and parameters of above irregular structures. The parameters and the schemes for determining the scope of structures with different kind of irregularity considering seismic action in two directions simultaneously are proposed, some conclusions have been get as following:①It is indicated by the analysis results that torsion effect has no dependency relation with eccentric torsional angel adopted in Code for Seismic Design for Buildings. It is suggested that the eccentric torsional angel should not be take as the parameter of torsion effect of structures considering seismic action in two directions simultaneously; ②For the structures with regular plan and elevation and with eccentricity in one direction due to irregular mass layout, the relative eccentricity can be take as the parameter of torsion effect of structures considering seismic action in two directions simultaneously;③For the structures with regular plan and elevation and with eccentricities in two directions due to irregular mass layout, the ratio of relative eccentricities in two directions and the angel of them can be take as the parameters of torsion effect of structures considering seismic action in two directions simultaneously;④It is indicated by the analysis results that the torsion effect has no dependency relation with re-entrant corner ratio adopted in Code for Seismic Design for Buildings asparameter of structure with re-entrant corner irregularity, so it is not radical to stipulate for all structures with re-entrant corner irregularity to consider torsion effect. It is suggested taking 15% as critical re-entrant corner ratio instead of 30% and procedure in conjunction with a three-dimensional model to determine the lateral seismic forces for design;⑤It is necessary to consider torsion effect for structures with small diaphragm effective width and consider real stiffness of diaphragm for structure with diaphragm discontinuity. Torsion effects of structures with diaphragm discontinuity can be included by referring to the schemes of the structures with eccentricities in one or two directions.Finally, the disadvantages of the present study are discussed and the directions of further developing of this field are prospected.