Seismic evaluation and behavior of irregular structures is one of the most important steps in the retrofitting process of such structures.The earliest provisions for seismic resistance were the requirement to design for a lateral force equal to a proportion of the building weight (applied at each floor level). This approach was adopted in the appendix of the 1927 Uniform Building Code (UBC), which was used on the west coast of the United States.
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It later became clear that the dynamic properties of the structure affected the loads generated during an earthquake Irregularity can be in the elevation or in the plan of a structure. Irregularity in plan shape which is due to the difference between the position of the center of stiffness and the mass center of a structure caused by architectural requirements is usually inevitable.. The guidelines and the methodology of the Indian standard of practice IS 1893 (Part I): 2002 is used to analyze the structures.
Irregularity results from the uneven distribution of mass, strength or stiffness along the elevation of a building structure. Mass irregularity results from a sudden change in mass between adjacent floors, such as mechanical plant on the roof of a structure. Stiffness irregularity results from a sudden change in stiffness between adjacent floors, such as setbacks in the elevation of a building. Plan Irregularity Asymmetric or plan irregular structures are those in which seismic response is not only translational but also torsional, and is a result of stiffness and/or mass eccentricity in the structure. A regular structure may actually be asymmetric if the structure has masonry infill walls or stiffer lateral resisting systems on one side of the structure that has not been taken into consideration in the analysis.
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Plan-irregular structures suffer higher levels of earthquake damage than their regular counterparts due to torsional response. This observation lends weight to the necessity of an advanced damage index which provides a quantitative measure of the susceptibility to torsional effects. In this paper, a new damage parameter is derived and shown to be capable of accounting for the effect of torsional imbalance on structural damage. Starting from the concept of planar decomposition of the complex 3D frame, the procedure uses relative weighting of the contribution of each (decomposed) frame line to the overall torsional response. The derived damage parameter, herein terms Demand-to-Capacity Ratio (DCR) is applicable to not only RC frames with planar irregularities but also steel, concrete and composite frames. It is also representative of the torsional sensitivity of moment-resisting and braced frames as well as frame-wall structures. Through an application example, it is demonstrated that the proposed DCR yields convincing results that match damage limit states more accurately than existing indices. Owing to its simplicity and clear application rules, the new damage measure is recommended for use in seismic assessment of structures. While the limit states previously suggested by researchers can be directly used for symmetric cases, their application to structures with planar irregularities should be accompanied with the new damage monitoring methodology suggested in this paper.
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