Fast Checking of Drift Demand in Multi-Storey Buildings with Asymmetry

Abstract

Buildings possessing an asymmetrical arrangement of structural elements are torsionally unbalanced and can be vulnerable in a seismic event. Building codes of practices typically recommend the use of three-dimensional dynamic analysis to determine the seismic demands of a multi-storey building. Whilst most design practices are well equipped with commercial software for undertaking such analyses, designers often find it difficult to verify results. Much of the published technical articles present findings for buildings based on an idealised single-storey model. As a result of challenges in dealing with real multi-storey buildings, there has been very limited uptake of research findings in design practices. This article presents a three-tiered approach of estimating the displacement behaviour of the building in term of 3D/2D displacement ratio. The estimate can be used for verifying results reported from a computer package conveniently. The quick method provides predictions of the 3D/2D ratio and only requires the gross plan dimensions of the building to be known. The refined method requires knowledge of the torsional stiffness properties to be known, whereas the detailed method requires the eccentricity properties to be known as well. The proposed methodology is robust and reliable, as is demonstrated by case studies undertaken on six real multi-storey buildings.

Simplified Method of Determining Torsional Stability of the Multi-Storey Reinforced Concrete Buildings

“…The first method which is the shear and bending combination method (SBCM) is proposed in this paper to manually calculate the 'b r ' value. The second method, known as the static analysis method (SAM), has been developed by the authors in [16,22] and is used for the verification of the proposed method. These methods are discussed in details in the following sections.…”

“…The static analysis method (SAM) is the robust method of determining elastic radius ratio 'b r ' based on the static analysis of the building performed in the computer. The method has been developed by the authors in [16,22], and the procedure is summarized below. a.…”

“…Out of these three parameters, 'b r ' which represents the degree of disposition of the lateral load resisting elements from the center of rigidity (CR), has been used as the benchmark parameter to determine the torsional stability or stiffness of the buildings. In a recent study, Khatiwada et al [16] also observed that 'e r ' is only influential to the torsional response of low-rise buildings and has small to no impact on medium-rise, and high-rise buildings; whereas, 'b r ' was found to be equally influential in all three types of buildings. Similarly, Lam et al [17], observed that increasing 'b r ' by less than 10% has an equal impact on torsional resistance as decreasing 'e r ' by more than 50%.…”

“…Despite being complex and laborious, this method cannot be used directly for the asymmetrical buildings in which the coupling of the translational and torsional responses is dominant due to the eccentricity. As finding uncoupled frequencies of such buildings is very difficult [21], recently, the authors have developed a more robust "static analysis method (SAM)" also known as "generalized force method" [16,22] applicable for symmetric as well as asymmetric buildings. This method requires a two-step static analysis of the building.…”

“…Figure6. Structural details of the asymmetrical buildings: (a) Model 'k'[16], (b) Model 'l'[27], and (c) model 'm'[16].…”