Effects of coupled vertical stiffness-strength irregularity due to modified interstorey height

Abstract

Structures may have vertical stiffness or strength irregularity for many reasons. In many practical cases, a change in storey stiffness, results a change in strength at the same storey. In this paper, the effect of a change in interstorey height is quantified. In order to do this, relationships between storey stiffness and strength resulting due to a modified interstorey height for a few common lateral force resisting systems was considered. It was applied to simple shear-type structures of 3, 5, 9 and 15 storeys, assumed to be located in Wellington. All structures were considered to have a constant mass at every floor level. Both regular and irregular structures were designed in accordance with the Equivalent Static method of the current New Zealand seismic design Standard, NZS 1170.5. Regular structures were designed to either (i) produce a constant target interstorey drift ratio at all the storeys simultaneously or (ii) to have uniform stiffness distribution over the height of the structure, with the target interstorey drift ratio at the first storey. An “interstorey height ratio” was defined as the ratio of modified to initial interstorey height, and applied separately at the first storey, mid-height storey and at the topmost storey by amounts between 0.5 and 3. The modified structures were then redesigned until the target interstorey drift ratio was achieved at the critical storey/storeys. Design structural ductility factors of 1, 2, 3, 4 and 6, and target (design) interstorey drift ratios ranging between 0.5% and 3%, were used in this study. Inelastic dynamic time-history analysis was carried out by subjecting these structures to code design level earthquake records, and the maximum interstorey drift ratio demands due to each record were used to compare the responses of regular and irregular structures.
 It was found that structural types in which only the storey stiffness was modified due to a change in the interstorey height produced the maximum increase in drift demands rather than structural forms with other stiffness-strength coupling cases. Shorter structures having an increased first storey height, and taller structures with an increased middle storey height generally produced greater interstorey drift demands than regular structures. For cases of increased storey stiffness due to decreased storey heights, the shorter structures with a decreased middle storey height resulted in higher median peak ISDR due to irregularity. A simple equation describing the maximum increase in response due to modifications to a storey height was developed. The equation was used along with the realistic correlations between storey stiffness and strength to obtain the governing code regularity limit.

Nonlinear Response Analysis of Plan and Vertical Asymmetric Reinforced Concrete Buildings under Directional Seismic Loadings

“…And the reduction in strength by 20% improved the ductility demand by 100-200%. Sadashiva et al (2011) explored stiffness irregularities in 3, 5, 9 and 15 storied buildings with variation in height of the lateral load resisting elements, leading to reduction in stiffness at different floors by performing nonlinear time history analysis under the seismic ground motions. The inter-story drift ratio was found higher in the case of stiffness reduced floors as well as in stiffness increased floors especially at middle floors when compared with regular structure.…”

Seismic Performance Evaluation of RC Buildings Using Irregularity Based Indices

“…Therefore, a substantial amount of research noted has been received to appraise the performance of vertical irregular buildings. Valmundsson and Nau 1997 [5]; Rutenberg 2002 [6]; Chintanapakdee and Chopra 2004 [7] Alba et al 2005 [8]; Fragiadakis et al 2005 [9]; Karavasilis et al 2008 [10]; Sadasiva et al 2009 [11]; Le-Trung et al 2010 [12]; Gerasimidis et al 2012 [3]; Pirizadeh and Shakib 2013 [13]; Georgoussiset al 2015 [14]; Farsangi et al 2015 [15]; Sudharasana Murthy and Balachandran 2016 [16]; Farsangi et al 2016 [17]; Priyanka et al 2018 [18]; Kassem et al 2019 [19]; Nazri et al 2019 [20]; Kassem et al 2020 [21]; Rathnasiri et al 2020 [22]; Akhil and Sajebb 2020 [23]; Kassem et al 2021 [24] are some of the past studies that evaluate the performance of the irregular buildings against the earthquake loading.…”

ارزیابی عملکرد لرزه‌یی سازه‌های فولادی با سیستم باربر جانبی دوگانه با نامنظمی غیرهندسی در امتداد ارتفاع

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