Investigation on deflection amplification factor for special moment resisting frames with vertical mass irregularity

Document Type : Original Research

Authors
M. Yakhchalian 1
S. Abdollahzadeh 2
1 Assistant Professor, Department of Civil Engineering, Qazvin Branch, Islamic Azad University, Qazvin, Iran
2 Msc. in Structural Engineering, Department of Civil Engineering, Qazvin Branch, Islamic Azad University, Qazvin, Iran,
Abstract
In design of structures using force-based methods applied in current seismic codes, to obtain the nonlinear displacements of structures under the design earthquake, deflection amplification factor (Cd) is applied. In other words, the displacements obtained from elastic analyses under the reduced seismic forces are amplified by Cd to obtain the inelastic displacements under the design earthquake. Research studies showed that using a constant coefficient for estimating the inelastic displacements may lead to considerable overestimation or underestimation of the displacements in different stories of structures. Generally, in regular structures the inelastic maximum interstory drift ratio (IMIDR) occurs in lower stories. Investigating the seismic performance of structures with irregularity in their heights showed that the inelastic responses of these types of structures can differ significantly from the inelastic responses of regular structures. The present study investigates Cd for estimating IMIDR and inelastic maximum roof drift ratio (IMRDR) for steel special moment resisting frames (SMRFs) with vertical mass irregularity under the design earthquake. In addition, the variation of Cd with the variation of the location of the heavier story in the structural height, and mass ratio (i.e., the ratio of the mass of the heavier story to the mass of the adjacent story) is studied. For producing a heavier story, the dead and live loads of the story are multiplied by 2.0 and 3.0. Three different locations (i.e., bottom, mid-height and top story) for the heavier story, are assumed. For investigating the effects of mass irregularity, two regular 5- and 10-story structures are also considered. Therefore, 14 structures (i.e., two mass ratios × two building heights (5 and 10 stories) × three locations for the heavier story + two regular structures) are considered. To perform nonlinear dynamic analyses, 67 ground motion records are applied. The records are scaled such that the mean of the pseudo acceleration response spectra exceeds the design response spectrum for the period range of 0.2T1 to 1.5T1. The results show that using Cd = 5.5 recommended by Standard No. 2800 and ASCE 7 for steel SMRFs underestimates the IMIDR in most of the structures considered and their stories, under the design earthquake. When the heavier story is located in the first story, the lowest mean Cd is obtained in the first story. Because, increasing the mass of the story leads to an increment in the stiffness and strength demand of the story. When the heavier story is located at the roof, the lowest mean Cd is obtained for the top story. While the mean Cd in the first story increases significantly. Moreover, it is shown that Cd = 5.5 underestimates the IMRDR in the structures considered. Investigating the consideration of different values for Cd shows that using Cd = 7.5 leads to the lowest error in the estimation of IMIDR in the structures considered. In the case of estimating IMRDR, the displacement amplification factor is termed Cd Roof, and it is shown that using Cd Roof = 6.5 leads to the lowest error in the estimation of IMRDR. Therefore, Cd = 7.5 and Cd Roof = 6.5 are respectively proposed for more precisely estimating IMIDR and IMRDR in steel SMRFs with vertical mass irregularity.

Keywords

Subjects

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Modares Civil Engineering journal
Volume 20, Issue 6
November and December 2020
Pages 163-173