Download citation:
BibTeX | RIS | EndNote | Medlars | ProCite | Reference Manager | RefWorks
Send citation to:

---

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 (C_d) is applied. In other words, the displacements obtained from elastic analyses under the reduced seismic forces are amplified by C_d 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 C_d 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 C_d 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 C_d = 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 C_d 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 C_d is obtained for the top story. While the mean C_d in the first story increases significantly. Moreover, it is shown that C_d = 5.5 underestimates the IMRDR in the structures considered. Investigating the consideration of different values for C_d shows that using C_d = 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 C_{d Roof}, and it is shown that using C_{d Roof} = 6.5 leads to the lowest error in the estimation of IMRDR. Therefore, C_d = 7.5 and C_{d Roof} = 6.5 are respectively proposed for more precisely estimating IMIDR and IMRDR in steel SMRFs with vertical mass irregularity.
 

Keywords: deflection amplification factor, steel special moment resisting frames, vertical mass irregularity, interstory drift ratio, roof drift ratio

Full-Text [PDF 818 kb]   (1369 Downloads)    

Add your comments about this article : Your username or Email:

---

---