PSI - Issue 14

Siva Naveen E et al. / Procedia Structural Integrity 14 (2019) 806–819 Siva Naveen E, Nimmy Mariam Abraham, Anitha Kumari S D / Structural Integrity Procedia 00 (2018) 000–000

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2. Methodology In the present work, seismic response of frames having different configurations are obtained numerically using a finite element based software, ETABS. The major inputs are geometry of the frame including dimensions of storeys and columns, total mass of each floor, modulus of elasticity, damping ratio and earthquake data. The modulus of elasticity for the material is taken as 20000 MPa. Rayleigh damping is assumed with a damping ratio of 4%. It is also assumed that the structure starts from rest on load application. The results are structural response in the form of storey displacement, storey drift, base shear and overturning moment. In order to validate the results, response obtained for the regular configurations is compared with the results reported in literature. For both validation and subsequent analysis, same methodology is adopted. 3. Validation Moehle (1984) has conducted experiments on a small scale nine-storyed test structure subjected to El-Centro North-South 1940 ground motion (scaled). The results reported are used to validate the model developed for the present study. Two frames having nine storeys and three bays were placed opposite and parallel to each other. The frames together carry a total weight of 460 kg at each floor level. The typical storey height is 0.229 m. The frame has three bays in the direction of length and one bay in the direction of width. The dimension of each bay in the direction of length and width are 0.305 m and 0.914 m respectively. At the base, the frames were subjected to simulated earthquake base motion in horizontal direction parallel to the plane of the structure. The same test structure is modeled and analyzed numerically using ETABS. The first three natural frequencies and the top storey displacement of the structure obtained numerically are compared with the experimental results reported by Moehle (1984). The same is tabulated in Table 1.

Table 1. Validation of the model. Particulars

Literature (Moehle,1984)

Obtained

1 st natural frequency (Hz) 2 nd natural frequency (Hz) 3 rd natural frequency (Hz) Top storey displacement (mm)

4.5

2.4

14.4 28.3 16.1

15.3 29.5 16.9

4. Analysis of frames with regular and irregular configurations A structure is said to be irregular, when certain structural parameters exceed the limits specified by standards. Table 2 shows the limits for mass (M), stiffness (S), vertical geometric (VG), re-entrant corner (REC) and torsional (T) irregularities prescribed by IS1893:2016 (Part I).

Table 2. Irregularity limits prescribed by IS 1893:2016 (Part I) (i = storey number, a = adjacent storey number, Δ max = maximum deformation and Δ avg = average deformation). Type of irregularity Classification Limits Mass (M) Vertical irregularity M i < 1.5M a Stiffness (S) Vertical irregularity S i < S i+1

Vertical geometry (VG) Re-entrant Corner (R) Torsion (T)

Vertical irregularity Horizontal irregularity Horizontal irregularity

VG< 1.25 VG a R i <= 15% Δ max <= 1.5Δ avg

A nine-storey scaled frame with a storey height of 0.229 m is considered for the study. The frame has six bays in the direction of length and three bays in the direction of width. The dimension of each bay in the direction of length and width are 0.305 m and 0.914 m respectively. Each floor carries a lumped mass of 2760 kg. The irregularities are