Comparative Seismic Analysis of Fixed-Base on Base-Isolated G+20 Storey RC Building with X-Type Steel Bracing as per IS 1893 (Part 1): 2016

1. ESA, RSA and NLTHA were performed on 3-D ETABS models of Models A and B per IS 1893 (Part 1): 2016. Peak roof displacement rose from 30.79-31.52 mm (RSA) to 130.28 mm (NLTHA, Model A), with a non-monotonic storey-shear profile (peak at Storey 10, 64% of base shear) reflecting higher-mode effects that ESA and RSA do not capture; the maximum drift ratio (0.00284) remained within the IS 1893 limit of 0.004 under all methods, confirming code compliance but establishing NLTHA as essential for this building class. 2. The fundamental period elongated only marginally, from 1.511 s (Model A) to 1.521 s (Model B, +0.66%), well short of the three- to four-fold elongation expected of effective isolation; modal mass participation (66.54-66.65% at Mode 1) was essentially unchanged, indicating energy dissipation remained governed by superstructure hysteresis rather than isolator damping. 3. Base isolation reduced NLTHA base shear by only 0.98-2.03% and drift by 3.9% relative to Model A, while peak roof displacement stayed unchanged at 130.28 mm - isolation redistributes demand to the isolator level rather than reducing it, lowering member-level damage but leaving global displacement demand essentially unaddressed. 4. Models A and B both exceed the H/500 = 120 mm limit at 130.28 mm; Model C reduces this to 117.56 mm (9.8% reduction, H/500-compliant) and cuts critical drift by 44.1%, at an 11.7% base-shear cost relative to Model B. The supplementary pushover/IDA study confirms this pattern: Model A reaches Collapse Prevention drift at 0.40 g, while Models B/C are isolator-governed, reaching their 200 mm design displacement at a much lower, tightly clustered 0.08 g, with superstructure drift held well below Immediate Occupancy. 5. Model C is the only configuration to satisfy both the drift and H/500 criteria while reducing base shear relative to the fixed-base case, establishing the hybrid isolation-plus-bracing strategy as the recommended configuration for tall RC buildings in high-seismic zones such as Gujarat, Seismic Zone V.

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