A Review: SSI Analysis of G+6 RCC Building on Stiff Soil Using ETABS and Safe

Abstract

Soil-structure interaction (SSI) significantly influences dynamic response of multi-storey RCC buildings, yet routine practice assumes fixed base ignoring soil flexibility. For G+6 RCC buildings on stiff soil, this simplification questions accuracy of period, base shear, and displacement estimates. This review analyses G+6 RCC residential building using ETABS for superstructure and SAFE for foundation, comparing fixed-base versus SSI responses on stiff soil conditions. Three-dimensional ETABS model incorporates IS 1893:2016 seismic loads with soil springs representing stiff soil subgrade modulus. SAFE evaluates raft foundation performance extracting ETABS reactions. Key parameters include fundamental period, base shear, storey drift, and contact pressures. Expected outcomes show 15-25% period increase, 10-20% shear reduction, 25-40% higher displacements due to SSI. Findings establish practical SSI modelling guidelines ensuring code compliance while optimizing mid-rise RCC design on stiff soil sites.

Introduction

Soil–Structure Interaction (SSI) represents the two-way interaction between a structure and its supporting soil, affecting stiffness, damping, and dynamic response. Traditional RCC building design often assumes fixed foundations, ignoring soil flexibility. While acceptable for low-rise buildings on very hard ground, this can lead to inaccurate predictions of fundamental period, base shear, and storey drift for mid-rise buildings (G+6) even on stiff soils, which deform slightly under loads. SSI introduces additional base flexibility and damping, typically lengthening the natural period, redistributing inertial forces, increasing lateral displacements, and influencing foundation pressures, bending moments, and settlements. Ignoring SSI may result in unsafe underestimation of drift/settlement or overdesign of foundations.

Modern tools like ETABS and SAFE enable practical SSI analysis:

• ETABS models the G+6 RCC superstructure as a 3D moment-resisting frame with soil springs representing base flexibility.

• SAFE models the raft foundation, assigning subgrade stiffness to simulate soil support and extract pressures, moments, and settlements.

Indian Standards provide guidance for seismic (IS 1893:2016), concrete (IS 456:2000), and gravity loads (IS 875 Parts 1 & 2), including drift limits and load combinations, but do not explicitly define when fixed-base assumptions are unconservative for mid-rise buildings or how to select soil spring stiffness for ETABS–SAFE modeling.

Literature Review Highlights:

• SSI generally increases natural period and lateral displacement and reduces base shear compared to fixed-base models.

• Softer soils amplify these effects, but even stiff soils show noticeable differences for mid-rise RCC buildings.

• Prior studies often neglect foundation bending, raft behavior, and soil pressure distribution, particularly for regular G+6 buildings on stiff soils.

• ETABS-based spring models and SAFE raft analysis allow more realistic SSI modeling within Indian code frameworks.

Methodology:

1. Building and Soil Modeling: Regular G+6 RCC frame with M25 concrete and Fe500 reinforcement; stiff soil represented via equivalent subgrade modulus for raft foundation.

2. ETABS Superstructure Analysis: Two models—fixed-base and SSI (with vertical/rotational soil springs)—analyzed under dead, live (IS 875), and seismic (IS 1893:2016, Zone III) loads. Extract period, base shear, storey displacement, and drift.

3. SAFE Raft Foundation Analysis: ETABS SSI reactions applied to raft; stiff soil modeled via subgrade modulus; results yield contact pressures, bending moments, shear forces, and settlements.

4. Comparison: Quantify differences in fundamental period, base shear, top storey displacement, inter-storey drift, soil pressures, and settlement between fixed-base and SSI cases.

Expected Results:

• SSI on stiff soil increases fundamental period and lateral displacement, while base shear may decrease slightly.

• Raft foundation exhibits non-uniform but controlled pressures and settlements within permissible limits.

• Even for stiff soils, SSI modeling provides a more realistic and economical assessment of structural and foundation performance.

Conclusion

The review of SSI studies and the proposed ETABS–SAFE methodology shows that soil–structure interaction can significantly alter the seismic response of G+6 RCC buildings even when they rest on stiff soil. Allowing base flexibility is expected to lengthen the fundamental period, reduce design base shear, and increase lateral displacements and drift compared with fixed-base assumptions, indicating that rigid support idealization may not always be conservative for serviceability checks. For foundation design, modelling a raft on stiff soil with appropriate subgrade modulus introduces realistic soil pressures and settlements that are absent in fixed-base analysis, enabling better assessment of raft thickness, reinforcement, and differential settlement control. The integrated ETABS–SAFE workflow therefore provides a practical framework to incorporate SSI effects in routine design of mid-rise RCC buildings under Indian standards. Overall, the study highlights that fixed-base models may be adequate only for preliminary sizing, whereas explicit SSI modelling is recommended whenever drift limits, non-structural performance, or foundation behavior are critical design considerations on stiff soil sites. Adopting calibrated soil springs and raft subgrade models helps designers achieve code-compliant and economical solutions for G+6 RCC buildings, bridging the gap between simplified assumptions and realistic soil–foundation–structure interaction.

References

[1] P. M. Kulkarni and Y. M. Ghugal, “Dynamic analysis of multistorey RCC buildings incorporating soil–structure interaction under different soil conditions,” International Journal of Civil and Structural Engineering, vol. 12, no. 2, pp. 1–10, 2022. [2] Deshmukh, “Influence of soil–structure interaction on vertically irregular RC buildings,” International Journal of Research in Engineering and Technology, vol. 9, no. 3, pp. 112–120, 2022. [3] U. T. Jagadale, S. S. Patil, and R. V. Kulkarni, “Seismic performance evaluation of RC buildings considering soil flexibility using ETABS,” International Journal of Engineering Research and Technology, vol. 10, no. 4, pp. 50–58, 2021. [4] S. A. Zaidi, M. W. Khan, and R. Singh, “Study on the effects of seismic soil–structure interaction on RC buildings,” Materials Today: Proceedings, vol. 42, pp. 1234–1240, [5] Jain and S. R. Parekar, “Comparative seismic behaviour of RC frame buildings with fixed and flexible base conditions,” Indian Concrete Journal, vol. 95, no. 7, pp. 34–45, 2021. [6] K. M. Ahmed and M. Basha, “Soil–structure interaction analysis of RC building with raft foundation under different soil conditions,” International Journal of Engineering Research and Technology (IJERT), vol. 11, no. 6, pp. 250–257, 2022. [7] M. Roopa, S. Kumar, and P. V. Reddy, “Soil–structure interaction effects on multistorey framed structures using equivalent soil springs in ETABS,” Journal of Structural Engineering (India), vol. 47, no. 2, pp. 89–102, 2020. [8] S. Sharma and R. Das, “Study of soil–structure interaction on RCC building using ETABS,” Journal of Emerging Technologies and Innovative Research (JETIR), vol. 7, no. 8, pp. 299–305, 2020. [9] J. Li, X. Chen, and Y. Zhou, “Evaluation of seismic performance and soil–structure interaction of low-rise RC frame buildings on different soil profiles,” Shock and Vibration, 2021, Article ID 7876389. [10] K. Pitilakis, S. Karapetrou, and G. Mylonakis, Soil–Structure Interaction for Building Structures (NIST GCR 12-917-21), National Institute of Standards and Technology, Gaithersburg, USA, 2012.

Copyright

Copyright © 2025 Harsh Patle, Aaquib R. Ansari. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.