Soil-Structure Interaction in Nonlinear Pushover Analysis of RC Buildings

Abstract

The seismic performance of reinforced concrete (RC) buildings is significantly influenced by soil–structure interaction (SSI). Traditional pushover analysis, based on FEMA 356 and ATC-40, assumes a fixed base and neglects foundation flexibility, potentially leading to unsafe designs. This study presents a nonlinear pushover analysis incorporating SSI for a four-storey RC frame using SAP2000. Different soil conditions—soft, medium, and hard—were modeled using equivalent spring stiffness. Performance was evaluated in terms of base shear, roof displacement, hinge formation, and performance points. Results indicate that flexible soil drastically reduces base shear (by over 80% in soft soil compared to fixed base) and increases displacements, significantly altering hinge formation patterns. The study concludes that neglecting SSI may result in overestimation of seismic capacity. Recommendations for future work include torsional analysis, higher-mode effects, and experimental validation.

Introduction

• Earthquake forces impose complex demands on reinforced concrete (RC) buildings, primarily influenced by the distribution of mass, stiffness, and strength.

• Pushover analysis is widely used for seismic performance evaluation but often assumes a rigid (fixed) foundation, ignoring soil deformability.

• Soil–Structure Interaction (SSI) significantly affects structural behavior by altering global stiffness, damping, and seismic demand.

???? Literature Review Highlights

• Bybordiani et al. (2019): Dynamic SSI changes seismic demand in steel frames.

• Matale & Khedekar (2018): SSI reduces base shear in RC frames.

• Zubair & Shilpa (2018): Flexible soils lead to increased displacement.

• Hasan et al. (2002), Gehlot & Sharma (2013): Pushover analysis used for performance-based design, but typically with fixed-base assumptions.

• Gap: Limited studies integrate SSI into nonlinear pushover analysis for RC frames.

???? Methodology

• Model: 4-storey, single-bay RC frame (6 m × 6 m, 3.5 m storey height), modeled in SAP2000.

• Structural details:

  • Columns: 500×500 mm (M30 concrete, Fe500 steel)

  • Beams: 230×500 mm

  • Slab: 150 mm thick

  • Live loads: 3.0 kN/m² (floors), 1.5 kN/m² (roof)

• Soil Conditions Modeled as Springs:

  • Soft soil (E = 12,000 kN/m²)

  • Medium soil (E = 35,000 kN/m²)

  • Hard soil (E = 200,000 kN/m²)

• Pushover Analysis:

  • Load pattern: First mode shape

  • Hinge types: M3 (beams), P-M3 (columns) per FEMA 356

  • Performance levels: IO (Immediate Occupancy), LS (Life Safety), CP (Collapse Prevention)

  • Capacity Spectrum Method used to determine performance points.

???? Results & Discussion

???? Capacity and Demand Curves

• Soft soils produced greater displacements and earlier hinge formation.

• Hard soil behavior was closest to the fixed-base model.

???? Base Shear & Roof Displacement

• Soft soil showed lowest base shear and highest displacement, pushing the structure past Life Safety (LS) levels but still within Collapse Prevention (CP).

???? Hinge Formation

• Soft soils caused:

  • More hinges at LS level

  • Fewer hinges at CP level

  • Indicating partial improvement in ductility and redistribution of seismic demands.

Conclusion

This study demonstrates that SSI significantly impacts nonlinear pushover analysis of RC buildings. • Soft soil reduced base shear by >80% compared to fixed base. • Roof displacements increased with soil flexibility. • Hinge patterns shifted toward LS level under flexible bases. • Neglecting SSI may lead to unsafe overestimation of seismic capacity. Future Work: Consider torsional effects, higher modes, and experimental validation to strengthen design guidelines.

References

[1] Bybordiani, M., Kaazemzadeh Azaad, S. (2019). Optimum design of steel braced frames considering dynamic SSI, Journal of Structural Engineering. [2] Matale, A., Khedekar, A. (2018). Soil-structure interaction of multistorey RCC frame, IJARIIE, 4(2). [3] Zubair, M., Shilpa, B.R. (2018). A parametric study of soil-structure interaction of raft foundation by dynamic analysis, IJESIRD. [4] Prakash, M.Y., Ghugal, Y.M., Wankhade, R.L. (2016). Study on soil-structure interaction: A review, IJER, 5, pp. 737–741. [5] Hasan, R., Xu, L., Grierson, D.E. (2002). Pushover analysis for performance-based seismic design, Computers & Structures, 80, pp. 2483–2493. [6] Gehlot, S.S., Sharma, R.K. (2013). Application of performance-based design to upgrade unsymmetrical RC building, Int. J. Struct. Civ. Eng. Res., 2, pp. 82–89.

Copyright

Copyright © 2025 Akshay Singh Gurjar, Dr. Rakesh Gupta, Dr. Mukesh Kumar Pandey. 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.