Comparative Study of Seismic Analysis of Vertically Irregular R.C. Frame Using Indian and Euro Code

Abstract

The present study conducts a comparative evaluation of the seismic performance of reinforced concrete (RC) building frames designed in accordance with two globally accepted design codes: the Indian Standard (IS 1893:2016) and the European Standard (Eurocode 8). Structures with vertical irregularities are known to be more susceptible to seismic damage, particularly when appropriate strength and detailing measures are not implemented. High-rise buildings, especially those exhibiting geometric discontinuities, require a comprehensive assessment of their seismic response to ensure safety and resilience. This research investigates a G+22 storey RC building characterized by vertical geometric irregularities. The seismic analysis is carried out using ETABS software, employing the Response Spectrum Method (RSM) to evaluate structural response parameters, including storey drift, overturning moment, storey shear, and lateral displacement in both principal directions. The study highlights the practical differences between the two codes by comparing their design implications. In particular, it is observed that structures analyzed under IS 1893 tend to exhibit relatively lower response values that remain within acceptable limits, whereas Eurocode 8 analysis results in higher seismic demand due to its detailed spectral definitions and consideration of site-specific soil characteristics.

Introduction

I. Introduction

India is highly seismically active, with a history of damaging earthquakes, making earthquake-resistant design essential—especially for high-rise buildings with vertical irregularities (sudden changes in mass, stiffness, or geometry along a building’s height). These irregularities increase the risk of structural failure during earthquakes.

A major concern is the "soft storey" effect, common in buildings where the ground floor is left open for parking, weakening the base structure. Events like the Bhuj and Latur earthquakes exposed the vulnerability of such designs.

II. Need for Code-Based Design

India’s IS 1893:2016 and Europe’s Eurocode 8 provide seismic design guidelines. While IS 1893 covers fundamental criteria, Eurocode 8 offers more detailed rules, especially for irregular structures. Modern tools like ETABS support simulation using both codes, with techniques like Response Spectrum Analysis (RSA) helping analyze parameters like base shear, drift, and displacement.

III. Literature Review

• Vertical irregularities significantly affect seismic response.

• IS 1893 identifies five types: soft storey, mass irregularity, vertical geometry change, in-plane discontinuity, and weak storey.

• Eurocode 8 emphasizes regularity, uniform stiffness, and load paths to ensure structural stability.

• Case studies from past earthquakes (e.g., Kashmir 2005, Christchurch 2011) show catastrophic failures in buildings with vertical irregularities.

• Research comparisons reveal that Eurocode predicts higher base shear, and bracing and symmetry help in reducing vulnerability.

IV. Methodology

The study analyzes a G+22 reinforced concrete (RC) building with vertical irregularities, modeled in ETABS 2016, using both IS 1893 and Eurocode 8:

• Analysis Methods:

  • Equivalent Static Analysis (ESA) – simplified method for lateral forces.

  • Response Spectrum Analysis (RSA) – dynamic method using building vibration modes.

  • Time History Analysis (THA) and Pushover Analysis are also discussed for validating seismic performance.

• Design Parameters and Codes Used:

  • Indian Codes: IS 1893:2016, IS 456:2000, IS 875

  • European Codes: Eurocode 2, Eurocode 8

• IS 1893 Parameters:

  • Zone factor (Z): 0.10 to 0.36 (India Zones II–V)

  • Importance factor (I): 1.0–1.5

  • Response reduction factor (R): 3.0 (OMRF), 5.0 (SMRF)

• Eurocode 8 Parameters:

  • Behavior factor (q): 1.5 to 5.85, reduced by 20% for irregularities

  • Importance factor (I): 0.8 to 1.4

  • Ground classifications from rock (Type A) to soft soils (Type E)

• Drift Control:

  • IS 1893: Max drift = 0.004 × storey height

  • Eurocode 8: Allows up to 0.015 × storey height

V. Structural Model Description

• Building Specs:

  • Plan: 30m × 30m

  • Height: 69.2m

  • Storeys: G+22

  • Irregular vertical profile with 5 bays in each direction

• Load and Material Details:

  • Concrete: M35, Density = 25 kN/m³

  • Steel: Fe500 (longitudinal), Fe250 (transverse)

  • Live Load: 3 kN/m² + 1 kN/m² floor finish

• Modeling Software: ETABS 2016 is used for simulation, applying both IS and Eurocode criteria to evaluate displacement, base shear, story drift, and overturning moments.

Key Findings and Objectives

• The study aims to compare seismic responses of the building using IS 1893 and Eurocode 8.

• It focuses on how vertical irregularities affect performance.

• Results are intended to inform future design practices and bridge the gap between Indian and international standards for safer, more resilient buildings.

Conclusion

This research focuses on the dynamic analysis of structural characteristics affecting a building\'s durability, stability, and safety by comparing seismic performance across IS 1893:2016 and Eurocode 8. The analysis shows that the base shear calculated under Eurocode 8 is approximately 67% higher than that determined by IS 1893:2016. This difference stems from the higher response reduction factor applied in the Indian standard. Despite this, storey shear values remain consistent between the two codes in both directions. Storey displacements are more significant under Eurocode due to higher base shear demands, with the difference increasing along the height of the structure. Similarly, the storey drift predicted using Eurocode is up to 65% higher than that under IS 1893. The structure analyzed under IS 1893 demonstrates favorable results in terms of displacement, drift, and shear, maintaining values well within codal limits. Maximum drift is consistently observed at the first storey level in both code-based models. Overall, IS 1893:2016 delivers comparatively better performance in this study context, suggesting its effectiveness in ensuring safety and serviceability for vertically irregular high-rise buildings. Scope for Future Work To broaden the understanding of seismic behavior, future studies may incorporate advanced nonlinear techniques such as Pushover and Time History Analysis. Additional investigations could consider combined vertical and plan irregularities to examine interaction effects. Moreover, exploring structural enhancements through lateral load-resisting components such as bracings and shear walls can further improve seismic resilience and stability.

References

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Copyright

Copyright © 2025 Akash Vijay Pandey, Er. Bajrang Ambhore, Prof. Pallavi N. Bhende. 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.