Evaluation of Horizontal and Vertical RC Jacketing Strategies for Seismic Retrofitting of Mid-Rise Frame Building

Abstract

A considerable number of mid-rise reinforced concrete (RC) frame structures constructed prior to the adoption of modern seismic codes have critical deficiencies in ductility detailing, lateral load resistance and capacity design principles making them vulnerable structures under seismic based design ground motions. RC column jacketing is a widely adopted structural rehabilitation technique to increase the axial strength, flexural capacity, shear resistance and ductility of members by providing composite concrete casing around the existing columns. However, the effect of jacketing configuration, especially the vertical extent and the distribution of intervention across the horizontal direction on global structural performance is still not well quantified at the system level. This study presents a comparative analytical evaluation of six different RC jacketing configurations which are applied to a commercial RC moment resisting frame modelled and analyzed using Response Spectrum Analysis. Performance is evaluated in terms of PMM interaction-based demand capacity (D/C) ratio and storey response plot. Results show that the best and most consistent performance among all strategies is found in dual-floor horizontal zone strategy in terms of D/C ranging from 0.415-0.796, 46.2% reduction in first floor drift, 6.5% improvement in displacement. Vertical continuous jacketing created the largest deficiencies in D/C (1.171-1.419), indicating that vertical distribution strategies are not effective substitutes for horizontal zone-based intervention. The remaining configurations showed moderate improvement but retain residual weak points. Nonlinear time history analysis and hybrid retrofitting systems in addition to damping and bracing should be tested for future work.

Introduction

The study investigates the effectiveness of reinforced concrete (RC) column jacketing as a seismic retrofitting technique for older RC buildings that were designed before modern earthquake-resistant codes. Many existing buildings, particularly in India's seismic Zones III, IV, and V, lack adequate ductility, joint strength, and capacity design, making them highly vulnerable to earthquake damage. Previous research has shown that RC jacketing significantly enhances the axial load capacity, flexural strength, shear resistance, stiffness, and ductility of existing columns, although the effectiveness of retrofitting depends on the distribution of strengthened members and the resulting stiffness of the building. Improperly concentrated strengthening may create stiffness irregularities and shift seismic demand to unretrofitted storeys.

The research addresses this limitation by developing a performance-based analytical methodology to evaluate different RC jacketing configurations for a six-storey reinforced concrete moment-resisting frame. Using ETABS and Response Spectrum Analysis according to IS 1893:2016, the study compares six retrofit layouts with varying jacket thicknesses, column selections, and vertical extents. The numerical model incorporates cracked section stiffness, rigid diaphragm action, P-Delta effects, and PMM interaction-based capacity checks to realistically simulate seismic behavior. Structural performance is assessed using demand-to-capacity (D/C) ratios, storey displacement, inter-storey drift, base shear, and stiffness redistribution.

A review of previous numerical and experimental studies confirms that RC jacketing consistently improves seismic performance by reducing inter-storey drift by approximately 34–49%, increasing lateral stiffness by 18–57%, and enhancing base shear capacity by 22–130%. Recent developments, including fiber-reinforced jackets, further improve confinement efficiency while reducing jacket thickness. However, most previous studies focus on individual member behavior rather than the influence of retrofit configuration on the overall structural response.

The analytical results demonstrate that the HZ-GF+FF configuration, in which all columns on the ground and first floors are jacketed, provides the most effective overall seismic performance. This configuration maintains ground-floor demand-to-capacity ratios well below unity while significantly reducing first-floor deficiencies, ensuring safe structural performance during design-level earthquakes. In contrast, strengthening only the ground floor (GF-FULL) successfully improves ground-floor capacity but leaves first-floor columns vulnerable, with D/C ratios exceeding unity. Selective jacketing schemes improve the capacity of retrofitted columns but leave unretrofitted members susceptible to failure, while vertically continuous jacketing (VR-C16) proves less effective because it fails to adequately strengthen the most critical lower-storey regions where seismic demands are highest.

Overall, the study concludes that the distribution of RC jacketing is more important than jacket thickness alone. Extending retrofitting over both the ground and first floors provides better stiffness redistribution, reduces inter-storey drift, improves seismic demand distribution, and eliminates weak-storey mechanisms. The findings support a configuration-sensitive, performance-based retrofit strategy for upgrading gravity-designed RC buildings in moderate seismic zones, offering a practical and cost-effective solution for enhancing earthquake resilience.

Conclusion

1) HZ-GF+FF generated the most consistent response of the structural system in the range of 0.415 to 0.796 with reduction by 6.5% on ground floor and first storey drift reduction by 46.2%. 2) The vertically continuous (VR-C16), though within five storeys (ground to fourth floor), was still insufficient at the ground level, and D/C ratios of 1.171-1.419. This ascertains that vertical continuity cannot be applied solely or suffice to the demand concentration at storey of critical height of the structure and might not be sufficient to ensure enough local capacity. 3) Inter storey drift in every arrangement was within the limit 0.004 in IS 1893:2016, with the highest value of 0.001924. The increasing base shear of 8.8% in HZ-GF +FF is associated with an increase in global stiffness and the better involvement of lateral forces as opposed to an increase in negative demand. 4) The findings indicate that prescriptive and standardized retrofitting methods are structurally inefficient. Configuration in performance differences were large (D/C: 0.415-1.476), and hence configuration specific retrofitting strategies that are demand oriented are necessary. 5) GF FULL/GF-8C strengthening at the ground floor only enhanced local capacity but resulted in concentration of stiffness leading to an initial: first floor D/C ratio of 1.258-1.398. This behavior is an indication of the need to create more intervention to at least the first floor to prevent demand transfer and create balanced seismic behavior. 6) The 16 column strengthening layout, (GF-FULL) (ground floor D/C = 0.486-0.572), provides relatively better structural adequacy based on the consistent increase of stiffness and axial-flexural strength at the critical storey of peak seismic demands; others display demand concentration based on selective column intervention (SEL-8-80 / SEL-8-100) and limited (VR-C16), where vertical continuity is insufficient to prevent peak ground-storey interactions of axial-moment demand.

References

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Copyright

Copyright © 2026 Nosato Neikha, Vinod Kumar Sothwal, Harish Chandra Arora. 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.