Extending Reinforced Concrete Structures: A Comparative Retrofitting Approach

Abstract

In densely populated urban regions, vertical extension of existing reinforced concrete (RC) buildings provides an efficient solution to land scarcity and urban expansion challenges. This study investigates the structural feasibility of upgrading an existing G+3 RC building to G+4 using two retrofitting techniques: concrete jacketing and steel jacketing. Numerical modeling and nonlinear seismic analysis were carried out using SAP2000 under gravity and seismic loading conditions in accordance with IS 1893 (Part 1): 2016 and ATC-40 guidelines. The study evaluated structural performance through modal analysis, pushover analysis, and nonlinear time-history analysis. The addition of the extra story increased the structural mass by approximately 22%, causing the natural period of the original structure to increase from 0.98 s to 1.14 s. After retrofitting, the natural period reduced to 0.95 s for concrete jacketing and 0.91 s for steel jacketing, indicating improved structural stiffness. Pushover analysis revealed that steel jacketing achieved nearly 15% higher base shear capacity than concrete jacketing, while time-history analysis demonstrated 10%-12% lower interstory drift and reduced residual displacement in the steel-jacketed model. Concrete jacketing improved stiffness significantly but resulted in comparatively lower ductility. The study further compares both retrofitting methods in terms of constructability, durability, seismic performance, and serviceability. Validation of analytical results with published experimental and analytical studies confirmed that the obtained numerical trends were consistent with previous research findings. The results indicate that steel jacketing provides superior seismic performance and ductility for vertical extension projects, whereas concrete jacketing remains an economical and structurally compatible strengthening technique.