Seismic Response of RC, Steel and Plan-Hybrid Steel-Concrete Frames

Abstract

Steel–concrete hybrid buildings are usually studied with the two materials stacked along the height, while frames that combine reinforced concrete (RC) and steel side by side in plan have received little attention. This paper compares the linear seismic response of six five-storey frames on an 18 m × 18 m plan: an all-RC frame, an all-steel frame and four plan-hybrid frames in which one to four of the five column lines are steel (steel grid ratio 0.2–0.8). Each layout is modelled with storey heights of 3 m and 5 m, designed to the Indian codes for seismic zone III without bracing or shear walls, and analysed in ETABS by the equivalent static method in both principal directions. Base shear, storey shear, roof displacement, inter-storey drift and secant lateral stiffness are compared. Replacing RC column lines with steel lowers the design base shear by up to 59%, because the frame becomes lighter, but it also lowers the lateral stiffness by up to 73%, so the roof displacement rises by up to 56%. The hybrid frames generally respond between the two single-material frames, and the frame with one steel line deflects slightly less than the RC frame while attracting 17% less base shear. The minor-axis orientation of the steel columns makes the hybrids 7–17% more flexible in the Y-direction. Raising the storey height from 3 m to 5 m increases the roof displacement 1.8–1.9 times and cuts the base shear by about 40%. All peak drifts stay below 0.09%, well inside the 0.4% limit of IS 1893 (Part 1):2016.

Introduction

This study investigates the seismic behaviour of hybrid reinforced concrete (RC) and structural steel buildings in which RC and steel frames are placed side by side within the same floor plan. The main objective is to evaluate how the proportion of steel framing and storey height affect important earthquake-response parameters, including base shear, storey shear, lateral stiffness, roof displacement, and inter-storey drift.

The study examines 12 building models using ETABS software. Six different plan layouts are considered, ranging from fully reinforced concrete to fully structural steel, with four hybrid configurations in between. Each layout is analysed with storey heights of 3 m and 5 m using the equivalent static seismic analysis method specified in Indian Standard IS 1893:2016.

The results show that replacing RC column lines with steel column lines generally reduces the building's seismic weight and design base shear. However, this reduction also decreases lateral stiffness and increases roof displacement and inter-storey drift. The effect is more pronounced in the Y-direction because the steel columns resist bending about their weaker minor axis in that direction. The study also finds that a hybrid frame with a small proportion of steel can achieve lower seismic weight while maintaining comparable displacement performance to an entirely RC frame in the X-direction.

Increasing the storey height from 3 m to 5 m generally increases roof displacement and drift while reducing lateral stiffness and design base shear. Although the distribution pattern of storey shear remains similar across the different layouts, the magnitude of the seismic response varies with the steel grid ratio and loading direction.

Conclusion

Twelve unbraced five-storey RC, steel and plan-hybrid frames, with storey heights of 3 m and 5 m, were compared under the equivalent static design loads of IS 1893 (Part 1):2016. The main conclusions are as follows. 1) Replacing RC column lines with steel lowers the design base shear almost in proportion to the steel grid ratio, by 54–55% at a ratio of 0.8, because the frame becomes lighter. The all-steel frame attracts 3–4% more base shear than the 1RC_S4 frame in X. 2) The lateral stiffness falls faster than the base shear, by 65–73% for the all-steel frame. The roof displacement therefore rises with the steel grid ratio, by up to 44% in X and 56% in Y. 3) The frame with one steel line (4RC_S1) is the best compromise in this set. It attracts about 17% less base shear than the RC frame, deflects about 2% less in X and only 5–6% more in Y. 4) The drift peaks in the second storey above the plinth in every frame. Its largest value, 0.081%, is about one fifth of the 0.4% limit, so drift does not govern the design of these low-rise unbraced frames in zone III. 5) Raising the storey height from 3 m to 5 m multiplies the roof displacement by 1.8–1.9 and the peak drift ratio by 1.08–1.13. It lowers the base shear to 57–60% and the stiffness to about one third. The ranking of the six layouts is the same at both heights. 6) Every frame containing steel is more flexible in Y, where the steel columns bend about their minor axis. The Y/X displacement ratio of the hybrids reaches 1.17, above the 1.09–1.12 of the all-steel frame, which is consistent with torsion from the stiffness eccentricity of the mixed plan. Torsional irregularity should therefore be checked in the direction normal to the RC–steel boundary. The study is limited to linear static analysis of regular low-rise frames. Nonlinear static and time-history analyses, a direct measure of the torsional response, and the detailing of the RC–steel interface are needed before the trends can be generalised to taller or irregular plan-hybrid buildings.

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Copyright © 2026 Finhasali Navsariwala, N. K. Solanki. 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.