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

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.