SEISMIC PERFORMANCE OF FRAMED TUBE STRUCTURES

Abstract

Tall buildings are subjected to high lateral loads due to wind and earthquake forces, which would lead to high costs and the design would be economically unfeasible using traditional structural systems. Hence, in order to overcome these, many structural systems have been developed, one of them is the framed tube structural system. These are efficient in withstanding lateral loads as well as economical for tall buildings. But there are a few problems with this system namely shear lag effect, which is the non-linear distribution of axial stresses in the columns causing high stresses in corner columns and low stresses in middle columns there by reducing efficiency. In the present study, behaviour of RC framed system and its performance under gravity and lateral loads is studied using linear and nonlinear analysis. Also, the contribution of carious modes of vibration in seismic response of the building has been studied considering different number of modes. The shear lag variation of the framed tube building is studied under different types of loading and it is found that the variation of shear lag along the height of the structure depends on the type of lateral load, relative stiffness of the beams and columns of the structure, the number of storeys and the number of bays. It is found that increasing the stiffness of the interconnecting beams significantly reduces the shear lag., mimicking the ideal tube behaviour. The shear lag behaviour is studied during nonlinear static and dynamic response as well. From the corresponding results, it is concluded that the nonlinearity in the structure did not affect the shear lag to a significant extent. The seismic behaviour of the frame-tube structure is studied using nonlinear static and dynamic analyses. It is found that nonlinear static analysis, which considers only the fundamental mode of the structure does not depict the true behaviour of the structure. A different approach to obtain the actual behaviour of tall structure from nonlinear static analysis considering higher modes (Modal Pushover Analysis, MPA) as well is used and compared with the response obtained from nonlinear dynamic analysis. It is found that the MPA procedure gives quite accurate results for storey displacements and inter-storey drift ratios but is not suitable to obtain the plastic hinge rotations in the structure. The difference between the response of the structure obtained from the MPA and the nonlinear time history analysis is studied and it is concluded that in tall structures, the contribution of higher modes is important in assessing the seismic