OPTIMIZATION OF FRAMED TUBE SYSTEMS

Abstract

Despite a considerable degree of shear lag and ineffective participation of the web frame, the framed tube systems have got wide applications as an economical system in high rise construction till date. This merits stiffening of the system to reduce the shear resisted by the tube and enhance the effective tubular response, there by, decreasing the shear lag effect. In this dissertation, the relative efficiency of modified concrete frame tubes systems for 30, 40 and 60 stories square buildings and 40 storied rectangular with aspect ratio 1:1.4 and 1:2 buildings have been studied by augmenting the simple framed tube system by either providing braces in facade panels or making it monolithic with interior rigid frame. A linear static and dynamic analysis using a 3D program has been employed to capture the true structural responses of these systems. In order to determine the lateral forces each structural system would attract during wind or earthquake, a free vibration analysis is performed. The linear structural response of the modified system with respect to framed hollow tube has been investigated. The 30, 40 and 60 storied square framed tube-buildings with different alternative stiffening systems have been designed and their corresponding costs have been worked out and compared. Later a push over analysis has been performed to know the non linear response of the 40 storied buildings with aspect: ratio 1:1 for the framed hollow tube and the framed tube with rigid frames has been done.. and their results compared. It has been observed that the lateral stiffness of the braced framed tube system is higher, thus yielding a lower time period. Consequently the system attracts more earthquake loads. However, it is seen in all the cases of aspect ratio 1:1 that the wind load remained to be the governing design lateral load. However the design lateral load in the length direction of the braced framed with aspect ratio 1:1.4 and 1:2 and the framed hollow tube with aspect ratio 1:2 has been found to be the earth quake load. This is due to the high lateral stiffness of the system that attracting higher seismic force and lower surface area of the building thus attracting lower wind forces. The parametric study has depicted that the framed tube system stiffened by braces or interior rigid gravity frame helped to iii reduce the shear lag effects and the shear carried by the tube. It is observed that allowing the interior gravity frame to participate in the lateral load resistance by a rigid connection with the outer tube has significantly reduced shear lag effect. This is in spite of the fact that the design of the interior columns is still governed by gravity load resistance only. The braces, which have been provided in the central bays on each face of the tube along with belt trusses connecting it to the tube, appears to be efficient in combating lateral displacement and inter-story drift It has also shown some improvement in enhancing cantilever mode of response. The web frame of the braced framed tube has also participated more efficiently in resisting over turning moments. It is also observed from the study that the column eccentricity is very small in the case of framed tube with rigid frames and braced framed tube. However it is relatively higher in framed hollow tube. Furthermore, it can be seen that the axial force in the columns of framed hollow tube are much higher than that in the corresponding columns of framed tube with rigid frames or braced framed tube. The design and cost analysis of the chosen 30, 40 and 60 storied square buildings with different alternative systems: has -revealed. that the framed tube with rigid frame gives a substantial economy than the rest of the systems. The braced framed• tube has shown a moderate reduction of structural cost and consumption of materials. Thus, a significant economy can be achieved in the design of such buildings if framed tube with rigid frames or braced framed tube systems is adopted. The non linear response has depicted the high ductility and redundancy of the framed _tube with rigid frames which undergo a higher deformation before failure. A relatively sudden type of failure has been observed in the case of framed hollow tube. However both systems have been classified under immediate occupancy during MCE level of ground shake. It has also been observed that the systems could perform well up to some level beyond MCE.