Case studies of the structural response of a number of framed-tube high-rise buildings

A number of existing tall buildings of 19 to 36 stories in north Tehran are redesigned keeping their architectural form intact while modifying their structural framing system to framed-tubes. Original structures have different load resisting systems including reinforced concrete frame with shear walls and steel frame with shear wall and cross-braces. Modified systems are tubular, with the external tube carrying the lateral forces and the core carrying the gravity loads. Tubular systems are analysed by commercial software and designed according to AISC and ACI standards for strength and serviceability. After the initial design phase, dynamic response of tubular systems to earthquake loads are calculated and compared to those of the original structures. Parameters compared include natural frequencies, story shears, overturning moments and lateral displacements. Buildings are also compared with respect to their weight, material usage and total cost. It is found that in steel systems, the framed-tube is a lot more flexible than its original counterpart resulting in larger lateral deformations. In the reinforced concrete system, however, the framed-tube plays stiffer while the stiffness disparity increases with an increase in the aspect ratio (here defined as the height squared divided by the plan area). In all cases, the structural efficiency of framed-tubes found to be better than the conventional systems.