In the design process of tall buildings, there is usually a higher demand for structural performances of the system during seismic actions. In this case, adding to the strength and stiffness of the building is not considered helpful in many cases because it may add to the lateral acceleration of the system and causes damage to non-structural parts of the building including the façade assembly. Considering the brittle behavior of cladding materials in façade frames, the main expectation of designers is to prevent the possibility of any breakage in façade assembly and suppress its broken materials to fall over pedestrians during earthquake action. Recently, some new techniques have been proposed to enhance the ability of façade frame in dealing with lateral acceleration and inter story drift during earthquakes incidents. To mention a few, using multiple Mass Dampers and other kinds of energy dissipating devices in design of façade assembly are recommended in this case (e.g. Fu, T.S., & Zhang, R., 2016). In current study it is proposed to take into account the façade framework in the design process of tall buildings using Partial Mass Isolation technique (PMI) described elsewhere (Ziyaeifar, M., & Noguchi, H. 1998). In this new approach (Facade Isolation) the cladding assembly is considered as a structural subsystem isolated from the main structural system using a flexible isolation layer with large energy dissipation potential. The cladding assembly in this approach is expected to have less lateral drift and acceleration responses during earthquake motions. In the meantime, considering interaction between façade frame and the main structural system, a marginal improvement in seismic performances of the main structural system is also anticipated. In FI a higher weight for cladding assembly is considered beneficial for some buildings due to its role in reducing earthquake-induced responses for the main structural system. This feature can revive the idea of using heavy facade slabs or blocks for facade frame of tall buildings to enhance their aesthetic values and insulation properties without negative effects on their seismic performances.
A two-dimensional model, developed in current study, is used for investigation on façade isolation technique in a 15-story building equipped with three types of façade frame configurations. The results of numerical studies on these models are compared with those of the same structural system without isolation. Modal analyses on a FI building has shown the ability of this technique in improving damping characteristics of the system by increasing damping ratios for all modal shapes of the structure. A large number of time Integration studies have been carried out on the model subjected to 14 of earthquake records. In comparison with the same structural system without isolation, the results of analyses have shown remarkable improvement in behavior of façade frame and the main structural system during seismic actions. It was shown that using FI in its simplest layout caused 60% reduction in lateral drift of the façade frame and 40% reduction in its acceleration responses. On the other hand, the main structural system has also enjoyed from 20% reduction in its lateral drift. Such notable enhancement in seismic performances of buildings, defines a new role for façade frame to be incorporated in the design process of tall buildings as an integrated part of the structural system.