Hybrid Structural System for High Rise Buildings

Abstract

Advances in construction technology, materials, structural systems and analytical methods for analysis and design facilitated the growth of high rise buildings. Structural design of high rise buildings is governed by lateral loads due to wind or earthquake. Lateral load resistance is provided by interior structural system or exterior structural system. Usually shear wall core, braced frame and their combination with frames are interior system, where lateral load is resisted by centrally located structural elements. While framed tube, braced tube structural system resist lateral loads by structural elements provided on periphery of structure. It is very important that the selected structural system is such that the structural elements are utilized effectively while satisfying design requirements.

Recently diagrid structural system is adopted in tall buildings due to its structural efficiency and flexibility in architectural planning. Compared to closely spaced vertical Columns in framed tube, diagrid structure consists of inclined columns on the exterior face of building. Due to inclined columns lateral loads are resisted by axial action of the diagonals compared to bending of vertical columns in framed tubular structure. Diagrid structures generally do not require core because lateral shear can be carried by the diagonals on the periphery of building.

Outrigger structural system consists of a central core consisting of either of braced frames or shear walls as well as horizontal cantilever outrigger trusses or deep girders connecting the core to the periphery columns. Having central core there is more freedom around core so that view and proper natural light can be obtain. In case of lateral loading, the rotation of the central core will be restrained by the deep outrigger by producing tension in the windward columns and compression in the leeward columns.

The objective of the present study is to understand the behavior of the hybrid structural systems of high-rise building and to design structural systems. Hybrid structural system is combination of Diagrid and Outrigger & Belt system. Periphery of buildings has diagrids, while at core it has steel plate shear wall along with boundary elements, outrigger & belt truss at mid height. Diagrid and Outrigger system has advantages in their own ways. Hence by combining both the systems making it hybrid, results in better performance.

Hybrid structural system for G+60 storey building is modeled and analysed using ETABS. Analysis results in terms of time period, base shear, storey displacement and inter storey drift are compared for Diagrid structural system, Outrigger & belt structural system and hybrid structural system. Structural elements are designed considering critical load combinations. Further shear lag effect in hybrid structural system is also studied.

From four structural system considered for G+60 storey building, hybrid structural system proves better alternative. Parametric studies of hybrid structural system shows that top storey displacement and interstorey drift for varying steel plate shear wall thickness and diagrid diameter satisfies the permissible top storey displacement and inter storey drift, hence hybrid structural system can be further optimized for achieving economy. From the comparison on steel usage, diagrid structural system require minimum steel among all four structural system i.e. 36.10% less steel than Steel Plate Shear Wall (SPSW) structural system, while hybrid structural system require 33.15% less steel than SPSW structural system. This study shows existence of shear lag effect in hybrid structural system. Maximum shear lag ratio 1.37 is observed at base of building and after 1/4th height of building, negative shear lag effect is observed.