Study of Braced Tube Structural System for High-Rise Buildings

Abstract

Tall building developments have been rapidly increasing worldwide because of rapid growth of the urban population, high cost of land and the need to preserve important agricultural production. Lateral forces due to wind or seismic loading have to consider for tall buildings along with gravity forces. Very often the design of tall buildings is governed by lateral load resistance requirement in conjunction with gravity load. High wind pressures on the sides of tall buildings produce base shear and overturning moments. Two most important design requirements for any building structural de- sign are strength and stones, and for very tall buildings with a large height-to-width aspect ratio, stiffness constraint generally governs the design. The lateral resisting systems that are widely used are the following: rigid frames, braced frames, belt and outrigger truss systems and framed tube structures. A core or a system of cores which provides additional stiffness to the structure, is usually present in the structural system and used for utilities and the elevator shaft. A combination of the lateral resisting systems or a variation of the concept such as the braced tube system or the bundled tube system has resulted in high-rises. In the tubular system, closely spaced periphery columns make very stiff moment resisting frames that form a "tube" around the perimeter of the building. The efficiency of framed tube can further be improved by providing bracings on outer faces of buildings. It is known as braced tube system and buildings in which these systems are used are known as braced tube buildings. The case studies of these types of braced tube steel and concrete buildings are presented. The aim of study about braced tube structural system is to understand the behaviour of this system under gravity loading, lateral loading and combine gravity and lateral loading. The effectiveness of braced tube structural system for reduction in the ax- ial force variation(shear lag effect) under the effect of lateral loading in periphery columns are also studied. The 70-storey steel building with plan dimension 50 m x 50 m is taken for analysis and design. The static earthquake load and static and dynamic wind load are applied as lateral forces along with gravity loads. Total five building cases are considered for comparison of various parameters. One is unbraced building and other four are braced buildings with single diagonal braces, double diagonal or X braces, V braces and inverted V braces. Analysis and design of all five buildings are carried out using structural software ETABS. The slab and secondary beams are designed as composite sections. The main beams and braces are fabricated I-section and the column sections are hollow box section. Various sections are used for main beam and column sections for various storeys. The lateral load and storey shear distribution along the height is presented and various parameters like, storey displacement, storey drift, time period, base moment in columns and weight difference are compared between all five buildings. The axial force variation(shear lag effect) in flange and web frames are also presented. The analysis results show that compare to unbraced building, all braced buildings are very effective for resisting lateral loads. The braced tube buildings are very stiff compare to unbraced building. All parameters are reduce in case of single diagonal braced building compare to unbraced building and are further reduce in case of X braced, V braced and inverted V braced buildings. The check and design of beam and column sections are also presented for unbraced and X braced buildings as per IS 800-1984. The moment resistant connection design between beam and column is also presented.