A Study on Outrigger System in a Tall R.C Structure with Steel Bracing Abdul Karim Mulla PG Student Department of Civil Engineering Alpha College of Engineering Bengaluru, India. Srinivas B. N Assistant Professor Department of Civil Engineering Alpha College of Engineering Bengaluru, India. Abstract—Tall building development has been rapidly increasing worldwide introducing new challenges that need to meet through engineering judgment. As the height of the building increases the stiffness of the building reduces. Therefore to improve the performance of the building under seismic loading, outrigger system is proposed in the present study of work. In the present work, contains a comparative study on regular building with and without outrigger and irregular building with and without outrigger with centrally rigid shear wall and steel bracings as outrigger. The modeling of the structure is done using “ETABS” program. The analysis of the model is carried out by equivalent static method and response spectrum method. The stiffness and efficiency characteristics of the structure is measured in terms of lateral displacement, drift, base shear and fundamental natural period for different types of buildings to provide stiffness against static and dynamic loads. The parameters should be minimized to prevent damage to the buildings. Keywords—Tall building; regular and irregular building; lateral displacement; base shear; fundamental natural period I. INTRODUCTION The development of tall building has always fascinated mankind from the ancient times. From the past, tall structures have always seen as a symbolic example of power and development. The challenging task in the construction field is to assemble the tall building. The design of tall building is based on analysis of models with experience and fundamental mechanics. As the height of the building increases the risk of horizontal and vertical load forces also increases. The moment resisting frames and braced core at certain height becomes inefficient to provide stiffness against wind and seismic loads. The lateral deflections due this load should be prevented for both structural and nonstructural damage to achieve the building strength and also stiffness against lateral loads in the analysis and design of tall building. To increase stiffness action against wind and seismic load outriggers are provided by the shear core with exterior frames in tall buildings. The effective depth of the structure is increased, when the outriggers are placed. The work is carried out to know the behavior of outriggers in the tall buildings. The building is analyzed for 20 storey building mainly four types of buildings are used and they are regular building with and without outrigger and also vertically irregular building with and without outriggers are used. In the proposed building instead of core wall shear wall is provided with columns inserted in it. The outrigger is made up of steel bracing. The analysis is carried out on building with varying zones using ETABS software (Engineering three dimensional analysis of structure) and results are generated with various parameters. The equivalent static analysis method and dynamic analysis method is adopted for analysis. The results generated are tabulated and graphed. The values are compared with different type of buildings for the lateral stiffness by seismic forces. The behavior of different types of building is known with various zones and also the effect of lateral load on the building when outriggers are provided. The lateral stiffness of the outrigger building is shown in this work. The primary purpose of the structural system is to effectively transfer the gravity loads without causing damage to the buildings. The gravity loads are mainly dead load, live load and snow load which affect the tall buildings. Apart from these loads the building is also subjected to horizontal lateral loading caused by the action of wind and earthquake forces. These lateral loads leads to huge damage to tall building by producing high stresses by causing vibration or sway movement. Therefore it becomes important that the tall buildings should be provided by necessary strength by installing these structural systems. The tall structure is provided with shear walls which have columns; these columns are installed in the shear wall primarily to carry the gravity loads. The shear wall in the building will provide strength and lateral stiffness to the building in the direction of building orientation by reducing the lateral sway and damage to the structure. The outriggers with steel bracings are provided which increase the axial stiffness with the peripheral columns to resist the overturning moments. The system is very effective to resist the lateral loads. As the concept of outrigger and bracings are combined it decreases the bending moment in beams and shear forces in columns by increasing the column axial compression. The structure consists of central core comprises shear wall with horizontal girders or cantilever type trusses called outriggers made up of steel bracing, connecting to the outer columns of building. The shear wall is located centrally by extending outriggers on both sides of the columns. When the horizontal loads acts on the building the outriggers resist the rotation of the core wall or shear wall by causing the lateral deflection and minimizing the effect of the loads on the building. The effective depth of the building is increased when it acts as a vertical cantilever by inducing compression in the leeward columns and tension in windward columns. The outriggers International Journal of Engineering Research & Technology (IJERT) ISSN: 2278-0181 www.ijert.org IJERTV4IS070459 (This work is licensed under a Creative Commons Attribution 4.0 International License.) Vol. 4 Issue 07, July-2015 551