Optimum Position of Outrigger System for Tall Vertical Irregularity Structures

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1 IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE) e-issn: ,p-ISSN: X, Volume 12, Issue 2 Ver. II (Mar - Apr. 2015), PP Optimum Position of Outrigger System for Tall Vertical Irregularity Structures Shivacharan K 1, Chandrakala S 2, Karthik N M 3, 1 (P G Student Department of civil engineering, SJCIT/VTU, India) 2 (Assistant Professor Department of civil engineering, SJCIT/VTU, India) 3 (Assistant Professor Department of civil engineering, CMRIT/VTU, India) Abstract : Analysis of the structure is carried out to study the behavior of outrigger and its efficiency for its optimum at the first and second location. The three dimensional models of outrigger and belt truss system are subjected to wind and earthquake load, analyzed and compared to find the lateral displacement reduction related to the outrigger and belt truss system location. The present work is to study the use of outrigger and belt truss placed at different location subjected to wind and earthquake load. The design of wind load was calculated based on IS 875 (Part 3) and the earthquake load obtained using IS 1893 (Part-1): The location of outrigger and belt truss for reducing lateral displacement and building drift. To evaluate the performance of vertical irregularities of outrigger structure, a linear static analysis has been conducted by using ETABS To achieve this objective, the vertical irregularities structures with 30 stories 7X7bay from 1 st to 10 th floor 5x5bay from 11 th to 20 th floor, 3x3bay from 21 st to 30 th floor with outriggers and belt truss at different stories were analyzed. Compression study of the displacement and drift of the structure is studied. Maximum Storey Shear and Axial Load of different columns are observed for the structure with first and second location of the outrigger. Keywords - vertical irregularities, outrigger, linear static analysis Wind and earthquake load. I. Introduction Mankind is always fascinated for height. At present the symbol of economic power and leadership is the skyscraper. There has been a demonstrated competitiveness that exists in mankind to proclaim to have the tallest building in the world. The design of skyscrapers is usually governed by the lateral loads imposed on the structure. As buildings have gotten taller and narrower, the structural engineer has been increasingly challenged to meet the imposed drift requirements while minimizing the architectural impact of the structure. In response to this challenge, the profession has proposed a multitude of lateral schemes that are now expressed in tall buildings across the globe. The design of tall and slender structures is controlled by three governing factors, strength (material capacity), stiffness (drift) and serviceability (motion perception and accelerations), produced by the action of lateral loading, such as wind. The overall geometry of a building often dictates which factor governs the overall design. As a building becomes taller and more slender, drift considerations become more significant. Vertical Geometric Irregularity Vertical geometric irregularity shall be considered to exist where the horizontal dimension of the lateral force resisting system in any storey is more than 150 percent of that in its adjacent storey (Table 5, Page 18, IS Part-1). Fig.1 represents the vertical (elevation) irregularities with abrupt change in geometry. Fig1: Elevation irregularities with abrupt change in geometry DOI: / Page

2 Optimum Position of Outrigger System for Tall Vertical Irregularity Structures Outriggers The lateral bracing system consisting of core with outriggers is one of the most efficient systems used for high rise construction to resist lateral forces caused by wind and earthquakes. Outrigger beams connected to the core and external columns are relatively more complicated and it is understood that the performance of such coupled wall systems depends primarily on adequate stiffness and strength of the outrigger beams. Therefore overall rigidity is imperative in tall buildings in order to control lateral deflection and inter-storey drift. The integration of the outrigger to the concrete core can be further optimized by guaranteeing concentrated core forces into the outriggers. Fig2: Core with Outrigger. The optimum location and number of the outriggers still remains a crucial but also pending question. This project implements a basic design optimization technique of tall steel structures for lateral loads, mainly wind, into trying to find the optimum locations and number of outriggers for a specific high-rise building. The structure is analyzed for an Earthquake and wind loading. Then the structure is analyzed with all the possible outrigger locations monitoring important factors, such as the drift of the building or the moments on the core. II. Methodolgy The Vertical Irregularities building was designed to the gravitational load in a three dimensional model to the structural elements and was analyzed by Etabs software. The concrete core is assigned in the middle of the building, which is surrounded by columns. As of the location of the outriggers, one of them was fixed at the top of the building, incorporating a basic design decision in agreement with the concept of this structural scheme. The goal of this Project is to find the optimum location for the outrigger and that is why the Structure was analyzed moving the second outrigger along the height of the building and placing it in all the possible locations. It must be noted here that outrigger trusses are normally one floor high and coincide with mechanical levels, so that they do not interfere with usable space elsewhere. The basic structural idea behind this scheme is the following. As the lateral loads deform the concrete core into a cantilever, the stiffness of the outrigger levels forces the participation of the exterior columns with the development of a couple of axial forces. The deformation of the core and in extent the drift at the top of the building is restrained by the system of columns and outriggers. The columns act as a couple of axial forces while the outriggers act as very stiff trusses transferring the bending moment of the core into the system of the columns. This is obvious from the comparison of the free cantilever deflection line and the real structure deflection line. The degree of stiffening and the level of drift control depend on the number of the outrigger trusses. For medium rise and high rise buildings, mechanical floors are generally provided once in 25 stories and provide opportunities for these trusses at those intervals. Although the rule of thumb for the building of is to place the second outrigger at the mid-height of the building, there is no clear indication why this rule applies or where exactly the outrigger should be designed. Details Of The Model: Table 1: Model Dimensions STOREY DESCRIPTION X 7 Bay Spacing 5.5m X 5 Bay Spacing 5.5m x 3 Bay Spacing 5.5m DOI: / Page

3 Optimum Position of Outrigger System for Tall Vertical Irregularity Structures Section Properites Beam Details: Breadth 230 mm Depth 450 mm Slab Details: Thickness 150 mm Live load - 3 kn/m 2 Floor Finish 1 kn/m 2 Shear Wall: Thickness 300 mm Column Details: 1 st and 2 nd storey 800 x 800mm 3 rd to 5 th storey 700 x 700mm 6 th to 10 th storey 600 x 600mm 11 th to15 th storey 500 x 500mm 16 th to 30 th storey 400 x 400mm Load Combinations: (DL X LL) X 1.5 (DL + LL + FL) X 1.5 Outrigger Property: 300 x 300mm with Belt truss Concrete Grade: M 40. Steel: Fe 500. Wind load: (IS: 875(Part 3) -1987) Bhuj Design Speed 50 m/s Terrain Category 3 Class B Diaphragms Rigid Earth Quake Load: (1893(Part 1): 2002) Bhuj Zone V 0.36 Importance factor 1 Type of soil Medium Soil Reduction Factor 5 Mass Source Definition Dead Load - 1 Floor Finish- 1 Live Load Fig 3: Plan of the Structure. DOI: / Page

4 Optimum Position of Outrigger System for Tall Vertical Irregularity Structures III. Results And Discussion Case 1: Bare Frame Analysis and Design Case 2: Analysis of Bare Frame with outrigger system for the first optimum location. Outrigger with Belt truss at 0.25 Position Outrigger with Belt truss at 0.33 Position Outrigger with Belt truss at 0.5 Position Outrigger with Belt truss at 0.67 Position Outrigger with Belt truss at 0.75 Position Outrigger with Belt truss at top Position Case 3: Analysis of Bare Frame with outrigger system for Second keeping first common at Outrigger with Belt truss at 0.25 Position Outrigger with Belt truss at 0.33 Position Outrigger with Belt truss at 0.5 Position Outrigger with Belt truss at 0.75 Position Outrigger with Belt truss at top Position DOI: / Page

5 Outrigger Sto ry Optimum Position of Outrigger System for Tall Vertical Irregularity Structures Table 2 Deflection for Case 1 and Case 2 Outrigger Position Bare frame Fig 8: Graph shown for table 2 Table 4: Story shear for Case 1 and case 2 Structure type Base Shear (kn) Bare frame DOI: / Page

6 Sto ry Optimum Position of Outrigger System for Tall Vertical Irregularity Structures Fig 9: Graph of table 4 Fig 10: Graph Shown for table5 Fig 11: Graph Shown for table Table 5: Deflection for Case 1,Case 2 and Case 3 Outrigger Position Bare frame First optimum DOI: / Page

7 Outrigger Sto ry Optimum Position of Outrigger System for Tall Vertical Irregularity Structures Table 3 Drift for Case 1 and case 2 Outrigger Position Bare frame Table 4: Story shear for Case 1 and case 2 Structure type Base Shear (kn) Bare frame Fig 9: Graph of table 4 DOI: / Page

8 Sto ry Sto ry Optimum Position of Outrigger System for Tall Vertical Irregularity Structures Fig 10: Graph Shown for table5 Table 5: Deflection for Case 1,Case 2 and Case 3 Outrigger Position First Bare frame optimum Table 6 : Drift for Case 1,Case 2 and Case 3 Outrigger Position Bare frame First optimum DOI: / Page

9 Outrigger Optimum Position of Outrigger System for Tall Vertical Irregularity Structures Table 7: Base Shear For Case 1, Case 2 and Case 3 STRUCTURE TYPE Base Shear (kn) Bare frame Fig 12: Graph of table 7 Table 8 Deflection of Outrigger Position optimum Deflection Bare DOI: / Page

10 Optimum Position of Outrigger System for Tall Vertical Irregularity Structures Fig 13: Graph of table 8 IV. Discussion And Conclusion The most significant basic parameter monitored throughout the whole analysis process was drift and deflection of the building. The following fig 7 and fig 8 shows the variation of drift and deflection: It is observed that 28.58% and 27% of the deflection and drift is controlled by providing one outrigger at 0.67 height compared to Bare frame. 37.7% and 36.11% of the Deflection and drift is controlled by providing outrigger with belt truss at 0.67 and 0.5 when compared with bare frame % and 11.5% of the deflection and drift is controlled by comparing first outrigger system and second of outrigger system of the building. The following conclusions are made from the present study 1- The use of outrigger and belt truss system in high-rise buildings increase the stiffness and makes the structural form efficient under lateral load. 2- The maximum drift at the top of structure when only core is employed is around mm and this is reduced by suitably selecting the lateral system. The placing of outrigger at 0.67 height is mm. 3- Using second outrigger with 0.67h gives the reduction of 11.5% and 12.78% for drift and deflection. The optimum location of second outrigger is middle height of the building. 4- It can be conclude that the optimum location of the outrigger is between 0.5 times its height. 5- For the second optimum of outrigger base shear is significantly high compared to first optimum and bare frame with shear wall.(fig 12) shear wall stress and axial load in the columns to the opposite side of the earthquake direction. References [1] P.M.B. Raj Kiran Nanduri, B.Suresh, MD. Ihtesham Hussain (2013) Optimum Position of Outrigger System for High-Rise Reinforced Concrete Buildings Under Wind And Earthquake Loadings [2] Mohd Irfan Moinuddin& Mohd Afroz Khan Conducted (2013) A Study for the Optimum Location of Outriggers for High-Rise Concrete Buildings. [3] Gerasimidis S., Efthymiou E. & Baniotopoulos C. C. (2009) Optimum outrigger locations of high-rise steel buildings for wind loading. [4] N. Herath, N. Haritos, T. Ngo & P. Mendis (2009) Behaviour of Outrigger Beams in High rise Buildings under Earthquake Loads. [5] Karthik.N.M, N.Jayaramappa (December 2014) optimum of Outrigger system for High Raised RC Buildings using etabs (push over analysis). [6] WIND LOAD (IS: 875(Part 3) -1987). [7] EARTHQUAKE LOADS (1893(Part 1): 2002). DOI: / Page