A Study of Efficient Outrigger Structural Systems for Tall Buildings

Proceedings of International Conference on Advances in Architecture and Civil Engineering (AARCV 2012), 21 st 23 rd June 2012 282 A Study of Efficient Outrigger Structural Systems for Tall Buildings Kiran Kamath, N. Divya and Asha U Rao Abstract--- This paper presents a study on efficient outrigger structural system for tall buildings. An investigation has been performed to examine the behavior of various alternative 3D models using ETABS software for reinforced concrete structure with central core wall with outrigger and without outrigger by varying the relative flexural rigidity (EI o /EI), (where E is the modulus of elasticity of the core and the outrigger concrete, I o is moment of inertia of outrigger and I is moment of inertia of core) from 0.25 to 2.0 with step of 0.25. Also the position of outrigger has been varied along the height of the building by considering a parameter relative height of outrigger, H s /H (where H s position of outrigger from base and H is the total height of the building), from 0.975 to 0.4. The parameters discussed in this paper include lateral deflection, peak acceleration; inter-storey drifts for static and dynamic analysis for a 3-dimensional model for various values of relative rigidity and relative height. From the analysis of the results obtained it has been found that performance of the outrigger is most efficient for relative height of the outrigger equal to 0.5. T I. INTRODUCTION HE major factor that affects the design of tall structures is its sensitivity to the lateral load. Several criteria for the design of tall buildings to be satisfied are lateral drift at top. The acceptable drift limit(top deflection in tall building) for wind load analysis (according to the IS-875-part3 (1987)) is 1/500 of the building height. The acceleration is also an important factor which actually brings about the feel of drift to human notice. Action of fluctuating winds can cause oscillatory movements and induce a wide range of response in building occupants, mild discomfort to acute nausea, buildings become undesirable. No universal accepted international standards for comfort criteria are available. However it is generally agreed that acceleration is the predominant parameter in determining human response to vibrations. Generally accepted criteria for acceleration are 20mg or 0.196m/s2 for commercial buildings and 15mg or 0.147m/s2 for residential buildings. The use of core-wall system has been Kiran Kamath, Professor, Department of Civil Engineering, Manipal Institute of Technology, Manipal-576105, India. E-mail: kiran.kamath@manipal.edu N. Divya, P.G Student in Structural Engineering, Department of Civil Engineering, Manipal Institute of Technology, Manipal-576105, India. E- mail: divinagraj2@gmail.com Asha U. Rao, Associate Professor, Department of Civil Engineering, Manipal Institute of Technology, Manipal-576105, India. E-mail: asha.prabhu@manipal.edu a very effective and efficient structural system used in reducing the drift due to lateral load (wind and earthquake loads). But as and when the height of the building increases, the core does not have the adequate stiffness to keep the wind drift down to acceptable limits. For such tall structures a structural system known as outriggers may be introduced. Outrigger is nothing but a deep, stiff beam which connects the central core to the exterior most columns which helps in keeping the columns in their position in turn reducing the sway. This system helps in reducing the movement of the core when compared to the system with freely standing core without outriggers. The restrain caused by the outrigger reduces the lateral drift at top. The stiffness is increased by 20 to 30 percent by introducing the outrigger structural system (Taranath (1988)).The resistance offered by a core system alone to the overturning is adequate, however drift increases approximately to the cube of the height of the building. This makes the core system inefficient as and when the height of the building increases. The outrigger which connects the two elements together provides additional stiffness to resist the overturning forces. The use of outriggers in high-rise buildings started about 5 decades ago. The first outrigger building (Palace Victoria Building) is in Montreal Canada, completed in 1962. When an outrigger-braced building deflects under wind or seismic load, the outrigger which connects to the core wall and the exterior columns, makes the whole system to act as a single unit in resisting the lateral loads. Earthquake ground motion can occur anywhere in the world and the risk associated with tall buildings especially under severe earthquake should be given particular attention since tall buildings often has thousands of occupants. The structural collapse of such buildings can lead to disasters of unacceptable proportions. II. LITERATURE REVIEW Iyengar (1995) documented the use of outrigger and belt trusses. He used outrigger and belt steel trusses in a two dimensional model of an 85 storey structure and presented that by using steel core there was about 75% reduction in lateral displacement with outrigger trusses. When concrete core was used, there was about 68 % reduction in lateral displacement with the same outrigger trusses, and exterior steel columns. Taranath (1998) demonstrated that the optimum location for a single outrigger is approximately in the middle height of the building for minimizing the top lateral displacement under wind load, however such location may or may not be available. Kian (2001) studied the use of outrigger and belt truss system for high-rise concrete building subjected to wind

Proceedings of International Conference on Advances in Architecture and Civil Engineering (AARCV 2012), 21 st 23 rd June 2012 283 or earthquake load. Eight 40 storey two dimensional models of outrigger and belt truss system are subjected to wind load, and five 60 storey three dimensional models are subjected to earthquake load, analyzed and compared to find the lateral displacement reduction related to the outrigger and belt system location. For the two dimensional 40 storey model, 65% maximum displacement reduction can be achieved by providing first outrigger at the top and second outrigger at the middle of the structure height. For the three dimensional 60 storey structural model subjected to the earthquake load, about 18 % reduction in maximum displacement can be achieved with optimum location of the outrigger truss placed at the top and the 33rd level. Zhang et al. (2006) presented the restraining moments of outriggers acting on the core wall and the equation of the horizontal top deflection based on a simplified outrigger model. One case study was carried out to analyze the horizontal top deflection and the change in the restraining moments caused by the variation of outrigger location. Gerasimidis et al. (2009) carried out the analysis to find the optimum location for the second outrigger by changing the position of second outrigger along the height of the building and placing it in all the possible locations, i.e. on every floor and analyzing. Herath et al. (2009) presented a study on behavior of outrigger beams in high rise building under earthquake loads. It has been shown from this study that the structure is optimized when the outrigger is placed between 0.44 0.48 times its height (from the bottom of the building). Fawzia et al. (2010) investigated the deflection control by effective utilization of belt truss and outrigger system on a 60-storey composite building subjected to wind loads. He performed a three dimensional Finite Element Analysis with one, two and three outrigger levels. The reductions in lateral deflection are 34%, 42% and 51% respectively as compared to a model without outrigger system. Kamath et al. (2012) presented a study on performance analysis of outrigger structural system for tall buildings and found that the optimum position of outrigger subjected to wind load is about mid height of the structure. In the present work a study on the effect of relative stiffness of outrigger with respect to core wall and effective position of outrigger subjected to Seismic load corresponding to time history analysis of earthquake of California region scaled down to region considered (Bangalore region) has been carried out. The 2-dimensional analysis is carried out using application software ETABS. III. MODELS CONSIDERED FOR ANALYSIS In the present study a three-dimensional 40 storey building with 7mX8m central shear wall is considered (Figure.1a & Figure.1b). The typical floor height is 3.5m giving a total height of 140m. The beams, columns, shear walls and outriggers are assumed as concrete structure. Column and beam sizes considered in the analysis are 0.75mX0.75m and 0.23mX0.45m respectively. A total of 6 different arrangements of outriggers by varying Hs/H ratio from 0.975 to 0.4 having relative stiffness (EI O/EI) between 0.25 and 2 has been modeled and analyzed (Table 1) Figure 1a: Elevation Figure 1b: Plan Table 1: Description of the Models Used for the Analysis IV. ANALYSIS OF THE BUILDING Lateral wind load calculation is done manually according to IS-875-part3 (1987) and the loads are applied as point loads at different height of the building. The acceleration time histories used for the lateral analysis are taken from records of past historical earthquakes occurred in the California region. The first two accelerograms, LA03 (El Centro Array 5, James Road)( LA stands for Los Angeles) and LA06 (El Centro Array 6) are taken from the1940 El Centro earthquake with a

Proceedings of International Conference on Advances in Architecture and Civil Engineering (AARCV 2012), 21 st 23 rd June 2012 284 peak ground acceleration (PGA) of 0.386g and 0.23g respectively, the third accelerogram LA14 (Northridge,LA County Fire Station) is from the 1994 Northridge Earthquake with a PGA of 0.64g. The ground motions are part of historical recordings from M6 - M7.3 range earthquakes which were scaled to match the 10% probability of exceedance in 50 years uniform hazard spectrum for Los Angeles, California. The dynamic analysis of structure has been carried out by response spectrum method using ETABS. The core wall & outrigger has been modeled as shell element with meshing, beams & columns are modeled as beam elements. The spacing between the frames has been considered as 8m (Figure.1b). V. RESULTS AND DISCUSSIONS The results obtained from analysis are compared and discussed as follows. Table 2: Lateral Displacements due to Wind Loads 1 0.309 0.309 0.309 0.309 2 0.1867 0.1339 0.1066 0.1159 3 0.1877 0.1355 0.109 0.1185 4 0.1885 0.1368 0.1108 0.1205 5 0.1891 0.1378 0.1123 0.1221 6 0.1896 0.1386 0.1135 0.1234 7 0.1901 0.1394 0.1146 0.1246 8 0.1904 0.14 0.1155 0.1257 9 0.1908 0.1704 0.1164 0.1266 Table 3: Lateral Displacement due to Static Earth Quake Load 1 0.4945 0.4945 0.4945 0.4945 2 0.3281 0.2087 0.1785 0.2053 3 0.3301 0.2116 0.1826 0.2096 4 0.3316 0.2138 0.1856 0.2129 5 0.3329 0.2156 0.188 0.2155 6 0.334 0.2171 0.1901 0.2177 7 0.3349 0.2184 0.1919 0.2197 8 0.3357 0.2195 0.1935 0.2214 9 0.3365 0.2206 0.1949 0.2229 Figure 3: Variation of Lateral Displacements for Static Earth Quake Loads Table 4: Lateral Displacement for LA03 Earthquake 1 0.9274 0.9274 0.9274 0.9274 2 0.6676 0.4404 0.352 0.3736 3 0.6713 0.4459 0.3596 0.3817 4 0.6741 0.45 0.3653 0.3878 5 0.6763 0.4534 0.3699 0.3926 6 0.6783 0.4562 0.3737 0.3967 7 0.68 0.4587 0.3772 0.4004 8 0.6815 0.4609 0.3802 0.4036 9 0.6828 0.4628 0.3828 0.4064 Figure 2: Variation of Lateral Displacement for Wind Loads All title and author details must be in single-column format and must be centered. From Table.2 it is observed that the lateral displacement is reduced by 37% by adding the outrigger at the top and it is reduced up to 61% by adding outrigger at mid height. From Table.3 it is observed that the lateral displacement due to earthquake load is reduced by 34% when the outrigger is placed at the top and it is reduced by 64% by placing the outigger Figure 4: Variation of Lateral Displacement With different locations of outrigger for LA06

Proceedings of International Conference on Advances in Architecture and Civil Engineering (AARCV 2012), 21 st 23 rd June 2012 285 From Table.4 it is observed that the lateral displacement is reduced by 30% when the outrigger is placed at the top of the From Table.5 it is observed that the lateral displacement is reduced by 29% when the outrigger is placed at the top of the Table 5: Lateral Displacement for LA06 Earthquake 1 0.671 0.671 0.671 0.671 2 0.4831 0.3186 0.2543 0.2699 3 0.4858 0.3225 0.2598 0.2757 4 0.4878 0.3255 0.2639 0.2801 5 0.4894 0.3279 0.2672 0.2836 6 0.4908 0.33 0.27 0.2866 7 0.4921 0.3318 0.2725 0.2892 8 0.4932 0.3334 0.2746 0.2916 9 0.4941 0.3348 0.2766 0.2936 03. Figure.6: Variation of Lateral Displacement Table 7: Inter Storey Drift for LA03 Earthquake inter storey drifts (xe-3) 1 78 78 78 78 2 14.29 18 34.57 42.86 3 14.29 18 34.57 42.86 4 14.29 18.29 35.14 43.43 5 14 18.86 35.43 43.71 6 14.29 19.14 36.43 43.71 7 14.29 19.43 36 44.29 8 14.29 19.71 36.29 44.57 9 14 20 36.29 44.57 Figure 5: Variation of Lateral Displacement With Different Locations of Outrigger for LA06 Table 6: Lateral Displacement for LA14 Earthquake 1 0.5301 0.5301 0.5301 0.5301 2 0.3811 0.2539 0.205 0.2181 3 0.3831 0.2571 0.2093 0.2226 4 0.3847 0.2595 0.2125 0.226 5 0.386 0.2614 0.2151 0.2287 6 0.3871 0.263 0.2173 0.231 7 0.3881 0.2645 0.2192 0.233 8 0.3889 0.2657 0.2209 0.2348 9 0.3897 0.2668 0.2224 0.2364 With different locations of outrigger for LA06 From Table.6 it is observed that the lateral displacement is reduced by 29% when the outrigger is placed at the top of the From Figure.7 it is noticed that the inter storey drift is minimized at the position where outrigger is present for LA Figure 7: Drift Index for LA03 Similar graphs may be obtained for LA06 and LA14. The following graph (Figure.8) shows the typical variation of top displacement for different location of outrigger.

Proceedings of International Conference on Advances in Architecture and Civil Engineering (AARCV 2012), 21 st 23 rd June 2012 286 From Fig.8 it is observed that the top displacement is minimum with Hs/H=0.5 for the model considered. Table 8: Peak Acceleration Due to Wind Loads peak acceleration m/s 2 1 0.261 0.261 0.261 0.261 2 0.186 0.205 0.3 0.27 3 0.186 0.206 0.253 0.27 4 0.186 0.2062 0.2534 0.2701 5 0.186 0.207 0.2538 0.269 6 0.1862 0.2067 0.2534 0.2701 7 0.186 0.2074 0.2538 0.2689 8 0.186 0.207 0.253 0.269 9 0.186 0.251 0.2531 0.2695 From Table.8 it is observed that the peak acceleration is reduced up to 30% by adding the outrigger at top (Hs/H=0.975). REFERENCES [1] Kian.P.S., Siahaaan.F.T (2001), The Use of Outrigger and Belt Truss System For High-Rise Concrete Buildings. Dimensi teknik sipil,vol.3, No.1, Maret 2001, 36-41 ISSN 1410-9530 [2] Taranath.B (1998), Structural Analysis & Design of Tall Buildings. New York, McGraw Hill. [3] Iyengar (1995), Hal, Composite and Steel High Rise Systems. Habitat and the High-Rise, Tradition & Innovation. In Proceedings of the Fifth World Congress. 14-19 May 1995. Amsterdam, the Netherlands, Bethlehem, Pa: Council on Tall Building and Urban Habitat, Lehigh University. [4] Gerasimidis et al. (2009), Optimum Outrigger Locations of High-rise Steel Buildings for Wind Loading, Institute of Metal Structures, Department of Civil Engineering, Aristotle University of Thessaloniki. [5] Herath et al. (2009), Behavior of Outrigger Beams in High Rise Buildings Under Earthquake Loads, The University of Melbourne, Parville, Victoria 3010, Australian Earthquake Engineering Society, 2009 Conference. [6] Zhang et al. (2006), :Safety Analysis of Optimal Outriggers Location in High-rise Building Structures (Department of Civil Engineering and Mechanics, Huazhong University of Science and Technology, Wuhan 430074, China) [7] Fawzia.S. and Fatima.T (2010), Deflection Control in Composite Building by Using Belt Truss and Outriggers Systems, World Academy of Science, Engineering and Technology 72 [8] Kamath et al. (2012), Performance Analysis of Outrigger Structural Systems for Tall Buildings, proceedings of National conference on Advanced Trends in Civil Engineering, Karpagam University. [9] IS 1893 (part 1):2002 Provision on Seismic Design of Buildings, Bureau of Indian Standards, New Delhi. [10] IS 875 (part 3):1987 Code of Practice for Design Loads (Other than Earthquake) for Buildings and Structures, Bureau of Indian Standards, New Delhi. VI. CONCLUSIONS From structural analysis results, followed by discussion the following conclusion are drawn; The optimum relative position of the outrigger considering the criteria for reduction in roof displacement for wind and seismic loading is 0.5 with relative rigidity of 0.25, irrespective of the type of analysis (static or dynamic) carried out. However, it has been noticed that if the peak acceleration needs to be controlled for human comfort levels, the optimum position is at top where there is substantial reduction in peak acceleration of about 30% when compared structure without outriggers. The response of the structure does not show any particular trend with peak acceleration component in seismic time history, but the lateral displacement of roof is least for outrigger structure with relative height of 0.5 for all earthquake history VII. ACKNOWLEDGMENTS The authors thanks The Director, and H.O.D, Civil engineering, Manipal Institute of Technology, Manipal for providing necessary facilities required for the present study.