Analysis of High Rise Building with Outrigger Structural System

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1 e-issn Volume 2 Issue, May 16 pp Scientific Journal Impact Factor : Analysis of High Rise Building with Outrigger Structural System Sarfaraz I. Bhati¹, Prof. P. A. Dode², Prof. P. R. Barbude³ ¹Department of Civil Engineering, DMCE, Navi Mumbai, bhatisarfaraz@gmail.com ²Department of Civil Engineering, DMCE, Navi Mumbai, padode@rediffmail.com ³Department of Civil Engineering, DMCE, Navi Mumbai, prbarbude@yahoo.com Abstract- This research work is an attempt to study the effect of provision of concrete outriggers in high rise building. Static and dynamic behavior of a 42 storey RCC model was examined for earthquake and wind loadings using ETABS software. Parameters of earthquake and wind loading has been defined as per IS 1893 (Part-1):2 and IS 87 (Part-3):1987 respectively. Linear dynamic analysis has been carried out by response spectrum analysis. For the various models generated (one without outrigger and others with outriggers placed at different storey); comparative study has been carried out to observe the change in parameters such as lateral storey displacements, storey drifts and base shear. From the results, it was concluded that provision of outrigger is effective in reducing the displacements and drifts significantly, while base shear of the building showed not much change with the introduction of outriggers. Keywords- Outriggers, response spectrum analysis, lateral displacement, storey drift, ETABS I. INTRODUCTION Mankind had always been fascinated for height and throughout our history; we have constantly sought to metaphorically reach for the stars. From the ancient pyramids to today s modern high rise structures, a civilizations power and wealth has been repeatedly expressed through spectacular and monumental structures. There has been a demonstrated competitiveness that exists in mankind to proclaim to have the tallest building in the world. Today, high rise tall structures are considered the symbol of economic power and leadership. As the buildings have gotten taller and narrower, the structural engineers have 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. For buildings taller than a certain height, moment resisting frame structures, shear wall structures, braced frame structures, tubular structures etc. may not provide adequate stiffness to resist lateral wind and earthquake loads. In this case the lateral stiffness can be increased by tying the exterior frames and shear core together by outrigger trusses or girders. In recent decades, outrigger structural systems have been widely utilized in tall buildings in order to decrease structure s deformation and increase its resistance in lateral loads. II. OUTRIGGER STRUCTURAL SYSTEM Outriggers are deep and rigid horizontal beams designed to enhance building overturning stiffness and strength by connecting the core shear wall or core braced frame to the distant peripheral column. The basic idea is to make the whole system to act as a single unit in resisting the lateral load. The core may be centrally located with outriggers extending on both sides or the core may be located on one side of the building with outriggers extending to the building column on the other side. Outriggers increase the effective height of the structure. When the outrigger braced structures are subjected to lateral loads, the exterior column and the outrigger battle the rotation of the central core and thus considerably reduce the lateral deflection and base moments, which would have arisen in free core All rights Reserved 421

2 Outriggers can be made of steel trusses or concrete beams or composite construction. Outrigger system can be effectively used for 1 stories height and possibly more. It should be noted that while the outrigger system is very effective in increasing the structures flexural stiffness, it doesn t increase its resistance to shear, which has to be carried mainly by the core. III. Figure 1. Outrigger structural system MODEL SPECIFICATION A 42 storey RCC model has been considered for analysis. The building dimensions are such that the building is intentionally kept slender, which is a requirement for the study. The building plan is symmetrical along both X and Y axis, so as to facilitate the ease in the comparative study of seismic parameters. ETABS v has been used for analysis purpose. 3.1 Geometry of the model Details related to geometry and dimensioning of the structure is discussed here. Table 1. Geometry of the model Model Geometry 1. Number of bays in X-direction :7. Typical storey height :4 m 2. Number of bays in Y-direction : 6. Bottom storey height : m 3. Largest dimension of building :26 m 7. Total height of bldg. :169 m 4. Least dimension of building :17 m 8. Aspect ratio H/B All rights Reserved 422

3 Figure 2. Grid plan of the building Element Dimensioning Table 2. Element Details Concrete Grade Remarks Slabs 1 mm thick M Two-way Slab Central shear wall core Beams Columns Mega-Columns (Modeled as Shear Walls) mm thick M4 Two C-shaped lift core (4 m X 3 m each) a) 2 mm X 6 mm M Replicated on all floors b) mm X 6 mm M Replicated on all floors a) b) 4 mm X mm M Base to th Floor mm X 1 mm M 11 th Floor to th Floor mm X mm M 21 st Floor to th Floor mm X 9 mm M 31 st Floor to Terrace mm X mm M Base to th Floor mm X mm M 11 th Floor to th Floor 4 mm X 4 mm M 21 st Floor to th Floor mm X mm M 31 st Floor to Terrace 37 mm X 19 mm M4 Base to th Floor 37 mm X 18 mm M4 11 th Floor to th Floor mm X 18 mm M4 21 st Floor to th Floor mm X 17 mm M4 31 st Floor to Terrace It should be noted that mega-columns on which outriggers are to be connected from central core have been modeled as shear walls; as the size of mega-columns was larger hence they All rights Reserved 423

4 been dimensioned in a way that their width does not bring in obstruction in the occupiable space in adjacent rooms. The depth of mega-columns is greater than 4 times its width, hence modeled as shear walls. 3.2 Static and dynamic loading Details related to static and dynamic loading are given below. Various parameters related to seismic and wind load cases are mentioned below, as they have been given as input in ETABS v Table 3. Static load cases Static Load Cases 1. Dead Load : 2 kn/m² 2. Live Load : 3 kn/m² 3. Earthquake in X direction : Auto generated as per IS 1893 (Part 1) Earthquake in Y direction : Auto generated as per IS 1893 (Part 1) - 2. Wind load in X direction : Auto generated as per IS 87 (Part 3) Wind load in Y direction : Auto generated as per IS 87 (Part 3) As per the provision of IS 1893 (Part-1):2, while defining mass source, mass multiplier for live load has been kept as. ; as only % of live load is to be considered for calculation of seismic weight for live load class upto 3 kn/m². Table 4. Dynamic load cases Dynamic Load Cases 1. Response Spectra in X- dir. : Auto generated as per IS 1893 (Part 1) Response Spectra in Y - dir. : Auto generated as per IS 1893 (Part 1) - 2 Table. Wind loading parameters Parameters of wind loading as per IS 87 (Part-3) : Structure class : C 2. Terrain Category : 2 3. Basic wind speed, V b : 44 m/s 4. Risk coefficient (k1 factor) : 1. Topography coefficient (k3 factor) : 1 6. Design wind speed (V z = V b k 1 k 2 k 3 ) : 3.24 All rights Reserved 424

5 Table 6. Earthquake loading parameters Parameters of seismic loading as per IS 1893 (Part-1) : 2 1. Seismic zone : III (Mumbai) 2. Seismic zone factor, Z : Importance factor, I : 1 4. Response reduction factor, R :. Tx =.9*(H/ D) : secs 6. Ty =.9*(H/ D) : secs 7. Soil Type : II (Medium) IV. RESULTS AND DISCUSSIONS The results studied for the 42 storey structure are discussed below. Response spectrum analysis has been carried out. The significant parameters monitored throughout the study were lateral storey displacement, inter-storey drift of the building and base shear. 4.1 Results of the bare frame without any outriggers Load Case Table 7. Maximum lateral displacements for bare frame without outriggers Maximum top lateral displacement Direction & position of displacement Earthquake in X-direction 9.3 mm. Along X-direction at top Earthquake in Y-direction mm. Along Y-direction at top Wind in X-direction mm. Along X-direction at top Wind in Y-direction mm. Along Y-direction at top Load Case Table 8. Maximum inter-storey drift for bare frame without outriggers Maximum interstorey drift Direction & position of drift Earthquake in X-direction.694 mm. Along X-direction at 21 st floor Earthquake in Y-direction 1.73 mm. Along Y-direction at 21 st floor Wind in X-direction 1.36 mm. Along X-direction at 1 th floor Wind in Y-direction mm. Along Y-direction at 16 th floor It should be noted that the building is more slender in Y-direction and hence displacements and drifts are considerably more for the load cases in Y-direction. Inter-storey drift is in control for all the cases as the actual drifts are much below the maximum allowable criteria of.4 times the storey height as given in IS 1893(Part-1):2. The top lateral displacement is in control in all the cases except Wind in Y-direction case ; as the displacement for the WY case is mm and maximum allowed is H/, which comes out to be 338 mm. Since the governing load case is wind in Y-direction for the particular structure under consideration. Hence for the All rights Reserved 4

6 Number of storeys the arrangement of outrigger system was decided in such a way that 8 number of outriggers were given in Y-direction and only 4 in X-direction. Figure 3. Plan showing outrigger layout Load Case Table 9. Base shear sharing between columns and shear walls Total base shear Base shear shared by columns Base shear shared by shear walls EQX 177 kn 113 kn 6.43 % 1644 kn 93.7 % EQY 1421 kn 173 kn % 1248 kn % 4.2 Result for models with single outrigger system Displacement result for the governing load case is given below. Result is given for a single outrigger system located at different heights along the structure namely.h,.33h,.h,.67h,.7h and at top. Each outrigger is mm thick and 1 storey deep (4 m.) and of M4 grade concrete. 6 Displacement in mm. for WY 1 - % Height 1 - % Height 1-7% Height 1 - % Height 1-33% Height 1-66% Height Figure 4. Displacement for wind y-direction load case for single outrigger system models As it can be seen that the displacement has come nowhere close to the limit (338 mm), we increase the number of outrigger systems for the All rights Reserved 426

7 Number of storeys 4.3 Result for models with double outrigger system Displacement result for the governing load case is given below. Result is given for double outrigger system located at different heights along the structure namely.h & 1H,.H & 1H,.7H & 1H,.33H & 1H,.66H & 1H,.H &.H,.H &.7H,.H &.7H,.33H &.66H. Each outrigger is mm thick and 1 storey deep (4 m.) and of M4 grade concrete Displacement in mm. for WY 2 - %+% 2 - %+% 2-7%+% 2-33%+% 2-66%+% 2 - %+% 2 - %+7% 2 - %+7% Figure. Displacement for wind y-direction load case for double outrigger system models As again it can be seen that the displacement has come nowhere close to the limit (338 mm), hence we further increase the number of outrigger systems for the structure. 4.3 Result for models with multiple outrigger system Displacement result for the governing load case is given below. Result is given for multiple outrigger system located at different heights along the structure. But as we increase the number of outriggers used we decrease the size of the outrigger. For this case each outrigger is mm thick and 2 m. deep and of M4 grade concrete. Table. Details of multiple outrigger system Number of outrigger storeys Positions of outrigger systems 3 H/3, 2H/3 & top ( 1/3 rd height interval) 4 H/4, H/2, 3H/4 & top (1/4 th height interval) 6 Floors: 7 th, 14 th, 21 st, 28 th, th & top 8 Floors: th, th, 1 th, th, th, th, th & top 11 Floors: th, 9 th, 13 th, 16 th, 19 th, 22 nd, th, 28 th, 32 nd, 36 th and All rights Reserved 427

8 Number of storeys Number of storeys Displacement in mm. for WY 3-33% Interval 4 - % Interval 6 Outriggers 8 Outriggers Figure 6. Displacement for wind y-direction load case for multiple outrigger system models As again it can be seen that the displacement has come in control to the limit (338 mm), for the model where 11 number of outriggers have been used. The maximum displacement for wind y- direction load case for the model with 11 number of outrigger is mm (< 338 mm). 4.3 Result for model with 11 number of outrigger floors across the structure height Given below are the results of the model with eleven number of outrigger system layout in comparison with the bare frame model without any outrigger system. The results include comparisons between top lateral displacement, inter-storey drift and base shear Displacement in mm. for WY Figure 7. Storey displacement in y-direction for wind load in All rights Reserved 428

9 Number of storeys Number of storeys Number of storeys Displacement in mm. for WX Figure 8. Storey displacement in x-direction for wind load in x-direction Displacement in mm. for EQY Figure 9. Storey displacement in y-direction for earthquake load in y-direction Displacement in mm. for EQX Figure. Storey displacement in x-direction for earthquake load in All rights Reserved 429

10 Number of storeys Number of storeys Number of storeys Drift in mm. for WY Figure 11. Inter-storey drift in y-direction for wind load in y-direction Drift in mm. for WX Figure 12. Inter-storey drift in x-direction for wind load in x-direction Drift in mm. for EQY Figure 13. Inter-storey drift in y-direction for earthquake load in All rights Reserved 4

11 Number of storeys Drift in mm. for EQX Figure 14. Inter-storey drift in x-direction for earthquake load in x-direction Load Case Direction of displacement Table 11. Reduction in maximum top lateral displacement Maximum top lateral storey displacement For model without outriggers For model with outriggers laid on 11 storeys Percentage reduction in displacement WY Y-direction mm mm 38. % WX X-direction mm 1.72 mm % EQY Y-direction mm mm.9 % EQX X-direction 9.3 mm 6.9 mm 33.4 % Load Case Direction of drift Table 12. Reduction in average inter storey drift Average inter storey displacement For model without outriggers For model with outriggers laid on 11 storeys Percentage reduction in displacement WY Y-direction 3.24 mm 2. mm % WX X-direction 1.13 mm.73 mm. % EQY Y-direction.88 mm.9 mm 32.9 % EQX X-direction.7 mm.39 mm 31.7 All rights Reserved 431

12 Base shear in Kn. Base shear in kn Without outriggers Base shear 18 With 11 number of outrigger storeys Figure 1. Base shear for EQX load case Without outriggers Base shear 1472 With 11 number of outrigger storeys Figure 16. Base shear for EQY load case Load Case EQX EQY Model Without outriggers With 11 number of outrigger storeys Without outriggers With 11 number of outrigger storeys Table 13. Base shear comparison Total Base shear Base shear shared by columns Base shear shared by shear walls 177 kn 113 kn 6.43 % 1644 kn 93.7 % 18 kn kn.76 % 171kN 94.24% 1421 kn 173 kn % 1248 kn % 1472 kn 1 kn.3% 1317 kn All rights Reserved 432

13 V. CONCLUSION The study assessed the behavior of outrigger braced structure under the influence of earthquake and wind loading from which the following conclusions can be drawn based upon the results shown above: The use of outriggers increases the stiffness of the building and makes it more efficient in resisting the lateral loads. The most critical lateral displacement for wind in y-direction loading was reduced by 38.% and brought under the limit to satisfy the criteria of Displacement < H/. Inter-storey drifts were also considerably reduced. Use of outriggers did not show any significant change in base shear, as the total force acting on the structure does not change with addition of outriggers. Small increment which is seen in base shear is due to the effect of increment in total seismic weight due to the addition of self weight of outriggers. Hence it can be concluded that outriggers are efficient in controlling the displacements, while they do not have noticeable effect on the lateral force acting on the structure. REFERENCES [1] P. N. Biradar and M. S. Bhandiwad, A Performance based study on Static and Dynamic behavior of Outrigger Structural System for Tall Buildings, International Research Journal of Engineering and Technology, Volume 2, Issue, 1. [2] Karthik and N. Jayaramappa, Optimum Position of Outrigger System for High Raised RC Buildings using Etabs 13.1., International Journal of Advanced Technology in Engineering and Sciences, Vol. 2, Issue No.12, 14. [3] Dr. K. S. Sathyanarayanan, A. Vijay, S. Balachandar, Feasibility Studies on the Use of Outrigger System for RC Core Frames, International Journal of Advance Innovations, Thoughts & Ideas, 12. [4] N. Herath, N, Haritos, T. Ngo and P. Mendis, Behavior of Outrigger Beams in High Rise Buildings under Earthquake Loads, 9, Proceedings of Australian Earthquake Engineering Society Conference. [] R. K. Nanduri, B. Suresh and I. Hussain, Optimum Position of Outrigger System for High-Rise Reinforced Concrete Buildings under Wind And Earthquake Loadings, 13, American Journal of Engineering and Research. [6] K. Shivcharan, S. Chandrakala and N. M. Karthik, 1, Optimum Position of Outrigger System for Tall Vertical Irregularity Structures, IOSR Journal of Mechanical and Civil Engineering, 1. [7] K. Kamath, N. Divya and A. U. Rao, Static and Dynamic Behavior of Outrigger Structural System, Bonfring International Journal of Industrial Engineering and Management Science, Volume 2, Issue 4, 12. [8] J. Ahmed and Y. Sreevalli, Application of Outrigger in Slender High Rise Buildings to Reduce Fundamental Time Period, Proceedings of 6 th IRF International Conference, All rights Reserved 433