International Journal of Research and Innovation (IJRI)
International Journal of Research and Innovation (IJRI) 1401-1402
RESPONSE OF LATERAL SYSTEM IN HIGH RISE BUILDING UNDER SEISMIC LOADS
Ahsan Mohammed Khan1, K. Mythili2, Shaik Subhani Shareef3 1 Research Scholar, Department Of Civil Engineering, Aurora's Scientific Technological & Research Academy, Hyderabad, India 2 Associate professor , Department Of Civil Engineering, Aurora's Scientific Technological & Research Academy, Hyderabad, India 3 professor , Department Of Civil Engineering, Aurora's Scientific Technological & Research Academy, Hyderabad, India
Abstract Tall building development has been rapidly increasing worldwide introducing new challenges that need to be met through engineering judgment. In modern tall buildings, lateral loads induced by wind or earthquake are often resisted by a system of coupled shear walls. But when the building increases in height, the stiffness of the structure becomes more important and introduction of outrigger beams between the shear walls and external columns is often used to provide sufficient lateral stiffness to the structure. In general, earthquake ground motion can occur anywhere in the world and the risk associated with tall buildings, especially under severe earthquakes, should be given particular attention, since tall buildings often accommodate thousands of occupants. However, there is an absence of scientific research or case studies dealing with optimum outrigger location under earthquake loads. This study aims to identify the optimum outrigger location in tall buildings under earthquake loads. A 50 storey building was investigated and three different peak ground acceleration to peak ground velocity ratios in each category of earthquake records were incorporated in this research study to provide a consistent level of approach. Response spectrum analysis was conducted and the behaviour of the building was determined considering response parameters such as lateral displacement and inter storey drift. It has been shown from this study that the structure is optimized when the outrigger is placed between 22-24 levels. Therefore it can be concluded that the optimum location of the structure is between 0.44 - 0.48 times its height (taken from the bottom of the building).The demands of taller structures are becoming imperative almost everywhere in the world in addition to the challenges of material and labor cost, project time line etc. The design of high-rise building is more often dictated by its serviceability rather than strength. Structural Engineers are always striving to overcome challenge of controlling lateral deflection and storey drifts as well as self-weight of structure imposed on foundation. One of the most effective techniques is the use of outrigger and belt truss system in composite structures that can astutely solve the above issues in High-rise constructions. *Corresponding Author: Ahsan Mohammed Khan, Research Scholar, Department Of Civil Engineering, Aurora's Scientific Technological & Research Academy, Hyderabad, India Published: October 25, 2014 Review Type: peer reviewed Volume: I, Issue : II
Citation: Ahsan Mohammed KhanScholar (2014) RESPONSE OF LATERAL SYSTEM IN HIGH RISE BUILDING UNDER SEISMIC LOADS
INTRODUCTION General High rise building is defined as a building 35 meters or more in height, which is divided at regular intervals in to occupiable levels. To be considered a high rise building a structure must be based on solid ground and fabricated along its full height through deliberate process. Cut off between high rise and low rise building is 35 meters. This height chosen based on an original 12 floor cut-off. There is no absolute definition of what constitutes a “tall building.” It is a building that exhibits some
element of “tallness” in one or more of the following categories: a. Height relative to context: It is not just about height, but about the context in which it exists. Thus whereas a 14-storey building may not be considered a tall building in a high-rise city such as Chicago or Hong Kong, in a provincial European city or a suburb this may be distinctly taller than the urban norm. b. Proportion: Again, a tall building is not just about height but also about proportion. There are numerous buildings which are not particularly high, but are slender enough to give the appearance of a tall building, especially against low urban backgrounds. Conversely, there are numerous big/large footprint buildings which are quite tall but their size/floor area rules them out as being classed as a tall building. c.Tall Building Technologies: Number of floors is a poor indicator of defining a tall building due to the changing floor to floor height between differing buildings and functions (e.g. office versus residential usage), a building of perhaps 14 or more stories (or over 50 meters/165 feet in height) could perhaps be used as a threshold for considering it a “tall building.” 8
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Structural Systems In the early structures at the beginning of the 20th century, structural members were assumed to carry primarily the gravity loads. Today, however, by the advances in structural design/systems and highstrength materials, building weight is reduced, and slenderness is increased, which necessitates taking into consideration mainly the lateral loads such as wind and earthquake. Understandably, especially for the tall buildings, as the slenderness, and so the flexibility increases, buildings suffer from the lateral loads resulting from wind and earthquake more and more. As a general rule, when other things being equal, the taller the building, the more necessary it is to identify the proper structural system for resisting the lateral loads. Currently, there are many structural systems that can be used for the lateral resistance of tall buildings. In this context, authors classify these systems based on the basic reaction mechanism/structural behaviour for resisting the lateral loads. Structural systems for tall buildings a. Rigid frame systems b. Braced frame and shear-walled frame systems c. Braced frame systems d. Shear-walled frame systems e. Outrigger systems f. Framed-tube systems g. Braced-tube systems h. Bundled-tube systems Introduction to Outriggers Mankind had always fascinated for height and throughout our history, we have constantly sought to metaphorically reach for the stars. Today, 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. This undying quest for height has laid out incredible opportunities for the building profession. From the early moment frames to today’s ultra-efficient mega braced structures, the structural engineering profession has come a long way. The recent development of structural analysis and design software couples with advances in the finite element method has allowed the creation of many structural and architecturally innovative forms Problems with Outriggers There are several problems associated with the use of outriggers, problems that limit the applicability concept in the real world.
members can be a major problem. b. Architectural and functional constraints may prevent placement of large outrigger columns where they could most conveniently be engaged by outrigger trusses extending out from the core. Benefits of an Outrigger System Outriggers are rigid horizontal structures designed to improve building overturning stiffness and strength by connecting the building core or spine to distant columns. Outriggers have been used in tall buildings acts as stiff arms engaging outer columns when a central core tries to tilt, its rotation at the outrigger level induces a tension-compression couple in the outer columns acting in opposition to the movement. The result is a type of restoring moment acting on the core at that level. The design depends on the relative stiffness. Need for Present Study Outriggers are a common method of stiffening and strengthening tall buildings. They work by connecting the inner core to the outer perimeter columns, much as a skier uses his arms and shoulders to hold onto ski-poles, providing extra stability. The method is very effective, and favored by many structural engineers. The method of analysis of the above mentioned system is based up on the assumptions that the outriggers are rigidly attached to the core: a. The core is rigidly attached to the foundation b. The sectional properties of the core, beams and columns are uniform throughout the height Tensional effects are not considered c. Material behavior is in linear elastic range d. The Outrigger Beams are flexurally rigid and induce only axial forces in the columns e. The lateral resistance is provided only by the bending resistance of the core and the tie down action of the exterior columns connected to the outrigger f. The rotation of the core due to the shear deformation is negligible. Aims and Objectives a. The objective of the present work is to study the use of outrigger and belt truss placed at different locations subjected to wind or earthquake load. b. The design of wind load was calculated based on IS 875 (Part-3) and the earthquake load obtained using IS 1893 (Part-1):2002. c. The location of outrigger and belt truss for reducing lateral displacement, building drift and core moments can be obtained.
a. The space occupied by the outrigger trusses places constraints on the use of the floors at which the outriggers are located. Even in the mechanical equipment floors, the presence of outrigger truss 9
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SHEAR WALLS
Load Calculations
General
Dead Load Calculations Sizes: Length of Building, L1 = 40.00 m Breadth of Building, L2 = 40.00 m Shear Core walls = 2. No’s Thick ness = 300 mm Size = 5000 x 5000 mm Column sizes for 15storey building = 800 x 800 mm Column sizes for 20storey building = 1000 x 1000 mm Column sizes for 25storey building = 1200 x 1200 mm Drop Size= 2500 x 2500 x 100 mm Outrigger Beams= 300 x 300 mm Flat Slab Thickness= 200 mm Self-Weight of Slab = 0.2 x 25 = 5.00 kN/m2 Floor Finish Loads = 1.00 kN/ m2
Shear wall is, concrete wall made to resist lateral forces acting on tall buildings. It is provided, when the centre of gravity of building area & loads acted on it differs by more than 30%. In order to bring the center of gravity in range of 30% concrete wall is provided. The Shear Wall sections are classified as six types. a. Box Section b. L – Section c. U - Section d. W – Section e. H - Section f. T – Section Structural Behaviour The multistorey building systems analyzed in this study are considered to be rigid frame structures. In such systems, all structural elements of the system are assumed to have infinitely rigid moment resistant connections at both ends. Another assumption about the structural system is the linear elastic structural system behavior, in which the deformations are proportional to the loads. It is widely used in structural analysis and leads to a very important simplification called superposition.
Live Load Calculations Live load for all floors = 2.00 kN/ m2 Wall Load Calculations Floor Height = 3.00 m Grade of concrete = M 40 Grade of steel = Fe 500
Analytical Models And Solution Procedures Introduction The model considered for this study is L shaped concrete building frame. The building represents a 25 storied office building. The Plan area of the Structure is 40 m x 40 m with columns spaced at 5.5m from center to center. The height of each storey is 3.00 m and all the floors are considered as Typical Floors. The location of the building is assumed to be at Hyderabad. An elevation and plan view of a typical structure is shown in fig. 5.5.5 and fig 5.5.6.
Plan of structures 1 & 2 at Basement level.: Plan of structures 3 & 4 at Basement level.
All wall piers are identical with a uniform wall thickness of 300mm over the entire height. The Bracing beams (outriggers) and all other beams are 300mm wide and 300mm deep, Grade 40 (Mix – M40) concrete is considered (Compressive strength 40 N/ mm²) throughout the height of the building. And number of stories considered for all the cases are 15, 20 and 25 stories and storey to storey height is 3.0 M. And the outer and inner columns sizes are considered as 800 x 800 mm and shear wall thickness is considered as 300 mm. 3D view of Structures 1 & 2 : 3D view of Structures 3 & 4
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Test Results And Discussions Introduction The analysis is carried out for study of rigid core and floor rigidity of 15, 20 & 25 storey L-shape Building for the following structures of different locations of outrigger beams and belt truss as shown in fig. Structure 1: Building frame outrigger beam locations as shown in fig Structure 2: Building frame same as structure1with belttruss. Structure 3: Building frame outrigger beam locations as shown in fig Structure 4: Building frame same as structure-3 with belttruss. Structure 5: Building frame without any outrigger beams as well as belttruss.
Graph Storey Level (No’s) vs. Shear Force (kN)
The analysis is carried with all the load combinations. But the wind load is governing, out of that, the load case (0.9 DL + 1.5 WL Y) is giving maximum values. Hence the above load case is considered for taking the values of forces, moments and the load case (D.L+0.8(LL+WLX) considered for taking the values of displacement and drift. Columns considered for comparison of analysis are C21, C23, C30, C38, C40, C43, C53 & C57.
Graph Storey Level (No’s) vs. CM displacement, Ux (m) for load case
Plan of structure 1 and structure 3
Graph Storey Level (No’s) vs. Storey Drift, Dx (m) for load case (D.L +EQXTP)
Graphs for 15- Storey Building
Graph Storey Level (No’s) vs. Storey Axial Force (kN)
Graph Storey Level (No’s) vs. Storey Drift, Dx (m) for load case (D.L+LL +EQXTP)
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International Journal of Research and Innovation (IJRI)
Due to symmetry the forces of C2 and C34 are equal as per table 6.2.1. Maximum Column Shear Forces V2 & V3 Considerations a.For Structure 4, Shear Forces, V2 in Column, C25 is 150 KN which is appreciably increased by -126.66%,-34.0%, 65.33%,-232.66% and 126.66% over C2, C4, C12, C31 and C34 Columns respectively. Due to symmetry the forces of C2 and C34 are equal as per table 6.2.2. Graph Storey Level (No’s) vs. Storey Drift, Dx (m) for load case (D.L+LL +WLX)
b.For Structure 4, Shear Forces, V3 in Column, C25 is -1402 kN which is appreciably increased by 6.63%, decreased by 55.42%, 46.0%, increased by 11.05% and 6.63% over C2, C4, C12, C31 and C34 Columns respectively. Due to symmetry the forces of C2 and C34 are equal as per table 6.2.3. Maximum Column Moments, M2 & M3 Considerations
Graph Storey Level (No’s) vs. Storey Drift, Dx (m) for load case (D.L+LL +WLY)
a.Moment (M2) depends on the shear force (V3) and effective length of the column, in addition with beam moments. The shear force (V3) act centre of column, as the column fixed on either ends. As the shear force varies from structure to structure corresponding moment also varies. b.For Structure 4, Moment (M2) in Column, C25 is -2785 KN-m which is appreciably increased by 10.89%, decreased by 17.88%, 17.09%, and 5.35% and increased by 10.89 % over C2, C4, C12, C31 and C34 Columns respectively. Due to symmetry the forces of C2 and C34 are equal as per table 6.2.4. c.For Structure 4, Moment (M3) in Column, C25 is 280 KN-m which is appreciably increased by -123.21%, -28.55%, 60.35%, -229.28% and 123.21% over C2, C4, C12, C31 and C34 Columns respectively. Due to symmetry the forces of C2 and C34 are equal as per table 6.2.5.
Graph Mode vs. Time period, T k
Results & Discussions Maximum Column Axial Forces Considerations a.C25 lies in 1st row 7th column, C2 and C34 are lies in 2nd row end columns, C31 lies in 3rd row 8th column and C4 and C12 are lies in 4th row 1st and 3rd columns. b.Considering the Structure 4: Double Core + Outrigger Beam + Increased stiffness of diaphragm at regular intervals with Load case (0.9 DL + 1.5WL Y). c.For Structure 4, Axial Forces in Column, C25 is -39,124 KN which is appreciably decreased by 30.08%, 73.29%, 73.92%, 39.55% and 30.08% over C2, C4, C12, C31 and C34 Columns respectively.
d.For structure 3, moments in corner columns C4 and C33 are less compared to the middle columns moments C2 and C29 by 26% and 0.8% respectively as per table 6.2.4. e.For structure 3, moments in outer periphery columns C12 and C20 are less compared to the moments in interior columns C6 and C18 by 21% and 16% respectively as per table 6.2.4. As per Clause 7.8.4.2 of IS 1893 ( Part I):2000, “The number of modes to be used in the analysis should be such that the sum total of modal masses of all modes considered is at least 90 percent of the total seismic mass and missing mass correction beyond 33Hz are to be considered. If modes with natural frequency beyond 33 Hz are to be considered, modal combination shall be carried out only for modes up to 33 Hz. 12
International Journal of Research and Innovation (IJRI)
As per Tables 6.2.15 & 6.2.16, the total sum of modal masses of all modes considered is more than 90 percent of the total seismic mass for all Structures. Results of Comparison of Structure: 4 (Double Core + Outrigger Beam + Increased stiffness of diaphragm at regular intervals) with Structure: 3 (Double Core + Outrigger Beam) & structure: 2 (Without Core + Outrigger Beam + Increased stiffness of diaphragm at regular intervals) as per table 6.1.18. a.The Maximum CM Displacement, Uy in Structure: 4 is 0.27 m which is appreciably less by 14.95% and 69.21% compared to Structure 3 and Structure 2 respectively. The limiting displacement is H / 500 i.e. =0.32 m. The maximum displacements of the structures 1 & 2 are 0.53m & 0.46m respectively and for structures 3 & 4 are 0.31m & 0.27m respectively as per Table 6.2.18. Hence structures 1 & 2 are not safe and structures 3 & 4 are safe. b.The Maximum Storey drift, Dy in Structure: 4 is 2.37 mm which is appreciably less by 2.90% and 97.35% for Structure 3 and Structure 2 respectively than structure: 4 (As per IS 1893 (Part1):2002 clause 7.11.1) limiting storey drift is 0.004 times storey height, i.e. 0.004 x 4.0 m = 0.016m or 16mm. The Maximum Storey Drift for all the structures is less than the limiting value as per table 6.2.18 i.e. (2.37, 2.44 & 4.68 < 16 mm). Hence safe. c.Storey Axial Force, P in Structure 4 is 972248 kN which is appreciably increased by 8.06% and 7.94% for Structure 3 and Structure 2 respectively than structure: 4 as per table 6.2.5 & 6.2.18. d.Maximum Storey Moment, Mx in Structure: 4 is 9349749 kN-m, which is appreciably increased by 11.32% and 11.14% for Structure 3 and Structure 2 respectively than structure: 4 as per table 6.2.9 & 6.2.18. e.Maximum Storey Moment, My in Structure: 4 is (-) 35000942 kN-m, which is appreciably increased by 8.06% and 7.94% for Structure 3 and Structure 2 respectively than structure: 4 as per table 6.2.10& 6.2.18. f.Concrete take off in the structure:4 is 960112 m3 , which is appreciably more by 8.07% and 9.12% compared to structure 3 and structure 2 respectively as per table 6.2.18. Conclusions The analysis is carried out for study of rigid core and floor rigidity of 15, 20 & 25 storey L-shape Building for the following structures of different locations of outrigger beams and belt truss as shown in fig. Structure 1: Building frame outrigger beam locations as shown in fig Structure 2: Building frame same as structure-
1with belttruss. Structure 3: Building frame outrigger beam locations as shown in fig Structure 4: Building frame same as structure-3 with belttruss. Structure 5: Building frame without any outrigger beams as well as belttruss From the analysis of the Data the following conclusions have been made •
Due to presence of the belttruss in Structure 2 and Structure 4, they are Stiffer Structures when compared to other three types. This is reflected in reduction of storey displacement and storey drift Values. • Column forces and moments are minimum in case of Structure 2 and Structure 3 for which drift and displacement are also comparatively less. Hence this is an optimum structural framing system. • Moments in Corner column are less compared to the middle column moments for all structures and moments in outer periphery columns are less compared to the moments in interior columns for all structures. • Outrigger beams help transfer lateral forces to core shear wall in Structure 1 and Structure 3. Hence the moments in columns nearer to core are reduced as compared to model without outrigger beam i.e. Structure 5. • The location of the outrigger beam has a critical influence on the lateral behaviour of the structure under earthquake load and the optimum outrigger locations of the building have to be carefully selected in the building design. • The use of outrigger and belt truss system in high-rise buildings increase the stiffness and makes the structural form efficient under lateral load. Scope of Further Study • • •
•
•
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Further study can be made by providing Shear Walls for further height of building. It can be studied by providing Shear Walls at other different locations and combinations of these. The use of outrigger structural systems in highrise buildings increases the stiffness and makes the structural form efficient under lateral load. Based on the analysis results obtained following conclusions made. When the criterion considered is lateral displacement then the optimum position of the outriggers is at mid height for both static and dynamic behaviour for the structure considered. The outrigger placed at the top of the building is about less efficient, however in many situations it may be more permissible to locate the outrigger at building top, therefore although not as efficient as when at mid height, the benefits of placing it at top are quite impressive resulting up to 50% reduction in drift. When the criterion for design is peak accelera13
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• •
tion the optimum position of outrigger is at top where it is reduced up to 30%. There is substantial reduction in forces in core, bending moment in particular when outrigger system is added to the structure. The outrigger structural systems not only proficient in controlling the top displacements but also play substantial role in reducing the inter store drifts.
References [1] IS-1893 (part 1), “Criteria for Earthquake Resistant Design of Structures” Bureau of Indian Standards, New Delhi, 2002. [2] IS: 456 - 2000 - Code of practice for plain and Rein forced concrete [3] IS: 875(part 1)–1987: Code of practice for design loads (Other than earthquake) for buildings and structures - Dead loads. [4] IS: 875 (part 2)–1987: Code of practice for design loads (Other than earthquake) for buildings and structures – Imposed loads. [5] IS: 875(part 3) - 1987: Code of practice for design loads (Other than Earthquake) for buildings and structures - Wind loads. [6] Taranath, B. S, Steel concrete and composite design of tall buildings (Second Edition, McGraw – Hill Publications, 2001) Chopra A.K. (2005):”-Dynamics of structures Theory and applications to Earthquake Engineering”, Second edition. [7] Zhang, Zhang, Zhao, Zhu and Zhou, “Safety Analysis of Optimal Outrigger Location in High-rise Building Structures, Journal of Zhejiang University Science A, Volume 8 (2), Page no. 264-269, 2007. [8] Minsik Bang and Jaehong Lee “An Analytical model for high-rise wall frame building structures”, Page No.1003-1009, for CTBUH 2004, October 1013, Seoul, Korea. [9] Shankar Nair, R , Belt Trusses and Basements as Virtual Outriggers for Tall Buildings, Engineering Journal , Fourth Quarter, Amercian journal of steel construction, 1998. [10] Moudarres, F.R, Outrigger Braced Coupled Shear Walls, Journal of Structural Engineering, ASCE, Vol. 110, No. 12, 1984. [11] Alex Coull and Otto Lau.W.H, Multi Outrigger Braced Structures, Journal of Structural Engineering, ASCE, Vol. 115, No. 7, 1989. [12] Hoenderkamp and Snijder, “Preliminary Analysis of High-rise Braced Frames with Facade Riggers”, Journal of Structural Engineering ASCE, May 2003. [13] Gerasimidis S. Efthymiou E. and Baniotopoulos C. C, Optimum outrigger locations for high rise steel buildings for wind loading, EACWE 5 Florence, Italy, 19th – 23rd July 2009. [14] Kenneth Arnott Shear wall analysis – New modelling, same answers”, CSC (UK) Ltd. [15] Computer programming by Ali Lame. [16] Herath, N., Haritos, N., Ngo, T., and Mendis, P. (2009), Behavior of Outrigger Beams in High Rise Buildings under Earthquake Loads, Australian
Earthquake Engineering Society Conference, 2009. [17] Wu and Li, “Structural Performance of Multi-Outrigger Braced Tall Buildings”, The Structural Design of Tall and Special Buildings, Volume 12, Page no. 155-176, 2003. [18] P.Jayachandran, “Design of tall buildings preliminary design and optimization” for National work shop on High rise & Tall buildings, University of Hyderabad, India, May, 2009, Keynote Lecture. [19] Tolga Aki, S, “Lateral load analysis of shear wall frame structures”, this is submitted to the Graduate school of Natural and Applied Sciences of the Middle East technical University, Janaury’2004. [20] Y. Chen, D. M. McFarland, Z. Wang, B. F. Spencer, J. R. L. A. Bergman “Analysis of tall buildings with damped outriggers, Journal of structural engineering, ASCE, 136(11), 1435-1443. Author
Ahsan Mohammed Khan, Research Scholar, Department Of Civil Engineering, Aurora's Scientific Technological & Research Academy, Hyderabad, India
K. Mythili, Associate professor,Department Of Civil Engineering, Aurora's Scientific Technological & Research Academy, Hyderabad, India
Shaik Subhani Shareef, professor , Department Of Civil Engineering, Aurora's Scientific Technological & Research Academy, Hyderabad, India
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