Analysis & Design of a High Rise Thin Building

Transcription

1 Analysis & Design of a High Rise Thin Building Ajitha B 1 & Nirupa N 2 1 Ajitha, Department of Civil Engineering, JNTUA College Of Engineering(Autonomous), Ananthapuramu, Andhra Pradesh, India. 2 Nirupa,Department of Civil Engineering, JNTUA College Of Engineering(Autonomous), Ananthapuramu, Andhra Pradesh, India. ABSTRACT Earthquake load is becoming a great concern in our country as because not a single zone can be designated as earthquake resistant zone. One of the most important aspects is to construct a building structure, which can resist the seismic force efficiently. Study is made on the different structural arrangement to find out the most optimized solution to produce an efficient safe earthquake resistant building. In the present analysis, a thin building is analyzed with a height of 60m, width of the building is taken as 15m the building is analyzed without bracings and with X bracings at optimum places and finding out the results of displacement, shear, moment, in Zone-3 in loose soil for finding the results in both static & response spectrum analysis. A commercial package of ETABS 2013 has been utilized for analyzing commercial building. The result has been compared using tables & graph to find out the most optimized solution. Concluding remark has been made on the basis of this analysis. Keywords:, Static analysis, Dynamic Analysis. 1. INTRODUCTION Mankind has always had a fascination for height and throughout our history we have constantly sought to metaphorically reach for the stars. From the ancient pyramids to today s modern skyscraper, a civilization s power and wealth has been repeatedly expressed through spectacular and monumental structures. 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 coupled with advances in the finite element method has allowed the creation of many structural and architecturally innovative forms. However, increased reliance on computer analysis is not the solution to the challenges that lie ahead in the profession. The basic understanding of structural behaviour while leveraging on computing tools are the elements that will change the way structures are designed and built. The design of skyscrapers is usually governed by the lateral loads imposed on the structure. As buildings have taller and narrower, the structural engineer has been increasingly challenged to meet the imposed drift requirements while minimizing the architectural 268 impact of the structure. In response to this challenge, the profession has proposed a multitude of lateral schemes that are now spoken in tall buildings across the globe. This study seeks to understand the evolution of the different lateral systems that have emerged and its associated structural behaviour, for each lateral scheme examined, its advantages and disadvantages will be looked at. Effect of Soils: The seismic motion that reaches a structure on the surface of the earth is influenced by the local soil conditions. The subsurface soil layers underlying the building foundation may amplify the response of the building to earthquake motions originating in the bedrock. Although it is somewhat difficult to visualize, it is possible that a number of underlying soils can have a period similar to the period of vibration of the structure. Greater structural distress is likely to occur when the period of the underlying soil is close to the fundamental period of the structure. Tall buildings tend to experience greater structural damage when they are located on soils having a long period of motion because of the resonance effect that develops between the structure and the underlying soils. If a building resonates in response to ground motion, its acceleration is amplified. It is possible that a number of underlying soils layer s can have a period similar to period of

2 vibration of the structure. Low-to mid-rise buildings typically have periods in the 0.10 to 1.0 sec range, whereas taller, more flexible buildings have periods between 1 and 5 sec or greater. Harder soils and bedrock will efficiently transmit short-period vibrations(caused by near field earthquakes)while filtering out longer-period vibrations (caused by distant earthquakes), whereas softer soils will transmit longer -period vibrations. As a building vibrates due to ground motion, its acceleration will be amplified if the fundamental period of the building coincides with the period of vibrations being transmitted through the soil. Natural period of soil is in the range of 0.5 to 1.0 sec. Therefore, it is entirely possible for the building and ground to have the same fundamental period. As per IS 1893 (Part I) 2002, soils classification can be taken as Type I, Rock or Hard soil: Well graded gravel and sand mixtures with or without clay binder and clayey sands poorly graded or sand clay mixtures, whose N (standard penetration value) should be above 30. Type II, Medium soils: All soils wit h N between 10 and 30, and poorly- graded sands or gravelly sands with little or no fines. Type III, Soft Soils: All soils other than whose N is less than 10. Objectives of the Study To investigate the different ways in which the tall structures can be stabilized against the effects of strong horizontal wind loading and seismic loading. Some other reasons why we use bracings are tall structures can be constructed which reduces the area used and we can accommodate a large population in that particular area. Other objective is to construct a cost effective structure in less period of time. This study helps in the investigation of behaviour of thin high rise buildings. The scope is to analyze the thin high rise building with bracings & without bracings in Zone-3 Type III, loose Soils in Static and Dynamic analysis. Firstly the model is implemented into known computer software and then it is analyzed based on the investigation of strength and stiffness. 2. LATERAL LOAD RESISTING SYSTEMS: A multi-storey building with no lateral bracing is shown in figure 2.1.When the beams and columns shown are connected with simple beam connections, the frame would have practically no resistance to the lateral forces and become geometrically unstable. The frame would be laterally deflect as shown in the below figure even under a small lateral load. Loading on tall buildings is different from low-rise buildings in many ways such as large accumulation of gravity loads on the floors from top to bottom, increased significance of wind loading and greater importance of dynamic effects. Thus, multi-storied structures need correct assessment of loads for safe and economical design. Excepting dead loads, the assessment of loads cannot be done accurately. Live loads can be anticipated approximately from a combination of experience and the previous field observations. But, wind and earthquake loads are random in nature. It is difficult to predict them exactly. These are estimated based on probabilistic approach. The following discussion describes the influence of the most common kinds of loads on multistoried structures. Figure 2.1 : multi- storey frame without lateral bracing 2.1 STRUCTURAL CONCEPTS: The key idea in conceptualizing the structural system for a narrow tall building is to think of it as a beam cantilevering from the earth (fig 2.2). Figure 2.1.1:Structural concept of tall building The laterally directed force generated, either due to wind blowing against the building or due to the inertia forces induced by ground shaking, tends both to snap it (shear), and push it over (bending). 269

3 Therefore, the building must have a system to resist shear as well as bending. In resisting shear forces, the building must not break by shearing off and must not strain beyond the limit of elastic recovery. Figure 2.1.2: Building shear resistance; (a) Building must not break (b) Building must not deflect excessively in shear. Figure 2.1.3: Bending resistance of building (a)building must not overturn (b)columns must not fail in tension or compression (c) Bending deflection must not be excessive. In the structure s resistance to-of bending-war ensues that sets and the shear building in motion, thus creating a third engineering problem; motion perception or Vibration. If the building sways too much, human comfort is sacrificed, or more importantly, non-structural elements may break resulting in expensive damage to the building contents and causing danger to the pedestrians. A perfect structural form to resist the effects of bending, shear and excessive vibration is a system possessing vertical continuity ideally located at the farthest extremity from the geometric centre of the building. A concrete chimney is perhaps an ideal, if not an inspiring engineering model for a rational super-tall structural form. The quest for the best solution lies in translating the ideal form of the chimney into a more practical skeletal structure LATERAL FORCE RESISTING SYSTEMS: There are several systems that can be used effectively for providing resistance to seismic lateral forces. Some of the more common systems are shown in figures below. All of the systems rely on a complete, three dimensional space frame; a coordinated system of moment frames, shear walls, or braced frames with horizontal diaphragms; or a combination of the systems. 1. In buildings where a space frame resists the earthquake forces, the columns and beams act in bending. During a large earthquake, storey to storey deflection may be accommodated within the structural systems without causing failure of columns or beams. However, the drift may be sufficient damage elements that are rigidly tied to the structural system such as brittle partitions, stairways, plumbing, exterior walls, and other elements that extend between floors. Therefore, buildings can have substantial interior and exterior non structural damage and still be structurally safe. Although there are excellent theoretical and economic reasons for resisting seismic forces by frame 2. A shear wall (or braced frame) building is normally more rigid than a framed structure. With low design stress limits in shear walls, deflection due to shear forces is relatively small. Shear wall construction is an economical method of bracing buildings to limit damage, and this type of construction is normally economically feasible up to about 15 stories. Notable exceptions to the excellent performance of shear walls occurs when the height-to-width ratio becomes great enough to make overturning a problem and when there are excessive openings in the shear walls. Also, if the soil beneath its footings is relatively soft, the entire shear wall may rotate, causing localized damage around the wall. 3. The structural systems just mentioned may be used singly or in combination with each other. When frames and shear walls interact, the system is called a dual system in the frame alone can resist 25% of the lateral load. Otherwise, it is referred to as a combined system

4 system is a fully triangulated truss. They undergo bending also when the braces are eccentrically connected to them. Because the lateral load on the building is reversible, braces are subjected in turn, to both compression and tension; consequently, they are most often designed for the more stringent case of compression. Figure : Lateral-force-resisting systems: (a) steel moment-resisting frame; (b) reinforced concrete moment-resisting frame; (c) braced steel frame; (d) reinforced concrete shear walls; (e) steel frame building with cast-in-place concrete shear walls; (f) steel frame building with infilled walls of non reinforced masonry. 3. BRACED FRAMES: Rigid frame systems are not efficient for buildings taller than about 30-stories because the shear racking component of deflection due to the bending of columns are girders causes the drift to be too large. A braced frame attempts to improve upon the efficiency of a rigid frame by virtually eliminating the bending of columns and girders. This is achieved by adding web members such as diagonals or chevron braces. The horizontal shear is now primarily absorbed by the web and not by the columns. The webs carry the lateral shear predominantly by the horizontal component of axial action allowing for nearly a pure cantilever behaviour. In simple terms, braced frames may be considered as cantilevered vertical trusses resisting lateral loads primarily through the axial stiffness of columns and braces. The columns act as a chord in resisting the overturning moment, with tension in the windward column and compression in the leeward column. The diagonals and girders work as the web members in resisting the horizontal shear, with diagonals in axial compression or tension depending upon their direction of inclination. The girders act axially, when the 271 Figure 3.1 : Braced frame deformation (a) flexural deformation (b) shear deformation; (c) Combined configuration The effect of axial deformation of configuration of the deflection with concavity downward and a maximum slope at the top (Fig-a). The axial deformations of the web members, on the other hand, cause a shear configuration at the top (Figb ). The resulting deflected shape of the frame (Fig-c) is a combination of the effects of the flexural and shear curves, with a resultant configuration depending on their relative magnitudes, as determined mainly by the type of bracings. Nevertheless, it is the flexural deflection that most often dominates the deflection characteristics. The role of web members in resisting shear can be demonstrated by following the path of the horizontal shear down the brace bent. Consider the brace frames shown subjected to an external shear force at the top level, the diagonal in each storey is in compression, causing the beams to be in axial tension; therefore, the shortening of the diagonal and extension of the beams give rise to the shear deformation of the bent. In the forces in the brace connecting to each beam-end are in equilibrium horizontally with the beam carrying insignificant axial load, half of each beam is in compression while the other half is in tension. the braces are alternatively in compression and tension while the beams remain basically unstressed. Finally, in

5 Fig3.6e, the end parts of the beam are in compression and tension with the entire beam subjected to double curvature bending. Observe that with a reversal in the direction of horizontal load, all actions and deformations in each member will also be reversed. 4. MATERIALS & METHODS: 4.1 Plan Considered: In this study an 35 storey building having same plan in different type of zones (as per IS 1893 (Part I): 2002) and different type of soils is taken. The tall building with X braces introduce in the central location in two bays is consider to study the effect of lateral deflection, bending moment, shear force caused due to lateral load. i.e. due to quake load (both static and dynamic). 4.4.Defining Frame Sections and Slab sections: After forming the grid we have to define frame sections which includes beam & column. In this software we can define a column with suitable reinforcement which can be edited if the provided reinforcement exceeds the limit. Beam defining & Column defining is shown in below figures. Figure : Showing Defining of Beam 4.2. Building Dimensions: The building is 15m x 60m in plan with columns spaced at 5m from centre to centre. A floor to floor height of 3.0m is assumed. The location of the building is assumed to be in Zone-3 and loose soils. Table 4.2.1: Size of Structural Members Sr. No. Contents Description Figure : Showing Defining of Column 1 Size of column From ground floor to fifteenth floor: 1000 mm X 900 mm From sixteenth floor to thirty fifth floor: 900 mm X 600 mm 2 Size of beam 400 mm X 600 mm Modeling : To create a model firstly we have to form a grid. Grid can be of uniform or non uniform. Uniform refers to spacing which are in equal distances in global x direction & equal distances in global Y direction. It is not compulsory to make same distances in both the global directions. 272 Thickness of Slab 4 Size of Brace Members 5 Materials 120 mm 230 mm X 230 mm Concrete (M30) and Reinforcement HYSD 500 Figure :Showing reinforcement details for column Figure : Showing defining of slab section 4.5 Loading: After the modeling is complete, ETABS generates code-based loading conditions for gravity, seismic, wind loads. We can specify an unlimited number of load patterns and combinations. Load patterns in ETABS software:

6 Figure:4.5.1 Showing loads to be applied on structure slab design software with post-tensioning (PT) capability, is one such option for export. CSI coordinated SAFE to be used in conjunction with ETABS such that engineers could more thoroughly detail, analyze, and design the individual levels of an ETABS model. While ETABS features a variety of sophisticated capabilities, the software is equally useful for designing basic systems. ETABS is the practical choice for all grid-like applications ranging from simple 2D frames to the most complex high rises. 4.8.PLAN AND ELEVATION OF MODEL: A simple plan of 150m X 60m is taken, with 5 bays of 8 m each as shown below. Figure4.5.2: Showing of defining Load Combinations And also model is analyzed in dynamic analysis by response spectrum method Response Spectrum Method: Response spectrum method is simple when compared to time history method, in this method model is placed in particular zone & soil and we see the behaviour of model or a building for that particular earthquake zone & soil. Earth Quake Data Is Referred from IS Table : Zone Factors in India ZONES ZONE-II 0.10 ZONE FACTOR Figure : Building plan dimension(common to all floors, all models, units m ). ZONE-III 0.16 ZONE-IV 0.24 ZONE-V Analysis & Design: Output and display formats are also practical and intuitive. Moment, shear, and axial force diagrams, presented in 2D and 3D views with corresponding data sets, may be organized into customizable reports. Also available are detailed section cuts depicting various local response measures. ETABS also features interoperability with related software products, providing for the import of architectural models from various technical drawing software, or export to various platforms and file formats. SAFE, the floor and foundation 273 Figure 4.8.2: Showing elevation view of high rise building

7 5.RESULTS: Table-1: Comparative values of Displacement in Zone-3 loose soil in Static Analysis Storey Without With Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey storey Storey Base 0 0 Table-2: Comparative values of Displacement in Zone-3 loose soil in Dynamic Analysis Without With Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey storey Storey Base 0 0 Graph 1 : Displacement variationin Zone-3 loose soil in Static Analysis 274 Graph 2 : Showing Displacement variation in Zone-3 loose soil in Dynamic Analysis

8 Table-3: Comparative values of Shear in Zone-3 loose soil in Static Analysis Storey Without With Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey storey Storey Base Table-4:Comparative values of Shear in Zone- 3 loose soil in Dynamic Analysis Storey Without With Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey storey Storey Base Graph 3 : Showing Shear variation in Zone-3 loose soil in Static Analysis Graph 4 : Showing Shear variation in Zone-3 loose soil in Dynamic Analysis

9 Table-5: Comparative values of Moment in Zone-3 loose soil in Static Analysis Storey Without With Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey storey Storey Base Table-6:Comparative values of Moment in Zone-3 loose soil in Dynamic Analysis Storey Without With Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey Storey storey Storey Base Graph 5 : Showing Moment variation in Zone-3 loose soil in Analysis 276 Graph 6 : Showing Moment variation in Zone-3 loose soil in Dynamic Analysis

10 277 (a) (b) Figure 6.1 : The variation of (a)shear (b)moment in 3D View 4. CONCLUSIONS: Based on the analysis: In static analysisthe structural performance is analyzed in two different models i.e. Without bracings, With X Bracing, the displacement of 40% is reduced when lateral systems are provided. Shear is also analyzed for both the models, Shear of 30% is reduced when the lateral systems i.e., X bracings are provided. Moment is also compared for both the models, moment of 60% is reduced when x bracings are provided. By providing the bracings the stiffness of the structure is increased and storey shear is decreased with increase in height of structure. Dynamic Analysis i.e.. Response Spectrum analysis is performed for all the models i.e. without bracings & with bracings. The displacement of 40% is reduced when X bracings are provided Dynamic Analysis i.e.. Response Spectrum analysis is carried out for all the models i.e. without bracings & with bracings. A shear of 30% is reduced when X bracings are provided. Dynamic Analysis i.e.. Response Spectrum analysis is carried out for all the models i.e. without bracings & with bracings. A moment of 40% is reduced when X bracings are provided. By providing lateral systems in the framed structures the reduction in the displacement, shear, moment thereby increasing the stiffness of the structure for resisting lateral loads due to earth quakes REFERENCES 1. Mahmoud R. Maheri, R. Akbari (2003) Seismic behaviour factor, R, for steel X-braced and knee-braced RC buildings Engineering Structures, Vol.25, 14 May 2003, pp J.C.D. Hoenderkamp and M.C.M. Bakker (2003) Analysis of High-Rise Braced Frames with Outriggers The structural design of tall and special buildings, Vol. 12, 10 July 2003, pp K.S.Jagadish, B.K.R.Prasad and P.V.Rao,"The Inelastic Vibration Absorber Subjected To Earthquake Ground Motions."Earthquake engineering and Structural Dynamics. 7, (1979). 4. Kim Sd, Hong Wk, Ju Yk"A modified dynamic inelastic analysis of tall buildings con- -sidering changes of dynamic characteristics" the structural design of tall Buildings 02/ J.R. Wu and Q.S.LI (2003) Structural performance of multi-outrigger-braced Tall Buildings. The structural design of tall and special buildings, Vol.12, October 2003, pp S.M.Wilkinson, R.A.Hiley "A Non-Linear Response History Model For The Seismic Analysis Of High-Rise Framed Buildings" september 2005, Computers and Structures. 7. V. Kapur and Ashok K. Jain (1983) Seismic response of shear wall frame versus braced concrete frames University of Roorkee, Roorkee April 1983 IS: 1893(Part I): 2002 Indian Standard Criteria for Earthquake Resistant Design of Structures Part I General provisions and buildings (Fifth Revision). 8. J.R. Wu and Q Structural..LIperformance (2003) of multi-outrigger-braced Tall Buildings. The structural design of tall and special buildings, Vol.12, October 2003, pp V. Kapur and Ashok K. Jain (1983) Seismic

11 response of shear wall frame versus braced concrete frames University of Roorkee, Roorkee April 1983 IS: 1893(Part I): 2002 Indian Standard Criteria for Earthquake Resistant Design of Structures Part I General provisions and buildings (Fifth Revision). 10. Pankaj Agarwal and Manish Shrikhande.(2010), Earthquake Resistant Design of Structures PHI Learning Private Limited. 11.Taranath B.S. (1988), Structural Analysis and Design of Tall Buildings McGraw-Hill Book Company. 12.E-Tabs 2013 training manuals. 278