SEISMIC ANALYSIS OF HIGH RISE BUILDINGS WITH PLAN IRREGULARITY

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 4, April 2017, pp , Article ID: IJCIET_08_04_154 Available online at aeme.com/ijciet/issues.asp?jtype=ijciet&vtyp pe=8&itype=4 ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed SEISMIC ANALYSIS OF HIGH RISE BUILDINGS WITH PLAN IRREGULARITY Albert Philip Post Graduate Student, Structural Engineering, School of Mechanical and Building Sciences, VIT University, Chennai, India Dr. S. Elavenil Professor, Structural Engineering Division, School of Mechanical and Building Sciences, VIT University, Chennai, India ABSTRACT A high rise building has to be designed to resist lateral loads due to wind or earthquake. Interior structural system or exterior structural system provides the lateral load resistance to the structure. The shape, structure and material used also influence the behaviour of structure against lateral loading. In this study three dimensional analytical models of G+12 storied buildings have been generated for regular and irregular buildings and analysed using CSI ETABS software (2015 version) for earthquake zone III in India. The objective of the project is to carry out seismic analysis (RSA) of regular and irregular reinforced concrete buildings and to carry out the ductility based design using IS Results of this analysis are discussed in terms of story displacements, story drifts, story shear and stiffness. From the results it is concludedd that story displacements increases linearly with height of the building; maximum storey drift is observed at second floor for irregular structure and at fourth floor for regular structure; maximum storey shear force was observed between ground floor and second floor for regular structure and at ground floor for irregular structure and the value decreases linearly with height; storey stiffness varies non - linearly for both the structures with maximum values at ground floor. Keywords: Response Spectrum Analysis, Seismic Analysis, Storey Drifts, Storey Shear. Cite this Article: Albert Philip and Dr. S. Elavenil, Seismic Analysis of High Rise Buildings with Plan Irregularity. International Journal of Civil Engineering and Technology, 8(4), 2017, pp IET/issues.asp?JType=IJCIET&VType=8&ITy ype= editor@iaeme.com

2 Albert Philip and Dr. S. Elavenil 1. INTRODUCTION 1.1. General A high-rise building is a multi-story structure between meters tall, or a building of unknown height having more than 12 floors used mainly as a residential and/or office building. Advances in construction technology, materials, structural systems and analytical methods for analysis and design facilitated the growth of high rise buildings. High-rise buildings became possible with the invention of the elevator and cheaper, more abundant building materials. The materials used for the structural system of high-rise buildings are reinforced concrete and steel. A building with fifty or more stories is generally considered as a skyscraper. High-rise structures pose particular design challenges for structural and geotechnical engineers, particularly if situated in a seismically active region or if the underlying soils have geotechnical risk factors such as high compressibility or soft soil. Meanwhile high-rise buildings have technical and economic advantages in areas of high population density and in regions where horizontal development is restricted or not possible and hence have become a distinctive feature of housing accommodation in virtually all densely populated urban areas around the world. Structural analysis is mainly concerned with finding out the behaviour of a physical structure when subjected to force. This action can be in the form of load due to weight of things such as people, equipment, wind, snow, etc. or some other kind of excitation such as an earthquake, shaking of the ground due to a blast nearby, etc. In essence all these loads are dynamic, including the self-weight of the structure because at some point in time these loads were not there. This distinction is made between the dynamic and the static analysis on the basis of whether the applied action has enough acceleration in comparison to the structure s natural frequency. If a load is applied sufficiently slowly, the inertia forces (Newton s first law of motion) can be ignored and the analysis can be simplified as static analysis. Structural dynamics therefore is a type of structural analysis which covers the behaviour of structures subjected to dynamic (actions having high acceleration) loading. A dynamic load is one which changes with time fairly quickly in comparison to the structure s natural frequency. Dynamic loads include people, wind, waves, traffic, earthquakes and blasts. Any structure can be subjected to dynamic loading. Dynamic analysis can be used to find dynamic displacements, time history, and modal analysis. This project is concerned with the study of high rise irregular building. The structural analysis of G+12 storeyed reinforced concrete regular and irregular frame building is done with the help of ETABS software. Response Spectrum Analysis (RSA) has also been carried out in the study. In response spectrum analysis the time evolution of response cannot be computed. A response spectrum is simply a plot of the peak or steady-state response (displacement, velocity or acceleration) of a series of oscillators of varying natural frequency that are forced into motion by the same base vibration or shock. This approach permits the multiple modes of response of a building to be taken into account. For each mode, a response is obtained from the design spectrum, corresponding to the modal frequency and the modal mass, and then they are combined to estimate the total response of the structure. In this the magnitude of forces in all directions is calculated and then the effects on the building are observed editor@iaeme.com

3 Seismic Analysis of High Rise Buildings with Plan Irregularity 1.2. Objectives and Scope The objective of the study is to study about the variations in building properties with different floor plans by carrying out response spectrum analysis. The scope of the study is extended to design and analysis of G+12 storied irregular and regular RC structures using CSI ETABS (2015 version) software. Two existing structures one with regular floor plan and other with irregular floor plan has been selected and modelled for the analysis. Storey displacements, storey drift, storey shear force, storey stiffness and storey overturning moments are the parameters obtained from the seismic analysis. 2. LITERATURE REVIEW Advancement in science and technology has made it possible to predict the behaviour of buildings during earthquake. In the past the buildings were designed just for gravity loads and recently many works is being carried out in seismic analysis of structures. Earthquake engineering is a much advanced area of engineering where lot of research works are being carried out. Ravindra N. Shelke et.al [1]studied the effects of various vertical irregularities on the seismic response of a structure. He concluded that, base shear and lateral displacement with height of the structure as the seismic intensity increases from zone-2 to zone-5 which indicates more seismic demand the structure should meet. Krishna G Nair et.al [2] conducted a seismic analysis of reinforced concrete buildings using static and dynamic analysis methods such as response spectrum method, time history method and push over analysis. She observed that for accurate and precise results non-linear dynamic analysis shall be carried out and response spectrum analysis gives better and accurate results as compared to time history analysis. Sakshi A. Manchalwar et.al [3] studied the effect of column stiffness, effect of number of bays and effect of building height by conducting seismic analysis of multistoried building (upto 14 story) subjected to earthquake force. She observed that as the stiffness of column increases the moment in the same floor level beam decreases and the moment in same floor level column increases continuously from negative value towards positive value. E. Hassaballa et.al [4] analyzed seismic analysis of a multi-story RC frame in Khartoum city under moderate earthquake loads to investigate the performance of existing buildings if exposed to seismic loads. He observed that the maximum values of compressive and tensile stresses in beams are approximately equal and bending moments in beams and columns due to seismic excitation showed much larger values compared to that due to static loads. Wakchaure M.R et.al [5] studied the effect of masonry walls on high rise building and carried out linear dynamic analysis on high rise building with different arrangements. Heobserved that infill walls reduce displacements, time period and increases base shear and hence it is essential to consider the effect of masonry infill for the seismic evaluation of moment resisting reinforced concrete frame Pardeshi Sameer et.al [6] in this work 3D analytical model of G+15 storied buildings was generated for symmetric and asymmetric building models and analyzed using structural analysis tool ETABS software. Finally it was concluded that the plan configurations of structure has significant impact on the seismic response of structure in terms of displacement, story drift, story shear. Sagar R Padol et.al [7] in this study seismic analysis of RCC buildings with mass irregularity at different floor level was carried out. He concluded that whenever a structure having different irregularity, it is necessary to analyse the building in various earthquake zones editor@iaeme.com

4 Albert Philip and Dr. S. Elavenil M. S. Aainawala et. al[8] in this paper static analysis for regular buildings up to 90m height in zone II and III was performed and it was observed that in G+12, G+25, G+38 Storey building constructing building with shear wall at corner location gives minimum drift and minimum displacement. Bahador Bagheri et. al [9] this study deals with the effect of the variation of the building height on the structural response of the shear wall building. It was concluded that the accuracy and exactness of Time History analysis in comparison with the most commonly adopted Response Spectrum Analysis and Equivalent Static Analysis. Romy Mohan et. al [10] in this work dynamic responses under prominent earthquake, El- Centro have been investigated and it was found that Equivalent Static Method can be used effectively for symmetric buildings up to 25 m height. For higher and unsymmetrical buildings Response Spectrum Method should be used. For important structures Time History Analysis should be performed. 3. DESIGN OF STRUCTURE 3.1. Designs for Shear and Flexure For design of structures, static earthquake analysis is done according to IS M30 concrete and Fe500 steel are used. Both models are designed according to IS 456:2000. The design is carried out using ETABS software. Two existing buildings (Seismic zone - III), one with regular floor plan and other with irregular floor plan have been modelled for the study. All beams and columns have cross sectional dimension of 300mm x 450mm and 450mm x 600mm respectively. Wall thickness is 220 mm, cover for beam is taken as 30mm and for columns 40mm.Approximate floor area of both the buildings is 700 m² and overall plan dimension is 25m x 28m. Foundation design is not done since it is not required. The following data are considered for the structural design. Table 3.1 Building Details PARAMETERS VALUE Dimension of Beam 300mm x 450mm Dimension of Column 450mm x 600mm Thickness of Slab 150mm Thickness of outside wall 220mm Height of each storey 3 m Height of bottom storey 3.5m Total height of Building at Roof level 45m Dimension of Building 25m x 28m Approximate Floor Area 700 m² Live Load 4kN/m² Floor Finish 1.5kN/m² Grade of Concrete M30 Grade of Reinforcing Steel Fe500 Density of Concrete 25kN/m² Seismic Intensity Moderate Importance Factor 1 Zone Factor 0.16 Damping Ratio 5% editor@iaeme.com

5 Seismic Analysis of High Rise Buildings with Plan Irregularity 4. ANALYTICAL INVESTIGATION The analysis and design of G+12 storeyed regular and irregular high rise reinforced concrete buildings are done using CSI ETABS software (2015 version). Two analytical models using one regular floor plan and one irregular floor plan of existing buildingswere developed using the software. The structure is assumed to be located in seismic zone III in India on a site with medium soil. Figure 4.1 Regular Floor Plan Figure 4.2 Regular Elevation editor@iaeme.com

6 Albert Philip and Dr. S. Elavenil Figure 4.3 Irregular Floor Plan Figure 4.4 Irregular Elevation Floor plans and elevations of regular and irregular buildings are shown in Fig. 4.1, 4.2, 4.3 and 4.4 respectively. Since the height of ground floor is 3.5 m and rest of the floors is 3m, there is stiffness irregularity as per IS 1893: Part 1. Stiffness of each column = 12EI/L³, therefore (stiffness of ground floor)/(stiffness of other floors) = (3/3.5)³ = < editor@iaeme.com

7 Seismic Analysis of High Rise Buildings with Plan Irregularity 4.2. Loading Details The gravity loads and earthquake loads will be taken for analysis. As per IS 1893 (Part1): 2002 Clause no: , the following load cases have to be considered for seismic analysis: DL (DL+ IL) (DL+IL + EL along X direction) (DL+IL + EL along Y direction) (DL+IL - EL along X direction) (DL+IL - EL along Y direction) (DL + EL along X direction) (DL + EL along Y direction) (DL - EL along X direction) (DL - EL along Y direction DL + 1.5EL along X direction DL + 1.5EL along Y direction DL - 1.5EL along X direction DL - 1.5EL along Y direction Where: DL Dead Load; IL Imposed /Live Load; EL Earthquake Load. Total 14 load combinations have been taken for the analysis. Wind loads are not considered. Gravity loads include dead load, live load and floor finish load (assumed as 1.5kN/m²). RESULTS AND DISCUSSIONS The results of the study are being illustrated using the graphs below (fig5.1 fig5.5) which explain the structural behaviour of both the structures (regular and irregular) in terms of storey displacements, storey drifts, storey shears, storey stiffness and storey overturning moments. Figure 5.1 Storey Displacements Fig. 5.1 shows the graph for storey displacements for both the buildings. Storey displacement is the displacement of one level of a multi-story building relative to the base of the building. Story displacement is linearly increasing from base to roof as illustrated in the graph. According to code, maximum or permissible story displacement should be equal to or less than 0.4% of total building height. Hence here the permissible maximum storey displacement = (0.4/100 x 45000) = 180 mm. For regular model, it is less than the limit editor@iaeme.com

8 Albert Philip and Dr. S. Elavenil (163.6 mm at roof level) while for irregular model it is just touching the limit (181.6 mm at roof level). Figure 5.2 Storey Drifts Fig.5.2 illustrates the variation of storey drifts between different floors of both the models. Storey drift is the drift of one level of a multi-storey building relative to the level below. Here the storey drifts varies in a similar manner for both regular and irregular structures. Maximum storey drift is observed at second floor ( ) for irregular structure and at fourth floor for regular structure ( ). Figure 5.3 Storey Shear Fig. 5.3 shows the shear force acting at different storey levels. Story shear is a force that acts on any storey in a direction perpendicular to its extension and is measured in kn. For both the structures it is highest at bottom and it decreases linearly towards top. For regular structure maximum story shear is kN (ground floor) and for irregular structure maximum story shear is kN (ground floor) editor@iaeme.com

9 Seismic Analysis of High Rise Buildings with Plan Irregularity Figure 5.4 Storey Stiffness Fig.5.4 illustrates the variation of storey stiffness at different floor levels of both the models. Earthquake loads in buildings generally increase with the density of the building. Yield strength is maximum horizontal load that can be applied to a building. Storey stiffness is the horizontal force distributed throughout a building divided by resulting lateral shear strain in the building (usually called drift). Here storey stiffness varies non - linearly for both the structures. For regular structure maximum story stiffness is 94.27kN/m (first floor and second floor) and for irregular structure maximum story stiffness is 78.75kN/m (first floor and second floor). Figure 5.5 Storey Overturning Moments Fig.5.5 illustrates the variation of storey overturning moments at different floor levels of both the models. Storey overturning moment of a building is the moment of energy capable of upsetting the storey; that is the point where the storey has been subjected to enough disturbances that it ceases to be stable, it overturns, capsizes, collapses, topples and eventually the structure fails. Here both the structures are safe, since there are no positive overturning moments and only negligible negative overturning moment towards base ( kN/m for regular structure at ground floor and kN/m for irregular structure at ground floor level) both of which can be approximated to zero. Hence it can be assumed that both the structures have zero overturning moments and are hence safe editor@iaeme.com

10 Albert Philip and Dr. S. Elavenil CONCLUSIONS On the basis of analytical results of the study, the following conclusions were drawn: 1. The floor plan of the structure has significant impact on the seismic behaviour of the structure in terms of displacement, storey drift and storey shears. 2. Storey displacement is linearly increasing (approx. by 2%) from bottom to top for both the structures and is more for irregular structure. The displacement values for both the structures are within the allowable limits (163.6mm at roof level for regular building and 181.6mm at roof level irregular building). 3. Maximumstorey drift is observed at second floor for irregular structure ( ) and at fourth floor for regular structure ( ). 4. From the seismic analysis, maximum storey shear force was observed at ground floor for both the structures (177.21kN for regular and kN for irregular) and the value decreases by 2% with height. 5. Storey stiffness varies non - linearly for both the structures with maximum values at first and second floor (94.27kN/m for regular and 78.75kN/m for irregular). 6. Overturning moments for all the stories of both the structures are approximately equal to zero ( kN/m for regular building and kN/m for irregular building), hence making the structure safe. 7. Compared to regular building, more displacements, storey drifts and storey shears were observed for the irregular building which implies that building with severe plan irregularity shows maximum displacement and storey drift. In general, the displacements increases linearly with height of the building; maximum storey drift is observed at second floor for irregular structure and at fourth floor for regular structure; maximum storey shear force was observed between ground floor and second floor for regular structure and at ground floor for irregular structure and the value decreases linearly with height; storey stiffness varies non - linearly for both the structures with maximum values at ground floor. Also there is negligible or zero overturning moments. REFFERENCES [1] Ravindra N. Shelke (2017) Seismic Analysis of Vertically Irregular RC Building Frames International Journal of Civil Engineering and Technology (IJCIET) 8(1), January 2017, pp [2] Krishna G Nair (2017) Seismic Analysis of Reinforced Concrete Buildings A Review International Research Journal of Engineering and Technology (IRJET) e-issn: Volume: 04 Issue: 02 Feb [3] Elavenil S (2015) Analytical Investigation on the Performance of Steel Frame with Solid and Hollow Sections, Romanian Journal of Social Sciences, Vol.1 No.1, pp [4] Sakshi A. Manchalwar (2014) Seismic Analysis of RC Frame A Parametric Study, International Journal of Engineering Research & Technology (IJERT) ISSN: Vol.3 Issue 9, September [5] Hassaballa A. E. (2013) Seismic Analysis of a Reinforced Concrete Building by Response Spectrum Method, IOSR Journal of Engineering (IOSRJEN) e-issn: , p-issn: Vol. 3, Issue 9 (September 2013), V3 PP [6] Wakchaure M.R (2012) Earthquake Analysis of High Rise Building with and without Infilled Walls, Int Journal of Eng and Innovative technology (IJEIT) 2(2), Aug [7] Elavenil S. (2011) Time History Response Prediction for Multi-Storied buildings under Earthquake Ground Motions, International Journal of Civil, Structural, Environment and Infrastructure Engineering Research and Development (IJCSEIERD) Vol-1 No.2, pp editor@iaeme.com

11 Seismic Analysis of High Rise Buildings with Plan Irregularity [8] Romy Mohan (2011) Dynamic Analysis of RCC Buildings with Shear Wall International Journal of Earth Sciences and Engineering ISSN , Volume 04, No 06 SPL, October 2011, Pp [9] K. S. Babu Narayan Seismic Performance Evaluation of RC Buildings with Vertical Irregularity Department of Civil Engineering, National Institute of Technology Surathkal, Karnataka, India. [10] Mahesh S. Comparison of Analysis and Design of Regular and Irregular Configuration of Multi-Story Building in Various Seismic Zones and Various Types of Soils Using ETABS and STAAD Department of Civil Engineering, V R Siddhartha Engineering College, India. [11] IS 1893 (Part I):2002 Criteria for Earthquake Resistant Design of Structures Bureau of Indian Standards, New Delhi, India. [12] IS-875 (Part 1):1987 Dead Loads on Buildings and Structures Bureau of Indian Standards, New Delhi, India. [13] IS-875 (Part 2):1987 Live Loads on Buildings and Structures Bureau of Indian Standards, New Delhi, India. [14] IS-456:2000 Plain and Reinforced Concrete Code of Practice - Bureau of Indian Standards, New Delhi, India. [15] IS-4326:1993(2005) Indian Standard Code of Practice for Earthquake Resistant Design and Construction of Building (Second Revision) - Bureau of Indian Standards, New Delhi, India. [16] IS-13920:1993 Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces - Bureau of Indian Standards, New Delhi, India editor@iaeme.com