EARTHQUAKE ANALYSIS OF RCC BUILDINGS ON HILLY AREAS WITH VARYING SLOPES

Transcription

1 EARTHQUAKE ANALYSIS OF RCC BUILDINGS ON HILLY AREAS WITH VARYING SLOPES N PAVAN KUMAR YADAV 1*, P RAJESH 2* 1. Student, Dept of CEL, St. Marys Group of Institutions, India. 2. Asst. Prof, Dept of CE, St. Marys Group of Institutions, India. ABSTRACT Highly seismic in India are in hilly areas. 3D analytical model of G+10 storied buildings have been generated for asymmetric building models and analyzed using structural analysis ETABS. To study the effect of varying height of columns in ground storey due to sloping ground, the plan layout is kept similar for sloping ground. The seismic forces are considered as per IS: The beaviour of a G+10 storey RCC building on varying slope angles i.e., 5 o to 15 o is studied. The analytical results of story displacement, story drift and story shear for varying sloped ground and flat ground are compared. 1. INTRODUCTION 1.1 GENERAL Multistoried R.C. framed buildings are getting popular in hilly areas because of increase in land cost and under unavoidable circumstances due to shortage of land in urban areas. Thus, many of them are constructed on hilly slopes. Setback multistoried buildings are frequent over level grounds whereas step back buildings are quite common on hilly slopes. Combinations of step back and setback buildings are also common on hilly slopes. At the location of setback stress concentration is expected when the building is subjected to earthquake excitation. These are generally not symmetrical due to setback and/or step back and result into severe torsion under an earthquake excitation. Current building code suggests detailed dynamic analysis for these types of buildings. Buildings in hilly areas are irregular and asymmetric and therefore are subjected to severe torsion in addition to lateral forces under the action of earthquake forces. Many buildings on hill slopes are supported by columns of different heights. The shorter columns attract more forces as the stiffness of the short columns is more and undergo damage when subjected to earthquakes. Buildings in hilly areas are subjected to lateral earth pressure at various levels in addition to other normal loads as specified on building on level grounds. Building loads transmitted at the foundation level to a slope create problem of slope instability and may result into collapse of the building. The soil profile is non uniform on the hilly slopes and result into total collapse of the building. The bearing capacity, cohesion, angle of internal friction, etc. may bedifferent at different levels. It may result into unequal settlement of foundations and local failure of the slope. Earthquake is the most disastrous due to its unpredictability and huge power of devastation. Earthquakes themselves do not kill people, rather the colossal loss of human lives and properties occur due to the destruction of structures. Building structures collapse during severe earthquakes, and cause direct loss of human lives. Numerous research works have been directed worldwide in last few decades to investigate the cause of failure of different types of buildings under severe seismic excitations. Massive destruction of high rise as well as lowrise buildings in recent devastating earthquake proves that in developing counties like India, such investigation is the need of the hour. Hence, seismic behavior of asymmetric building structures has become a topic of worldwide active research. Many Investigations have been conducted on elastic and inelastic seismic behavior of asymmetric systems to find out the cause of seismic vulnerability of such structures. The

2 purpose of the paper is to perform linear static analysis of medium height RC buildings and investigate the changes in structural behavior due to consideration of sloping ground. An important feature in building configuration is its regularity and symmetry in the plane and elevation. Buildings on hill slope are highly irregular and asymmetric in plan and elevation. One of the major contributors to structural damage during strong earthquake is the discontinuities and irregularities in the load path or load transfer. The lateral load such as earthquake is to be classified as live horizontal force acting on the structure depending on the building s geographic location, height, shape and structural materials. A building with an irregular configuration may be designed to meet all code requirements but it will not perform well as compared to a building with a regular configuration. 1.2 SEISMIC BEHAVIOUR OF BUILDINGS ON SLOPES IN INDIA: North and northeastern parts of India have large scales of hilly region, which are categorized under seismic zone IV and V. In this region the construction of multistory RC framed buildings on hill slopes has a popular and pressing demand, due to its economic growth and rapid urbanization. This growth in construction activity is adding increase in population density. While construction, it must be noted that Hill buildings are different from those in plains i.e., they are very irregular and unsymmetrical in horizontal and vertical planes, and torsionally coupled. Since there is scarcity of plain ground in hilly areas, it obligates the construction of buildings on slopes. During past earthquakes, reinforced concrete (RC) frame buildings that have columns of different heights within one storey, suffered more damage in the shorter columns as compared to taller columns in the same storey. One example of buildings with short columns in buildings on a sloping ground can be seen in the fig-1 given. Fig-1: Building Frame with Short Columns 1.3 CONFIGURATION OF BUILDINGS ON HILL SLOPES Buildings constructed in hilly areas have peculiar structural configurations. Successive floors of such buildings step back towards the hill slope and sometimes, the buildings also set back, as shown in the Fig-2. The stepping back of building towards hill slope results in unequal column heights in the same storey, which causes severe stiffness irregularities in along- and cross-slope directions. When subjected to lateral loads in cross-slope direction, even the buildings with symmetric rectangular configurations are subjected to significant torsional coupling due to varying lateral stiffness of uphill and downhill side frames. The torsional behavior of these buildings is much more complex than that of buildings on flat ground due to shifting of centre of stiffness and centre of mass with floor level. Under along-slope excitation, the buildings having a symmetric configuration are not subjected to torsion, but the shorter columns on uphill side of a storey take the major share of the storey shear, which is usually much higher than their capacity and may result in shear failure. Fig -2: (a) Buildings step back towards the hill slope, (b) Buildings also set back. 1.4: OBJECTIVES OF THE STUDY

3 The present thesis work is aimed at evaluating hypothetical existing RC framed building with the following objectives: Generation of 3D building model for both elastic and inelastic method of analyses. Determination of deflections and storey drifts at each storey using Response Spectrum method and Pushover analysis. Determination of performance level of building using static analysis. To study on the influence of varying sloped ground on the overall behavior of structure when subjected to lateral seismic forces. To study the effect of vertical irregularity on the fundamental natural period of the building and its effect on performance of the structure during earthquake for different building models selected. To find out the damage distribution in the structure due to earthquake loading. 2. REVIEW OF LITERATURE Chen and Constantinou (1998) studied that the practical system deliberately introduces flexibility to the sloping ground storey of structures was described. The system utilizes Teflon sliders to carry aportion of the superstructure. Energy dissipation is provided by the ground story ductile column s and by the Teflon sliders. Utilizing this concept the seismic response characteristics of a multistory frame are analyzed an discussed. The results show that it is possible to provide safely to the superstructure while maintaining the stability of the ground storey. Chandrasekaran and Rao (2002) investigated analysis and the design of multi storied RCC buildings for seismicity. Reinforced concrete multi storied buildings re very complex to model as structural systems for analysis. Usually, they are modeled as two dimensional or three dimensional frame,systems are in to plane and slope with different angles 5 10, and 15.Analyze multistoried buildings in the country for seismic forces and comparing the axial force, shear force, moment, nodal displacement, stress in beam and support reaction compared t o current version of th e IS: to the last version IS: Birajdar B.G. (2004) presented the results from seismic analyses performed on 24 RC buildings with three different co figurations like, Step back building; Step back Set back building and Set back building are presented. 3 D analysis including tensional effect has been carried out by using response spectrum method. The dynamic response properties i.e. fundamental time period, to p storey displacement and, the base shear action induced in columns have been studied with reference to the suitability of a building configuration on sloping ground. It is observed that Step back Set back buildings are found to be more suitable on sloping ground. Kadid A. and Boumrkik A. (2005) studied experimental pushover an alysis was carried out with a study the performance e of framed buildings u nder future expected earthquakes. Sloping ground are consider the three framed buildings with 5, 8 and 12 stories respectively were analyzed. The results obtained in these three buildings and compare the axial force, bending moment, nodal displacement, base shear and s haws that properly designed frames will perform well under seismic codes. Some of the conclusions made by the authors are the pushover analysis is a relatively simple way to explore the linear and non linear behavior of Buildings. Abu L. (201 0) studied the Analysis of earthquake resistant building using Site Response spectra method. According to the Indian standard for Earthquake resistant design (IS: 1893), the seismic force depends o n the zone factor (Z) and the average response acceleration coefficient (Sa/g) of the soil types at twenty meter depth with suitable modification depending upon the depth of foundation in plane and sloping ground. In the their studied an attempt has been made to generate response spectra using site specific soil parameters for some sites in seismic zone III and IV, study the variation of top storey displacement with respect to different sloping angle i. e. Arunachal Pradesh and Meghalaya are the generated response spectra is used to analyze some structures using commercial software staad pro. Saptadip S. (2010) studied the Design of Earthquake resistant multi stories RCC building on a sloping ground which involves the analysis of simple 2 D frames of varying floor heights and varying no of bays using a very popular software tool Staad PRo. Using the analysis results various graphs were drawn between the maximum axial force, maximum shear force maximum bending moment, maximum t ensile

4 force and maximum compressive stress being developed for the frames on plane ground and sloping ground. Balaji K.V.G.D (2011) studied the non linear analysis of various symmetric and asymmetric structures constructed on plain as well as sloping grounds subjected to various kinds of loads. Different structures constructed on plane ground and inclined ground of 30o slope is considered in the study. Various structures are considered in plan symmetry and also asymmetry with difference in bay sizes in mutual directions. The analysis has been carried out using SAP 2000 and staad pro software. Mohammed U. and Farooque P. (2012) studied the buildings on hill differ from other buildings. The various floors of such building steps back towards the hill slope and at the same time buildings may have setbacks also. Buildings situated in hilly areas are much more vulnerable to seismic environment. In this study the effect of varying height of columns in ground storey due to sloping ground and the effect of shear wall at different positions during earthquake. Seismic analysis has been done using Linear Static, Linear Dynamic method and evaluated using pushover analysis Eight Storied building. Keyvan Ramin (2013) studied the experimental modeling and numerical modeling for a four story reinforced concrete building that the analysis of simple 3 D frames of varying floor heights and varying number of bays with different slope angles using a very popular software tool Staad PRo. on both a sloping and a flat lot. Also Staad PRo software had been used to show that the displacement of floors is greater for a flat lot building than a sloping lot building. Pradeep Kumar Ramancharla (2013) studied the behavior of a building on varying slope angles i.e., 15, 30 and 45 is studied using shear wall in different location and compared with the same on the flat ground. Building is designed as per IS 456 and later subjected to earthquake loads. The salient objectives of the present study have been to study the effect of sloping ground on structural forces in critical horizontal reaction, bending moment in footing and critical axial force, bending moment in columns Rajeeva and Tesfamariam (2012) Fragility based seismic vulnerability of structures with consideration of soft -storey (SS) and quality of construction (CQ) was demonstrated on three, five, and nine storey RC building frames designed prior to 1970s. Probabilistic seismic demand model (PSDM) for those gravity load designed structures was developed, using non-linear finite element analysis, considering the interactions between SS and CQ. The response surface method is used to develop a predictive equation for PSDM parameters as a function of SS and CQ. Result of the analysis shows the sensitivity of the model parameter to the interaction of SS and CQ. Sarkar et al. (2010) proposed a new method of quantifying irregularity in vertically irregular building frames, accounting for dynamic characteristics (mass and stiffness). The salient conclusions were as follows: (1)A measure of vertical irregularity, suitable for stepped buildings, called regularity index, is proposed, accounting for the changes in mass and stiffness along the height of the building. (2) An empirical formula is proposed to calculate the fundamental time period of stepped building, as a function of regularity index. 3. ANALYSIS OF STRUCTURE: A software ETABS has been used to study the changes of the Structural Behaviour for different shapes of R.C Building on plan and on sloping ground under the lateral load effect such as earthquake load, According to IS 1893:2002, Both the equivalent lateral force procedure (static method) and response spectrum analysis procedure (dynamic method) lead directly to lateral forces in the direction of the ground motion component. The main differences between the two methods are in the magnitude and distribution of the lateral load over the height of the building. Most building codes prescribe the method of analysis based on whether the building is regular or irregular. Almost all the codes suggest the use of static analysis for symmetric and selected class of regular buildings. For buildings with irregular configurations, the codes suggest the use of dynamic analysis procedures such as response spectrum method or time history analysis. Seismic codes gives different methods to carry out lateral load analysis, while carrying out this analysis infill walls present in the structure are normally considered as non

5 structural elements and their present in the structure are normally considered as non structural elements and their presence is usually ignored while analysis and design. However even though they are considered as non-structural elements, they tend to interact with the frame when the structures are subjected to lateral loads. This present work deals with study of behavior of sloping ground building frames considering different inclination (5 o, 10 o, 15 o ) under earthquake forces. The comparison of sloping ground and plane ground building under seismic forces is done. Here G+ 8 storey is taken and same live load is applied in three the buildings for its behavior and comparison. The framed buildings are subjected to vibrations because of earthquake and therefore seismic analysis is essential for these building frames. The fixed base system is analyzed by employing in three building frames in seismic zone II by means of ETABS. The response of three the building frames is studied for useful interpretation of results. Description Of Structural Models BASIC DATA FOR BUILDINGS MODEL: Height of each storey: (3) m Number of storeys: G+ 10 storeys Dimension of Column: (450 X 230) mm Dimension of Beam : (230 X300) mm Walls Thickness: (230) mm thick Shear wall Grade of the concrete: M 25 Grade of the steel: Fe- 500 Type of Soil: Type II, Medium Soil Seismic Zone: II Building Frame Systems: Ordinary RC momentresisting Live Load on Typical Floor: (2.0) KN/m2 Wind speed: (44) m/s Support: Fixed Fig-1: General Plan View of G+10 Storey RCC Building Fig-2: Elevation of 5 o Sloped Ground Building.

6 Fig-3: Elevation of 10 o Sloped Ground Building. Story Shear (kn) Vx Vy STORY STORY STORY STORY STORY STORY STORY STORY STORY STORY Table-3: Story Shear for Building with 10 o Deg Slope. Fig-4: Elevation of 15 o Sloped Ground Building. 4. RESULTS AND DISCUSSION Seismic analysis is performed on the all models of the building, i.e. building with varying slopes. Response spectrum method is used for the analysis in ETABS. The parameters like storey shear; storey displacement, storey drift and lateral storey stiffness are calculated and compared in Earthquake zones II. 4.1 Analysis of Storey Shear: Table-1: Story Shear for Building with 0 o Deg Slope. Story Shear (kn) Vx Vy STORY STORY STORY STORY STORY STORY STORY STORY STORY STORY Table-2: Story Shear for Building with 5 o Deg Slope. Story Shear (kn) Vx Vy STORY STORY STORY STORY STORY STORY STORY STORY STORY STORY Table-4: Story Shear for Building with 15 o Deg Slope. Story Shear (kn) Vx Vy STORY STORY STORY STORY STORY STORY STORY STORY STORY STORY

7 STORY Table 6: Story Drift for Building with 5 o Deg Slope Fig-5: Story Shear Graph with Varying Slopes along X- Direction. Story Drift (mm) X-Dir Y-Dir Base STORY STORY STORY STORY STORY STORY STORY STORY STORY STORY Table 7: Story Drift for Building with 10 o Deg Slope Fig-6: Story Shear Graph with Varying Slopes along Y- Direction. 4.2 STORY DRIFT: Storey drift is the lateral displacement of the storey. It is the drift of one level of a multistory building relative to the level of below storey. Table 5: Story Drift for Building with 0 o Deg Slope Story Drift (mm) X-Dir Y-Dir Base STORY STORY STORY STORY STORY STORY STORY STORY STORY Story Drift (mm) X-Dir Y-Dir Base STORY STORY STORY STORY STORY STORY STORY STORY STORY STORY Table 8: Story Drift for Building with 15 o Deg Slope Story Drift (mm) X-Dir Y-Dir Base STORY STORY STORY STORY STORY STORY STORY STORY STORY STORY

8 STORY STORY STORY STORY STORY Fig-7: Story Drift Graph with Varying Slopes along X- Direction. Fig -9: Story Displacement Graph with Varying Slopes CONCLUSION Fig-8: Story Drift Graph with Varying Slopes along Y- Direction. 4.3 STORY DISPLACEMENT: Storey displacements are the vertical displacements of members, occurs due to dead and live loads. Table-9: Story Displacement for Building with Varying Slopes Vertical Displacement (m) 0 o 5 o 10 o 15 o Base STORY STORY STORY STORY STORY The following conclusions from this study are: The performance of reinforced concrete frames was investigated using the static linear analysis. These are the conclusions drawn from the analysis. In step back and set back frames, it is observed that extreme left columns, which are on the higher side of the sloping ground and are short, are most affected. So, special attention is required while detailing and designing these short columns. From this study we observed that as the storey height increases the base shear and displacement also increases. But according to the performance of structure for slope at an angle of 15 degree it suggests up to four storey as the height increases it leads to collapse state. As the height increases storey drifts also increases. From this analysis the observations are as the angle of slope increases base shear increases and target displacement decreases. The base shear acts more in longitudinal direction than in transverse direction.

9 As the angle of slope increases displacement and drifts decreases. REFERENCE [1]. Agarwal.P. and Shrikhande. M., 2006 Earthquake Resistant Design of Structures Prentice-Hall of India Private Limited, New Delhi, India. [2]. Arlekar N.J., Jain K.S., and Murthy C.V.R, 1997 Seismic Response of RC Frame Buildings with Soft First Storeys Proceedings of the CBRL Golden Jubilee Conference on Natural Hazards in Urban Habitat, New Delhi. [3]. Applied Technology Council, 1996: Seismic Evaluation and Retrofit of Concrete Buildings, ATC-40, Volume 1. [4]. Brick infills in RC frames: E-Conference Proceeding, January 28, [5]. IS: 1893 (Part-I) 2002 Criteria for Earthquake Resistant Design of Structures, Part 1 General provisions and buildings, fifth revision, Bureau of Indian Standards, New Delhi. [6]. Krawinkler H. and Seneviratna G.D.P.K, 1998: Pros and Cons of Pushover Analysis of Seismic Performance Evaluation. [7]. Kanitkar R. and Kanitkar V., 2004 Seismic Performance of Conventional Multi-storey Buildings with open ground storey for Vehicular Parking, Indian Concrete Journal. [8]. Lee H.S. and Woo W.S., 2001 Effect of Masonry infills on Seismic Performance of a 3 Storey RC Frame with non-seismic Detailing, John Wiley & Sons, Ltd. [9]. Murthy C.V.R. and Jain S.K., 2000 The Beneficial Influence of Masonry Infill Walls on the Seismic Performance of RC Framed Buildings, Indian Institute of Technology, Kanpur, 12th World Conference on Earthquake Engineering, Auckland, New Zealand. [10]. Naik. Pramodini, Dyavanal S.S., and Annigeri. S.A., 2006 Performance Evaluation of Multistoreyed Buildings Subjected to Earthquake Forces, Proceedings of National Conference on Earthquake Analysis and Design of Structures. [11]. Ravi Sinha et al: Earthquake Resistant Capacity of Reinforced Concrete Frame Buildings, Technical Project Report Volume 2, Indian Institute of Technology, Bombay [12]. Gupta A, Krawinkler H. Estimation of seismic drift demands for frame structures. Earthquake [13]. Gupta B, Kunnath SK Adaptive spectrabased pushover procedure for seismic evaluation of structures. Earthquake Spectra. [14]. Chopra AK, Goel RK A modal pushover analysis procedure to estimate seismic demands for Buildings: theory and preliminary evaluation. PEER Report 2001/03, Pacific Earthquake [15]. Chopra AK, Goel RK A modal pushover analysis procedure for estimating seismic demands for buildings. Earthquake Engineering Structural Dynamics.