International Journal of Engineering Science Invention Research & Development; Vol. II Issue VII January e-issn:

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1 PUSHOVER ANALYSIS OF MULTISTOREY BUILDINGS HAVING FLAT SLAB AND GRID SLAB Anuj Bansal 1, Aditi Patidar 2 Civil Engineering Department, IES, IPSA, Indore 1 anujb261@gmail.com, 2 vishal03arora@gmail.com Abstract In this modern industrial time we can see large construction activities happening everywhere, hence there will be a shortage of land space, so construction of tall structures has been initiated up to overcome this problem. There are numerous elements which are modified to make work faster and economical also like introducing flat slab and grid slab construction which reduces dead weight, and makes beams invisible, enhances floor area. The object of the present work is to compare the behaviour of multi-storey buildings having flat slab with that of having grid slab and to study the effect of base shear, storey drift and maximum displacement on it under seismic forces. For this purpose three cases of multi-storey buildings are considered with area 20 m x 20 m having 4 storey, 8 storey and 12 storey with 3.6 m storey height considered. All the three cases are considered having flat slab and grid slab, and also analyzed by using software SAP2000. Observation shows that pushover analysis is a simple way to explore the nonlinear behavior of building. Also pushover analysis is an approximation method based on static loading. It may not accurately represent dynamic phenomena. Performance points for flat slab are larger than in grid slab models. Resultant displacements for flat slab are quite larger than in grid slab models and also base shear in both types of slabs is almost similar. Present work provides a good source of information on the parameters of pushover analysis of multistory buildings. Keywords Flat Slab, Grid Slab, Performance Point, Base Shear, Displacement. I. INTRODUCTION In this modern industrial time we can see large construction activities happening everywhere, hence there will be a shortage of land space, so construction of tall structures have been initiated up to overcome this problem. There are numerous elements which are modified to make work faster and economical also like introducing flat slab and grid slab construction which reduces dead weight, and makes beams invisible, enhances floor area. We can t avoid future earthquakes, but awareness and safe building construction practices can certainly reduce the extent of damage and failure. With the purpose of strengthening and resisting the buildings for upcoming earthquakes, some actions have to be adopted. One of the methods is the static pushover analysis which is becoming a popular tool for seismic performance evaluation of existing and new structures. By conducting this push over analysis, we can know the weak zones in the structure and then we can make a decision that whether the particular part is retrofitted or rehabilitated as per requirement. II. LITERATURE REVIEW Research on the behaviour of the flat slab and grid slab with different models is described below: 2015, Gouramma G, Dr. Jagadish Kori G Seismic Performance Of Different RC Slab Systems For Tall Building, investigated analytically different types of RC slab taken as an example and performed the various analytical approaches (linear and nonlinear analysis) on the building to identify the seismic demand and also performed pushover analysis to determine the performance levels. The analysis of seismic behavior of different type of RC slab systems is done by using ETABS software v This study consists three systems, Conventional RC slab system (Model 1), ribbed slab system (Model 2), and Flat slab system (Model 3) in different height i.e. 10 storey, 15 storey, 20 storey and base is same for all the models(36 m x30 m). These three systems are the most attractive and commonly used floor systems, flat slab is flexible and it is Vulnerable in earthquake resistance especially in high-rise constructions. So certain modification is done, flat slab with edge beam system (Model 4),Flat slab with shear wall system (Model 5) the building is designed for gravity loads as per IS The tall building is located in seismic zone-ii, Anuj Bansal and Aditi Patidar ijesird, Vol. II (VII) January 2016/ 435

2 III, IV, V and soil type is II in accordance with IS (part-1). They concluded that pushover analysis is useful tool to performance based seismic engineering to study post yield behavior of structure. It is more complex than traditional linear analysis. 2015, Mohana H.S, Kavan M.R Comparative Study of Flat Slab and Conventional Slab Structure Using ETABS for Different Earthquake Zones of India, analyzed a G+5 commercial multistoried building having flat slab and conventional slab for the parameters like base shear, storey drift, axial force, and displacement. The performance and behavior of both the structures in all seismic zones of India has been studied. The storey shear of flat slab is 5% more than conventional slab structure, the axial forces on flat slab building is nearly 6% more than conventional building, the difference in storey displacement of flat and conventional building are approximately 4mm in each floor. The work provides reasonable information about the suitability of flat slab for various seismic zones without compromising the performance over the conventional slab structures. They concluded the following observations: 1. Storey shear of flat slab is 6% more compared to conventional slab structure, and storey shear is Maximum at base and least at top storey. 2. The design axial forces on flat slab are more compared to conventional structure the difference of forces is nearly 5.5%. 3. Storey displacement is Maximum at roof level than at base, and storey displacement of flat slab structure is greater than conventional structure, there will be an average 4mm displacement variation in each seismic zone for both structures. 4. As the seismic level increases all parameters like axial force, displacement, storey shear intensities are increases. 2015, Pradip S. Lande, Aniket B. Raut Seismic Behaviour of Flat Slab Systems, carried out a parametric investigation in order to identify the seismic response of systems a) flat slab building b) flat slab with parametric beams c) flat slab with shear walls d) flat slab with drop panel. e) Conventional building the aforementioned hypothetical systems were studied for two different storey heights located in zone v. and analysed by using ETABS Nonlinear version Linear dynamic analysis i.e. response spectrum analysis was performed on the system to get the seismic behaviour. On the basis of the results following conclusions have been drawn: 1. The storey displacement is found maximum for the flat slab building as compared to conventional RC building and flat slab with shear wall the maximum displacement of the flat slab building is due to the absence of lateral load resisting system. 2. The maximum storey drift found for G+6 building having a flat slab (As compared to its maximum limit i.e. 0.04% of height) 3. For all the cases considered drift values follow a parabolic path along storey height with maximum value lying somewhere near the middle storey. 4. It is found that flat slab structures exhibit higher flexibility compared to traditional frame structures. In order to limit deformation demands under the seismic excitations, combination with other stiffer structural systems as shear-walls is advisable. 5. The maximum storey drift also found for flab slab. Min displacement found for flat slab with shear wall. Flat slab displacement is found 28% more than of conventional building for G+ 6. And 49.49% for G+12 building. Therefore it is advisable for tall building to use the shear wall. 2015, Salman I. Khan, Ashok R. Mundhada Comparative study of Seismic Performance of multistoried R.C.C buildings with Flat slab & Grid slab, performed dynamic analysis of three different high rise buildings having 12, 15 & 18 stories using response spectrum method for all four seismic zones of India, as categorized by the Indian code for earthquake resistant structures. The assessment of the seismic response is based on the maximum inter-story drift, roof displacement, Time period and the base shear. E-TAB v9.7.3 software is used for the analysis. It is observed that the seismic performance of grid slab buildings was better as compared to that of flat slab buildings. Anuj Bansal and Aditi Patidar ijesird, Vol. II (VII) January 2016/ 436

3 Based on the results the following conclusions could be drawn: 1. The choice of the system for slab in the tall building is very important to resist the internal forces and stability. 2. The base shear will increase drastically as the height increases. Base shear of flat slab building is more than that of the grid slab building. The difference between the two varies from 3-4(%). 3. The lateral displacement (both Ux and Uy) is maximum at terrace level for all types of columns. Lateral displacement of Grid slab building is less than that of the flat slab building. The difference between the two is less if the building width is more. 4. Storey drift in buildings with flat slab construction is significantly more as compared to Grid slab buildings. As a result of this, additional moments are developed. Therefore, the columns of such buildings should be designed by considering additional moments caused by the drift. The difference between the two is less if the building width is more. 5. For improving drift conditions of flat slab in higher seismic zones, lateral load resisting system should be coupled with slab column frame or stiffness of columns should be increased. 6. The natural time period increases as the height increases. In comparison of the grid slab building and flat slab building, the time period is more for flat slab building than that of grid slab building. The difference between the two is about 23(%). 2014, A. E. Hassaballa, M. A. Ismaeil, A. N. Alzead, Fathelrahman M. Adam Pushover Analysis of Existing 4 Storey RC Flat Slab Building, analysed a four-story residential existing reinforced concrete building in the city of Khartoum-Sudan, subjected to seismic hazard. Plastic hinge was used to represent the failure mode in the beams and columns when the member yields. The pushover analysis was performed on the building using SAP2000 software (Ver.14) and equivalent static method according to UBC 97. The principles of Performance Based Seismic Engineering were used to govern the analysis, where inelastic structural analysis is combined with the seismic hazard to calculate expected seismic performance of a structure. Base shear versus tip displacement curve of the structure, called pushover curve, is an essential outcomes of pushover analysis. The pushover analysis is carried out in both positive and negative x and y directions. Default hinge properties, available in some programs based on the FEMA -356 and Applied Technology Council (ATC-40) guidelines were used for each member. One case study has been chosen for this purpose. The evaluation has proved that the four-story residential building is not seismically safe. The main output of a pushover analysis was in terms of response demand versus capacity. If the demand curve intersects the capacity envelope near the elastic range, then the structure has a good resistance. If the demand curve intersects the capacity curve with little reserve of strength and deformation capacity, then it can be concluded that the structure will behave poorly during the imposed seismic excitation and need to be retrofitted to avoid future major damage or collapse. 2013, Dhananjay D. Joshi, Dr. Pranesh B. Murnal Performance of Flat Slab Structure Using Pushover Analysis, performed push over analysis on flat slabs by using software SAP2000.For analysis they have considered a (G+7) frame having 5 bays. It is observed that when compared to beamcolumn connections, flat slabs are becoming popular and gaining importance as they are economical. From the analysis they concluded the following points: 1. The pushover analysis is a relatively simple way to explore the non linear behavior of buildings. 2. Base shear of conventional RCC building is more than the flat slab building. 3. The performance point of flat slab is more than the conventional structure due to its flexibility. 4. The behavior of properly detailed conventional building is adequate as intersection of the demand and capacity curves. 5. The results obtained in terms of demand, capacity gave an insight into real behavior of the structure. Anuj Bansal and Aditi Patidar ijesird, Vol. II (VII) January 2016/ 437

4 2012, Amit A. Sathawane, R.S. Deotale Analysis And Design of Flat Slab And Grid Slab And Their Cost Comparison, aimed to determine the most economical slab between flat slab with drop, Flat slab without drop and grid slab. The proposed construction site is Nexus point apposite to Vidhan Bhavan and beside NMC office, Nagpur. The total length of slab is m and width is m. total area of slab is sqm. It is designed by using M35 Grade concrete and Fe415 steel. Analysis of the flat slab and grid slab has been done both manually by IS and by using software also. Flat slab and Grid slab has been analyzed by STAAD PRO. Rates have been taken according to N.M.C. C.S.R It is observed that the Flat slab with drop is more economical than Flat slab without drop and Grid slabs. Also they concluded the following points: 1. Drops are important criteria in increasing the shear strength of the slab. 2. Enhance resistance to punching failure at the junction of concrete slab and column. 3. By incorporating heads in slab, we are increasing rigidity of slab. 4. The negative moment s section shall be designed to resist the larger of the two interior negative design moments for the span framing into common supports. 5. Concrete required in grid slab is more as compared to flat slab with and without drop. 6. Steel required in flat slab without drop is more as compared to flat slab with drop and grid slab. 7. Flat slab with drop is more economical than flat slab without drop and grid slab. 8. Rate per square meter of flat slab with drop (3524) was found more economical than flat slab without drop (3855) and grid slab (4005). 2012, K. Soni Priya, T. Durgabhavani, K. Mounika, M.Nageswari, P.Poluraju Non-Linear Pushover Analysis of Flatslab Building by using SAP2000, performed the push over analysis on flat slabs by using most common software SAP2000. According to this paper many existing flat slab buildings may not have been designed for seismic forces. Hence it is important to study their response under seismic conditions and to evaluate seismic retrofit schemes. But when compared to beamcolumn connections, flat slabs are becoming popular and gaining importance as they are economical. A typical flat slab with two stories is considered in the present study. It has not been detailed for earthquake loads but have been designed for wind loads. The height of the each storey is 3m. For the purpose of design, linear structural analysis of the building is carried out by the computer program SAP2000 (Structural Analysis Program, Version 12.00). The slab of the building is assumed to be acting like a rigid floor diaphragm. The building is designed as per IS: using limit state method of design. The load combinations used are as follows: 1.5 DL+1.5 LL 1.5 DL+1.5 LL+1.2 WL The materials used are M 20 grade concrete and Fe 415 grade steel. The thickness of the slab is 200 mm and no shear reinforcement is provided in the slabs. The columns are 250 mm square in section. Under the pressure of recent developments, seismic codes have begun to explicitly require the identification of sources of inelasticity in structural response, together with the quantification of their energy absorption capacity. Many existing buildings do not have been designed for seismic forces. It is important to study their response under seismic conditions and to evaluate seismic retrofit schemes. Hence push over analysis has been gaining importance for the strengthening and evaluation of the existing structures. By conducting the pushover analysis on flat slabs, pushover curve and demand curve can be obtained. Then, based on the results we need to decide whether to perform rehabilitation or retrofitting depending upon the seismic zone of the existing structures. M. A. Ismaeil, Pushover Analysis of Existing 3 Stories RC Flat slab Building, focused on the study of seismic performance of the existing hospital buildings in the Sudan. Plastic hinge is used to represent the failure mode in the beams and columns when the member yields. The pushover analysis was performed on the building using SAP2000 software (Ver.14) and equivalent static method according to UBC 97. The principles of Performance Based Seismic Engineering are used to Anuj Bansal and Aditi Patidar ijesird, Vol. II (VII) January 2016/ 438

5 govern the analysis, where inelastic structural analysis is combined with the seismic hazard to calculate expected seismic performance of a structure. Base shear versus tip displacement curve of the structure, called pushover curve, is an essential outcomes of pushover analysis. The pushover analysis is carried out in both X and Y directions. Default hinge properties, available in some programs based on the FEMA -356 [3] and Applied Technology Council (ATC-40) [4] guidelines are used for each member. One case study has been chosen for this purpose. The evaluation has proved that the three stories hospital building is seismically safe. The conclusions obtained from the analysis are as follows: 1. The pushover analysis is a simple way to explore the nonlinear behavior of building. 2. Pushover analysis can identify weak elements by predicting the failure mechanism and account for the redistribution of forces during progressive yielding. It may help engineers take action for rehabilitation work. 3. Pushover analysis is an approximation method based on static loading. It may not accurately represent dynamic phenomena. 4. The results show that design considering only gravity load is found inadequate. Therefore, a structural engineer should consider earthquakes in designing building. 5. The building that was analysed according to UBC is satisfactory. The performance point location is at IO (Immediate Occupancy) level. It means the design satisfies pushover analysis according to ATC -40. III. DESIGN PHILOSOPHIES A. PUSHOVER ANALYSIS Pushover analysis is an approximate analysis method in which the structure is subjected to monotonically increasing lateral forces with an invariant height-wise distribution until a target displacement is reached. Pushover analysis consists of a series of sequential elastic analysis, superimposed to approximate a force-displacement curve of the overall structure. Pushover analysis can be performed as force-controlled or displacement-controlled. In force- controlled pushover procedure, full load combination is applied as specified, that is, forcecontrolled procedure should be used when the load is known. Also, in force-controlled pushover procedure some numerical problems that affect the accuracy of results occur since target displacement may be associated with a very small positive or even a negative lateral stiffness because of the development of mechanisms and P-delta effects. A pushover analysis is performed by subjecting a structure to a monotonically increasing pattern of lateral loads, representing the inertial forces which would be experienced by the structure when subjected to ground shaking. Under incrementally increasing loads various structural elements may yield sequentially. Consequently, at each event, the structure experiences a loss in stiffness. Using a pushover analysis, a characteristic non linear force displacement relationship can be determined. A generalized force-displacement characteristic of a non-degrading frame element (or hinge Properties) is shown in Figure. Figure I Typical force deformation curve Point A corresponds to unloaded condition and point B represents yielding of the element. The ordinate at C corresponds to nominal strength and abscissa at C corresponds to the deformation at which significant strength degradation begins. The drop from C to D represents the initial failure of the element and resistance to lateral loads beyond point C is usually unreliable. The residual resistance from Anuj Bansal and Aditi Patidar ijesird, Vol. II (VII) January 2016/ 439

6 D to E allows the frame elements to sustain gravity loads. Beyond point E, the maximum deformation capacity, gravity load can no longer be sustained. IO stands for immediate occupancy, LS stands for life safety and CP stands for collapse prevention. B. APPLICATIONS OF PUSHOVER ANALYSIS Pushover analysis may be applied to verify the structural performance of newly designed and the existing buildings for the following purposes: 1. To verify the over strength ratio values. 2. To estimate the expected plastic mechanism and the distribution of damage. 3. To assess the structural performance of existing or retrofitted buildings. 4. As an alternative to the design based on linear analysis. IV. PROBLEM DEFINITION AND ANALYSIS The object of the present work is to compare the behaviour of multi-storey buildings having flat slab with that of having grid slab and to study the effect of base shear, storey drift and maximum displacement on it under seismic forces. For this purpose three cases of multi-storey buildings are considered. All the three cases are considered having flat slab and grid slab, and also analyzed by using software SAP2000. Case-I : Four storey building Case-II : Eight storey building Case-III: Twelve storey building Following specifications are considered for all the cases: Total plan area: 20 m x 20 m Grid size : 5 m x 5 m Column size : Slab thickness : Storey height : 0.4 m x 0.4 m for 4 storey 0.5 m x 0.5 m for 8 storey 0.6 m x 0.6 m for 12 storey 0.15 m 3.6 m Figure II Models of Flat Slab Figure III Models of Grid Slab Anuj Bansal and Aditi Patidar ijesird, Vol. II (VII) January 2016/ 440

7 No. Of Storeys No. Of Storeys TABLE I PERFORMANCE POINTS Performance Points Flat Slab Grid Slab S a S d S a S d TABLE III BASE SHEAR VS. DISPLACEMENT Base Shear and Displacement Flat Slab Grid Slab Displacement Base Displacement Shear Base Shear V. CONCLUSIONS Under the pressure of recent developments, seismic codes have begun to explicitly require the identification of sources of inelasticity in structural response, together with the quantification of their energy absorption capacity. Many existing buildings do not have been designed for seismic forces. It is important to study their response under seismic conditions and to evaluate seismic retrofit schemes. Hence push over analysis has been gaining importance for the strengthening and evaluation of the existing structures. By conducting the pushover analysis on flat slabs and grid slabs, pushover curve and demand curve can be obtained which can easily be compared for all the considered models. Within the scope of present work following conclusions are drawn: 1. The pushover analysis is a simple way to explore the nonlinear behavior of building. 2. Pushover analysis can identify weak elements by predicting the failure mechanism and account for the redistribution of forces during progressive yielding. It may help engineers take action for rehabilitation work. 3. Pushover analysis is an approximation method based on static loading. It may not accurately represent dynamic phenomena. 4. The building that was analysed according to UBC is satisfactory. The performance point location is at IO (Immediate Occupancy) level. It means the design satisfies pushover analysis according to ATC The performance point is obtained as per ATC 40 capacity spectrum method. Performance points for flat slab are larger than in grid slab models. 6. Resultant displacements for flat slab are quite larger than in grid slab models. 7. Base shear in both types of slabs is almost similar. 8. For seismic loadings flat slab is deflected more than that of grid slab models for all the three storeys considered. 9. Lateral displacement in flat slab model is more than that in grid slab model. REFERENCES , Basavaraj H S, Rashmi B A, Seismic Performance of RC Flat Slab Building Structural Systems , Gouramma G, Dr. Jagadish Kori G, Seismic Performance Of Different RC Slab Systems For Tall Building , Mohana H.S, Kavan M.R, Comparative Study of Flat Slab and Conventional Slab Structure Using ETABS for Different Earthquake Zones of India , Pradip S. Lande, Aniket B. Raut, Seismic Behaviour of Flat Slab Systems , Salman I. Khan, Ashok R. Mundhada, Comparative study of Seismic Performance of multistoried R.C.C buildings with Flat slab & Grid slab , A. E. Hassaballa, M. A. Ismaeil, A. N. Alzead, Fathelrahman M. Adam, Pushover Analysis of Existing 4 Storey RC Flat Slab Building , Dhananjay D. Joshi, Dr. Pranesh B. Murnal, Performance of Flat Slab Structure Using Pushover Analysis , Amit A. Sathawane, R.S. Deotale, Analysis And Design of Flat Slab And Grid Slab And Their Cost Comparison , K. Soni Priya, T. Durgabhavani, K. Mounika, M.Nageswari, P.Poluraju, Non-Linear Pushover Analysis of Flatslab Building by using SAP M. A. Ismaeil, Pushover Analysis of Existing 3 Stories RC Flat slab Building. Anuj Bansal and Aditi Patidar ijesird, Vol. II (VII) January 2016/ 441