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

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1 PUHOVER ANALYI OF MULTITOREY BUILDING HAVING FLAT LAB AND GRID LAB Anuj Bansal 1, Aditi Patidar 2 Civil Engineering Department, IE, IPA, Indore 1 anujbansal531@gmail.com, 2 er.aditipatidar@gmail.com Abstract In this modern industrial time we can see large buildings for upcoming earthquakes, some actions construction activities happening everywhere, hence there will be have to be adopted. One of the methods is the static a shortage of land space, so construction of tall structures has pushover analysis which is becoming a popular tool been initiated up to overcome this problem. There are numerous elements which are modified to make work faster and economical for seismic performance evaluation of existing and also like introducing flat slab and grid slab construction which new structures. By conducting this push over reduces dead weight, and makes beams invisible, enhances floor analysis, we can know the weak zones in the area. The object of the present work is to compare the behaviour of multi-storey buildings having flat slab with that of having grid structure and then we can make a decision that slab and to study the effect of base shear, storey drift and whether the particular part is retrofitted or maximum displacement on it under seismic forces. For this purpose three cases of multi-storey buildings are considered with rehabilitated as per requirement. 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 II. LITERATURE REVIEW having flat slab and grid slab, and also analyzed by using Research on the behaviour of the flat slab and software AP2000. Observation shows that pushover analysis is a simple way to grid slab with different models is described below: explore the nonlinear behavior of building. Also pushover Gouramma, et.al (2015) analysis is an approximation method based on static loading. It eismic Performance Of Different RC lab may not accurately represent dynamic phenomena. Performance ystems For Tall Building, investigated points for flat slab are larger than in grid slab models. Resultant displacements for flat slab are quite larger than in grid slab analytically different types of RC slab taken as an models and also base shear in both types of slabs is almost example and performed the various analytical similar. Present work provides a good source of information on the approaches (linear and nonlinear analysis) on the parameters of pushover analysis of multistory buildings. building to identify the seismic demand and also performed pushover analysis to determine the Keywords Flat lab, Grid lab, Performance Point, Base performance levels. The analysis of seismic hear, Displacement. behavior of different type of RC slab systems is I. INTRODUCTION done by using ETAB software v This study In this modern industrial time we can see large consists three systems, Conventional RC slab construction activities happening everywhere, system (Model 1), ribbed slab system (Model 2), hence there will be a shortage of land space, so and Flat slab system (Model 3) in different height construction of tall structures have been initiated up i.e. 10 storey, 15 storey, 20 storey and base is same to overcome this problem. There are numerous for all the models(36 m x30 m). These three elements which are modified to make work faster systems are the most attractive and commonly used and economical also like introducing flat slab and floor systems, flat slab is flexible and it is grid slab construction which reduces dead weight, Vulnerable in earthquake resistance especially in and makes beams invisible, enhances floor area. high-rise constructions. o certain modification is We can t avoid future earthquakes, but awareness done, flat slab with edge beam system (Model and safe building construction practices can 4),Flat slab with shear wall system (Model 5) the certainly reduce the extent of damage and failure. building is designed for gravity loads as per I 456- With the purpose of strengthening and resisting the 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 I storey heights located in zone v. and analysed by (part-1). They concluded that pushover using ETAB Nonlinear version Linear analysis is useful tool to performance based seismic dynamic analysis i.e. response spectrum analysis engineering to study post yield behavior of structure. was performed on the system to get the seismic It is more complex than traditional linear analysis. behaviour. Mohana, et.al (2015) Comparative On the basis of the results following tudy of Flat lab and Conventional lab tructure conclusions have been drawn: Using ETAB for Different Earthquake Zones of 1. The storey displacement is found maximum for India, analyzed a G+5 commercial multistoried the flat slab building as compared to building having flat slab and conventional slab for conventional RC building and flat slab with the parameters like base shear, storey drift, axial shear wall the maximum displacement of the force, and displacement. The performance and flat slab building is due to the absence of lateral behavior of both the structures in all seismic zones load resisting system. of India has been studied. The storey shear of flat 2. The maximum storey drift found for G+6 slab is 5% more than conventional slab structure, building having a flat slab (As compared to its the axial forces on flat slab building is nearly 6% maximum limit i.e. 0.04% of height) more than conventional building, the difference in 3. For all the cases considered drift values follow a storey displacement of flat and conventional parabolic path along storey height with building are approximately 4mm in each floor. The maximum value lying somewhere near the work provides reasonable information about the middle storey. suitability of flat slab for various seismic zones 4. It is found that flat slab structures exhibit higher without compromising the performance over the flexibility compared to traditional frame conventional slab structures. They concluded the structures. In order to limit deformation following observations: demands under the seismic excitations, 1. torey shear of flat slab is 6% more compared combination with other stiffer structural systems to conventional slab structure, and storey shear as shear-walls is advisable. is Maximum at base and least at top storey. 5. The maximum storey drift also found for flab 2. The design axial forces on flat slab are more slab. Min displacement found for flat slab with compared to conventional structure the shear wall. Flat slab displacement is found 28% difference of forces is nearly 5.5%. more than of conventional building for G torey displacement is Maximum at roof level And 49.49% for G+12 building. Therefore it is than at base, and storey displacement of flat slab advisable for tall building to use the shear wall. structure is greater than conventional structure, Khan, et.al (2015) there will be an average 4mm displacement Comparative study of eismic Performance of variation in each seismic zone for both multistoried R.C.C buildings with Flat slab & Grid structures. slab, performed dynamic analysis of three different 4. As the seismic level increases all parameters high rise buildings having 12, 15 & 18 stories using like axial force, displacement, storey shear response spectrum method for all four seismic intensities are increases. zones of India, as categorized by the Indian code for Lande, et.al (2015) earthquake resistant structures. The assessment of eismic Behaviour of Flat lab ystems, carried the seismic response is based on the maximum out a parametric investigation in order to identify inter-story drift, roof displacement, Time period and the seismic response of systems a) flat slab building the base shear. E-TAB v9.7.3 software is used for b) flat slab with parametric beams c) flat slab with the analysis. It is observed that the seismic shear walls d) flat slab with drop panel. e) performance of grid slab buildings was better as Conventional building the aforementioned compared to that of flat slab buildings. hypothetical systems were studied for two different Anuj Bansal and Aditi Patidar ijesird, Vol. II (VII) January 2016/ 436

3 Based on the results the following conclusions where inelastic structural analysis is combined with could be drawn: the seismic hazard to calculate expected seismic 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(%). 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 3. The lateral displacement (both Ux and Uy) is Technology Council (ATC-40) guidelines were maximum at terrace level for all types of used for each member. One case study has been columns. Lateral displacement of Grid slab chosen for this purpose. The evaluation has proved building is less than that of the flat slab that the four-story residential building is not building. The difference between the two is less seismically safe. if the building width is more. The main output of a pushover analysis was in 4. torey drift in buildings with flat slab terms of response demand versus capacity. If the construction is significantly more as compared demand curve intersects the capacity envelope near to Grid slab buildings. As a result of this, the elastic range, then the structure has a good additional moments are developed. Therefore, resistance. If the demand curve intersects the the columns of such buildings should be capacity curve with little reserve of strength and designed by considering additional moments deformation capacity, then it can be concluded that caused by the drift. The difference between the the structure will behave poorly during the imposed two is less if the building width is more. seismic excitation and need to be retrofitted to 5. For improving drift conditions of flat slab in avoid future major damage or collapse. higher seismic zones, lateral load resisting Joshi, et.al (2013) system should be coupled with slab column Performance of Flat lab tructure Using frame or stiffness of columns should be Pushover Analysis, performed push over analysis increased. on flat slabs by using software AP2000.For 6. The natural time period increases as the height analysis they have considered a (G+7) frame having increases. In comparison of the grid slab 5 bays. It is observed that when compared to beambuilding and flat slab building, the time period column connections, flat slabs are becoming is more for flat slab building than that of grid popular and gaining importance as they are slab building. The difference between the two is economical. From the analysis they concluded the about 23(%). following points: Hassaballa, et.al (2014) 1. The pushover analysis is a relatively simple way Pushover Analysis of Existing 4 torey RC Flat lab Building, to explore the non linear behavior of buildings. 2. Base shear of conventional RCC building is analysed a four-story residential existing reinforced concrete building in the city of more than the flat slab building. 3. The performance point of flat slab is more than Khartoum-udan, subjected to seismic hazard. the conventional structure due to its flexibility. Plastic hinge was used to represent the failure mode 4. The behavior of properly detailed conventional in the beams and columns when the member yields. building is adequate as intersection of the The pushover analysis was performed on the demand and capacity curves. building using AP2000 software (Ver.14) and 5. The results obtained in terms of demand, equivalent static method according to UBC 97. The capacity gave an insight into real behavior of principles of Performance Based eismic the structure. Engineering were used to govern the analysis, Anuj Bansal and Aditi Patidar ijesird, Vol. II (VII) January 2016/ 437

4 athawane, et.al (2012) column connections, flat slabs are becoming Analysis And Design of Flat lab And Grid lab popular and gaining importance as they are And Their Cost Comparison, aimed to determine economical. A typical flat slab with two stories is the most economical slab between flat slab with considered in the present study. It has not been drop, Flat slab without drop and grid slab. The detailed for earthquake loads but have been proposed construction site is Nexus point apposite designed for wind loads. The height of the each to Vidhan Bhavan and beside NMC office, Nagpur. storey is 3m. For the purpose of design, linear The total length of slab is m and width is structural analysis of the building is carried out by m. total area of slab is sqm. It is the computer program AP2000 (tructural designed by using M35 Grade concrete and Fe415 Analysis Program, Version 12.00). The slab of the steel. Analysis of the flat slab and grid slab has building is assumed to be acting like a rigid floor been done both manually by I and by diaphragm. The building is designed as per I: 456- using software also. Flat slab and Grid slab has 1978 using limit state method of design. The load been analyzed by TAAD PRO. Rates have been combinations used are as follows: taken according to N.M.C. C..R It is observed that 1.5 DL+1.5 LL the Flat slab with drop is more economical than Flat 1.5 DL+1.5 LL+1.2 WL slab without drop and Grid slabs. Also they The materials used are M 20 grade concrete concluded the following points: and Fe 415 grade steel. The thickness of the slab is 1. Drops are important criteria in increasing the 200 mm and no shear reinforcement is provided in shear strength of the slab. the slabs. The columns are 250 mm square in 2. Enhance resistance to punching failure at the section. junction of concrete slab and column. Under the pressure of recent developments, 3. By incorporating heads in slab, we are seismic codes have begun to explicitly require the increasing rigidity of slab. identification of sources of inelasticity in structural 4. The negative moment s section shall be response, together with the quantification of their designed to resist the larger of the two interior energy absorption capacity. Many existing negative design moments for the span framing buildings do not have been designed for seismic into common supports. forces. It is important to study their response under 5. Concrete required in grid slab is more as seismic conditions and to evaluate seismic retrofit compared to flat slab with and without drop. schemes. Hence push over analysis has been 6. teel required in flat slab without drop is more gaining importance for the strengthening and as compared to flat slab with drop and grid slab. evaluation of the existing structures. By conducting 7. Flat slab with drop is more economical than flat the pushover analysis on flat slabs, pushover curve slab without drop and grid slab. and demand curve can be obtained. Then, based on 8. Rate per square meter of flat slab with drop the results we need to decide whether to perform (3524) was found more economical than flat rehabilitation or retrofitting depending upon the slab without drop (3855) and grid slab (4005). seismic zone of the existing structures. oni, et.al (2012). Non-Linear M. A. Ismaeil, Pushover Analysis of Existing AP2000, performed the push over analysis on flat 3 tories RC Flat slab Building, focused on the slabs by using most common software AP2000. study of seismic performance of the existing According to this paper many existing flat slab hospital buildings in the udan. Plastic hinge is buildings may not have been designed for seismic used to represent the failure mode in the beams and forces. Hence it is important to study their response columns when the member yields. The pushover under seismic conditions and to evaluate seismic analysis was performed on the building using retrofit schemes. But when compared to beam- AP2000 software (Ver.14) and equivalent static method according to UBC 97. The principles of Performance Based eismic 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. 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. III. DEIGN PHILOOPHIE Figure I Typical force deformation curve A. PUHOVER ANALYI Pushover analysis is an approximate Point A corresponds to unloaded condition analysis method in which the structure is subjected and point B represents yielding of the element. The to monotonically increasing lateral forces with an ordinate at C corresponds to nominal strength and invariant height-wise distribution until a target abscissa at C corresponds to the deformation at displacement is reached. Pushover analysis consists which significant strength degradation begins. The of a series of sequential elastic analysis, drop from C to D represents the initial failure of the superimposed to approximate a force-displacement element and resistance to lateral loads beyond point curve of the overall structure. 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, L stands for life safety and CP stands for collapse prevention. B. APPLICATION OF PUHOVER ANALYI 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 ANALYI 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 AP2000. 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 : 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 lab thickness : 0.15 m torey height : 3.6 m Figure II Models of Flat lab Figure III Models of Grid lab Anuj Bansal and Aditi Patidar ijesird, Vol. II (VII) January 2016/ 440

7 TABLE I PERFORMANCE POINT Performance Points No. Of Flat lab Grid lab toreys a d TABLE III BAE HEAR V. DIPLACEMENT Base hear and Displacement No. Of Flat lab Grid lab toreys Base Base Displacement Displacement hear hear V. CONCLUION 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. a d 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. REFERENCE , Basavaraj H, Rashmi B A, eismic Performance of RC Flat lab Building tructural ystems , Gouramma G, Dr. Jagadish Kori G, eismic Performance Of Different RC lab ystems For Tall Building , Mohana H., Kavan M.R, Comparative tudy of Flat lab and Conventional lab tructure Using ETAB for Different Earthquake Zones of India , Pradip. Lande, Aniket B. Raut, eismic Behaviour of Flat lab ystems , alman I. Khan, Ashok R. Mundhada, Comparative study of eismic 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 torey RC Flat lab Building , Dhananjay D. Joshi, Dr. Pranesh B. Murnal, Performance of Flat lab tructure Using Pushover Analysis , Amit A. athawane, R.. Deotale, Analysis And Design of Flat lab And Grid lab And Their Cost Comparison , K. oni Priya, T. Durgabhavani, K. Mounika, M.Nageswari, P.Poluraju, Non-Linear Pushover Analysis of Flatslab Building by using AP M. A. Ismaeil, Pushover Analysis of Existing 3 tories RC Flat slab Building. Anuj Bansal and Aditi Patidar ijesird, Vol. II (VII) January 2016/ 441