BEHAVIOUR OF RCC FLAT SLAB STRUCTURE UNDER EARTHQUAKE LOADING

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1 ISSN -8 IJESR/July / Vol-/Issue-/8-89 P. Srinivasulu et. al./ International Journal of Engineering & Science Research BEHAVIOUR OF RCC FLAT SLAB STRUCTURE UNDER EARTHQUAKE LOADING ABSTRACT P. Srinivasulu*, A. Dattatreya Kumar Post Graduate student, Department of Civil Engineering, VRSEC, Vijayawada (A.P), India. Asst. Prof, Department of Civil Engineering, VRSEC, Vijayawada (A.P), India. The flat slab system is currently widely used in construction. It permits flexibility in architecture, more clear space, low building height, easier formwork, and shorter construction time. However, Flat slab building structures are significantly more flexible than traditional concrete structures as beams are not present. They are becoming more vulnerable to earthquakes. The objective of this paper is to investigate the behaviour of flat slab in different cases as I).flat slab structure without drop, II). Flat slab structure with column drop, III). Flat slab structure with shear wall, IV). Flat slab structure with column drop and shear wall together, through response spectrum method, by using ETABS software. The behaviour of the flat slab is investigated in terms of story displacements, frequency, base shear, story level accelerations. And also most severe problem in flat slabs is punching shear failure. During the earthquake, unbalanced moments can produce significant shear stresses that causes slab column connections to brittle punching shear failure. This paper also investigates on which type of combination produces less punching shear at slab column joint. Introduction Flat slab construction is a developing technology in India. A slab constructed without supporting beams resting directly on columns, such slab is called flat slab. Provision of thickened portion of slab around column is called drop panel, drops are proved to increase shear strength of slab and to reduce negative reinforcement in the slab column connections. Column heads are flared profile around column is also provide to increase the perimeter of critical section for shear. Slabs of constant thickness which do not have drop panels or column capitals are called as flat plates. The strength of the flat plate structure is often limited due to punching shear action around columns. So they are predominantly used in low seismic areas. The performance of flat slab building under seismic loading is poor as compare to frame structure due to lack of frame action which leads to excessive lateral deformation. This leads to instability in the structure. And also Transfer of lateral displacement induces moment at slab column connection which is of complex -dimensional behaviour. Despite the advantages of flat slab, it fails to gravity loads by punching shear. It can be overcome by providing column drops in low seismic areas. But when these flat slab structures situated in seismic zones, the moments transferring from slab to column through shear increases further more and becoming more tendency to punching shear failure during earthquakes. Due to the flexibility of flat slab buildings, they must be combined with a stiffer lateral force resisting system in high seismic regions like shear walls, braces to reduce lateral loads on structural frame. When flat slab is used in combination with bracings, shear wall for lateral load resistance, the column in building can be designed for only % of the design seismic force.thus the behavior of a structure for dynamic loads can be determined by model analysis. And dynamic behaviour can be examined by considering the parameters as storey drift, base shear, time period and acceleration of model. PROBLEM FORMULATION Here, we are focusing on the behaviour of flat slab RCC structure in four different types. One is with drops and another one is without drop and these two models are modeled with shear walls at corners. As it is clear from *Corresponding Author 8

2 previous literature that flat slab structure are unstable for seismic forces, we are analytically investigating the behaviour of flat slab during the earthquakes(zoneiii) and checked for increase of punching shear from gravity loads to earthquake loads by taking one center column and one exterior column in intermediate frame in model and also checked for tendency of punching shear failure in flat slab through checking punching shear stress ( v) variation at various places in prescribed models.response spectrum method is considered to analyze the structure by using ETABS software. Here, models were created and all are analyzed for seismic loads. Those are. Flat slab structure without drop. Flat slab structure with column drops. Flat slab structure with shear wall. Flat slab structure with column drops and shear wall together. MODELING AND ANALYSIS OF STOREY OFFICE BUILDING Grade of concrete= M Grade of steel =Fe Slab thickness =. m Number of stories = () G+ Number of bays along X-direction = Number of bays along Y-direction = Storey height =.meters Bay width along X-direction = 8m Bay width along Y-direction = 8m Column =.x.m Edge beam Drop size slab thickness at drop Shear wall thickness =.x.m =.x.m =.m =.m Loading specifications. Wall load for the outer side = kn/m Wall load for the inner side = 9 kn/m Wall load for the terrace Dead load of slab =. kn/m = kn/m Live load = kn/m Earthquake load for the building has been calculated as per IS-89: i. Zone (Z) = III ii. Soil =medium iii. Response Reduction Factor ( RF ) = iv. Importance Facto = Copyright Published by IJESR. All rights reserved 8

3 v. Damping Ratio =. For Seismic loading only % of the imposed load is considered. Fig. : Working plan. Fig. : Model (Flat slab structure without drop) Fig : Model (Flat slab structure with column drop only) Copyright Published by IJESR. All rights reserved 8

4 Fig. : Model (Flat slab structure with shear wall) Response spectrum method Fig. : Model (Flat slab structure with drop and shear wall together) Response-spectrum analysis is useful for decision making to select structural type, before designing a structure. It gives the dynamic performance of a structure. Structures of shorter period experience greater acceleration, whereas those of longer period experience greater displacement. The number of modes to be considered in analysis should be such that the sum of total of model mass of all the modes considered is not less than 9% of total seismic mass of structure. By considering modes mass participation of flat slab building is achieved up to 9%.Therefore modes are considered for all models. Center of mass & centre of rigidity coincides, due to regularity in the plan, mass and stiffness of the building. so providing shear walls at all corners symmetrically may not affect center of mass and center of rigidity. RESULTS Table: Comparison of frequencies of mode shapes in all models Mode. No MODEL (Hz) MODEL (Hz) MODEL (Hz) MODEL (Hz) Copyright Published by IJESR. All rights reserved 8

5 Graph : Graph for fundamental mode of frequencies fundamental mode of frequency fundamental mode of frequency(hz) MODEL MODEL MODEL MODEL Graph : Graph for fundamental time period Time period (sec).. FUNDAMENTAL TIME PERIOD MODEL MODEL MODEL MODEL Table : comparison of design storey shear in all models Height of building Story MODEL MODEL MODEL MODEL (m) (KN) (KN) (KN) (KN). Plinth STORY STORY.... STORY STORY STORY STORY Graph : Graph shown for storey shear in all models storey shears MODEL MODEL storey no. storey shear(kn) MODEL MODEL Copyright Published by IJESR. All rights reserved 8

6 Table : Comparison of storey displacements in x-direction in models Story MODEL (mm) MODEL (mm) MODEL (mm) MODEL (mm) STORY... STORY.... STORY STORY STORY STORY.... Graph : Graph shown for comparison of storey displacements in x-direction in all models storey displacements storey no MODEL MODEL MODEL MODEL 8 displacements (mm) PUNCHING SHEAR FAILURE IN FLAT SLAB BUILDINGS Table : Comparison of shear stresses in model, corresponding to M x moments in column C (center column) in all stories Story DD+LL DD+LL+EQX Graph : Comparison of shear stresses in modelcorresponding to M x moments in column C in all stories DD+LL DD+LL+EQX storey no shear stress (N/mm ) Copyright Published by IJESR. All rights reserved 8

7 Table : Comparison of shear stresses in model, corresponding to M x moments in column C (exterior column) Story DD+LL DD+LL+EQX Graph : Comparison of shear stresses corresponding to M x moments in column C storey no DD+LL DD+LL+EQX shear stress (N/mm ) Table : Comparison of punching shear stresses in column C (center column) corresponding to models MODEL MODEL MODEL STOREY NO MODEL Graph : Comparison of punching shear stresses in column C corresponding to models storey no MODEL MODEL MODEL MODEL shear stress (N/mm ) Copyright Published by IJESR. All rights reserved 8

8 Table : Comparison of punching shear stresses in column C (exterior column) corresponding to models STOREY NO MODE MODE MODE MODEL Graph 8: Comparison of punching shear stresses in column C (exterior column) corresponding to models storey no MODEL MODEL MODEL MODEL.. shear stress (N/mm ) CONCLUSION Within the scope of present work, following conclusions are drafted Fundamental mode of frequencies of a flat slab structure increase % when drops panels are present, as further increasing of stiffness by providing shear walls those values increases to 9%. Base Shear values increases from model to model. As weight of structure increases from model to model Flat slab attracts more shear value, when flat slab provided with shear wall rather than flat slab having column drops. Providing column drops to flat slab, storey displacements reduces slightly, as stiffness increases slightly. But when flat slabs combine with shear walls, these displacements reduces tremendously as stiffness of shear walls increases overall lateral stiffness of structure. For inner columns, punching shear stresses are increasing linearly from top stories to bottom stories. As earthquake moments are increasing from top stories to bottom stories. But the punching shear variation due to the gravity loads are not much changes from storey to storey. This shows that earthquake moments are more effective in producing punching shear at bottom stories. Due to the effect of exterior panel moments and earthquake moments, punching shear stresses varying slightly irregular in exterior columns. In exterior columns punching shear stress is more in columns at top stories than the columns in the bottom stories. Punching shear failure occurs, more in flat plate. On provision of column drops it s punching shear stress decreases unto %. Provision of shear walls may not effective in reducing punching shear on intermediate storey s but effective in top and bottom storey s as shear wall attracts lateral moments from columns. Copyright Published by IJESR. All rights reserved 88

9 REFERENCES [] Paz M, Leigh W. Structural Dynamics, Fifth Edition. [] Lelekakis GE, Ioannis A. Tegos Aristotle University of Thessaloniki, Department of Civil Engineering, Thessaloniki, Greece. Applications of flat-slab RCC structures in seismic regions. [] Bhunia D. Solution of Shear Wall Location in Multi-Storey Building: [] Megally S, Ghali A. Punching shear design of earthquake resistant slab column connections. ACI Structural Journal, Title No. 9 S. [] Gupta U, Ratnaparkhe S, Gome P. Seismic Behaviour of Buildings Having Flat Slabs with Drops. Journal of IJERT ; (). [] IS - Indian standard plain and reinforced concrete code of practice. [] Agarwal P, Shrikhande M. Earthquake Resistant Design of Structures. [8] Pan A. Lateral Displacement Ductility of Reinforced Concrete Flat plates. ACI Structural journal, Title No. 8 S. [9] Durrani AJ. Seismic Resistance of Nonductile slab Column Connections in Existing Flat Slab Buildings. ACI structural Journal, Title No. 9 S. Copyright Published by IJESR. All rights reserved 89