STUDY ON EDGE SLAB-COLUMN CONNECTION IN FLAT SLAB

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 9, September 2017, pp , Article ID: IJCIET_08_09_008 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed STUDY ON EDGE SLAB-COLUMN CONNECTION IN FLAT SLAB T. S. Viswanathan, V. Sairam and K.Srinivasan School of civil and chemical engineering, VIT University, Vellore, Tamil Nadu, India Giriraj Mannayee School of Mechanical Engineering, VIT University, Tamil Nadu, Vellore, India ABSTRACT The major concern in flat slabs is the concentration of high stresses in the slab column connection resulting in punching shear failure of the entire slab system. Hence in the present study an attempt has been made to study and analyze the effect of edge column in a flat slab. A numerical investigation was conducted using the finite element software ABAQUS to evaluate their ability to resist punching shear in a flat slab. The nonlinear characteristics of concrete were achieved by employing the concrete damaged plasticity model in the finite element program. Linear and nonlinear analysis of edge slab-column connection was carried out for slab of depth 150mm and 200 mm. Remarkable reductions in deflection is observed when the depth of slab is increased. Keywords: Flat slab system, punching shear, edge slab-column connection, plastic damaged plasticity model, Abaqus. Cite this Article T. S. Viswanathan, V. Sairam, K. Srinivasan and Giriraj Mannayee, Study on Edge Slab-Column Connection in Flat Slab, International Journal of Civil Engineering and Technology, 8(9), 2017, pp INTRODUCTION Flat slab is a reinforced slab which does not have beams or girders and in which the slab directly rests on the column. Its load transfer mechanism includes transfer of load directly from slab to supporting columns. Flat slab provides more head room as there are no beams and hence provides more working area. The minimum overall thickness of flat slab is 125mm and maximum is 250mm. The columns are provided with flared portion at the top which is known as a column head which in turn increases the resistance of the slab against punching shear. A portion of the slab thickened in the vicinity of the column is known as a drop. The absence of drops and column heads in a flat slab constitutes a flat plate. The major concern in flat slabs is the concentration of high stresses in the slab column connection resulting in punching shear failure of the entire slab system. This may be due to improper design for shear especially in exterior columns. Punching shear induces a cone shaped perforation starting 53 editor@iaeme.com

2 T. S. Viswanathan, V. Sairam, K. Srinivasan and Giriraj Mannayee from the top surface of the slab and leads to progressive collapse of the slab system. Being a brittle failure mode, it requires high safety class in structural design. The three possible types of shear action are (i) One way shear of the slab (wide beam action), (ii) Two way shear around columns (punching shear action) and (iii) Shear caused by moment transfer. The finite element method is a numerical technique widely used in the engineering field. With the advancement of the understanding of the material properties of concrete, various constitutive laws and failure criteria have been developed to model the behaviour of concrete. Therefore, an increasing number of researchers are using finite element analysis to study the response of reinforced concrete structures. Finite element modeling of a flat-plate system requires that the punching shear failure of the slab column connections is reproduced properly. Such a simulation has been the focus of many numerical studies using various elements. ABAQUS is well-established commercial finite element software. Its constitutive models treat concrete as a continuous isotropic linearly elastic-plastic strain hardening fracture material. The software provides the capability of simulating damage using three crack models (Fig 1) for reinforced concrete elements: (i) the smeared crack concrete model (ii) the brittle crack concrete model and (iii) the concrete damaged plasticity model (CDP). The CDP model was selected for the present study because this technique has the ability to present complete inelastic behavior of concrete both in compression and tension, including damaged characteristics. The concrete damaged plasticity model assumes that the two main failure mechanisms in concrete which are tensile cracking and compression crushing. Figure 1 Different Crack models for RCC Elements 2. RESEARCH SIGNIFICANCE Many studies have been carried out on punching shear effect of interior columns on flat slab. Not much study has been done on effect of punching shear on edge slab-column connection. Hence in the present study an attempt has been made to study and analyze the effect of edge column in a flat slab. The present study is carried out to achieve two main objectives: To evaluate punching shear effect of edge slab-column connection. To compare behaviour of edge slab-column connection in slabs of varying depth. Figure 2 Punching Shear 54 editor@iaeme.com

3 Study on Edge Slab-Column Connection in Flat Slab 3. MODELLING To study and find the percentage reduction in stress with increase of depth of slab, three slabs of different depths were modelled in Abaqus. Slab 1 was modelled with a depth of 150mm; slab 2 was modelled with depth of 200 mm. The size of the slab model is similar to the one used by Aikaterini S. Genikomsou (1). Slabs with a size of 1350 mm x 1000 mm x 150 mm are considered. Reinforcement bars of diameter 12mm are used in the modelling. The concrete is assumed to be homogeneous and isotropic. The slab is simply supported along the three sides and load of 5 kn/m 2 applied at a column stub area of 250 mm x 250 mm (Fig 3). The column stub is represented as a uniform load applied over an area 250 mm x 250 mm equivalent to the area of the stub, as is generally used in this kind of slab analysis. In the model Hex type of mesh is considered of size 30 mm (Fig 4). Figure 3 Support conditions simply supported at edges Figure 4 Meshing of slab 4. INPUT PARAMETERS The default parameters in ABAQUS and the parameters proposed in other publications are used. The parameters needed to describe the plastic properties of concrete are as following: 4.1. Dilation Angle (ψ): The ratio of volume change to shear strain is called the dilation angle. The range of dilation angle is taken 30-40ᵒ Eccentricity: This value is used to get a soft curvature of the potential flow and gives almost the Similar dilation angle for a wide range of confining pressure values. an eccentricity of 0.1 is used editor@iaeme.com

4 T. S. Viswanathan, V. Sairam, K. Srinivasan and Giriraj Mannayee 4.3. σb o /σc o Parameter: σb o /σc o Parameter is the ratio of the initial biaxial compressive strength to the uniaxial compressive strength. The default value of 1.16 is used in this model Viscosity Parameter: It is required when a convergence problem is caused by softening behaviour. As flat-slab models cause convergence difficulties, the viscosity parameter is assumed to be K c Parameter: The value of the K c parameter is to be determined considering the yield surface in the deviatory plane (Fig 5). K c is the ratio of the second stress invariant on the tensile stress meridian (T.M.) to the second stress invariant on the compressive stress meridian (C.M.). The value 2/3 is used in the calculations. 5. INELASTIC PROPERTIES Figure 5 K c Parameters 5.1. Compressive behaviour: An accurate model of compressive behaviour is necessary for the analysis. Values of yield strength, ultimate strength and compressive damage were taken from the ABAQUS verification manual (Table 1), which assumes that the yield strength is 74% of the ultimate strength and the plastic strain at failure is 0.12%. Table 1 Compressive Stress-Strain Value for Concrete Sl. No. Stress Inelastic Compression (N/mm 2 ) Strain damage factor editor@iaeme.com

5 Study on Edge Slab-Column Connection in Flat Slab 5.2. Tensile Behaviour The concrete damaged plasticity model allows determination of post failure behaviour in tension by defining strain, crack opening or fracture energy towards plastic tensile stress. These three options are related to one another and the choice of them depends on the knowledge of structural behaviour and material. The input values required for tensile behaviour were taken from the ABAQUS verification manual (Table 2). Tensile behaviour of steel and other material properties (Table 3&4) are also from the ABAQUS manual. Table 2 Tensile Stress-Strain Values for Concrete Sl. No. Stress (N/mm 2 ) 6. RESULTS AND DISCUSSION Cracking Strain Tensile damage factor Table 3 Material Properties Poisson s Elastic modulus Material ratio (MPa) Concrete Steel Table 4 Tensile Stress-Strain Values of Steel Yield Stress (MPa) Plastic strain Linear analysis: To understand the basic behaviour of slab, linear analysis was performed. Shear stress distribution around edge slab column was analysed (Table 5) which is within the admissible limit. According to IS 456 permissible shear stress is 0.25 fck. Deflection of 150 mm depth slab (Fig 7) is 0.095mm which is within the permissible limit editor@iaeme.com

6 T. S. Viswanathan, V. Sairam, K. Srinivasan and Giriraj Mannayee Table 5 for Slab of 150mm Shear Stress (N/mm2 ) Node no Figure 6 Shear stress distribution of slab Figure 7 Deflection for 150mm slab 6.2. Non-Linear analysis: Non- linear analysis was performed for two different thickness of slab. Slab was analyzed for shear stress distribution around slab column connection. Shear stress distributions around slab column connection at the same node for the two slabs are compared. Shear stress distributions along the width as well as radial direction are considered for comparison (Table 6 &7). Shear stresses are appreciably reduced for 200mm depth of slab compared to 150mm in all direction of slab column connection editor@iaeme.com

7 Study on Edge Slab-Column Connection in Flat Slab Table 6 Shear stress along width of slab (N/mm 2 ) Slab of 150mm Slab of 200mm Figure 8 Shear stress for Non-linear analysis of slab Table 7 Shear stress along radial direction of slab (N/mm 2 ) Slab of 150mm Slab of 200mm editor@iaeme.com

8 T. S. Viswanathan, V. Sairam, K. Srinivasan and Giriraj Mannayee 7. CONCLUSIONS: Linear and nonlinear analysis of edge slab-column connection was carried out for slab of depth 150mm and 200mm. A stress reduction of up to 26-28% is observed by increase in depth of the slab. For reducing shear and moment, provision of drop/column head or shear reinforcements in the column is recommended. The unbalanced shear and moments in the slab near the column caused varying stresses which needs to be studied further. REFERENCES [1] Investigation of Shear Stud Performance in Flat Plate Using Finite Element Analysis, T.S. Viswanathan, G. Mohan Ganesh & A.S. Santhi, Journal of engineering and science, Nov [2] Defining parameters for concrete damage plasticity model, Yusuf Sümer, Muharrem Aktaş, Journal of structural mechanics, [3] A Plastic-damage Model For Concrete, J. Lubliner, J. Uliver, S. Uller and E. Uñate, Int. J. Solids Structures Vol. 25, No. 3, pp , [4] Finite element analysis of punching shear of concrete slabs using damaged plasticity model in ABAQUS, Aikaterini S. Genikomsou, Maria Anna Polak, Elsevier, [5] Abaqus/CAE Material Nonlinearity Manual, version [6] Kamal Padhiar, Dr. C.D. Modhera and Dr. A. K. Desai, Comparative Parametric Study for Post-Tension Flat Slab and Flat Slab with Drop System. International Journal of Civil Engineering and Technology, 8(5), 2017, pp [7] Sandeep G S and Gururaj Patil, Comparative Study of Lateral Displacement and Storey Drift of Flat Slab and Conventional Slab Structures In Different Seismic Zones, International Journal of Civil Engineering and Technology, 8(7), 2017, pp [8] Pooja M and Dr. Karthiyaini S, Investigation of Flat Slab Structures with and Without Expansion Joints for Thermal Stresses. International Journal of Civil Engineering and Technology, 8(4), 2017, pp editor@iaeme.com