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STUDY ON EDGE SLAB-COLUMN CONNECTION IN … · (MPa) Material 0.167 26480 Concrete 0.300 200000...
Transcript of STUDY ON EDGE SLAB-COLUMN CONNECTION IN … · (MPa) Material 0.167 26480 Concrete 0.300 200000...
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International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 9, September 2017, pp. 53–60, Article ID: IJCIET_08_09_008
Available online at http://http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=8&IType=8
ISSN Print: 0976-6308 and ISSN Online: 0976-6316
© 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. 53–60.
http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=9
1. 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
T. S. Viswanathan, V. Sairam, K. Srinivasan and Giriraj Mannayee
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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
Study on Edge Slab-Column Connection in Flat Slab
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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/m2 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.
4.2. 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.
T. S. Viswanathan, V. Sairam, K. Srinivasan and Giriraj Mannayee
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4.3. σbo/σco Parameter:
σbo/σco 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.
4.4. 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 0.05.
4.5. Kc Parameter:
The value of the Kc parameter is to be determined considering the yield surface in the
deviatory plane (Fig 5). Kc 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.
Figure 5 Kc Parameters
5. INELASTIC PROPERTIES
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
(N/mm2)
Inelastic
Strain
Compression
damage factor
1 24.00 0.0000 0.000
2 29.20 0.0004 0.129
3 31.70 0.0008 0.242
4 32.30 0.0012 0.341
5 31.76 0.0016 0.426
6 30.37 0.0020 0.501
7 28.50 0.0024 0.566
8 21.90 0.0036 0.714
9 14.89 0.0050 0.824
10 2.95 0.01 0.969
Study on Edge Slab-Column Connection in Flat Slab
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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/mm2)
Cracking
Strain
Tensile
damage
factor
1 1.780 0.0000 0.00
2 1.450 0.0001 0.30
3 1.113 0.0003 0.55
4 0.960 0.0004 0.70
5 0.800 0.0005 0.80
6 0.536 0.0008 0.90
7 0.359 0.0010 0.93
8 0.161 0.0020 0.95
Table 3 Material Properties
Poisson’s
ratio
Elastic modulus
(MPa) Material
0.167 26480 Concrete
0.300 200000 Steel
Table 4 Tensile Stress-Strain Values of Steel
Yield Stress
(MPa) Plastic strain
200.2 0
246 0.02374
294 0.04784
374 0.09436
437 0.13880
480 0.18140
6. RESULTS AND DISCUSSION
6.1. 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.
T. S. Viswanathan, V. Sairam, K. Srinivasan and Giriraj Mannayee
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Table 5 for Slab of 150mm
Shear Stress
(N/mm2 ) Node no.
0.101 807
0.107 871
0.113 935
0.109 999
0.107 1063
0.104 1126
0.102 1190
0.070 1254
0.011 1318
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.
Study on Edge Slab-Column Connection in Flat Slab
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Table 6 Shear stress along width of slab (N/mm2)
Slab of 150mm Slab of 200mm
2.203 1.789
1.642 1.314
1.393 1.073
1.135 0.891
0.986 0.756
0.872 0.664
0.755 0.571
0.643 0.493
0.554 0.414
0.375 0.309
0.282 0.269
Figure 8 Shear stress for Non-linear analysis of slab
Table 7 Shear stress along radial direction of slab (N/mm2)
Slab of 150mm Slab of 200mm
1.934 0.825
1.548 0.648
1.473 0.619
1.2 0.556
0.743 0.434
0.546 0.342
0.332 0.278
0.324 0.235
0.321 0.22
0.361 0.195
0.268 0.189
T. S. Viswanathan, V. Sairam, K. Srinivasan and Giriraj Mannayee
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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
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Viswanathan, G. Mohan Ganesh & A.S. Santhi, Journal of engineering and science, Nov.
2014.
[2] Defining parameters for concrete damage plasticity model, Yusuf Sümer, Muharrem
Aktaş, Journal of structural mechanics, 2015.
[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. 299-326, 1989.
[4] Finite element analysis of punching shear of concrete slabs using damaged plasticity
model in ABAQUS, Aikaterini S. Genikomsou, Maria Anna Polak, Elsevier, 2015.
[5] Abaqus/CAE Material Nonlinearity Manual, version 6.11.
[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
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[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,
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