behaviour of shear wall slab junction with...

IJREAS VOLUME 6, ISSUE 6(June, 2016) (ISSN 2249-3905)

International Journal of Research in Engineering and Applied Sciences (IMPACT FACTOR – 6.573)

International Journal of Research in Engineering & Applied Sciences

Email:- [email protected], http://www.euroasiapub.org

98

BEHAVIOUR OF SHEAR WALL SLAB JUNCTION WITH SFRC SUBJECTED TO IN PLANE BENDING

R. Thanaykumar1,

Post graduate Student,

Department of Civil Engineering,

The National Institute of Engineering, Mysore.

T.H. Sadashiva Murthy2

Associate Professor,

Department of Civil Engineering,

The National Institute of Engineering, Mysore.

Abstract

An analytical study has been carried out to understand the interaction between laterally loaded shear wall and floor slabs in building with fiber reinforced concrete at the junction of shear wall and slab. To carry out the analytical investigation, a ten floor building with cross wall system is analysed using ETABS and the shear wall slab junction was modelled using ANSYS 14.5. The models are sorted into 5 groups including control model with normal concrete and four other models with different steel fiber ratios from 0.5 to 2 percent. Non-linear analysis of models subjected to in plane and out of plane bending. The performance of the junction in terms of load-deflection, ultimate load, post cracking behaviour are compared. Fiber reinforced concrete performed well for in plane bending, increasing the ultimate load and also resistance to crack, and better post cracking behaviour.

Key words: cross wall, in plane bending, out of plane bending, non-linear analysis, ANSYS

Introduction

Construction of high rise buildings has become very common solution to overcome many issues such as population growth, shortage of space, high cost of land in urban areas are few among them. One major structural characteristic of tall buildings is that the effect of wind and seismic load becomes more pronounced with the increase in the height of the building. To resist these lateral forces shear walls are commonly used, slabs acts as diaphragm through which the lateral forces are transferred to the shear walls. Shear wall-slab junction acts as a rigid joint due to this the stresses are concentrated at the junction while transferring the lateral loads, hence it is important to study the behaviour of wall-slab junction in order to ensure better load transfer to prevent the failure of the junction.

Literature

Greeshma.s and Jaya.k.p analytically studied the response of the shear wall-slab junction with different patterns of detailing at the junction subjected to earthquake response spectrum. Three types of detailing were studied namely 900 bent configuration, 450 bent configuration and U-hook configuration. Finite element method was used to analyse these models, software used





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was ANSYS. And the parameters considered are absolute maximum displacement and von-mises stress. Solid65 and link-180 elements are used to model the concrete and reinforcement respectively. All these three models were subjected to three different earthquake loadings to evaluate the performance at the junction. Conclusion made by author was that the U-hook configuration performed well. Coull and wong studied the effect of local elastic wall deformations on the interaction between slabs and shear walls. Analytical study is carried out by finite element method for laterally loaded flanged shear wall and floor slabs to understand the interaction at the junction considering the influence of local elastic wall deformations on the effective width and coupling stiffness of the slab. Design curves were presented to allow a rapid assessment to be made of these quantities for preliminary design purposes. Anthony J. Wolanski, B.S (2004) did research on the flexural behavior of reinforced and prestressed concrete beams using finite element analysis. The two beams that were selected for modeling were simply supported and loaded with two symmetrically placed concentrated transverse loads. Mustafa Gencoglu and Iihan Eren carried out an experimental investigation to study the effect of steel fiber reinforced concrete on the behaviour of the exterior beam-column joints subjected to reverse cyclic loading. To avoid the congestion of reinforcement, alternate solution was given in the literature. Steel fibers of aspect ratio 75 were added at a 1% volume fraction. Tests were conducted by applying reverse cyclic loading and mode of failure and energy dissipation capacity was observed. It was concluded that steel fibers can be used as an alternative for the increase in confining reinforcement so as to minimize the congestion of reinforcement at beam column joint and hence reduce the problem of consolidation of concrete.

Objectives:

1. To model a full scale shear wall-slab junction of control model with conventional reinforced concrete and various test models with different volume fraction of fibers at junction using FEM.

2. To determine the load deflection curve, load carrying capacity, and cracking pattern analytically.

3. Comparing the results of control and test models.

Design and detailing of shear wall slab connection

In order to obtain the design forces, moments and the detailing of the shear wall slab-junction, a ten story residential building is analysed and designed using the ETABS software. The building is designed by considering all the load combinations which are pre-coded in the software. Seismic zone considered is zone-II and the wind speed in the region is 33m/s. material properties are M25 concrete and Fe415 steel. One of the exterior shear wall slab junction of ground floor is considered for the further analysis.

Dimensions and reinforcement details of the model: The dimensions of the shear wall considered for this study is shown in Figure 1 and the reinforcement detailing of slab, shear wall and junction are shown in Figure 2, Figure 3 and Figure 4 respectively.

Thickness of the shear wall : 0.3m Width of the shear wall : 2m Thickness of the slab : 0.2m Width of the slab : 2m

Models description. For the current study 5 different kinds of models are considered, labelled as A, B, C, D, and

E. Geometry of all 5 models is same as shown in Figure 9. Model A is treated as the control model for this study. The difference between these 5 models is the fiber content at the junction





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of the slab. Model A has no fiber content whereas the models B, C, D and E has 0.5%, 1%, 1.5% and 2% of steel fibers by volume at the junction of shear wall and slab. Fiber reinforcement region is 300mm form the face of shear wall and slab. The region of fiber reinforcement is shown in Figure 6

Finite element modeling

Geometry

The geometry of the model as mentioned in the previous sections was modeled using the graphical user interface (GUI) of ANSYS R14.5. Volumes are created corresponding to the dimensions of the shear wall and slab as shown in Figure 8. The volume is divided into 3 parts as shown in the Figure 8. The volume at the junction (shown in purple colour) is the region of fiber reinforcement. And the rest is meshed with normal concrete.

Figure 1 Geometry of full scale model

Figure 3 Reinforcement detail of slab

Figure 2 Reinforcement detail at the

junction





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Concrete modeling An eight-node solid element, Solid65 was used to model the concrete. The solid element

has eight nodes with three degrees of freedom at each node – translations in the nodal x, y, and z directions. The element is capable of plastic deformation, cracking in three orthogonal directions, and crushing. The geometry and node locations for this element type are shown in Figure 5. The Solid65 element requires linear isotropic and multilinear isotropic material properties to properly model concrete. Table 1 lists concrete properties within Solid65 element, prior to initial yield surface, beyond that concrete parameters are shown in Table 3. Solid65 element is capable of cracking in tension and crushing in compression shear. These properties are listed in Table 2

Table 1 concrete properties prior to initial yield

Table 3 Multilinear stress strain properties for concrete

Table 2 shear transfer coefficients

Steel reinforcement in concrete elements: The steel reinforcement is incorporated in concrete using either discrete model, embedded model or smeared model depending on the

Material Material

model

Modulus of

elasticity

Poisson’s ratio

concrete Linear elastic 25000 0.2

SL No Stress Strain 1 0 0 2 6.714346 0.000274 3 9.615385 0.0004 4 17.24138 0.0008 5 22.05882 0.0012 6 24.39024 0.0016 7 24.89431 0.001824 8 23.07692 0.003

Open shear transfer coefficient ( )

0.3

Closed shear transfer coefficient ( )

0.7

Uniaxial cracking stress 3

Uniaxial crushing stress 25

Figure 5 Solid65 3D element.

Figure 4 reinforcement detail of shear wall





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geometry of the system. In the discrete model, spar or beam elements with geometrical properties similar to the original reinforcing elements are connected to concrete mesh nodes and hence the concrete and the reinforcement mesh share the same nodes. For complex reinforcement details, this model is advantageous. However, this model increases the number of nodes and degrees of freedom which increases the run time and computational cost. The smeared model assumes that the reinforcement is uniformly spread throughout the concrete elements in a defined region of the FE mesh. The effect of reinforcing is averaged within the pertaining concrete element. This approach is used for large-scale models where the reinforcement does not significantly contribute to the overall response of the structure. The features of the above techniques are schematically shown in Figure. 6. The discrete modeling approach provides an accurate and true representation of the field reality. In this study discrete reinforcement is used for modeling the reinforcement of slab and shear wall and smeared reinforcement is used for modeling steel fiber reinforced concrete.

A Link-180 element, shown in Figure 7, was used to model the steel reinforcement. Two nodes are required for this element. Each node has three degrees of freedom, – translations in the nodal x, y, and z directions. The element is also capable of plastic deformation. To ensure the bonding between concrete and the reinforcement, the rebar is provided such that it shares common nodes with the concrete element. This type of reinforcement is called as discrete reinforcement. The geometry and node locations for this element type are shown in Figure 7 material properties used for steel are shown in table 4. The material property for steel fibers is listed in table 5 and real constant sets used for solid65 are shown in table 6

Table 4 material properties for steel Table 5 material properties for steel

Fiber reinforced concrete modeling. Solid65 element also has the provision of providing reinforcement in the form of volume ratios in three orthogonal directions. The fibers are modeled as smeared-reinforcement embedded in the Solid65 element. The volume ratio of the fiber reinforced concrete is assumed

Youngs modulus of steel 210000Mpa

Poissons ratio 0.3

Yield stress 415Mpa

Tangent modulus 830Mpa

Youngs modulus of steel 210000Mpa

Poissons ratio 0.3

Yield stress 500Mpa

Tangent modulus 830Mpa

Figure 6 Idealization of reinforcement in

concrete element

Figure 7 Link-180 beam element





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Figure 8 Model showing fiber

reinforcement region Figure 9 Load applied in the negative global Z

direction

to be equally aligned in three orthogonal direction. By defining the real constant sets of solid65 for various amount of volume fraction of steel fibers, SFRC is modeled.

Table 6 Real constant sets for Solid65 and link180 elements

Boundary conditions: All the loads acting on the shear wall are previously determined and are explained in the

previous sections. All these loads and boundary conditions are applied to the FE model and are referred as static loads. After applying these loads the model is then subjected to in-plane cyclic loading.

Displacement boundary conditions used for in plane bending are as fallows

Real constant set

Element type

1 Solid65 (normal

concrete)

Real

constant for rebar 1

Real constant for rebar 2

Real constant

for rebar 3 Material no 2 2 2

Volume ratio 0 0 0

Orientation angle(θ) 0 0 90

Orientation angle(Φ) 0 90 0

2

Solid65 (1% fiber reinforced concrete)

Material no 2 2 2

Volume ratio 0.00333 0.00333 0.00333

Orientation angle(θ) 0 0 90

Orientation angle(Φ) 0 90 0





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Base of the shear wall is fixed in all direction. The slab end is fixed in global Y and global X direction and is free to translate in global Z

direction ensuring in-plane bending

Axial load and moment acting on the shear wall is distributed to the nodes at the top end of the shear wall.

Cyclic loading for in plane bending

In addition to the static loads the slab end is loaded with positive and negative loads in global z direction. The loads are applied in load steps by creating LS-files.

Cyclic loading is done using the command line prompt data (set of commands for loading). The commands are written into a text file and are then executed. The commands given are such that when it is executed it forms a finite number of load steps which are analysed in order. First load step will gradually apply a 5KN load, in the second load step the load is brought back to zero and in third load step a -5KN load is applied at the other end of the slab again in forth load step the load is brought to zero. This cycle is continued until the concrete fails with an increment of 5KN for each cycle.

Results and discussion

This study was intended to find the behaviour of steel fiber reinforced concrete compared to conventional concrete at the junction of shear wall and slab. The models were analysed for in plane cyclic loading. Results obtained from the analysis are presented in this section.

Ultimate load and moment carrying capacity The ultimate load carrying capacity is calculated as the average of ultimate positive and ultimate negative loads.

The ultimate moment is a product of ultimate load and distance between the centre line of the shear wall and loading location at the slab end in this case the distance works out to be 1.75m.

The ultimate load and moments for different volume fraction are tabulated in the table 7.

Table.7 Ultimate load and ultimate moment of models

SL NO Volume fraction

In %

Average ultimate load

Average ultimate moment

Cracking load

1 0 80 140 45

2 0.5 97.5 170.625 55

3 1 112.5 196.875 65

4 1.5 155 271.25 75

5 2 175 306.25 85





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Load deflection behaviour

Deflection of the slab end corresponding to cyclic loading are plotted for all the five models as shown in Figure 10. Comparison of cracking load and ultimate loads are shown in figure 11. Crack patterns at ultimate loads are shown in table 3 for all five models.

0

20

40

60

80

100

120

140

160

180

200

-3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5

0% 0% 0.50% 0.50% 1%

1% 1.50% 1.50% 2% 2%

negative diflection(mm) positive deflection(mm)

Load

(KN

)

Figure 10 Load deflection curve for in plane bending

0

20

40

60

80

100

120

140

160

180

200

0 0.5 1 1.5 2 2.5

Load

(kN

)

volume fraction

Average

ultimateload

Figure 11 Variation of ultimate load and cracking load for different volume fraction of steel fibers





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Model E at

175kN

Model D at

155kN

Model A at

80kN

Model B at

97.5kN Model C at

112.5kN

Table 8 Crack patterns

Side view of the junction showing crack pattern at the ultimate load of the Models

From the crack pattern Figures in Table 8 it is observed that the initial crack patterns are similar for all the models all the cracks appear at the bottom junction, but occur at different load steps. The appearance of crack at left corner in all models is due to the load application pattern in which the increment of load in each cycle is in the negative global Z direction.

It can be seen that the cracks at the ultimate load of model “A” (control model) are right at the face of shear wall slab junction and at the bottom face of the slab. The cracks extend up to entire depth of slab and shear wall. Whereas for fiber reinforced models the cracks are little different, they do not extend up to entire depth of slab or shear wall. Another observation is that the fiber reinforced concrete develops a diagonal crack that penetrating upwards into the shear wall from the bottom face of the slab.

Further it is also observed that the crack growth is arrested in the fiber reinforced concrete. The vertical crack and horizontal crack length are gradually reduced for the models with higher volume fraction of steel fiber. But the length of diagonal crack increases with the increase in fiber volume fraction. This indicates that the region of stress concentration due to lateral loads is at the junction for model A. And on the adding the steel fiber this stress concentration region has moved into the shear wall. Hence the stresses developed at the wall slab junction are less.

Tensile stress variation in concrete

In this section the tensile stress variation in concrete is plotted both along the shear wall and along the slab at the critical line. The stress variations plotted along the lines A-A and line B-B. The selected lines are depicted in the Figure 12. The selected lines are at the mid surface of the slab. The line AA is right along the junction.

The stress variation along the line is extracted using path operation from ANSYS for load approximately equal to 25%, 50% 75% and 100% of the ultimate load. The selected load points are such that they induce the tension along the line BB, for that the load has to be acting at the slab end point B. Figures 5.24 to 5.28 represents stress variation along line BB and Figure 5.29 to 5.33 represents stress variation along line AA.





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Figure 12 path considered for stress variation

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000

Stre

ss

Distance from shear wall A25% A50% A75% A100% JUNCTION

Figure 13 Stress variation along line BB of model A





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0.0

0.2

0.5

0.7

1.0

1.2

0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000

Stre

ss

Distance from shear wall B25% B50% B75% B100% JUNCTION

0.0

0.3

0.6

0.8

1.1

1.4

0 500 1,000 1,500 2,000

Stre

ss

Distance from shear wall C25% C50% C75% C100% JUNCTION

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 500 1,000 1,500 2,000

Stre

ss

Distance from shear wall D25% D50% D75% D100% JUNCTION

0.0

0.5

1.0

1.4

1.9

2.4

0 500 1,000 1,500 2,000

Stre

ss

Diatance from shear wall E25% E50% E75% E100% JUNCTION

Figure 14 Stress variation along line BB of model B

Figure 15 Stress variation along line BB of model C

Figure 16 Stress variation along line BB of model D

Figure 17 Stress variation along line BB of model E





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The variation of tensile stress in the direction of global X along the line BB is shown In the Figure 13 to Figure 17. The stress are plotted for the 25%, 50%, 75% and 100% of the ultimate load, stress variation of model A shows that the tensile strength of the elements at the junction is reduced and this happens because of the cracking. Once the element cracks in tension its stiffness approaches zero. The tensile strength is reduced at the 75% of the loading itself at the junction whereas the tensile strength of steel fiber reinforced concrete is much persistent.

The stress distribution of fiber reinforced concrete shows that the concrete is capable of withstanding the tensile stresses even after cracking. The cracking load for the fiber reinforced concrete is around 60% of the ultimate load as discussed in the earlier section, even after cracking the fibers in the concrete acts as the bridge for the cracks and are able to take the additional tensile stress.

Conclusions

This study was intended to understand the behaviour of normal and fiber reinforced concrete in terms of load deflection, crack patterns and stress variation at the shear wall-slab junction when it is subjected to cyclic loading. In this section the conclusions are drawn based on the results obtained by finite element analysis of wall slab junction

Addition of steel fibers significantly increases the lateral load bearing capacity of shear

wall-slab junction, ultimate load bearing capacity increases almost linearly with the

volume fraction of steel fibers, it was found that the percentage increase of ultimate

strength is 21%, 40%, 93%, and 118% for in plane bending corresponding to 0.5%, 1%,

1.5%, and 2% of steel fibers by volume respectively.

Addition of steel fiber comparatively enhances the ability of concrete to withstand loads

without cracking. Addition of every 0.5% of steel fibers by volume increases the

cracking load by approximately by 20% for in plane bending

Post cracking behaviour of the steel fiber reinforced concrete is much better compared

to the conventional concrete for in plane bending.

Conventional concrete develops much deeper cracks both horizontally and vertically at

the junction whereas the fiber reinforced concrete does not develop much deeper

cracks. In addition the crack propagates diagonally into shear wall rather than at the

edge.

Stress variation graphs plotted for in plane bending show that the fiber reinforcement

reduces the stress concentration in concrete at the junction along the slab, and along the

shear wall





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REFERENCES

1. ANSYS, “ANSYS User’s Manual Revision 10”, ANSYS,Inc.2013

2. Antonio F. Barbosa and Gabriel O Ribeiro, “Analysis of Reinforced Concrete Structures

Using ANSYS Non Linear Concrete Model”, Computational Mechanics, New Trends and

Applications, Barcelona, Spain. 2000

3. Coull. A and Wong. Y. C., “Structural Behaviour of Floor slabs in Shear Wall Building,

Advance in concrete slab Technology R. K. Dhir and J.G.L. Munday Pergamon Press, pp.

301-312. 1980

4. Fanning P, “Nonlinear Models of Reinforced and Post-tensioned Concrete Beams”,

Electronic Journal of Structural Engineering. Vol 2, pp 111 -119. 2001

5. Kwak, Y.-K., Eberhard, M. O., Kim, W.-S., and Kim, J., “Shear Strength of Steel Fiber-

reinforced Concrete Beams without Stirrups”, ACI Structural Journal, 99(4), pp 530-538,

2002.

behaviour of shear wall slab junction with...

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