Enhancement of flat flabs' shear resistance using gabion mesh

TABLE OF CONTENTS

ACKNOWLEDGEMENT

LIST OF FIGURES

LIST OF TABLES

NOTATIONS

ABSTRACT

Chapter-1: -INTRODUCTION
1.1 The background
1.2 Statement of the problem
1.3 Objectives of the study
1.3.1 General objective
1.3.2 Specific objective
1.4 Scope of the study

Chapter-2:-LITERATURE REVIEW
2.1 Introduction
2.2. Behavior of slabs falling by punching
2.2.1Experimental investigations
2.2.2 Code provision
3.2.3 Finite element analysis
2.3 Enhancing the punching shear strength of flat slabs

Chapter-3:- FINITE ELEMENT MODELING AND METHODS
3.1. Overview of abaqus
3.2. Material modeling
3.2.1 Concrete
3.2.2. Reinforcement bar and gabion mesh
3.3. Selected geometry and modeling of flat slab
3.1. Concrete slab
3.2. Reinforcement
3.4. Gabion
3.4. Material properties
3.4.1. Concrete
3.4.2. Reinforcement bar
3.5. Interaction
3.6. Element type selection
3.6.1. Concrete
3.6.2. Reinforcement bar and concrete
3.7. Loading and boundary condition
3.8. Mesh convergence

Chapter -4:- RESULTS AND DISCUSSIONS
4.1. Introduction
4.2. Ultimate load capacity
4.3. Effect of gabion arrangements and their ultimate load prediction
4.4. Load-deflection relationships
4.5. Failure mode and failure location
4.6 Effect of gabion for flexural resistance
4.7 Effect of column size on percentage enhancement of gabion for punching
4.8 Effect of thickness of slab on punching resistance
4.9 Effect of drop panel for punching resistance
4.10 Comparison of Numerical Results with euro Code Provision

Chapter:-5-CONCLUSION AND RECOMMENDATION
5.1. Conclusion
5.2. Recommendations

REFERENCE

Appendix-A: SOME CALCULATIONS ON SLAB-COLUMN CONNECTION OF RC FLAT SLAB
A1. Calculation of Punching resistance and load
A1.1. Punching shear resistance calculation
A1.2. Punching shear area and punching force calculation
A2. Detailing calculation

Appendix-B: PARAMETERS USED FOR THE ANALYSIS

Appendix-C: SOME SELECTED ABAQUS OUTPUT RESULTS

ACKNOWLEDGEMENT

First and foremost I would like to thank the Almighty God for being with me throughout my life.

I wish to express my deepest gratitude to my advisor Dr. Medhanye Beidebrhan for his encouragement, guidance and recommendations from the initial to the final level enabled me to assemble and finish the thesis effectively.

I would also like to thank all my friends who have supported and encouraged me throughout my thesis. They have directed me through various situations, allowing me to reach this accomplishment.

The support received from the School of Civil Engineering, ERA funded project at EIT-M is gratefully acknowledged.

Finally, my family has supported and helped me in every condition of my life and my thesis by giving encouragement and providing the moral and emotional support I needed. To them, I am eternally grateful.

LIST OF FIGURES

Figure 2-1 experimentally punching failure mode by Guandalini

Figure 2-2 Euro code punching failure mode and critical perimeter

Figure 2-3 ACI code punching failure mode and critical perimeter

Figure 2-4 Modeling of flat slab by T.S. Viswanathan, 2014

Figure 2-5 Rankin punching strength against reinforcement ratio

Figure 3-1 flat slab concrete model

Figure 3-2 typical finite element model hexagonal gabion mesh

Figure 3-3 typical finite element model Rectangular Welded wire mesh

Figure 3-4 FE model of Slab reinforcement detailing

Figure 3-5 FE model for Reinforcement and gabion mesh detailing

Figure 3-6 modified Hongstad stress strain relationship for concrete

Figure 3-7 embedded interaction reinforcement bars in concrete

Figure 3-8 Element types for concrete

Figure 3-9 C3D8R with integration point

Figure 3-10 Truss elements

Figure 3-11 typical finite element model of the plate with loading and boundary condition

Figure 3-12 Mesh convergence

Figure 3-13 meshing the concrete slab

Figure 4-1 load deflection response for RC flat slab

Figure 4-2 comparative load deflection response for gabion at different depths

Figure 4-4 comparative load deflection response for flat slab with different layers

Figure 4-5 Comparative load deflection of hexagonal gabion mesh and equivalent rectangular welded wire mesh

Figure 4-6 comparative load deflection response of hexagonal gabion mesh, welded wire mesh and equivalent reinforcement

Figure 4-7initial plastic strain cracks at 45.85KN loading

Figure 4-8 crack patterns of punching shear failure at 231.1KN loading

Figure 4-9 plastic strain at failure

Figure 4.10 comparative load deflection responses for 25cm column flat slab with different layers

Figure 4-11 comparative load deflection responses for 30cm column flat slab with different layers

Figure 4.12 comparative load deflection responses for 35cm column flat slab with different layers

Figure 4.13 comparative load deflection responses for 40cm column flat slab with different layers

Figure 4.14 load deflection response of slabs with different thickness

LIST OF TABLES

Table 3-1mesh convergence values

Table 4-1comparative load deflection response for gabion at different depths

Table 4-2comparative value of flat slab with and without gabion

Table 4-3 comparative values of effects of gabion for different layers

Table 4-4 comparative abaqus output values for hexagonal and rectangular welded mesh gabions

Table 4-5comparative load deflection values of hexagonal gabion mesh, welded wire mesh and equivalent reinforcement

Table 4-6effect of gabion on central deflection

Table 4-7Percentage enhancement of gabion for punching at different column size

Table 4-8effect of flat slab for punching

Table 4-9 ultimate load comparative values between FE and euro code

NOTATIONS

Reinforcement ratio

Compressive strength of concrete

Yield strength of flexural reinforcement

Effective depth of slab

Punching perimeter at 2d

Dimension of column along y and z direction

Load proportion factor

Finite element

Partial safety factor of concrete

Nominal punching strength

Modulus of elasticity of concrete

Young's modulus of reinforcement

Poisson’s ratio

8 node 3 dimensional solid element

2 node 3 dimensional truss element

Code predicted ultimate load

Ultimate load finite element

Deflection along axis 2-2 (vertical deflection)

ABSTRACT

Punching strength is a critical point in the design of flat slabs and due to the lack of a theoretical method capable of explaining this phenomenon, empirical formulations presented by codes of practice are still the most used method to check the punching resistance of slab-column connections. This thesis presents study of punching shear capacity of flat slab-column junctions. A three dimensional nonlinear finite element program based on 8 node solid elements was used to carry out the nonlinear analysis of flat-slab models with and without gabion-mesh.

The effect of gabion arrangements for punching and the ultimate load prediction for each was presented in this thesis. The results obtained from abaqus were compared to code prediction results, and the failure mode also compared to experimental and code predicted failure modes. The predicted mode of failure and other responses are in a good correlation to euro code predicted values. In addition to punching gabion has greater resistance to flexure by increasing the stiffness of the slab.

Finally it is concluded that using hexagonal gabion mesh at tension part is easy, effective and can solve construction difficulty of drop panels and one layer gabion can reduce 10mm of slab thickness.

Key words: gabion mesh , flat slab, Concrete Damaged Plasticity model, Finite Element Analysis, Punching shear.

Chapter-1:-INTRODUCTION

1.1 The background

Flat slab is a reinforced concrete slab supported directly by concrete columns without the use of beams. This type of slab is appropriate for most floor situations and also for irregular column layouts. Because of its aesthetical view, simplicity for construction, reduction of foundation cost, this becomes very common and competitive structural system for cast-in-place slabs in buildings. Flat plates allow easy and flexible partitioning of space and reduce the overall height of tall buildings. But since the load is directly transferred from slab to column due to high localized force at the column punching effect or punching shear failure is critical. This type of failure is catastrophic because no visible signs are shown prior to failure.

To increase the punching resistance of the flat slab several methods have been used, such as drop panel, column capital, column head and shear reinforcements such as shear stud and stirrups. In our country Ethiopia the first three mechanisms are used to increase the resistance of punching shear in flat slabs but shear reinforcements are being used in other countries such as America and British. This is come in to practice due to several experimental and numerical researches. The researchers finally conclude that the shear strength of a flat slab depends on several parameters such as concrete strength, column size, amount of tension reinforcement, support condition and shear reinforcement. Two methods are available to assess the punching shear capacity and behavior of flat slabs. The first is by testing of models or the prototypes. The second is numerical analysis using computer software the second one is selected for analysis of flat slab with gabion mesh and finally compared to the code predictions

1.2 Statement of the problem

In the design of reinforced concrete flat slabs, the regions around the column always pose a critical analysis problem. Column tends to punch through the flat slabs, because of the shear stresses, which act in them around the perimeter of the columns. Shear failure, punching type, may be considered more dangerous than flexure failure because of greater uncertainty in predicting shear failure, which is likely to occur suddenly with no advance warning of distress. Large research efforts have been made in the past and are still being made to develop methods for a reliable prediction of the punching shear capacity. Numerous tests have been carried out to evaluate the punching shear strength of slabs. Several theories and analysis have been put 1 forward to predict the strength observed in these tests and analytical results. To increase the punching resistance of the flat slab several methods have been used, such as drop panel, column capital, column head and shear reinforcements such as shear stud and stirrups. But in most countries the first three mechanisms are used to increase the resistance of punching shear in flat slabs. The presence of these increases the overall coast of the structure. So to use another economical solution for the punching resistance another material should be used. So gabion may strengthen the punching resistance by increasing the bond between aggregates and by reducing concrete tensile cracks causing the slab to fail in punching and is easy to use. That is why I decided to analyze the resistance of gabion for punching.

1.3 Objectives of the study

1.3.1 General objective

The aim of this research is to address a better analytical understanding of punching of flat slabs with gabion. Thereby, the focus should be set on the analysis of the maximum increase in strength and deflection capacity due to gabion. Therefore, the principal aim is the nonlinear finite element analysis of flat slabs with the use of gabion by varying its layers and positions to increase shear strength of the slab.

1.3.2 Specific objective

- To predict the ultimate load capacity of slabs with and without gabion
- To investigate load deformation response with and without gabion
- To see the effect of gabion arrangements on punching resistance
- To investigate the failure mechanism of the flat slab in punching

1.4 Scope of the study

The research is mainly concerned on the analytical method of determining the punching behavior of flat slabs reinforced with gabion. Non-linear finite element has been used to predict the ultimate load capacity and the behavior of punching shear failure. This research only considers the behavior of interior slab-column connections using gabion mesh.

Chapter-2:-LITERATURE REVIEW

2.1 Introduction

Punching shear is one of the most critical phenomena for flat plate building systems due to the brittle nature of this failure mode. The region of a slab in the vicinity of a column could fail in shear by developing a failure surface in the form of a truncated cone or pyramid. This type of failure, called a punching shear failure, is usually the source of collapse of flat plate and flat slab buildings. Therefore the shear tends to reduce the ultimate load of the structure below its flexural capacity. It is one of topics of intensive research in recent years by various concrete structure researchers. Numerous tests have been carried out to evaluate the punching shear strength of slabs. Several theories have been put forward to predict the strength observed in these tests. Punching failure is brittle failure its occurrence can easily lead to progressive failure of the structure because the failed slab can easily fall down on to the next floor, causing cascading effect.

2.2. Behavior of slabs falling by punching

2.2.1 Experimental investigations

In experimental work on punching shear failure of interior slab-column connections, the slabs are loaded at the center through steel plates or column stubs and are simply supported around their edges. This section describes experimentally observed behavior of slab-column connections loaded at the center.

(Marko Bartolac, 2015), held an experimental investigation on punching shear with and without shear reinforcement and conclude that the failure mechanism of flat slabs is truncated cone and the cracks forms an angle of 15-45 degrees to the horizontal.

(Guandalini S., 2009) The test series consisted of 11 reinforced concrete square slabs representing internal slab-column connections by varying steel ratio from 0.25% to 1.5% the final failure mode for all slabs with different steel ratio was punching shear, with a clearly delimited punching cone.

Abbildung in dieser Leseprobe nicht enthalten

Figure 2-1experimentally punching failure mode by Guandalini

(P. V. P. SACRAMENTO, 2012)Determines the failure load and mode of failure of 74 test results of flat slabs without shear reinforcement and they compared to the code predictions of punching resistance. Generally they conclude that EC2 presented satisfactory and safety results, being registered average results for the ratio M/r of less than 1.19. ACI’s recommendations are meant to be safe, but underestimate the punching strength of flat slabs in about 37% for those 74 slabs.

The experimental program conducted by Marzouk and Hussein was based on a series of practical configurations of a conventional slab-column system. Seventeen specimens were subjected to concentric vertical loading. The specimens were simply supported along all four edges to simulate the lines of contra flexure. Such specimen represented the region of negative bending moment around an interior column. These specimens were divided into four groups in order to investigate important parameters of flat slabs. Reinforcing bars consisted of Grade 400 steel with actual yield strength of 490 MPa. The concrete mix was designed to produce 28-day strength of 70 Mpa. The specimens were loaded monotonically through the center stub column. The load was applied with a hydraulic actuator that has a maximum capacity of 670 KN. During the test, the slabs were carefully inspected. Extensive measurements of the strains were used at key locations on the flexural bar and surface of concrete slabs. Hussein chose four series of specimens in his experiment with four variables, including the concrete strength, the reinforcement ratio, the slab thickness and the load area. Hussein analyzed the experimental data and investigated the behavior of HSC flat slabs, including load-deflection characteristics, ductility and energy absorption characteristics, concrete strains, steel strains, cracking and failure characteristics, and modes of failure.

2.2.2 Code provision

2.2.2.1 Euro code provision

According to euro code (2004) , the critical section for punching shear follows the basic control perimeter (ui) located at a distance 2d from the periphery of the concentrated load. According to this code the punching failure at ultimate limit state is shown in the following figure. The punching shear strength provided by the concrete is also given by the following equation;

Abbildung in dieser Leseprobe nicht enthalten

Figure 2-2Euro code punching failure mode and critical perimeter

3.2.2.2 ACI code provisions

Abbildung in dieser Leseprobe nicht enthalten

Figure 2-3ACI code punching failure mode and critical perimeter

3.2.3 Finite element analysis

In finite element method, the slab is divided into a number of elements or sub-regions. The shape of these elements can be hexahedral, tetrahedral or quadrilateral in shape. They are considered interconnected only at discrete points, called nodes, these nodes are at the corners of the individual elements in the case of linear interpolation and at the corners and at the middle of each edge in the case of quadratic interpolation.

Proper finite element formulation is one way of predicting ultimate load capacity and behavior slab­column connections. (T.S. Viswanathan, 2014)Modeled a simply supported flat slab supported along the four sides and load was applied at a column stub area of 200 mm x 200 mm (Figure 2.4). He used a concrete damage plasticity model with an element type of C3D8R and T3D2 for the reinforcement bars. The FEM results of the punching shear strength were compared with the predictions based on the equations specified in ACI-318M-08, and Euro code. His model agrees well with ACI prediction compared to euro code prediction. The predicted values obtained from euro code are less than the actual carrying capacity of the structure. He also tried to determine Crack pattern of punching shear failure.

(Mahmoud, 2015),has developed a three dimensional finite element model (FEM) through Ansys 10 computer software, to carry out the nonlinear analysis of 16 flat-slab models with and without punching shear reinforcement. He modeled a 3mx3m simply supported flat slab and loaded a pressure load at the column. Several important parameters were incorporated in his analysis, namely the column size, the slab thickness and the punching shear reinforcement system in order to study their effects on the flat slab behavior.

Abbildung in dieser Leseprobe nicht enthalten

Figure 2-4Modeling of flat slab by T.S. Viswanathan, 2014

2.3 Enhancing the punching shear strength of flat slabs

The punching shear resistance of reinforced concrete flat slabs can be enhanced by various means. In our country when we design flat slabs if the punching shear is greater than the punching resistance we use different techniques to enhance the punching resistance of the flat slab. Such as drop panel, column capital, column head or combination of them. Several researches also held numerically and experimentally to enhance the punching resistance of flat slabs.

The following are some research outputs of enhancing punching resistance of flat slabs

1. Ratio of flexural reinforcement

The flexural reinforcement ratio (p) is defined as the ratio between the area of tensile flexural reinforcement (As) and the area of concrete (Ac) increasing the flexural reinforcement ratio raises the compression zone, reducing cracking in the slab-column connection.

(P. V. P. SACRAMENTO, 2012), and (Marko Bartolac D. D., 2015) conclude that the punching resistance increases with increasing the flexural reinforcement ratio.

Abbildung in dieser Leseprobe nicht enthalten

Figure 2-5Rankin punching strength against reinforcement ratio

2. Strength of Concrete

The shear failure of flat slab without shear reinforcement is governed by concrete strength. Increasing concrete grade will increase the punching resistance of flat slab.

3. Using shear reinforcement ratio

(Ahsanul Kabir), observed the effect of compression mat and shear reinforcement on concrete flat plate. He finally concludes the punching shear strength and the ductility of the slab increased with the addition of shear reinforcement.

Mahmoud studied on the influence of varying shear reinforcement system. He observed that the strength and the rotation capacity significantly increase if shear reinforcement is provided. Also, the slabs with studs showed a higher strength and a larger rotation capacity than slabs with stirrups. A reduction in his proposed normalized rotation values of approximately 35% compared to the experimentally obtained.

Chapter-3:- FINITE ELEMENT MODELING AND METHODS

3.1. Overview of abaqus

Abaqus is a suite of powerful engineering simulation programs, based on the finite element method, which can solve problems ranging from relatively simple linear analyses to the most challenging nonlinear simulations. Abaqus contains an extensive library of elements that can model virtually any geometry. It can simulate the behavior of most engineering materials including metals, rubber, polymers, composites, reinforced concrete, crushable and resilient foams, and geotechnical materials such as soils and rock. Abaqus offers a wide range of capabilities for simulation of linear and nonlinear applications. Problems with multiple components are modeled by associating the geometry defining each component with the appropriate material models and specifying component interactions. In a nonlinear analysis Abaqus automatically chooses appropriate load increments and convergence tolerances and continually adjusts them during the analysis to ensure that an accurate solution is obtained efficiently. Abaqus consists of two main analysis products—Abaqus/Standard and Abaqus/Explicit. Abaqus/CAE is the complete Abaqus environment that includes capabilities for creating Abaqus models, interactively submitting and monitoring Abaqus jobs, and evaluating results. Abaqus/Viewer is a subset of Abaqus/CAE that includes just the post processing functionality.

Abaqus/Standard:

Abaqus/Standard is a general-purpose analysis product that can solve a wide range of linear and nonlinear problems involving the static, dynamic, thermal-stress, mass diffusion and electrical response of components. Abaqus/Standard solves a system of equations implicitly at each solution increment.

Abaqus/Explicit:

Abaqus/Explicit is a special-purpose analysis product that uses an explicit dynamic finite element formulation. It is suitable for modeling brief, transient dynamic events, such as impact and blast problems, and is also very efficient for highly nonlinear problems involving changing contact conditions, such as forming simulations.

Abaqus/CAE:

Abaqus/CAE (Complete Abaqus Environment) is an interactive, graphical environment for Abaqus. It allows models to be created quickly and easily by producing the geometry of the structure to be analyzed and decomposing the geometry into mesh able regions. Physical and material properties can be assigned to the geometry, together with loads and boundary conditions. Abaqus/CAE contains very powerful options to mesh the geometry and to verify the resulting analysis model. Once the model is complete, Abaqus/CAE can submit, monitor, and control the analysis jobs. The Visualization module can then be used to interpret the results.

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