Term Paper Slab

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Contents

Chapter-1
INTRODUCTION
1. General
1.1 Objective of Study
1.2 Organization of the dissertation

Chapter-2
LITERATURE REVIEW

Chapter-3
MODELLING OF FLAT SLAB BUILDINGS
3.1.General
3.2 Types of flat slab
3.3 Different components of flat slab
3.3.1 Use of column head
3.3.2 Use of drop panel
3.4 Advantages of flat slab
3.4.1 Flexibility in room Layout
3.4.2 Prefabricated welded mesh
3.4.3 Buildable scores
3.5 Behavior of flat slab building during earthquake
3.6 Modeling of flat slab
3.6.1 Finite element approach
3.6.2 Equivalent frame method
3.6.3 Modified equivalent frame method
3.6.4 Extended beam- width procedure
3.6.5 Extended equivalent column method
3.6.6 Extended equivalent slab method

Chapter-4
SEISMIC SAFETY OF FLAT SLAB BUILDING
4.1 Building data
4.2 Comparison of cost
4.3 Materials of construction

Chapter-5
METHODS OF ANALYSIS
5.1 Static analysis
5.2 Frequency analysis
5.3 Response spectra
5.4 Design spectra
5.5 Push over analysis
5.6 Time history

Chapter-6
RESULTS AND DISCUSSION
6.1 Static analysis and validation of model
6.2 Frequency analysis
6.3 Response spectra
6.4 Pushover Analysis
6.4.1 Modeling Approach
6.4.2 Modeling of Frame
6.4.3 Material nonlinearity
6.4.4 Geometric nonlinearity
6.4.5 Plastic deformation curve

Chapter-7
STABILITY OF FLAT SLAB- A CASE STUDY
7.1 Structural configuration
Fig-7.2 FEM model of city mall building
7.2 Loads on the building
7.2.1 Earthquake Load (EL)
7.3 Results and discussions

Chapter-8

CONCLUSIONS

PUBLICATIONS

Chapter-1

INTRODUCTION

1. General

Common practice of design and construction is to support the slab by beam and beams by column. This may be called as beam-column construction. The beam reduces the available net clear ceiling height. The aesthetically this type of construction is poor but performance of those buildings are quite good. In recent practice slabs are directly put on the column for aesthetic and architectural point of view. The load transmission path changes due to deletion of beams. But the safety of those building is to be checked. Seismic codes are also silent on the design of flat slab. But from the past history it can be understood that the flat is very vulnerable in earthquake point of view. Figure 1.1 shows the flat slab failure in an earthquake.

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Fig-1.1 Flat slab failure in earthquake at Tropicana casino parking

Keeping the failure in mind in this dissertation the performance of the flat slab building over a similar conventional building is estimated. Frequency analysis, response spectrum analysis, non-linear pushover analysis and non-linear time history analysis has performed to ensure the stability of flat slab building.

1.1 Objective of Study

i). To check the feasibility of flat slab in high seismic zone.
ii). To compare the cost effectiveness of flat slab over a conventional building.
iii). To compare the response parameters of flat slab and conventional building by using IS:1893-1984 and euro code method.
iv). To compare of deflection of flat slab building and conventional building.
v). To perform the non-linear pushover analysis to detect the failure pattern.
vi). To execute the time history analysis to estimate the story drift of the flat slab and conventional building.

1.2 Organization of the dissertation

The dissertation has been described in eight chapters. Chapter-1 covers the introduction and objectives of the thesis. Chapter-2 describes the related literature review. Chapter-3 deals with the modeling of flat slab. Chapter-4 explains the seismic safety of flat slab. Chapter-5 discusses the methods of analysis a flat slab. Chapter-6 deals with the results and discussions. And in chapter-7 a case study has given. Chapter-8 gives a conclusion of the thesis.

Chapter-2

LITERATURE REVIEW

Seismic response of flat slab building has been a subject of discussion since many decades. A lot of research work has taken place in this field addressing all relevant issues pertaining to the modeling, analysis and construction of flat slab structures. Literature survey for seismic safety of flat slab buildings have also been covered.

Park et al. (2008) found that Equivalent Frame method is not appropriate in accurately predicting the response of two-way slab systems under lateral loads. The equivalent frame method was for two-way slab as a simple approximate method. Currently design code, ACI 318-05[2.1] permit the EFM for the analysis of two-way slab system under gravity loads s well as lateral loads such as seismic loads.

Subramanian (2005) has found that to enhance the punching shear strength of lat slab, the shear stud reinforcement is found to provide economic and aesthetic solution. They not enhance the shear capacity but also result in flexural failure of the slab and thus increasing the ductility of flat slab, which is very important in earthquake prone zone.

Carl Erik Broms (1990) has proposed a method for the prediction of punching shear strength of the flat slab. This method accurately predicts the failure loads and the deformation in the whole range of failure modes, from the overall flexure failure to brittle punching failure. This method also account for the size effect, increasing brittleness at high strength, symmetrical and unsymmetrical loading and reinforcement ratio. He concluded flat slab has two disadvantages i) poor capacity of slab to distribute the negative support moment and ii) the risk of brittle punching failure. But this disadvantage can be overcome by using shear reinforcement with the slab in the accurate fashion.

Meghally and Ghali (2005) has proposed the value of the unbalanced moment to be used in punching shear design. The upper limit for the design value of the unbalanced moment was based on the flexural capacity of the slab. Use of the upper limit of the design moment ensure at punching shear failure is prevented. And also the use of shear reinforcement, particularly shear stud with mechanical anchored significantly enhanced the ductility of slab column connection

1. Brittle punching failure can occurred due to the transfer of shearing force and unbalanced moment between slab and column. During an earthquake, the unbalanced moment can produced significant shear stress in the slab.
2. Through drop panels and shear capital enhance the punching strength of slab, they do not improve the ductility, which is an essential requirement of earthquake resistant structure.
3. Shear capital do not enhance the punching strength when column transfer large moment reversals to the slab.

Corley and Jirsa (1970) first developed “Equivalent Frame Method (EFM)” for design of all types of slab system in 1970. That method did not have any limitation like direct design method. They compared the moment calculated by EFM with those measured in test slab and the moment shown the satisfactory agreement. They also provided the list of constants for calculating stiffness, fixed end moments and carry over factor for beam element.

Kim and Lee (2005) has purposed an improved analytical method that can consider the stiffness degradation effects in the slab depending on the lateral drifts using super element for the efficient and accurate analysis of flat slab structure. The super elements and fictions beam used for the efficient analysis and the accuracy and the efficiency of the proposed method were investigated through the analysis of three example structures. The major observations and findings could be summarized as follows.

1. Structural analysis of the flat slab structure having irregular plan or slab with openings can be performed and stress distribution of floor slab can be easily represented the finite element method if the stiffness degradation in the slab could be consider proper ly
2. The stiffness degradation in the flat slab system could be represented by the reduced modulus of elasticity of floor slab.
3. The proposed method using super elements developed by introducing fictions beams could reduced the computational and memory significantly in the analysis.
4. The stress distribution in the slab could be represented approximately by the proposed method.

Hwang and Moehle (1993) carried out an experimental study on nine panel model comprising a slab supported without beams, drop panels, slab shear reinforcement. A portion of the slab was design for gravity and wind load in accordance with ACI 318-83, where the remainder was designed for the moment distribution not permitted by the code.

Dovich and Wight (2005) developed an effective slab width model to describe the lateral behavior of the reinforced concrete flat slab frame with in a two dimensional non linear frame analysis. Based on the results of this study, the following can be drawn

1. The exterior connections contribute little to lateral strength and stiffness.
2. If an inelastic push over analysis had been used at the design stage, it could have predicted that slab punching shear possible at drifts reasonably expected to occur during a moderate to severe earthquake.

Chapter-3

MODELLING OF FLAT SLAB BUILDINGS

3.1.General

Flat slab are preferential by both architects and clients because of their aesthetic and economic advantages. Though this form of reinforced concrete construction gives several advantages over frame structure, they also present some disadvantages because of brittle punching failure and large deformation. Many researches propose that flat slab should be designed to resist only gravity loads when used in higher seismic zones and lateral loads should be carried by lateral resist in system.

This study is inclined towards the seismic response of flat plate-braced frame system or stud rail. Braces are used as structure elements and they do resist lateral forces with substantial structural action and stud rail resist punching shear.

Slab-column connections are the first point of yielding in case of building without braces or stud rail. The shear capacity of this connection reduces drastically in the presence of lateral loading and hence, in such type of buildings should be carefully designed.

Flat slab can be supported by column and by a column capital and drop panel. The flat slab system has been adopted in many buildings constructed recently due to the advantage of reduced floor heights to meet the economical and architecture demands. The design of flat slabs is typically governed by the punching shear strength at failure.

3.2 Types of flat slab

Flat slabs can be classified as per the slab column junction. There are four types of flat slabs are commonly used in buildings. They are as follows

- Slab without drop and column with column head
- Slab with drop and column without column head.
- Slab without drop and column with column head.
- Slab without drop and column head

The various slabs are shown in Fig-3.1.

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Fig-3.1 Slab with various drop panels

3.3 Different components of flat slab

The main portion of interest in the flat slab building is the slab column junction. To ensure the safety drop panels and column heads are provided.

3.3.1 Use of column head

- It increase the punching shear strength between column and slab
- It increases the moment capacity of the slab.
- It provided more thick supporting area.

3.3.2 Use of drop panel

- Drop panel enhance punching shear strength of flat slab
- Drop panel Increase negative moment caring capacity of flat slab
- Drop panel increase stiffness of the slab and hence reduce deflection

3.4 Advantages of flat slab

- The layout of the building part like room, kitchen, are more flexible
- The building height of flat slab building is more as compare to conventional building due to low story height.
- Time in construction of flat slab building is less.
- Fitting of electrical and mechanical devises are very easy.
- Welded mesh can use in flat slab building to increase the speed of construction.

3.4.1 Flexibility in room Layout

- Allow architect in introduce partition walls anywhere require
- Allows owner to change the size of the room layout
- In Flat slab building no need to provided false ceiling and finish soffit of slab with skim coating.
- Flat plate design will make possible the use of big table formwork to boost output.

3.4.2 Prefabricated welded mesh

- Prefabricated in standard size
- Minimize installation time
- Better quality control

Prefabricated welded mesh construction is shown in Fig-3.2.

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Fig-3.2 Reinforcement placing of flat slab

3.4.3 Buildable scores

Allows standardizes structural member and pre fabricated section to be integrated in to the design for ease of construction. This process will make the easy and fast construction, reduce the number of site workers and increase the productivity at site.

3.5 Behavior of flat slab building during earthquake

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. In flat slab building the most vulnerable part is slab column joint. Extensive research has been done to find out the behavior of flat slab column connection. The failure mode depends upon the type and extent of loading. Punching shear strength of slab column connection is of importance which very much depends on the gravity shear ratio. Punching failure of flat slab can occur as a result of transfer of shearing force and unbalanced moment between slab and column.

The behavior and design of flat slab plat structure for gravity loads are well established. Transfer of lateral displacement induces moment at slab column connection which is of complex three dimensional behaviors. Due to the flexibility of flat plate building, they must be combined with a stiffer lateral force resisting system in high seismic regions. When flat slab is used in combination with braced frames, shear wall for lateral load resistance, the column in building can be designed for only 25% of the design seismic force.

3.6 Modeling of flat slab

Flat slab can be modeled in various techniques. They are

i). Finite element approach
ii). Equivalent frame approach
iii). Modified equivalent frame approach
iv). Effective beam width procedure
v). Extended equivalent column method
vi). Extended equivalent slab method

3.6.1 Finite element approach

Slab behavior can be modeled directly using finite element method, typically involving plate bending element. Hwang and Moehle (1993) conclude after comparative analysis by test result that finite element solution is nearly equal to theoretical solution. Polak (2005) used degenerated shell element for modeling the behavior of reinforced concrete slab subjected to high transverse stress. The model was able to predict the capacity of slab indicated the type of failure (failure or shear). The parametric study indicate that the analysis is able to predict the influence of several of the most commonly considered parameters on the shear behavior of linear static analysis is questionable when calculated slab stresses exceed cracking value. For these reasons, direct use of finite element approaches is not discussed further here. The finite element model of a flat slab building is shown in Fig 3.3.

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Fig -3.3 FEM model of the building

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