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ABSTRACT

This work presents the finite element analysis (FEA) of the requirements of compression reinforcements in raft foundations using ABAQUS. The model helps to confirm and provide a valuable supplement to the theoretical design. For validation, a reinforced concrete raft foundationis modeled whichis conventionally designed according to Eurocode 2 (EN 1992-1-1:2004). The result indicates that there is differential settlement within the raft foundation based on the settlement and stress patterns obtained from the finite element model (FEM). This is followed by the addition of compression reinforcement, from 0.1% to 0.9% of the cross sectional area of the raft slab, until uniform settlement is obtained. The results suggest that a suitable percentage of the concrete cross sectional area of raft slab foundations should be used as compression reinforcement, when designing conventionally using Eurocode 2, in order to prevent differential settlements. The required area of compression reinforcement is 0.9% of the cross sectional area of the concrete section.

 

 

TABLE OF CONTENTS

 

TITLE PAGE …………………………………..………………………………… i
DECLARATION ……………..………………………………………………… іi
CERTIFICATION ………..……………………………………………………. iii
DEDICATION………………….…………………….…………………………iv
ACKNOWLEDGEMENT……………………………………………………….v
ABSTRACT…………………………………………………………………..…. vi
TABLE OF CONTENTS……………………………………………..……….. vii
LIST OF FIGURES………………………………………………………………. x
NOMENCLATURE ……………………………………………………………. xiii
CHAPTER ONE:INTRODUCTION …………………………………………. 1
1.1 Preamble…………………………………….…………………………….1
1.2 Justification For The Study……………….……………………………..2
1.3 Aim and Objectives ……………………….………………………………3
1.4 Methodology……………………….………………………………………4
1.5 Scope and Limitation………………….………………………………….5
CHAPTER TWO: LITERATURE REVIEW…………………………………6
2.1 Site Investigation……………………………….…………………………6
2.1.1 Bearing capacity of foundations ……………….….……………………7
2.1.2 Total and differential settlements …………………………..…………..7
2.1.3 Soil horizontal variability ………………………………..…………….8
2.1.4 Other uncertainties involved in site investigation ……………..……….9
2.2 Raft Foundations………………………….……………………………..10
2.2.1 Need for raft foundations ………………….……….…………………10
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2.2.2 Types of raft foundations ………………………………..……………11
2.2.3 Design of raft foundations ……………………………..……………..12
2.2.4 Concrete under compression ……………………..……………………12
2.3 Finite Element Analysis…………………….……………………………14
2.4 Overview of the ABAQUS Program…………….……………………..15
CHAPTER THREE: RESEARCH METHODOLOGY……………………16
3.1 Introduction …………………………….………………………………..16
3.2Design of Raft Foundation……………….……………………………..17
3.3 Finite Element Analysis…………………….……………………………17
CHAPTER FOUR: RESULTS……………………………………………….25
4.1 Design of Raft Foundation According To Eurocode 2……….……….25
4.1.1 Design of a simple raft foundation ………………………..…………..26
4.1.2 Design of a simple raft foundation with additional 0.1%
compression reinforcement ……………………..…..………………… 28
4.1.3 Design of a simple raft foundation with additional 0.2%
compression reinforcement ……………………………..……………. 29
4.1.4 Design of a simple raft foundation with additional 0.3%
compression reinforcement ……………………………..……………. 30
4.1.5 Design of a simple raft foundation with additional 0.4%
compression reinforcement ……………………………..……………. 32
4.1.6 Design of a simple raft foundation with additional 0.5%
compression reinforcement ……………………………..……………. 33
4.1.7 Design of a simple raft foundation with additional 0.6%
compression reinforcement ………………..…………………………. 35
4.1.8 Design of a simple raft foundation with additional 0.7%
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compression reinforcement ……………………………..……………. 36
4.1.9 Design of a simple raft foundation with additional 0.8%
compression reinforcement …………………………………………… 38
4.1.10 Design of a simple raft foundation with additional 0.9%
compression reinforcement ………………………………..………… 40
4.2Stress Patterns in The Raft Foundation……….………………………54
4.3Settlement of The Raft Foundation…………………………………….62
CHAPTER FIVE: DISCUSSIONS…………………………………………..70
5.1 Stress Patterns in The Raft Foundation……………………………….70
5.2 Settlement of The Raft Foundation…………………………………….70
CHAPTER SIX:SUMMARY, CONCLUSIONS AND RECOMMENDATION ………………………………………………………………………….72
6.1 Summary …………………………………………………………………72
6.2Conclusions………………………………………………………………72
6.3 Recommendation…………………………………………………………73
REFERENCES…………………………………………………………………74
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Project Topics

 

CHAPTER ONE

INTRODUCTION
1.1 Preamble
The raft foundation was invented in the 19th century (Paul, 2010). Its development was necessitated by engineering requirements to build tall buildings (Paul, 2010). Initially, raft foundations were used for commercial and industrial developments (Paul, 2010). However, once the advantages of the concept were realised, the raft foundation became popular within residential developments (Paul, 2010).
A raft foundation is usually used when building in low soil bearing conditions to spread the load from a structure over a large area, normally the entire area of the structure (UWE, 2012). They are used when column loads or other structural loads are close together and individual pad foundations would interact (UWE, 2012). Raft foundations may be used for buildings on compressible ground such as very soft clay, alluvial deposits and compressible fill material where strip, pad or pile foundations would not provide a stable foundation without excessive excavation (Stephen and Christopher, 2010). The reinforced concrete raft is designed to transmit the load of the building and distribute the load over the whole area under the raft, reducing the load per unit area placed on the ground (Stephen and Christopher, 2010). Distributing the loads this way causes little, if any, appreciable settlement (Stephen and Christopher, 2010).
Structurally, raft foundations resting directly on soil act as a flat slab or a flat plate, upside down, i.e., loaded upward by the bearing pressure and downward by the concentrated column reactions (Mahdi, 2008). The raft foundation develops the maximum available bearing area under the building (Mahdi, 2008). Raft foundations are designed as inverted beam and slab system (Singh and Singh, 2006). The weight of the raft is not considered in the structural design (Singh and Singh, 2006). If all the loads transferred to the raft foundation are equal, raft may be a simple flat slab type, without any beam (Singh and Singh, 2006). In case loads are not equal, slab and beam system is usually adopted (Singh and Singh, 2006). Differential and total settlements usually govern the design (GEO, 2006).
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Finite element analysis or elastic continuum method is preferred for the design of raft foundations (French, 1999; Poulos, 2000).Subgrade reaction models are often not appropriate (Eurocode 7, 2004). More precise methods, such as finite element computations, should be used when ground-structure interaction has a dominant effect (Eurocode 7, 2004).
After laying a mat or raft foundation on the soil, soft soil for example, there is tendency of cracks developing in areas between columns (lower part) and in areas near and under columns (upper part) (Babak, 2011). Then there is need usually to reinforce upper part of foundations near columns and lower part between columns (Babak, 2011). Compression reinforcement is usually not applied in foundations. However, there is need to apply a minimum amount of reinforcement in the upper part of the foundation due to practical points of view. Then the additional minimum compression reinforcement may heighten the center of compression and increase the resisting moment provided by the section.
In the case of an under-design, there is a very high risk of potential failure, which if occurs, amounts to greater financial costs due to refurbishment, redesign and reconstruction. While in the case of an over design, the initial financial costs of design and construction will be higher with less financial risks of failure occurring.
1.2 Justification for The Study
The design of a raft foundation is prone to significant uncertainties. Many of such uncertainties are related to the estimation of suitable soil properties. The sources of uncertainties for soil properties are classified into three main components: inherent soil variability, measurement error and transformation model uncertainty (Filippas et al., 1988). Other uncertainties are associated with the site investigation and settlement technique. Also, the variation of elastic modulus of soil and presence of rock media plays a significant role and affects the moments and deformations of raft foundation (Venkatesh et al., 2009). The effect of the spatial variation of soil properties which induces foundation stresses and/or
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displacements that cannot be predicted when assuming soil homogeneity (Niandou et al., 2006), the variation of elastic modulus of soil, the presence of rock media and the design uncertainties may give rise to differential settlement within the raft foundation and subsequently its structural failure.
Eurocode 7 2004 specifies that more precise methods, such as finite element computations, should be used when ground-structure interaction has a dominant effect. This implies that the conventional method of design has little precision and should be complemented with a more advanced design method. This work presents the FEA modeling of the requirements of compression reinforcements in raft foundations using ABAQUS. The model helps to confirm and provide a valuable supplement to the conventional design.
1.3 Aim and Objectives
The aim of this research is to use finite element analysis in the optimum design of reinforced concrete raft foundations. The detailed objectives are to:
a. Design a simple reinforced concrete raft foundation structure using the conventional method of design. The design will then be subjected to deformation using finite element analysis in order to obtain the stress pattern and settlement.
b. Identify the need to provide additional compression reinforcement to the design at different percentages of the reinforcement ratio based on the cross sectional area of the raft slab and hence determine its effectiveness in providing resistance against differential settlement.
c. Appreciate the need to use finite element analysis in the design of reinforced concrete raft foundations.
1.4 Methodology
The structural design of the raft foundation will be carried out using the conventional method of design (i.e. hand calculation) and finite element analysis. The conventional design will be carried out according to Eurocode 2 (EN 1992-1-
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1:2004), which is to specify the depth of foundation, area and amount of reinforcement, and all the necessary checks needed in the design calculations.
The finite element analysis (FEA) will be carried out with the aid of a computer program. The program that will be used is SIMULIA ABAQUS 6.10. ABAQUS is a finite element analysis software that is used in a wide range of industries like automotive, aerospace etc., and also is extensively used in academic and research institutions due to its capability to address non-linear problems (Manjunath, 2009). The ABAQUS program can be used to model reinforced concrete structures analyze and generate test results using a state of the art 3D modeling and finite element technology.
The finite element analysis (FEA) will be used to test the designed reinforced concrete raft foundation. Other models will also be designed and tested which have compression reinforcement at various percentages of the reinforcement ratio based on the cross sectional area. This is to determine the effect of the compression reinforcement in providing resistance against differential settlement.
The raft foundation consists of a regular arrangement of eight column loads with four corner and four internal loads. All the corner columns carry a load of 458.33 KN each and the internal columns carry 666.66 KN each. Each column is 0.5 m by 0.5 m. Bearing capacity of the soil will be taken as 100 KN/m2. The characteristic strengths of the concrete and steel to be used in the design are 45 MPa and 500 MPa respectively. The Poisson ratio and density of the concrete will be taken as 0.2 and 2400 kg/m3 respectively while the Poisson ratio and density of the steel will be taken as 0.3 and 7850 kg/m3 respectively.
During the modeling in ABAQUS, the analysis parts for the soil, slab, and reinforcement will be created and assigned material and section properties. The embedded element option will be used to embed the reinforcements in the slab. The elastic foundation option will be used to model the soil surface to make it act as springs to ground which includes the stiffness effects of a support (such as the soil under a building) without modeling the details of the support. The parts will
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then be assembled together, the loads and boundary conditions will be imposed and the job executed to obtain the results.
1.5 Scope and Limitation
This research work is limited to the design, modeling and analysis of the flat reinforced concrete raft foundation without any experimental study

 

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