Optimum Design of Reinforced Concrete Raft Foundations using Finite Element Analysis

Optimum Design of Reinforced Concrete Raft Foundations using Finite Element Analysis.

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
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%
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

INTRODUCTION  

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.

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).  

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StudentsandScholarship Team.