FINITE ELEMENT MODELLING OF A FIXED SANDCRETE MASONRY WALL UNDER BLAST LOADING

ABSTRACT

Terrorism, with explosions both in and outside buildings, has become a significant problem in the world. There are already several houses of unreinforced masonry, some of which are historical. But under blast loading they do not function well. Retrofitting methods have been developed to protect masonry buildings. Some research work has been carried out in recent years to study the impact of retrofitted URM walls; these experiments require usually significant time and funds. Also, with the attacks being carried out by Boko Haram in Nigeria, a study is needed to check the behaviour of sandcrete masonry under blast loading as it is widely used in Nigeria.
Unreinforced masonry walls are vulnerable to collapse when they are subjected to explosives of out-of-plane and in-plane loads, as masonry houses were historically designed with little to no consideration to the impact of explosive loads [1] . Because of the lack of strengthening, many of these structures mostly fail in out-of-plane bending. In addition, the key cause of death was found in the latest explosive attacks was unreinforced masonry buildings (i.e. bursting into debris) [2] . In the field of structural engineering, the upgrade of such structures has become a priority [3] .
In order to mitigate out-of-plan bending, this study was conducted to develop and test innovative fiber reinforcement techniques in the retrofitting of masonry walls. A model was made using finite element analysis computer software called ABAQUS, to study the behaviour of masonry wall subjected to the blast load. Also, CFRP material was applied to the wall to improve the resistivity of the masonry wall to explosions and was modelled as well to check if it’s has enough strength to withstand an explosive load.

TABLE OF CONTENTS
DECLARATION II
CERTIFICATION III
ACKNOWLEGEMENT IV
ABSTRACT V
LIST OF FIGURES VIII
LIST OF TABLES IX

CHAPTER ONE 1
INTRODUCTION 1
1.1 BACKGROUND 1
1.2 RESEARCH PROBLEM(S) 3
1.3 MOTIVATION(S) 3
1.4 AIMS AND OBJECTIVES 3
1.5 THESIS OUTLINE 4

CHAPTER TWO 5
2.1. INTRODUCTION 5
2.2 BLAST LOADING 5
2.3 BLAST LOADING TYPES 6
2.3.1 Unconfined Explosions 8
2.3.2 Confined Explosions 9
2.4. BASIC PARAMETERS OF THE EXPLOSION 9
2.5. BLAST WAVE INTERACTIONS 10
2.6. STAND-OFF 11
2.7. FAILURE MODES OF BLAST-LOADED STRUCTURES 12
2.7.1. Global Structural Behaviour 12
2.7.2. Localized Structural Behaviour 12
2.7.3 Pressure-Impulse (P-I) Diagrams 13
2.8. REVIEW OF PREVIOUS WORKS 14
2.9 DESIGN GUIDELINE 16

CHAPTER THREE 17
3.1. FINITE ELEMENT ANALYSIS 17
3.1.1 INTRODUCTION 17
3.1.2 MASONRY MODELLING STRATEGY 18
3.2 SANDCRETE BLOCK 20
3.3. FIBRE REINFORCED POLYMER (FRP) 22
3.3.1 FRP STRENGTHENED MASONRY STRUCTURES 23
3.4 FAILURE CRITERIA 26
3.5 MESH CONVERGENCE 27

CHAPTER FOUR 28
RESULT AND DISCUSSION 28
4.1 INTRODUCTION 28
4.2 ABAQUS DESIGN 28
4.2.1 ASSEMBLY 28
4.2.2 MATERIAL PROPERTIES 29
4.2.3 LOADING 30
4.2.4 BOUNDARY CONDITION 31
4.2.5 MESH 31
4.3 MESH CONVERGENCE STUDY 32
4.4 DISPLACEMENT RESULTS 33
4.4.1 FIRST AND SECOND MODEL 33
4.4.2 THIRD MODEL 36
4.4.3 FOURTH AND FIFTH MODEL 37
4.4.4 FIRST MODEL WITH CFRP 39
4.4.5 SECOND AND THIRD MODEL WITH CFRP 40
4.4.6 FOURTH AND FIFTH MODEL WITH CFRP 42

CHAPTER FIVE 45
CONCLUSION AND RECOMMENDATION 45
5.1 CONCLUSION 45
5.2 LIMITATIONS 45
5.3 RECOMMENDATION 45
REFERENCES 46

 

CHAPTER ONE

INTRODUCTION

1.1 BACKGROUND

Over the course of the last decade, public locations, high valued building and military bases have been the major center of terrorist attacks and blast explosions[4] . Most attacks tend to be done in a place of high human and economic losses. Most of these attacks are focused mainly in areas like Schools, hospitals, public government buildings because they are easy targets i.e. free movement[5] . Such attacks have led to significant loss of lives, injuries and catastrophic damage to structures worldwide. Both structural and non-structural members can be damaged depending on the distance between the blast explosion and the structure. Blast explosions have a huge dynamic force which when in contact with a structure, could cause significant damage or even lead to the collapse of the structure[6] . For several years, structural engineers have been concerned with how to cater for blast explosions in the design of structures. Originally, for accidental explosions that may occur in factories, military depot, chemical manufacturing plants or even refineries, research, analysis have been conducted to protect structures against such occurrences. Recently over the world, the design philosophy has been shifted due to the increase in terrorist attacks from designing for expected explosive events of known magnitude and size (i.e. military weapon testing) to designing for unexpected explosive events. These intentional explosions could lead to intense structural damage and possibly loss of lives if not properly designed for. To avoid the loss of lives and reduce the damages sustained by a structure, research, experiments and analysis have been conducted to provide mitigation solutions. The weight of the charge(W), distance of the blast (i.e. the stand-off distance), location of the blast (i.e. geometrical orientation e.g. air or surface blast), or orientation of the structure (i.e. direction of the blast) are the major factors influencing the blast loading and its effects on structures. Although the threat of an explosion is random in nature, depending on the combination of these factors, the structural response will differ. When assessing the vulnerability of structures, it is vital to identify how each factor influence the blast loading and its effect on the structure, in doing so, the analysis becomes complex.

Terrorist bombings have demonstrated the need for a thorough analysis and improvement to the design standard and guidelines. Bombing events of high structures such as the World Trade Centre in New York in 1993, U.S. embassies in Nairobi, Kenya, and Dar es Salaam, the Murrah Federal Building in Oklahoma in 1995, the Khobar Towers military barracks in Dhahran, Saudi Arabia in 1996 and many others are reasons enough for structural engineers to pay attention to behaviour of structures and adjust the design and construction codes to provide adequate protection against blast explosions. High strain rate, non-linear elastic material behaviour and time-dependent finite deformations are difficulties that arise with the intensity of the problem, which has been the cause of various approximations and assumptions to easy the analysis of these models. These models span the full range of sophistication from a single degree of freedom systems to general-purpose finite element programs such as ABAQUS, ANSYS, and ADINA, etc. [7] . Based on the knowledge of pressure and impulse values exclusively, the severity of damage sustained by a structure cannot be determined, understanding the characteristics of the blast loaded building is vital, especially with regard to the dynamic properties of the building material and structural form. Composite materials such as FRP (Fibre Reinforced Polymer/ Plastics) among innovative and modern methods of intervention on structures are continuously being considered for fixing and enhancing of masonry structures (both historic and modern) and structural elements (i.e. beams, columns, walls, etc.).

A comprehensive description of the blast design is available from several valuable resources. A summary on the structural steel designing is discussed in a question and answer format by Longinow and Alfawakhiri [8] . The questions are categorized into the science of general explosion, structural design, physical security, progressive collapse and methods and techniques of analysis. A summary on the effects of blast loadings on structures and discussions on methods and techniques of predicting blast explosion and behaviour of the structure is provided by Ngo et al. [9] . Further details than Longinow and Alfawakhiri [8] are available on this paper as well in Q&A format.

Although it’s in its early stages, typical design guidelines utilize a “Single Degree of Freedom (SDOF)” model to analyze blast explosion and retrofitted masonry design (Biggs 1964). The SDOF system is easy to deploy and numerically accurate but has a range of inconveniences. It cannot, for example, capture a modification in mechanical characteristics of the cross-sectional area together with its component, at the same time not withstand shear and bending deformation, and cannot allow spatially and partial distributions of blast loading. Although the finite element analysis requires further development to analyze the dynamic response of retrofitted masonry elements against blast loads, all the above variations from the SDOF can be accommodated. Studying the behaviour of blast loaded retrofitted URM walls and develop effective methods of retrofitting to improve the ability of a structure to withstand blast explosions.

 

1.2 RESEARCH PROBLEM(S)

In light of the recent events in Nigeria, such as the bomb attacks by the Boko haram insurgency, commercial buildings and private properties were damaged by the attacks. High valued buildings like the UN building, where lives were lost and a lot of people injured, was damaged in an attack by the insurgency group in 2011. Also, a lot of private buildings nationwide have been damaged by similar attacks. The lack of provision for blast resistance in the design of structures will cause significant damage to structures in the cause of such events.

 

1.3 MOTIVATION(S)

The lack of blast resistance design practice in the nation and the enthusiasm for structural design prompt me to conduct this study and analysis to improve the design of structures to mitigate the amount of damage a building sustains and the injuries of the humans in the buildings.

 

1.4 AIMS AND OBJECTIVES

The aim of this study is to investigate the effects of blast loads on a masonry wall and further, to see the dynamic properties and response of the masonry wall under high pressure and strains typically produced by the blast loads.

Specific objectives:

• Identify and choose the most appropriate method for modelling of a masonry wall and also the types of fibre-reinforced polymers to be used for mitigation from the literature review.
• Study and investigate the dynamic response and damage of a masonry wall subjected to external blast loading including; the threat definition, properties of the materials and blast wave parameters.
• Modelling and finite element analysis of the masonry wall in Abaqus/CAE.
• Modelling and finite element analysis of the masonry wall reinforced with FRP in Abaqus/CAE.

 

1.5 THESIS OUTLINE

This thesis is divided into five chapters:

• In Chapter 1, the background, research problem, motivation and aims and objectives of this project are introduced. The summary of this thesis will be presented in the following content in this chapter.
• Chapter Two explains the explosions definition, blast effects on buildings, and the relevant literature review required. Furthermore, design guidelines to mitigate are also reviewed in this Chapter.
• Chapter Three deals with the blast analysis methods, material type of the masonry wall and modelling of the masonry wall under blast loading
• Analysis and discussion of the modelling results are presented in Chapter Four.
• Finally, the conclusions drawn from this thesis work and the recommendations for future work are presented in Chapter Five.

FINITE ELEMENT MODELLING OF A FIXED SANDCRETE MASONRY WALL UNDER BLAST LOADING

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