Carbon Fiber-Reinforced Polymer (CFRP) composite materials have provenvaluable properties and suitability to be used in the construction of new buildings and in upgrading theexisting ones. One of the objectives of this study is to laminate solid timber columns with varying thicknesses of CFRP laminates to check the patterns of the stresses, displacements and reactions of the solid timber columns. An analytical model for CFRP strengthened timber columnswith length of 3700mm, width of 150mm, depth of 200mm and varying thicknesses of 0.2mm, 0.4mm, 0.6mm, 0.8mm and 1 mm with coded specified axial applied load for three different timber species of Abura, Afromosia and Confusa. The secondlayer of CFRP is consideredmost effective wrapping scheme due to the cost effectivenessof increase in axial applied load. This research findings have shown that CFRP laminates and sprays are not only effective in restoring the lost capacity of damaged timber columns sections, but are also quite effective in strengthening of timber columns sections to sustain higher loads,extend their fatigue life and reduce crack propagation. The reliability assessment of composite timber columnswith varioustimber species was also executed using MATLAB programming. Adequate safety indices were obtained for different load ratio, MOE of CFRP, thickness of CFRP and strength classes of timber species..
TABLE OF CONTENTS
TITLE PageCover page i
Table of content vii
List of Figure vii
List of Tables x
List of Appendices xiii
CHAPTER ONE: INTRODUCTION 1
1.1 General 1
1.2 Background of Study and Statement of the Problem 3
1.2.1 Background of study 3
1.2.2 Statement of the problem 5
1.3. Justification of the Study 6
1.4Scope and Limitation 7
1.4.1 Scope 7
1.4.2 Limitations 7
1.5 Aim and Objectives 8
1.5.1 Aim 8
1.5.2 Objectives 8
CHAPTER TWO: LITERATURE REVIEW 9
2.1 General 9
2.2 Timber 12
2.2.1Failure Modes of Timber 12
2.2.2Tensile Failure in Timber 13
2.2.3Compressive Failure in Timber 13
2.2.4Structural Behavior of Timber 14
2.2.5Stress-Strain Behaviour 14
2.3Fibre Reinforced Polymers (FRP) 15
2.3.1Types of Carbon Fibers 16
2.3.3Strengthening of Structures 17
2.3.4Manufacture of Carbon Fiber Reinforced Polymer (CFRP) 18
2.3.5Properties of Carbon Fiber Reinforced Polymer (CFRP) 19
2.3.6Failure Modes 19
2.4Composite Timber Structures 20
2.5Reinforcing With Fibre Reinforced Plastics (FRP) 21
2.6CFRP Strengthening of Channel Columns 21
2.7Design of Circular Hollow Steel Sections with CFRP 21
2.8Design of Square Hollow Sections with CFRP 22
2.9Increases in Compressive Strength of Timber Column 22
2.10Wrap Orientation 23
2.11 Fiber Reinforced Polymer Spray for Structural Strengthening 23
2.12Protection from Corrosion Damage 26
2.13Deterministic and Probabilistic Design Approaches 26
2.14Behaviour of Columns 27
2.15Eccentricity of loading 27
2.16Design Criteria of Timber Columns using Eurocode 5 (2004) 27
CHAPTER THREE: METHODOLOGY 33
3.1 General 31
3.2 Timber 31
3.3First Order Reliability Method (FORM) 34
3.4Computation of Reliability Indices 36
3.5 Derivation of Limit State Equations 40
3.5.1 Compressive Resistance of Solid Timber Column 40
3.6 Compressive Resistance of Composite Solid Timber Columns in CFRP Laminates and Sprays 41
3.6.1 Column Geometric Properties 41
3.6.2 Timber Properties 42
3.6.3 Partial Safety Factors 42
3.6.4 Actions 42
3.6.5 Modification factors 43
3.6.6 Compression strength of column 43
3.6.7 Composite solid timber column in CFRP laminate 44
3.7 Finite-Element Method 46
3.8 Finite Element Modeling 47
CHAPTER FOUR; RESULTS AND DISCUSION 53
4.1 General 53
4.2 Analytical Results 53
4.3 Performance Model of Bonded CFRP Timber Columns 55
4.3.1 Stress pattern of solid timber column sections 64
4.3.2 Axial displacement of solid timber column sections 66
4.3.3 Reactions pattern of solid timber column sections 68
4.4 Program Development 70
4.5 Reliability Analysis of Solid Timber Column with CFRP Laminates and Sprays 71
CHAPTER FIVE; SUMMARY, CONCLUSION AND RECOMMENDATION 79
5.1 Summary 79
5.2 Conclusions 80
5.3 Recommendation 81
Timber have for ages remained among the major structural materials for building construction worldwide due to their renewable nature, availability in various sizes, shapes and colours, affordability, relatively high fatigue resistance and specific strength, ease of joining, durability, and aesthetic appeal. In Europe, timber, have been successfully utilized in both simple and complex structures(Ratief and Holicky, 2005). In Nigeria however, the only area where timberreceived wide acceptance is in roof framings. The utilization of the material in the engineered design of residential and commercial building receives little or no attention.
NCP 2 (1973), which is the timber design code in Nigeria since 1973 is based on permissible stress design philosophy. The code made references to the CP 112(1967). CP 112 (1967) had passed through several revisions since its inception and up to the current BS 5268 (2002). EC5 (2004) is a limit state design code for timber structures which currently co-exist with BS5268 (2002), which was fully replaced in 2010 (Tord, 2001; TRADA, 2009). BS 5268 (2002) is fully deterministic, while the EC5 (2004) is semi-probabilistic (Chanakya, 2009). The use of limit state design, instead of permissible stresses, enables differentiation of partial safety factors for permanent and variable loads, and makes it possible to reach a more even nominal safety level in all structures (Alpo, 2004). As a recently developed and formulated structural design standard, EC5 (2004) provides a wide range of consistent and up to date models and procedures that can be considered for the development of a local code. Revision of theEC5 (2004) design requirements based on the data on the properties of Nigerian timber species will go a long way in providing a background for the adaptation of advances in technology in the
developed countries of Europe to local design practice of timber structures in Nigeria, just as it is being done in other developing countries like South Africa (Ratief and Holicky, 2005).
These work is governed by structural economics, reliability, and so these costs must be considered as vital parts of engineering management. The concept of rehabilitation to increase structures stability should be considered as an alternative to complete reconstruction which can be uneconomical and timely due to jacking up of the structure, while repairs occur. The replacement of structural components would normally require the use of hardwood which is generally stronger than softwood, due to its superior density. This is typically an expensive process (Emerson, 2004), also creating sustainability issues due to the difficulty of obtaining old growth mature forest sawn timber (Walter, 1996). For softwood to be used in an outside environment, it generally has to be treated with toxic preservatives to protect it against fungi and insect pests. Although considered weaker, softwood timber forests are sustainable due to the speed at which the trees grow. If the softwood timber were able to be sufficiently protected, it could make for a viable option environmentally. Due to these concerns, column strengthening using alternative construction methods must be considered.
Carbon Fibre Reinforced Polymers (CFRP) have the capability to strengthen timber components in compression. Limited research has been conducted on layers of CFRP as a protective method from environmental degradation, but some promising results have been demonstrated.
The benefits of confining timber have mostly been documented from experimental research. North American and European standards establish that CFRP confinement of steel and reinforced concrete has structural benefits such as increased strength and ductility, comparable to the limited timber research conducted. The advantages of using CFRP confinement for reinforcement are that it is durable, corrosion resistant, easy to use and transport due to its high
strength to weight ratio and is more flexible than steel (Micheal, 2006). In a circular or rectangular timber column, the axial capacity may be increased, enhancing the compressive strength. Comparably to steel plate bonding, an alternative strengthening system in CFRP confinement, requires less manpower and scaffolding as it is a light material, saving time and money (Heslehurst, 2008).
1.2 Background of Study and Statement of the Problem
1.2.1 Background of Study
CFRP confinement as a strengthening system has been found to increase the load capacity of structures. The uncertainty of the mechanical properties of timber can be dissipated through the use of CFRP wraps. The laminates are carbon fibre reinforced polymers. The fibres, which are highly anisotropic (Pearce, 1970) provide the stiffness and strength of the system, while the polymer matrix holds the fibres in place. As timber does not have equal properties in all directions, confinement with CFRP can help to mitigate the random character of wood (Kasal and Heiduschke, 2010). Fibres are used from a mass of materials, a significant increase in strength and decrease in brittleness occurring when the materials undergoes an extrusion-like process (Heslehurst, 2008). The bond of the confinement of timber with CFRP is of great significance. The epoxy matrix displays excellent adhesion and strength, forcing the individual and flexible fibres to cooperate in the same direction, transfer loads between the fibres and protect the fibres from environmental factors (Pearce, 1970). The epoxy matrix is cured by the addition of a hardener, the mix forming a chemical bond. When cured, the once flexible and workable fibres become very stiff. This is important for compressive loads and the avoidance of buckling. Strengthening of structures is a requirement for rehabilitation of structures in order to increase its structural capacity. Compressive strength, ductility and environmental protection are three of the factors considered to be benefited from confinement. The benefits vary
depending on wrap number and orientation, and produce differing failure mode from the reference samples (Zhang et al., 2012; Heiduschke and Haller 2012; Kasal and Heiduschke 2010). Compressive forces are jointly transferred to the timber and the carbon fibre reinforcement polymers, thereby increasing the strength of the columns. Tests have shown that with the combination of different orientations and types of wraps, as well as number of layers, different benefits can be found including increase in strength, ductility and stiffness (Zhang et al., 2012; Heiduschke and Haller, 2012). Dissimilar failure modes were also observed in comparison to the reference specimens. The notion of CFRP for rehabilitation of timber structures has not been examined to satisfied extent. The benefit of confining timber has mostly been documented from experimental research.
There are so many advantages derived from wrapping timber with CFRP. Some of the main advantages of CFRP wrapping of timber are (Najm et al., 2007; Heiduschke and Haller, 2012; Zhang et al., 2012 Jimenez et al., 2011; Tsakania and Mouzakis, 2010; Webber and Yao, 2001): (i) Increase in compressive strength; (ii) Protection from environmental degradation; (iii) Restoration of historical building or structures; (v) Can be designed or manufactured to meet specific mechanical properties. Emerson (2004) found that transverse reinforcement increased the strength of the column so that it exceeded that of the design value of the column. Additional compressive and bending strength was provided by longitudinal reinforcement. In both of these cases, full wraps were utilized, which has shown to produce the most promising results. Longitudinal cracks in timber are a common concern and reduce compressive strength in columns. Cai et al.,(2012) found that with wraps 50 mm wide and 115 mm apart that eccentric load capacity could be increased, and that the CFRP could contain further crack opening and confine local ruptures. Najm et al.,(2007) compared the behavior of full wraps and spirals, finding that the full wrapping had greater benefits than those with spiral reinforcement. This coincides with research conducted by Zhang et al.,(2012), a main variable being CFRP
spacing and width along the length of the timber columns. The benefits increased with smaller spacing between wraps and an increase in the width of the sheets. The number of layers also had an effect with Zhang et al.,(2012) finding that results became stable after three layers of CFRP were applied. The use of three layers produced the best results Najm et al.,(2007), they found that tests fully confined with CFRP showed an increase in load capacity as more layers were added. A favorable failure mode occurs when deformation is observed before failure. A sudden failure can cause sudden collapse without warning, potentially resulting in the loss of lives. The failure modes observed by Heidushke and Haller (2012) of reinforced tubes demonstrated ductile behavior, the wood fibres crushing parallel to the grain, in conjunction with local buckling. Song et al.,(2010) found that two types of failure modes were prevalent, significant changes in the columns occurring in both types around 70% of the maximum loads. For the first types of failure, compression wrinkles become apparent at mid-height of the specimens which then propagated until maximum load, while in the second type, significant crushing deformation occurred near the ends of the cylinders, the deformations propagating until maximum load.
1.2.2 Statement of the Problem
Timber of historical structures may experience significant damages and cracks during their service life due to fungal decay, moisture changes or external loading. These damages and cracks may significantly affect the compressive strength or behaviour of timber column and it carrying capacity. According to Sousa et al.,(2011), the material properties of a timber element vary both in different parts of the same cross-section as well as along the element itself. As such, timber structures are better analysed using probabilistic models. EC5 (2004) provides a wide range of consistent and up to date models and procedures that can be considered for the development of a local code.
EC5 (2004) does not contain any data on the material properties of Nigerian timber species.
According to standards such as probabilistic model code, JCSS (2006) and EN 408 (2003), the basic material properties of any timber species are determined through laboratory experiments, while other material properties can be determined using the JCSS (2006) and EN 408 (2000). This research work therefore has to come up with data through laboratory experiments that was used to develop the strength classes for the three (3) selected Nigerian timber species based on the recommendations of EN 338 (2009), which are: Mitragynaciliata (Abura), Afromosia elata (Afromosia) and Berlinia/confusa grandiflora (Berlinia).
CFRP is a strong and light fiber reinforced polymer which contains carbon fibers. It has high strength to weight ratio, recyclable, environmentally friendly and with good rigidity when compared to steel. It can also be used as an alternative instead of steel, on the onset of thisstudy. The safety of the timber columns in CFRP and thus of the whole structure depends mainly on the evaluation of thickness of timber column and laminates or sprays since the two materials involved are anisotropic.
1.3 Justification of Study
Timber is an organic material and thus is subject to deterioration with time. Trees are of immense importance in the ecosystem and felling of trees could exacerbate the problems of ozone layers depletion.
Timber benefits from its natural growth characteristics such as grain patterns, colors and availability in many species, sizes and shapes that make it remarkable, versatile and aesthetically pleasing materials. Timber can easily be shaped and connected using nails, screws, bolts and dowels or adhesively bonded together (Jack and Abdy, 2007). The limitations in maximum cross-sectional dimensions and length of solid sawn timber, due to available log sizes and natural defects, are overcome by the recent development in composite and engineered wood products.
Timber structures can be highly durable when properly treated, detailed and built, but solid timber is rapidly becoming scarce and expensive due to logging and the long period of time it takes for most trees to grow to maturity. Timber is an excellent choice for any sort of wood work but good quality timber with minimum flaws comes with a bit of extra cost due to the reasons above. It is for these reasons that it becomes necessary to study the behavior of timber columns laminated with CFRP in economical way and how to maximally use, while it meets up with its design life structurally in service.
1.4 Scope and Limitation
This research work considers a reliability analysis using EC 5 (2004) of axially loaded timber columns of three different species of timber columns,fixed-free end and fixed-fixed end laminated with CFRP and sprays and to use Finite Element Analysis (FEA) as coded in ABAQUS software.
The limitations of this thesis include the following (Micheal, 2006).
(i.) There have been attempts to improve the ductility of CFRP with little or no success.
(ii.) Structural limitation of CFRP is that, it lacks fatigue endurance limit.
(iii.) CFRP columns when loaded in tension, exhibit a linear stress-strain behavior up to rupture.
(iv.) Many researches have been carried out using CFRP both for retrofitting and as an alternative to steel as reinforcement or pre-stressing materials. Cost remains an issue and long term durability questions still remain.
(v.) Only three species of Nigerian timber are catalogued in BS 5268 (2000).
(vi.) 1.5 Aim and Objectives
The aim of this work is to check the reliability assessment of the strength capacity of solid timber columns in Carbon Fibre Reinforced Plastics (CFRP) laminates and sprays.
The objectives of this study include:
(i.) To evaluate the structural reliability assessment of three timber species mentioned in BS 5268 (2000) and EC5 (2004) laminated with CFRP under compression.
(ii.) Use Finite Element Method as coded in ABAQUS software for structural safety assessment of the timber columns with CFRP laminates and sprays and
(iii.) Perform reliability analysis of the designed timber columns in FRP laminates and sprays using First Order Reliability Method in view to generating structural safety indices for varying thicknesses of FRP laminates and sprays in the designed structure.
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