RELIABILITY-BASED ANALYSIS OF COMPOSITE SOLID SLENDER TIMBER COLUMNS WITH ALUMINIUM LAMINATES USING SELECTED NIGERIAN TIMBER SPECIES

RELIABILITY-BASED ANALYSIS OF COMPOSITE SOLID SLENDER TIMBER COLUMNS WITH ALUMINIUM LAMINATES USING SELECTED NIGERIAN TIMBER SPECIES

Abstract:

Four timber species (Strombosia pustulata, Macrocarpa biquertii, Entandrophragma cylindricum and Nauclea diderichii) were laminated with aluminium sheets of varying thicknesses.Axial and flexural loads were applied to the composite columns to determine their behaviour under such loadings. Finite element analysis was carried out in Abaqus/CAE 6.10 (2010) and the reliability analysis was carried out using the First Order Reliability Analysis (FORM) in MATLAB (2007) on the laminated timber columns. The results for finite element analysis show the effect of the thickness of Aluminium laminates on the strength capacity of timber columns whereby deflection reduced from 3.28×10-2mm for timber column without laminate to 2.96×10-2mm with the introduction of 20mm aluminium laminate in the x-axis. Similarly, the deflection reduced from 5.18×10-2mm to 1.18×10-2mm in the y-axis, and 5.45mm to 1.093mm in the z-axis for the timber (Strombosia pustulata specie) column that is fixed-free end restraint condition. For the reliability analysis, bending, buckling and flexural buckling failure modes were considered. The results for compression mode of failure show that the column is safewith safety index values of 4.3 and 9.68 for load parameters and dead-to-live load parameters respectively without laminates. The study shows that the most critical failure mode for the column is flexural buckling and hence isnot safe for imposed loads that are greater than 20kN but made safe with the use of aluminium laminate of 8mm.

CHAPTER ONE

INTRODUCTION

1.1        General

Timber, as one of the materials used in construction, is a sustainable resource (Porteous and Kermani, 2007). It is one of the few natural and renewable construction materials that existand is used for construction, carpentry and upholstery. Also, timber is an organic material and thus is subject to deterioration with time (Robert, 2010). These limitations are basically related to the importance of trees in the ecosystem and how felling trees could exacerbate the problems of ozone layer depletion. Trees take very long time to be fully grown and mature, hence optimum use of timber will ensure adequate balance in logging and replacement. Timber from well-managed forests is one of the most sustainable resources available and it is one of the oldest known materials used in construction. It has a very high strength-to-weight ratio, it is capable of transferring both tension and compression forces, and is naturally suitable as a flexural member. Timber is a material that is used for a variety of structural forms such as beams, colums, trusses, girders, and is also used in building systems such as piles, deck members, railway sleepers and in formwork for concrete (Porteous and Kermani, 2007).

Extensive research over the past few decades has resulted in comprehensive information on material properties of timber and its reconstituted and engineered products and their effects on structural design and service performance (Gentile, 2000; Mamman, 2014; Mohammed, 2014; Abubakar and Nabade 2013b; Avent, 1985; Forest Product

laboratory, 1999). Centuries of experience of use of timber in buildings has shown the safe methods of construction, connection details and design limitations.

There are a number of inherent characteristics that make timber an ideal construction material. These include its high strength-to-weight ratio, its impressive record for durability and performance and good insulating properties against heat and sound. Timber also benefits from its natural growth characteristics such as grain patterns, colours and its availability in many species, sizes and shapes that make it a remarkably versatile and an aesthetically pleasing material. Timber can easily be shaped and connected using nails, screws, bolts and dowels or adhesively bonded together. Timber structures can be highly durable when properly treated, detailed and built. They can easily be reshaped or altered, and if damaged they can be repaired (Porteous and Kermani, 2007; Hollaway and Cadei, 2002).

The limitations in maximum cross-sectional dimensions and lengths of solid sawn timbers, due to available log sizes and natural defects, are overcome by the recent developments in composite and engineered wood products (APA, 2009; Zahn and

Rammer, 1995). Finger jointing and various lamination techniques have enabled timbers (elements and systems) of uniform and high quality in any shape, form and size to be constructed; being only limited by the manufacturing and/or transportation boundaries. 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.Aluminium, on the other hand, is a lightweight and durable metal. It is an abundant element found in the earth crust. It is the third most abundant element and the most abundant metal in existence. It is silvery in appearance when freshly cut, is a good conductor of heat and electricity and is easily shaped by moulding and extruding. The extraction process of aluminium is electrolysis (Grjotheim and Welch, 1988) which enables large amounts of the metal to be available for construction purposes.Aluminium has two main advantages when compared with other metals. Firstly, it has a low density, about one third that of iron and copper.

Secondly, although it reacts rapidly with the oxygen in air, it forms a thin tough and impervious oxide layer which resists further oxidation. This removes the need for surface protection coatings such as those required with other metals, in particular with iron.

There are many different timbers in the market that range in price, characteristics and strength. Timber is an excellent choice for any sort of woodwork but good quality timber with minimum flaws comes with a bit extra cost due to the reasons above. Hence, there is need to maximally use timber for efficiency in construction and to reinforce the slender timber columns withaluminium laminates to increase the stiffness and sectional properties of the slender columns.

Traditionally, in Nigeria, timbers have been used to a significant extent in construction purposes and particularly building constructions. Over 80% of the timber and timber products in Nigeria are utilized for different purposes (Ohagwu and Ugwuishiwu, 2011). The major uses of timber in building construction are roofing members, doors, frames, and staircases. They are also used for scaffolding and shuttering during construction. In road construction, large quantities of woods are also used for frameworks, pilling materials, road signboards, temporary shades, and road paving plants in temporary construction site. They are also used at petroleum exploitation sites (Ohagwu and Ugwuishiwu, 2011). However, the structural strength of solid timbers needs to be increased in order to sustain more structural loads. This can be done by using aluminium laminates to increase its strength and durability by increasing the stiffness and sectional properties of the timber. This is the work presented in this study and including the application of probability in order to obtain its optimum resistance strength in analysis and design.

 

1.2        Statement of Problem and Justification of Study

Trees are of immense importance in the ecosystem and felling of trees could exacerbate the problems of ozone layers depletion (Akimbo and Lawrence, 1996). The limitations in maximum cross-sectional dimensions and lengths of solid sawn timbers, due to available log sizes and natural defects make the use of solid timber to be limited to small construction and structural works. Solid timber is rapidly becoming expensive due to excessive logging and the long period of time it takes for most trees to grow to maturity.

Timber is a highly sustainable construction material that has high-strength-to weight ratio as compared to other construction materials, the environmental impact of processing and milling timber for construction purpose is at a low level due to the low energy use and low level off pollution associated with the manufacturing of timber. Timber has very great aesthetic properties owing to its natural growth characteristics such as grain patterns. Also, timber structures can be highly durable when properly treated, detailed and built. It is for these reasons that it becomes necessary to study the behaviour of timber laminated with aluminium laminates.

1.3        Aim and Objectives

1.3.1 Aim

The aim of this work is to carry out reliability-based evaluation of the structural performance of composite solid slender timber columns of some selected Nigerian timber species with aluminium laminates.

1.3.2 Objectives

The objectives are to:

1. Generate the limit state equations for columns according to Eurocode 5 (2004) requirements for reliability analysis,
2. Develop a program for reliability analysis in MATLAB R2011b (2011) for the composite columns, iii. Carry out finite element analysis, using Abaqus, of the composite columns under axial loads for different end restraint conditions, iv. Check the structural performance of the columns considering the properties of some selected Nigerian timber species under various failure criteria, and

v.Study the effects of aluminium laminate thickness on the load carrying capacity of the columns.

1.4       Scope and Limitations of Study

1.4.1 Scope of study

This study is based on the modification of the strength of solid timber columns with the introduction of aluminium laminates. The strength variation of the timber is assessed to determine the optimum strength with varying aluminium laminate thickness. A finite element analysis of the four timber species laminated with aluminium sheets have been carried out in Abaqus CAE 6.10 (2010) to determine the effect of the laminate on the structural capacity of solid timber columns. Also, reliability analysis of solid timber columns laminated with aluminium sheet of varying thickness were carried out

considering certain target reliability indices values. The reliability processes were carried out considering three failure modes which are bending, buckling and flexure. MATLAB R2011b (2011) was used to run the First Order Reliability Method analyses incorporating programs that were designed for the three failure modes. The selected Nigerian timbers used in this study are StrombosiaPustulata, MacrocarpaBequerti, Nauclea Diderrichii and Entandrophragma Cylindricum.

             

1.4.2    Limitations of study

This study is limited to the use of four selected Nigerian timbers which are

StrombosiaPustulata, MacrocarpaBequerti, Nauclea Diderrichii and Entandrophragma

Cylindricumwith their local names as Itako, Oporoporo, Opepe and Ijebu respectively. The timbers are used as slender columns which are laminated with varying aluminium laminate thickness and end-restraint condition viz-a-viz fixed-free, fixed-fixed and pinned-pinned end conditions.

             

RELIABILITY-BASED ANALYSIS OF COMPOSITE SOLID SLENDER TIMBER COLUMNS WITH ALUMINIUM LAMINATES USING SELECTED NIGERIAN TIMBER SPECIES

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