This work provides results for stochastic safetyevaluation of Nigerian timbers for bridge decks in accordance to American Association of State Transportation and Highway Officials (AASHTO) Load Resistance Factored Design (LRFD) design specifications.A timber bridge is modeled in accordance to AASHTO LRFD, 2010, to represent real life experiment in order to depict the structural behavior of planks when used as a bridge deck. This model was then subjected to some degree of entropy using Advanced Second Moment Reliability Assessment (ASMRA) method, which was subsequently analysed using JAVA library with the help of Flanagan polynomial.Experimental data collected from literature was comparedwith the current Nigerian Code of Practice, NCP 2, 1973 for timber, and were used for the assessment. It was observed that, strength classes, timber thicknesses and stringer spacing are the major factors among others influencing the structural behavior of Nigerian timber proposed as bridge decks. Therefore, the major classes of Nigerian timber recommended for bridge decks are timber within the strength classes N1 to N4 with dimensions ranging from 100 x 250mm to 150 x 300mm on stringers spaced not greater than 300mm, depending on the strength class adopted; with timber belonging to the higher strength classes taking the lower dimension. An exception to this, are timber which belong to the strength class N1, where the stringers can be spaced at 450mm using timber with dimension not lower then 100mm thick and width not less than 250mm.The recommended strength classes with associated material properties can be a source of sustainable bridge deck material over a reasonable period of time as indicated by the probability of failure as a result of damage due to load accumulation. In view of this, timber which is a locally available material can be used as substitute for the expensive concrete and steel which are the most commonly used materials.
TABLE OF CONTENTS
TABLE OF CONTENTS vi
LIST OF FIGURES ix
LIST OF TABLES x
CHAPTER 1 1
1.1 GENERAL OVER VIEW 1
1.2 JUSTIFICATION OF STUDY 3
1.3 AIM AND OBJECTIVES 5
1.4 SCOPE AND LIMITATION 5
CHAPTER 2 7
2.0 LITERATURE REVIEW 7
2.1 INTRODUCTION 7
2.2 CLEAR WOOD SPECIMENS AND THEIR PROPERTIES 8
2.2.1 Moisture Content 9
2.2.2 Shrinkage and Swelling 9
2.2.3 Density 10
2.2.4 Strength and Stiffness Properties 10
2.2.5 Moisture Content and Mechanical Properties 11
2.3 STRUCTURAL TIMBER 12
2.4 GROWTH IRREGULARITIES IN TIMBER STRUCTURAL COMPONENTS 13
2.4.1 Knots 13
2.4.2 Cross Grain 13
2.4.3 Distortion 14
2.4.4 Wane 14
2.4.5 Permanent Compressive Yield 14
2.5 NIGERIAN TIMBER 15
2.6 BRIDGES IN RETROSPECTIVE 15
2.7 BRIEF HISTORY OF TIMBER BRIDGES 17
2.8 DECLINE OF TIMBER BRIDGE BUILDING 19
2.9 REVIVAL OF TIMBER BRIDGE BUILDING 20
2.10 TIMBER BRIDGES 21
CHAPTER 3 28
3.0 RESEARCH METHODOLOGY 28
3.1 INTRODUCTION 28
3.2 ADVANCE SECOND MOMENT RELIABILITY ASSESSMENT METHOD (ASMRAM) 30
3.3 PLANK DECK DESIGN MODEL 39
3.3.1 LRFD AASHTO SPECIFICATIONS (2010) 40
3.3.2 TIRE CONTACT AREA 44
3.3.3 RELIABILITY ANALYSIS 46
3.3.4 DAMAGE ACCUMULATION MODEL 48
CHAPTER 4 51
4.0 DATA SOURCE, ANALYSIS AND RESULTS 51
4.1 INTRODUCTION 51
4.2 DATA SOURCE 52
4.3 ANALYSIS AND RESULTS 54
4.3.1 General reliability assessment 54
4.3.2 Reliability assessment of Nigerian timbers in relation to stringer spacing under varying loads 56
4.3.3 Reliability assessment of Nigerian timbers in relation to stringer spacing under constant loads 57
4.3.4 Reliability assessment of Nigerian timbers in relation to plank thickness at 0.3m stringer spacing 58
4.3.5 Reliability assessment of Nigerian timbers in relation to plank thickness at constant stringer spacing of 0.45m 59
4.3.6 Reliability assessment of Nigerian timbers in relation to plank width at constant stringer spacing 60
4.3.7 Damage accumulation reliability analysis results 61
CHAPTER 5 62
5.0 DISCUSSION, CONCLUSIONS AND RECOMMENDATIONS 62
5.1 DISCUSSION 62
5.2 CONCLUSIONS 66
5.3 RECOMMENDATIONS 67
Appendix I: Stochastic assessment data and reliability index 71
Appendix II: Java safety assessment programs 78
1.1 GENERAL OVER VIEW
The need for local content in construction of engineering infrastructure is now a serious engineering challenge in Nigeria. This is because vast quantities of local raw materials, which must be processed and used for cost effective construction abound. Construction activities based on these locally available raw materials are major steps towards industrialisation and economic independence for developing countries (Aguwa and Sadiku, 2011). Timber is one of the natural occurring raw materials which abound in Nigeria and it had been put to use as a building material for construction since prehistoric times. It is available inlarge quantities in the forested parts of the country. The extent of its usage by professionals in the buildingindustry is determined not only by their understanding of the material, but also by their perception of thematerial (Adedeji and Ogunsote, 2004).
The major use of timber in Nigeria and most part of the third world has been limited to domestic use as an alternative source of heat energy or household kitchen appliance. In this respect, timber which is supposed to be used to sustain the development of the economy is been burnt as coal. With critical analysis of our environment and careful exploration of the structural properties of timber, one can adequately establish and design an environmentally friendly structure which is cost effective. This is why Afolayan (1999), describes timber as a low density, cellular, polymeric composite which does not fall into any one class of materials; rather it tends
to overlap a number of classes. Because of its high strength performance and low cost, timber is found to be the world most successful fiber composite, (Afolayan, 1999). Timber, like other building materials, has inherent advantages that make it especially attractive in specific applications (Aguwa and Sadiku, 2011: Afolayan and Adeyeye, 1998). Structural timber is the timber used in framing and load-bearing structures, where strength is the major factor in its selection and use. Trees are the only sources oftimber, and those that carry naked seeds are called softwoods, while those with seeds inside a fruit are termed hardwoods(Aguwa and Sadiku, 2011). With the current dispensation and increasing global development, the public demands a safe and efficienttransportation system. With increasing traffic demand, it is clear that engineers and politicalleaders must find ways to upgrade bridges and doso with fewer resources at hand. The use of timber (which is a locally available material) as component for expensive civil engineering bridge project (for example in bridge decks), will be a cost effective bridge type for rural bridge rehabilitation and sustainability. The benefits of timber bridge are primarily derived from the use of native timber species which are renewable, reusable, replaceable recyclable, environmentally friendly, sustainable and maintainable. The main advantages of timber bridgesrelative to other bridge materials as pointed out by DelDOT(2005) which is a construction company in Delaware, include Ease of construction, Ease of maintenance, Pleasing appearance, Renewable resource, it‟s use in Construction is not weather dependent, it is Lightweight among others.
The use of this renewable lightweight natural resource as a bridge deck, will not only be a new strategy for development in the third world but also as a sustainable development which will help to overcome the exclusion of timber technology in modern times, thereby strengthening its inclusion through research and practical application. In view of the above,this work focuses on the possibility of using Nigerian timber for bridge decks. As a result, timber decks was proposed and evaluated to assess the possibility and use of timber bridge decks for the construction and or rehabilitation of road bridges both in rural and urban settings of Nigeria (and other parts of the country) in order to open up new roads while at the same time rehabilitating old and or abandoned bridges.
1.2 JUSTIFICATION OF STUDY
Due to the current dispensation and increased challenges in global development in all sectors of the economy, construction companies inclusive, there exist these motives to build bridges faster that will last longer, for less money and with aesthetics appeal which has led to the quest for the perfect bridge material.
Reinforced concrete and steel which has edged out timber as a construction component for bridges was reported by Bell (2007) as not been an everlasting material they were assumed to be. This is because many countries have experienced serious problems with concrete bridges built in the 1960‟s and 70‟s. This was backed with the assertions stated in Vermont (2008) local road fact sheet. It was clearly stated in this local fact sheet that, properly treated, timber is stable and durable under the most severe weather and site conditions, which is one of its attractive performance features for bridges as it is completely resistant to the deicing salts, decay and insect
attacks. It had also been noticed, that deicing salts have caused significant and surprisingly rapid deterioration of both steel and concrete bridges and components. It was reported too that, when a larger structural timber is exposed to fire, there is some delay as it chars and eventually flames. As burning continues, the charred layer has an insulative effect, and the burning slows to an average rate of about 1/40 inch (0.6mm) per minute (or 1 ½ (38mm) per hour), for average structural timber species. These slow rates of fire penetration means that timber structural members subjected to fire maintain a high percentage of their original strength for considerable periods of time. In contrast, structural steel becomes plastic when exposed to a heat of 1,000(340) and it yields almost immediately. Treated timber bridge decks are built in days not weeks, because materials are low energy, certified, reusable and renewable, where components are shop manufactured under controlled conditions to maintain quality (David,2011). As a result the use of timber will reduce detour times and construction inspection expense, because controlled shop conditions will improve quality and thus, reduce the need for site inspection. Considering the availability of timber as a renewable, sustainable natural resource which can be formed locally even with simple hand tools, remote parts of the country can easily be linked and reached through the construction of short span timber bridges at a reasonable low cost.
Currently, the use of timber as abridge deck or bridge component has not been a common practice in Africa, though with few physical examples associated with scarce historical documentation. The last two decades have seen a growing interest in timber bridges in many European countries (Bell, 2007). However, in 2005, DelDOT Construction Company Delaware,
reported that its construction of timber bridges has decreased in proportion to its use of other materials over the last 50 years.
1.3 AIM AND OBJECTIVES
The aim of this research is to assess the possibility of using various Nigerian timber species as sustainable bridge decks. In this regard, the objectives of the research include,
i. To develop a suitable stochastic model for the analysis of Nigerian timber species when used as bridge decks, taking appropriate variables into consideration.
ii. Identify the suitability of Nigerian timber for use as material for bridge decks.
iii. To estimate safety indices and probability of failure of using Nigerian timber for bridge decks
iv. To predict and or evaluate the long-term performance of Nigerian timber when use as a bridge deck material.
1.4 SCOPE AND LIMITATION
The scope of this research is to use a stochastic method of analysis to model a real world experiment consisting of limit states which occur randomly. In relation to timber, the major parameters which determine performance ofwood structures are modulus of rupture (MOR),modulus of elasticity (MOE), and loads.These parameters are subject to considerable variation andtherefore, they are treated as random variables.(Nowak and Saraf, 1996). Abejide (2007), and Martenson(1992), had both iterated that “the effect of load duration and moisture influence on the strength and formation of timber members in a structure affect the load carrying
capacity of structural timber”. A bridge deck is always subjected to impact and dynamic load of varying magnitude at random intervals of time and seasonal changes.These parameters will be subjected to a stochastic model in order to evaluate the structural behavior of Nigerian woods when used as a bridge deck. This work is limited to the structural behavior of plank decks when used as bridge decks. Girder or stringer designs are not included, so also are effects of preservatives and perish ability of the timbers are not included in this work. The stochastic data used for this work are based on real tests from existing literatures. The design considerations and properties of timber will be based on NCP 2 (1973),and LRFD AASHTO (2010).
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