Structural Grade Properties of Some’ Selected Nigerian Timber Species: Chlorophoral Excelsa (Iroko); Khaya Senegalensis (Mahogany); Ceiba Pentandra (Araba); and Daniellia Oliveri (Maje)
Chapter One
AIM AND OBJECTIVES OF THE STUDY
Aim
The aim of this research is to determine the structural grade properties of some selected Nigerian timber species used in buildings in Nigeria in accordance with EN338 (2008) and EN384 (2004).
Objectives
The objectives of this research work are:
- Determine the moisture content, density, modulus of elasticity and bending strength of some Nigerian timber species: These are Chlorophoral excelsa (Iroko), Khaya senegalensis (Mahogany), Ceiba pentandra (Araba) and Daniellia oliveri (Maje) and classify these species based on EN384 (2004)
- Generate data on the derived material properties (tension and compression strengths parallel and perpendicular to grain, shear strength and shear modulus of the species based on EN 384 (2004) and JCSS (2006) requirements.)
- Determine the characteristic values of both the reference and the derived material of each species in accordance with EN 384 (2004) with the aid of a specialised statistical analysis distribution fitting packages (Easy fit 2010).
- Perform strength grading in order to assign appropriate strength class to each species based on EN 338 (2008) recommendations.
CHAPTER TWO
LITERATURE REVIEW
GENERAL
Wood density is often considered to be the single most important wood property because of its strong influence on the quality of a wide range of solid wood and fiber products and has such received the most attention (Bunn, 1981). It was further suggested that density is probably the most important intrinsic wood property for most wood products. Bamber and Burley (1983) on the other hand pointed out that of all the wood properties, density is the most significant in determining end use. Similarly, Zobel and Van Buijtenan (1989) stated that density largely determines the value and utility of wood and overshadows the importance of other wood properties. However, the importance of density as an indicator of wood quality alone is debatable because variability is due to many intrinsic factors, defects and variations inherent in basic wood structures.
Another important element of wood quality is that of ‘’stiffness’’ or its modulus of elasticity . The end use of wood material especially for structural timber is strongly related to the modulus of elasticity, Piter and Zerbino (2003). Modulus of elasticity defines the relationship between stress and strain within the elastic region. Timber is considered to be an isotropic material with three mutually perpendicular axes in the longitudinal, radial and tangential direction. The longitudinal modulus of elasticity is a quantitative measure of the stiffness of the wood along the grain. It is the most commonly measured elastic property and the most important elastic constant. Elasticity implies that deformation produced by the low stress are completely recoverable after loads are removed, and this is the accepted criterion of stiffness (Abubakar et al, 2011).
Structural timber displays considerable strength variability between and within members. One way of describing the variability phenomena and their consequences for instance is the strength of a timber beam shown in a model introduced by Riberholt et al. (1979). It was assumed that timber is composed of localized weak zones connected by segments of clear wood and that failure was primarily initiated in these weak zones. The weak zones correspond to knots or groups of knots, which are distributed along the length at random. The length of a timber beam affects the apparent strength since there is a higher probability of having weak sections within a longer beam than in a shorter one. Furthermore, the apparent strength of a timber beam will depend on the nature of the moment distribution along the beam. For instance, a beam with a point load at mid span can be expected to have an apparent strength that is higher than a beam of the same length loaded with a constant bending moment along the entire length.
Grading of timber (Johansson, 1999), a pre- requisite for the use of timber in load bearing construction is that the strength and stiffness properties are known and can be controlled to stay within desirable limits. This can be achieved in the same way as for man- made products such as steel, concrete, plastics and wood fibre board where a certain material quality is obtained by changing the composition of the raw materials or by changing some of the environmental conditions (temperature, pressure etc). As far as the mechanical properties of wood are concerned the only realistic way of obtaining quality within desired limits is grading. There are presently two types of strength grading systems based on Eurocode.
CHAPTER THREE
MATERIALS AND METHODS
MATERIALS
Timber boards for the test were bought from Kaduna market in Kaduna State. A total of 40 specimens for each species were used for determining modulus of elasticity in bending, of which 20 specimens were used centre line for three – point method and another 20 specimens for four – point method. Test pieces of sizes 75 mm x 50 mm x 1500 mm were prepared after seasoning for about three months by exposure to open air drying. This gives a span /depth ratio of 18 as required (EN408, 2002). A total of 40 specimens for each species were also used for the density and moisture content tests, in which twenty pieces of sizes for each specie 60 mm x 40 mm x 40 mm were prepared. The materials were prepared for test as shown in plate 1.
CHAPTER FOUR
RESULTS AND DISCUSSION
RESULTS
The grade properties of the four Nigerian timber species were determined based on methods of EN 408 (2003), EN 384 (2004) and EN 1995 (2004). The four timber species considered were Chlorophoral excelsa (Iroko), Khaya senegalensis (Mahogany), Ceiba pentandra (Araba) and Daniellia oliveri (Maje).
The results obtained are as presented:
CHAPTER FIVE
CONCLUSION AND RECOMMENDATIONS
CONCLUSSION
The statistical models of the grade determining properties (density , modulus of elasticity and bending strength) of four selected Nigerian timber species: Chlorophoral excelsa,
Khaya senegalensis, Ceiba pentandra and Daniellia oliveri were generated from three- point and four – point bending tests as well as the oven dry method for the density and moisture content.
The generated statistical models include the means, standard deviations, coefficient of variations and theoretical distributions. These are the necessary inputs for the reliability analysis and design of engineering structures.
Chlorophoral ecxelsa, was assigned to strength class D70, Khaya senegalensis was assigned to strength class D70, Daniellia oliveri was assigned to strength class D50 and Ceiba pentandra was assigned to strength class C40.
RECOMMENDATIONS
Based on the finding of this research work, the following recommendation can be made.
- The four (4) classified Nigerian timber species are reliable structural materials that can be used based for structural design using Eurocode 5 (2004).
- NCP2 (1973) is recommended for revision in order to adopt the recent timber design methodology for all Nigerian timbers.
- Assigned statistical distribution models are also recommended for use in timber structural reliability analysis.
REFERENCES
- Abubakar, I., Mohammed, J. K. and Ejeh, S. P. (2011). Probabilistic Models of Grade Determining Properties of Some Common Nigerian Timber Species, NigerianJournalof Engineering, Vol. 18, No.1, pp. 74 – 81.
- Ang , A. H. Tang , T. H. (1975) Probability Concepts in Engineering Planning and Design. Vol.1 , Wiley series , New York.
- Antti, H. Ranta-Maunus, A. and Goran, T. (2006) ‘’ Potential of Strength Grading of Timber with Combined Measurement Technique. Report of Cambridge Project – Phase 1.
- ASCE Journal of Structural Engineering 108(7): 1494-1510.
- ASTM D-143-94 (2004) “ Standard Method of Testing Small Clear Specimens of Timber,’’ American Society for Testing and Materials, USA
- Baltrusaitis, A and Prankevicene, V. (2003) ‘’ Strength Grading of the Structural Timber’’ Materials Science Journal, Vol. 9, No. 3, PP. 284 – 287.
- Bamber, R. K. and Burley, J. (1983). The Wood Properties of Radiata Pine, Commonwealth Agricultural Bureau, England.
- Barrett, J. D. Lau, W.(1994) Canadian Lumber Properties, Canadian Wood Council, pp. 346
- Barrett, J. D. Foschi, R. O. (1978a). Duration of Load and Probability of Failure in Wood. Part 1. Modelling Creep Rupture. Canadian J. of Civil Engineering Vol. 5, No. 4, pp. 505 – 514
- Barrett, J. D. Foschi, R. O. (1978b). Duration of Load and Probability of Failure in Wood.
- Part 11. Constant, Ramp and Cyclic Loading. Canadian J. of Civil Engineering Vol. 5, No. 4, pp. 515 – 532
