COMPARATIVE ASSESSMENT OF THE STRENGTHS OF SOLID AND GLUED LAMINATED TIMBER

Timber isa construction material with unparalleledenvironmental credential. Howbeit, limitations of span and crossectional dimension, strength reducing defects and anisotropy limits its engineering application. Mechanical methods of jointing to address the earlier challenge has introduced serious wood fiber failure and increased the embodied energy in timber as a green material. However, developed societies have established in literature the possibility of overcoming these limitations to utilize timber beyond the traditional application in Nigeria being glueable for developing engineered wood products like glued laminated timber (glulam) using their timber species. Conversely, while little or no interest is shown in timber as a structural material in Nigeria, many of the few researches in timber have focused on solid timber elements leaving its limitations unattended. Consequently, its structural capabilities is yet unappreciated. Nevertheless, the fact remains that reconstituting natural timber as glulam is an effective way of optimizing this green material for limitless structural use. Whether these qualities are achievable with local timber is the main thrust of this research. The research therefore conducted laboratory experiments on selected timber species namely; Ire (Funtumia africana), Awun (Alstonia congensis) and Oriro (Antiaristoxicaria) being readily available and widely used species with no information on Oriro and Ire in NCP 2 of 1973. The aim was to assess their strength properties as glulam elements with the view to improving their structural capacity.It also set out to determine their glueability, the effects of temperature variation on compressive strength parallel to grain for glulam short columns by subjecting specimens ofequal dimension for the three species to different temperatures of 0 0C, 40 0C, 50 0C, 70 0C, 100 0C and room temperature for 4 hours in an electric oven prior to testing; to compare the mechanical properties of solid and glulam elements. In furtherance, specimens were prepared and tested for; static bending strength, compression parallel and perpendicular to grain, density and moisture content in line with ASTM D193, EN 408(2003) and EN13183-1(2002). The research established that the species are; structurally glueable, that due to temperature increase compressive strength is lost in glulam columns from control temperature (30 0C and 27.90C) to 100 was 41% 14.4% and 21.6% in Ire, Awun and Oriro. Results showed that glulam elements developed 55%, 95% and 143% of clear solid wood bending strength and that bending strength of 65.22N/mm2; 36.44N/mm2, 26.15N/mm2; 25N/mm2 and 14N/mm2; 20N/mm2 in solid and glulam in the species are structurally significant.The study has therefore demonstrated that the timber species studied can be engineered to load bearing glued laminated structural elements using polyvinyl acetate glue without severe loss of strength below and above room temperature.

The versatility of timber finds wide application in the construction industry spanning from simple framing in housing projects to large scale public facilities. However because sawnwood hasrestrictionsto spans and cross-sectional dimensions due to size of tree as well as strength reducing features which occur at growth, its value as a structural material for extensive structural applications is limited. Engineered wood products such as glue laminated timber (glulam) were thereforedeveloped to improve the use of natural timber beyond its natural limitations.