Significance of Density in Structural Timber Engineering and Density Profile of Bhutanese Softwood Timber

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Abstract

Given its favorable structural and environmental attributes, timber is witnessing a renaissance as aviable modern structural material. A vital property influencing the use of timber as a structuralmaterial is its density. Density not only dictates the self-weight of structures but is one of the keycharacterizing parameters for timber strength. Structural softwood species in Europe are graded intostrength class ranging from C14 to C50 according to the requirement established in EN338. A C14strength class, for instance, must have a characteristic density value of 290kg/m3 and a mean densityvalue of 350kg/m3 at 12% moisture content. More significantly, the strength of fasteners used forconnections in timber structures is greatly influenced by density. The embedment and withdrawalstrength of fasteners which affect the connection’s ultimate capacity and behavior are very much afunction of timber density. Further, the ease and cost of processing and fabrication and the charringrate of timber during fire are also dependent on its density. Timber, however, exhibits significantinter and intra species variability in its density. The Joint committee for Structural Safety (JCSS)recommends a probabilistic model for density variability for graded softwood in Europe with anormal distribution variation and a coefficient of variation (COV) of 0.1. Currently models torepresent the density variability for native timber species in Bhutan is nonexistent. This paperdiscusses the importance of density in timber structures and presents the density profile recordedtill date within the overall framework of developing the probabilistic model to represent the densityvariation in Bhutanese conifer timber species.

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