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

Abstract

Given its favorable structural and environmental attributes, timber is witnessing a renaissance as a
viable modern structural material. A vital property influencing the use of timber as a structural
material is its density. Density not only dictates the self-weight of structures but is one of the key
characterizing parameters for timber strength. Structural softwood species in Europe are graded into
strength class ranging from C14 to C50 according to the requirement established in EN338. A C14
strength class, for instance, must have a characteristic density value of 290kg/m3 and a mean density
value of 350kg/m3 at 12% moisture content. More significantly, the strength of fasteners used for
connections in timber structures is greatly influenced by density. The embedment and withdrawal
strength of fasteners which affect the connection’s ultimate capacity and behavior are very much a
function of timber density. Further, the ease and cost of processing and fabrication and the charring
rate of timber during fire are also dependent on its density. Timber, however, exhibits significant
inter 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 a
normal distribution variation and a coefficient of variation (COV) of 0.1. Currently models to
represent the density variability for native timber species in Bhutan is nonexistent. This paper
discusses the importance of density in timber structures and presents the density profile recorded
till date within the overall framework of developing the probabilistic model to represent the density
variation in Bhutanese conifer timber species.