Compressive strength parallel to the fiber of spruce with high moisture content

The research aimed to derive substantiated reduction factors for compressive strength and modulus of elasticity parallel to the fiber for structural sized spruce and fir (round) wood with moisture contents at and beyond fiber saturation. Such values, which at present are not given in Eurocodes 5 and 7, are essential in order to perform a reliable design of water/soil immersed piles as well as structural assessment of piles in embankments and foundations of historic, cultural heritage buildings and bridges. The experimental investigations, preceded by a comprehensive literature evaluation, comprised a total of 17 pairs of matched structural sized spruce (Picea abies) pile specimens with mid-length diameters of about 200 mm, with each pair being cut successively from individual logs. From each pair, one specimen was tested in the dry state (MC = 12%), and the other in the wet/green state (MCmean = 90%) according to EN 408 (2010). Further, for calibration purposes, matched small clear specimens with two different aspect ratios were sampled close to the sections from which the structural sized piles were cut. At the mean strength level, the pile compression tests parallel to the fiber gave a moisture reduction factor of \(k_{moist, green, mean}\) = 0.57, conforming well with the condensed result (0.54) of the literature evaluation on clear wood data, whereas the conducted matched clear wood reference tests gave a 15 % lower reduction factor. The moisture reduction at the 5 % quantile level of the pile compressive strength \(k_{moist, green, 05}\) = 0.65 was less expressed, and 30 % lower than that obtained for the small clear specimens. The moisture modification factor for the modulus of elasticity in compression parallel to fiber from dry to green state was found to be 0.89 for the pile specimens, which conforms closely to literature data on small clear specimens. Based on the experimental results for the moisture reduction and the present implicit provisions of EC 5 on duration of load, a modification factor \(k_{mod}\;=\;0.65 \times 0.6\;=\;0.4\) is proposed for structural sized spruce/fir piles as well as sawn lumber subjected to permanent load in the water saturated state.