Duration of load effect in tension perpendicular to the grain of glulam in different climates

Abstract

It is reported on results of a European research project on the DOL effect of structural sized glulam in tension perpendicular to the grain with special respect to different climates and volumes. The extensive experimental and theoretical investigations were performed jointly at VTT and FMPA — Otto-Graf-Institute in Finland and Germany, respectively. The test scheme comprised 80 curved beams and 180 prismatic tension specimens built-up of Nordic spruce laminations of grades C35 and C40 from one selected stand. The long-term investigations with stepwise loading regime were done in constant, cyclic and natural sheltered outdoor climates with constant stress volumes of 0.01 to 0.24 m3. As a major general result it can be stated that the findings from curved beams and tension specimens are well consistent. The results of ramp load reference and long term tests in constant climates validated the magnitudes of volume and pure time effect as specified in Eurocode 5. In variable climates a pronouncedly increased DOL effect was obtained which however is not bound to a specific material degradation but results from superimposed eigenstresses caused by the transient moisture gradients. Mean stress levels at failure normalized to ramp load strength in case of variable climates were in the range of 45%–66% with times to failure less than one month. The theoretical modeling of the long term damage forwarded that an effective stress concept based on transient Weibull stresses is a suitable approach to handle the non-linear superposition of applied loads and climate actions. The climate induced effective tension stress perpendicular to the grain can be in the range of 0.3 N/mm2 in case of severe service class 2 conditions. Wider cross-sections are relatively less influenced by moisture variations. With respect to design two alternative strategies to deal with the variable climate aspect are sensitive: introduction of additive climate bound design stresses or, more globally, a strength modification factor for accumulated time of loading, k mod, of about 0.55 for all load-duration classes in service class 2.