Tord Isaksson

The combined effect of wetting ability and durability on outdoor performance of wood: development and verification of a new prediction approach

Comprehensive approaches to predict performance of wood products are requested by international standards, and the first attempts have been made in the frame of European research projects. However, there is still an imminent need for a methodology to implement the durability and moisture performance of wood in an engineering design method and performance classification system. The aim of this study was therefore to establish an approach to predict service life of wood above ground taking into account the combined effect of wetting ability and durability data. A comprehensive data set was obtained from laboratory durability tests and still ongoing field trials in Norway, Germany and Sweden. In addition, four different wetting ability tests were performed with the same material. Based on a dose–response concept, decay rates for specimens exposed above ground were predicted implementing various indicating factors. A model was developed and optimised taking into account the resistance of wood against soft, white and brown rot as well as relevant types of water uptake and release. Decay rates from above-ground field tests at different test sites in Norway were predicted with the model. In a second step, the model was validated using data from laboratory and field tests performed in Germany and Sweden. The model was found to be fairly reliable, and it has the advantage to get implemented into existing engineering design guidelines. The approach at hand might furthermore be used for implementing wetting ability data into performance classification as requested by European standardisation bodies.

Moisture conditions of rain-exposed glue laminated timber members: the effect of different detailing: the effect of different detailing

ABSTRACT In performance-based durability design, relating the in-use conditions of wooden members to their moisture content is an important step. In the present study, the effects of detail design on the wood moisture content of glulam members are investigated experimentally. The moisture content of glulam members, designed with various connection details and structural protection, was monitored at 18 different measuring points (n = 3) by use of resistance-type moisture sensors for a period of a year. The effects of detail design are studied by comparing the moisture content of various details to that of a freely exposed beam. As expected, the design of the details was clearly reflected by their moisture content. Efforts to protect the wood were favourable in most cases, although only complete shelter kept the moisture content consistently below the level critical for the occurrence of decay. In order to relate the effects of moisture traps to the climate, a three-parameter empirical model was constructed and fitted to the experimental results. The model was able to capture the main features of the measurements and was used in order to characterize the performance of details in terms of their response to weather.

Simulation of load carrying capacity of sheathed parallel timber beam structure

Abstract A study on the load carrying capacity of a sheathed parallel timber beam structure taking into account load sharing between timber beams is presented. Monte Carlo simulations are used to generate systems and to evaluate the influence of different parameters on the system behaviour. The variability in bending strength within and between timber elements is accounted for using a model by Isaksson (1999). A tri-linear load deformation relationship is used giving the beam a reduced load carrying capacity after initial failure. Experiments indicate that system failure can be defined to occur when two beams next to each other or any three beams fail. The properties of the sheathing are assumed to be deterministic and the stiffness of the joint between beam and sheathing is varied. The simulations give the load capacity for the weakest beam in the system (with no load sharing and no capacity of the beam after initial failure) and for the system failure. The results show a 19-30 % increase in load capacity for the full model with load sharing compared to a system where the weakest beam with no load sharing defines the load capacity. The failure mode considered is bending failure in the solid timber beam.