INCLUDING MOISTURE INDUCED STRESSES IN THE SAFETY FORMAT OF TIMBER STRUCTURES

摘要

Many failures and malfunctions of timber structures are related to the special way timber or wood interacts with the surrounding climate. During the role of COST E55 Action it became apparent that present engineering best practice, namely codes and standards, do not account for the climate - timber interaction with sufficient resolution[1,2].It is a known fact that the surrounding climate, i.e. the airs relative humidity and temperature and the corresponding changes over time influence the load bearing behaviour of timber structural elements. Every attempt to grasp the many different ways how wood and its mechanical properties interact with its surrounding climate take basis in the understanding of the special way timber desorbs and adsorbs water, i.e. how moisture equilibrium in wood is reached depending not only on the state of relative humidity and temperature but also on the corresponding moisture state history of the timber. Despite of the complexity of these phenomena and the anticipated large sensitivity of timber structural integrity to moisture effects, the way how these aspects are handled in the present design codes appears oversimplified and insufficient to ensure the safety and serviceability of structures required. Climate exposure when described in Eurocode 5 [3] is only differentiated into three so called service classes (SC). The SC that are defined by corresponding timber moisture content (mc) values that are expected during the service life of the structure; SCI, the mc is higher than 12% only for a view weeks per year, SC2, the mc is higher than 20% only for a view weeks per year, SC3 the mc is higher as expected for SC2. The effect of the climate exposures on the load bearing capacity is accounted for in combination with the mechanical load effect on the corresponding timber structural element, i.e. similarly as for the climate, different discrete load duration classes are distinguished. For every combination of climate and load exposure class, factors for the modification of characteristic strength and stiffness properties, knfandknf respectively, are prescribed in the code.Moisture induced mechanical effects might be classified into:1. Restrained shrinkage as it is an issue for joint design and detailing; but also within solid cross sections;2. Mechano-sorption that will alter the stress state but is more connected to accelerated creep deformationsand;3. accelerated aging due to moisture changes.Effect 3 is interrelated with the duration of load effect and a common treatment for structural design as in the present Eurocode might be justified. Effect 1 on the other hand inducing stresses in the timber, especially in the weak direction of the wood perpendicular to the grain. The results can be seen as cracks in timber elements, visibly growing from the outside (drying) and invisibly growing from inside (wetting). These cracks might become critical if the residual contact area is too small to transfer shear stresses in beam elements or even more if a structural element or connection is stressed perpendicular to the grain due to external forces as in curved beams or in the surrounding of holes or notches. The stresses induced by moisture changes would be consistently treated in the design if the action inducing the stresses was accounted for as any other load described in the code.During the course of the COST E55 action strategies for how this change of aspect can be realized and implemented into future design codes was developed. This pursuit includes the following considerations: A description of the moisture exposure, structured for building classes and climatic regions (as e.g. for snow loads), a sound representation of equilibrium moisture states of wood in natural climates, models for moisture transport in seasoned timber in natural varying climates, the modelling of moisture induced strain (shrinkage and swelling) and finally a sound stress evaluation scheme. With the models describing the hygro-mechanical relations of wood it is readily seen that moisture induced stress is a response to spatial differences of moisture states within a wood volume. For a given variation of relative humidity with constant periodicity (Figure la) it can be estimated how this climate variation affects the moisture variation in transversal direction of a solid piece of timber (Figure lb). The upper and lower bound in Figure lb indicate the bounds within moisture content