Moisture-induced distortion in timber structures - examples based on partition walls

Doktorsavhandling, 2006

The shape of timber changes due to variations in moisture content (MC). In Scandinavia, producers normally dry timber to an MC of 18%. However, after the studs are built into the wall structure, for example, they may become distorted as a result of further drying in the structure. The final MC in a heated building in Scandinavia is about 8%. It is therefore important to produce and deliver timber products dried to the moisture content appropriate to the construction site. This project focuses on shape stability in timber studs built into wall structures and drying in in-service conditions. The timber studs were made of Norway spruce (Picea abies) and special interest was taken in the distortion modes of twist and spring. A relationship between force and deformation for the connection was obtained from the shear tests and pull-out tests of screws from gypsum wallboard. Tests on single studs (50 x 100 x 2500 mm) in restraint revealed the amount of torsional moment induced by twist when drying in in-service conditions from about 18% MC to about 8% MC. The tests resulted in torsional moment in the range of 10 Nm to 50 Nm for a restrained difference in “free” twist of between 2° and 7°. Four tests were conducted on whole wall structures. The walls were made of studs (45 x 70 x 2500 mm) conditioned to 18% MC, whereupon the structure dried to 8% MC in a normal indoor climate during a period of three months. Tests conducted on the two walls with gypsum cladding on both sides revealed that the free twist is significantly restrained by about 75%. The two walls with gypsum cladding on one side only reduce the twist by about 13%. However, the spring in the stud and thereby the curvature of the wall is sharply increased in this type of structure. A number of analytical models were produced. The first is a geometric model that explains in an illustrative way how twist is dependent on changes in MC, spiral grain angle, annual ring curvature and shrinkage properties. The second model provides a good explanation of the occurrence of excessive spring in the studs in a single-sided wall. It is based on the difference in stiffness and shrinkage properties between the stud and the cladding and shows that a relatively high modulus of elasticity and low shrinkage in the longitudinal direction of the stud reduce the curvature of the wall structure. The other models deal with how twist is restrained in the wall structure. They show that large bending stiffness in the cladding, as well as low torsional stiffness in the studs, is beneficial when it comes to producing a straight wall.