1991

Leivo, Mika

The purpose of this study is to model and experimentally verify factors influencing changes in the instantaneous, medium-term and long-term stiffness of nail plate joints, sawn timber and nail plate trusses. The experimental testing of the stiffness models was achieved with the aid of an extensive test series. In the instantaneous stiffness tests, 4 series of nail plate joints were loading in 20 different service conditions and 14 test trusses were loaded in 10 different services conditions. Long term tests were performed on sawn timber and nail plate joints in constant condition of 35 % RH and 90 % RH as well as in cyclically varying moisture conditions of 35 - 90 % RH. There were 2 sawn timber test series in each of the test conditions and 4 - 6 joint test series. 11 trusses were subjected to full scale loading in a test house. The duration of creep tests performed on the joints, sawn timber and nail plate trusses was about 1 year. The instantaneous stiffness of the nail plate truss is modelled with the aid of nail plate joint and sawn timber models, the contributions of these components to the overall truss deformation being weighted by the magnitude of their effects. A new joint model is proposed for the nail plate joint and the instantaneous stiffness of sawn timber is modelled by also taking account of the change occurring in the cross sectional shape as the moisture content of the wood changes. In addition to the constant humidity creep models described in the literature, new mechanosorptive creep models describing joint creep in both constant and variable humidity conditions are presented for the nail plate joint. The sawn timber creep models are experimental power functions in the case of constant humidity and mechanosorptive creep models in the case of variable humidity. The contribution of the joints to the creep of the trusses is emphasized and the presented creep model for the truss in variable humidity is formulated from the joint creepm model. An easy to use power function is presented for estimating the creep of deflection caused by the medium term loading of trusses.