Prediction models for creep behavior of nailed joints between Douglas-fir lumber and plywood

Abstract

To apply accurate procedures of structural analysis that are now available, the behavior of nailed joints in light-frame wood buildings under long-term loads needs to be studied. Such a behavior can best be evaluated by testing specimens under constant loads, which requires relatively simple testing arrangements. To provide for a practical use of the constant-load test results, theoretical models were developed that predict the behavior of nailed joints under varying loads that are subjected to wood structures in service. Existing models and principles were used to develop five new general models, all of which account for the nonlinear viscous-viscoelastic behavior of nailed joints. The models incorporated the modified superposition principle and strain hardening principle. Heaviside function and Fourier series were also incorporated to describe varying loads that can be either discrete or mathematically defined continuous function. To develop experimental data needed for the formulation and verification of the models developed, joints made of Douglas-fir lumber, plywood and 6d nails were tested under four constant and four varying loads. The experimental data for constant loads were used to formulate specific theoretical models which were further modified for varying loads. The comparison between the predicted and the corresponding test results shows a very good agreement for all the specific models. The models that include the modified superposition principle are the most accurate and the simplest to apply to nailed joints under discrete load functions. Fourier series representation of varying-load functions shows a great potential for practical applications, because it can represent the service loads more accurately than the discrete approximation. The specific models presented are limited to the type of joints used in this investigation. Other types of joints need to be tested under constant loads to develop appropriate equations that describe their creep behavior under varying loads.