Introduction. The models of wood resistance, formulated at the initial stages of development of the theory of limit states of building structures and based on the postulates accepted in classical mechanics of deformable solids, have undergone changes over time, dictated by the results obtained in modern research in the field of kinetic theory of strength, mesomechanics of solids and in other areas of research into the strength of materials and structures. The refinement of the rheological model of wood is assessed within the framework of a comparison of the capabilities of theories of limit states and the kinetic theory of destruction in the field of long-term resistance.

Aim. Substantiation of long-term strength of wood and wood structures based on the kinetic theory of strength, taking into account the wave effect of stresses on the strength of interatomic bonds of the elements of the structure of wood as a natural polymer.

Materials and methods. The analysis of the studies by Chernykh A.G., Zhurkov S.N.,

Ivanov Yu.M., Slavik Yu.Yu., Naichuk A.Ya. on the features of the long-term resistance of wood and wooden structures, damage accumulation, fatigue processes from the point of view of the kinetic theory of strength is performed. A hypothesis is proposed on the influence of wave propagation of deformations on the long-term strength. A decrease in the effect of reducing the long-term strength is theoretically substantiated due to the fact that the kinetics of crack growth is accompanied by counter kinetics of crack healing, limited by the energy imbalance, beyond which the broken interatomic bonds cannot be restored. An additional factor in reducing the effect is the variety of strength indicators – the required potential energy for breaking interatomic bonds – of highly oriented cellulose, which determines the strength of wood matter as a natural polymer, and which resists the development of microcracks in the wood of a wooden structure in different ways due to the anisotropy of properties, especially important for cross-laminated timber structures.

Results. The calculation method shows a reduction in long-term strength by 100 years, with a 10 % excess of the stress at rest of the loaded element, in the time period from zero to 120 years. The proposed hypothesis is confirmed by numerical experiments, it is established that the wave component of stresses in wooden elements exceeds the stress at rest during operation from 40 to 100 % or more.

Conclusions. The mechanism of reduction of long-term strength of wood under wave action of stresses can be explained by the proposed hypothesis. Rapid development of numerical modeling, development of new hypotheses in strength of materials in a wide range of external influences and conditions of external environments allow mutually complementing theories of limit states and kinetic theory of strength of solids.