History and solution of the issue of creep accounting in the analysis of timber structures

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Abstract

Introduction. Until now, in the analysis of timber structures, wood has been considered practically as an elastic material. Only in the latest edition of SNiP II-25-80 was an attempt made to account for creep in second-order analysis and stability design by introducing a reduced modulus of elasticity, E = 300 R. However, this provision turned out to be unclaimed, because residual relative deformations do not correspond to elastic ones and depend on the stress level at individual intervals of the bending element. Thus, in linear analysis, the integral modulus of elasticity of an element should be abandoned in favor of interval values of moduli that vary with the stress level. Aim. Improvement of the linear method for the analysis of bending and beam-column timber structures taking into account wood creep by using the long-term modulus of elasticity. Materials and methods. For the first time in timber engineering practice, a methodology has been developed and pilot long-term bending tests have been carried out to obtain the long-term modulus of elasticity depending on the stress level. Specimens were made of spruce and were held until the deflections stabilized. The stress increment step was 1.0 MPa. Results. The test duration at low stress levels was 1–1.5 months; at high levels, 10–12 months. A preliminary table of long-term moduli was compiled based on the deflection data. A comparison of structural analysis results using variable moduli showed a significant discrepancy with the results obtained by the standard method. Conclusions. The LIRA-SAPR software system enables the implementation of a two-stage linear approach for the analysis of bending and beam-column elements accounting for wood creep, using tabulated values of the long-term modulus of elasticity depending on the stress level. Reliable data for the table can be obtained from long-term tests of bending elements in adapted premises.

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