The current research is devoted to the investigation of relationships in creep behaviour of timber beams under natural environmental conditions with the purpose of detecting the factors significantly affecting such parameters as the variable moisture content of wood, temperature, stress level, span-depth ratio, macrostructure of wood and other factors, and to establish a mathematical model applicable for determining parameters related to prognosis of deflection of timber beams under variable loads. This study includes the analysis of experimental test results in static bending of 17 softwood (Pinus Sylvestris L.) beams with different span and span/depth ratios regarding a variation of moisture content, relative humidity and air temperature. The mathematical model examined in this study for the description of creep has been developed on the basis of the Burger body concept. A sufficiently good compatibility of average experimental and modelled strain values of timber beams was observed in the results. These results testify that the developed creep model may be used for predicting deformation of timber beams, and constants involved are applicable for natural environmental conditions. This study is part of extensive research that is aimed at contributing to the determination of accurate model parameters and to establishing an adequate and practically applicable mathematical model for more accurate predictions of final deflection of timber beams for design purposes. Keywords: timber beams, deflection in bending, development of creep under natural environmental conditions, mathematical model of creep.

Lilita Ozola, born 1950, received her Doctor degree in engineering from the Estonia University of Agriculture in 2005. Aivars BROKANS Doctoral Student Latvia University of Agriculture Jelgava, LATVIA coldzero@inbox.lv Aivars Brokans, born 1985, received his diploma engineer (2009) and Master degree in civil engineering from the Latvia University of Agriculture in 2011. SummaryThis paper presents the results on experimental research of the bending creep behaviour of softwood timber beams in uncontrolled microclimate conditions (unheated building, winter climate in the region near the Baltic Sea) under different levels of a constant load not exceeding the proportional limit. Experimental evidence is produced to corroborate the Eurocode 5 conditions for predictions of creep effects under variable load. Deflection values of beams in four point bending are measured daily during 3 months simultaneously performing monitoring of the moisture content of the wood, relative humidity and air temperature. It has been observed that at the initial period the character of creep development differs significantly comparing it with later period till the end of the test. The linear equations are found as sufficiently good approximations for relationship creep versus time at the moderate load levels not exceeding the proportional limit.

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Lilita Ozola, born 1950, received her Doctor degree in engineering from the Estonia University of Agriculture in 2005. Aivars BROKANS Doctoral Student Latvia University of Agriculture Jelgava, LATVIA Brokans.Aivars@inbox.lv Aivars Brokans, born 1985, received his diploma engineer (2009) and Master degree in civil engineering from the Latvia University of Agriculture in 2011. SummaryThis study is devoted to the development of the mathematical model applicable for prediction of final deformation of timber beams at the end of service life. The model presented in a new edition is based on the known Burger body concept. This is continued study by authors and deals with defining the limitations of numerical values for model's constants representing the influencing factors mentioned above with purpose to assess the fitness for use of model in design. Numerical values of strain obtained by treatment of theoretical model proposed are compared with the corresponding estimates from experiments performed in longterm (approximately 1.5 years) static loading of softwood (European Redwood -Pinus Sylvestris) beams in a four point bending under natural environmental conditions at the Baltic Sea region. It has been proved that the model proposed may be acknowledged as appropriate for prediction of deformations in static bending.