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The security in case of fire constitutes one of the demands of Eurocode 5 part1.2 (European prestandard, semi - probabilistic method): design of timber structures, general rules - structural fire design. The purpose of this prestandard is to define a uniform method for the evaluation of the fire resistance of timber structures throughout Europe.

The objective of our study is: to verify the stability of different types of commercial agricultural buildings whose span is from 11 m to 27 m approximately by using codes of calculation for ultimate limit and serviceability states and to determine consequences that result for these timber structures with an accidental situation of fire exposure.

The methodology consists in taking into account the charring rate and the decrease of strength and stiffness properties of the different parts of the frame and on evaluating their limit of strength (tension, compression, bending, shearing), buckling and deflection. We also consider: permanent loads and combinations of imposed loads, snow load (up to an altitude of 700m) and wind load (in rural sites). The constructions parts are best fitted taking into account a duration of resistance to the fire of 30 minutes. The impact of the calculation method is appreciated on the basis of a comparison of wood volumes necessary when account is taken of the fire resistance of these structures and when it is not allowed for.

Expected results : if one considers a fire resistance of 30 minutes, the best fit of the different parts of structures involves an increasing the volume of the structures of 10 to 47% (altitude 700 m). Correspondingly the implications of this method of calculation put a real economic cost problem for these agricultural wood buildings.