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Methods for fire resistance analysis

The fire resistance of timber members can be verified using two different design methods according to the chapter 4.2 of EN 1995-1-2: "Reduced cross-section method" and "Reduced properties method". The "Reduced cross-section method" is recommended in the standard, the "Reduced properties method" provides more economical results.

Reduced cross-section method

The effective cross-section is verified for this method. The effective cross-section is calculated by reducing the residual cross-section by the edge layer with the thickness k₀d₀, where k₀ is the factor, that grows linearly from 0 (time 0 min) to 1 (time 20 min or t_ch for t_ch> 20 min). Thickness d₀ is a constant with its value 7 mm. Design values of material characteristics are calculated using factor k_mod,fi equal to 1.0.

Reduced properties method

The residual cross-section is verified in this method, however the material properties are reduced by factor k_mod,fi. The factor k_mod,fi is calculated in a different ways for different properties and depends on the perimeter and area of the residual cross-section.

The following formula is used for bending strength:

For compressive strength:

For tensile strength and modulus of elasticity:

These values of k_mod,fi are valid for time t> 20 min. For time 0-20 min the value of k_mod,fi is increasing linearly from 0 to the described value.