Design effects of actions in the fire situation: which combination to use

Where the design effects are used

The check of a member exposed to fire compares the design effect in the fire situation with the design resistance at time t (EN 1993-1-2, section 4):

E_fi,d is the design effect of actions in the fire situation: the axial force N_fi,Ed, the bending moment M_fi,Ed or the shear force V_fi,Ed. The same E_fi,d gives the degree of utilisation μ₀ = E_fi,d / R_fi,d,0, from which the critical temperature is calculated.

Accidental combination of actions

Fire is an accidental design situation. EN 1990, 6.4.3.3, eq. (6.11b) gives the combination:

G_k,j is the characteristic value of permanent action j.

P is the relevant value of the prestressing action, if any.

A_d is the design value of the indirect fire action (the effects of restrained thermal expansion).

Q_k,1 is the leading variable action and Q_k,i are the other variable actions.

ψ₁ is the factor for the frequent value and ψ₂ is the factor for the quasi-permanent value.

The accidental combination has no partial factors on actions: G_k and Q_k enter with a factor of 1.0.

What EN 1991-1-2 recommends

Value of the variable action Q₁

EN 1991-1-2, 4.3.1(2) allows the frequent value ψ_1,1Q₁ or the quasi-permanent value ψ_2,1Q₁ and recommends ψ_2,1Q₁. The choice is a nationally determined parameter: check the National Annex of the country where the structure is built.

With the recommended value, all variable actions enter with ψ₂, and the combination becomes:

Because all variable actions have the same type of factor, you do not need to choose the leading variable action. You calculate a single combination.

Other actions in the fire situation

EN 1991-1-2, 4.2.1 does not require other independent accidental actions to be combined with the fire. Depending on the design situation, actions caused by the fire are taken into account, for example the impact from the collapse of structural members or heavy machinery.

Some National Annexes list which other actions are or are not taken into account in the fire situation. Check the National Annex that applies to your project.

Example: office beam

Simply supported beam, span L = 6 m, permanent load g_k = 12 kN/m, office imposed load (category B) q_k = 8 kN/m.

Fundamental combination, at normal temperature:

Combination in the fire situation, with ψ₂ = 0.3 (EN 1990, Table A1.1, category B, recommended value):

If the National Annex requires the frequent value ψ₁ = 0.5, the result is q_fi,d = 16.0 kN/m and M_fi,Ed = 72.0 kNm, 11% more. Use the ψ values set in your National Annex.

Design effects in FireSolver

FireSolver does not calculate the combination of actions. Enter the design effects in the fire situation (N_fi,Ed, M_y,fi,Ed, M_z,fi,Ed) in the app, calculated with the combination above. The app uses them for the degree of utilisation μ₀, the critical temperature and the member checks.

Informative material summarising the provisions of EN 1990 and EN 1991-1-2, with their recommended values. It does not replace the text of the standards, the National Annex, the fire safety regulations or verification by a qualified engineer.