Steel Buildings in Europe

Part 7: Fire Engineering 7 - 14 Time  g  max t* max heating phase cooling phase  g =20+1325(1-0,324e -0,2t* -0,2e -1,7t* -0,427e -19t* ) with t*= t.C where t is the time (hours) and )² R [O / b]² /(0.04 /1160  Main parameters: - Wall characteristics : thermal inertia   c b  - Opening characteristics: opening factor t v O A h / A   max =  g (t* max ) =  g (t max .  ) (°C) with t max = max{ (0.2.10 -3 q t,d / O).  / O, t lim } (hours) where t lim is a function of fire growth rate (according to building type): - t lim =25 min if slow fire growth rate - t lim =20 min if medium fire growth rate, - t lim =15 min if fast fire growth rate, - q t,d is the design value of the load density [MJ/m²]  g =  g (t*, t* max , x) (°C) =  max – 625.(t* - t* max .x) if t* max  0,5 =  max – 250.(3- t* max ).(t* - t* max .x) if 0,5 < t* max  2 =  max – 250.(t* - t* max ) if t* max > 2 with t*= t.  t* max = (0.2.10 -3 q t,d / O).  and x is a function of t max as follows: x = 1 if t max > t lim x = t lim .  / t* max if t max = t lim Figure 5.1 Parametric Fire (Annex A of EN 1991-1-2) The inputs for the parametric fire curves are the design fire load density, the fire growth rate, the ventilation conditions (described by the size and the position of the openings) and the thermal properties (heat capacity, the density and the conductivity) of walls to evaluate the heat losses which occur by convection and radiation at the compartment boundaries. For the fire load density, it is common practice in design to refer to the characteristic values given in EN 1991-1-2. Even though these parametric fire curves offer a significant improvement compared to the standard “ISO-fire”, the parametric fires are not yet able to provide a very accurate evaluation of the fire severity. Consequently, some European countries recommend their use only for pre-design calculation. 5.1.3 Localised fire EN 1991-1-2 provides simple approaches for determining thermal actions of localised fires in Annex C. Two situations are distinguished according to the height of the fire flame relative to the ceiling of the compartment: where the flame is not impacting the ceiling (based on Heskestad’s method); and where the flame is impacting the ceiling (based on Hasemi’s method).