Presentation for Dissertation - Eurocode 1


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Presentation for Dissertation - Eurocode 1

  1. 1. Department of<br />Civil and <br />Structural <br />Engineering<br />Annex B[informative]<br />THERMAL ACTIONS FOR EXTERNAL MEMBERS<br />Eurocode 1<br />Actions on structures <br />BS EN 1991 Part 1-2:2002 General actions – Actions on structures exposed to fire<br />
  2. 2. Thermal Calculation <br />Thermal action for external members - Simplified calculation method<br />The method is used to determine the maximum compartment fire temperature. <br />It gives the temperature and size of flame emanating from the openings.<br />For various parameters, the method considers steady–state condition. <br />This method is adopted for fire loads greater than 200 MJ/m2.<br />Thermal Calculation<br />
  3. 3. Usage conditions<br />Usage conditions<br />If there are more number of windows in a fire compartment, the weighted average height of openings heq, summation of width of windows (wt = ∑wi) and vertical openings total area Av are used. <br />The compartment dimensions limited to 70m, width to 18m and 5m height.<br /> All along the thickness and width of flame the flame temperature remains uniform.<br />Usage conditions<br />
  4. 4. Usage conditions<br />Usage conditions<br />If windows are present only on one side of wall 1, then the D/W ratio <br />D/W = W2/wt<br />If more number of windows are present on more than one wall of the fire compartment, then D/W ratio is given as <br />D/W = (W2 Av,1) / (W1 Av)<br />Where,<br />W1 - wall width on side 1, generally side containing greatest area of window<br />Av,1 - summation of window area on side 1<br />W2 - wall width perpendicular to side 1 <br />Usage conditions<br />
  5. 5. Wind effect<br />Mode of Ventilation<br />If there are windows present on either side or when the fire is fed with additional external supply of air, then the ‘forced draught’ method is used for calculations. Else, no forced draught conditions are used.<br />Deflection of flame by wind<br />Flames coming out of fire compartment are assumed to be in a direction perpendicular to the facade; At an angle 45⁰, due to deflection by wind effects.<br />Wind effect<br />45⁰<br />135⁰<br />wind<br />Flame deflection due to wind - Horizontal cross section<br />
  6. 6. No forced draught<br />Burning Rate<br />For most types of furniture found in buildings, τF is about 1200sec so that for a free burning fire of furniture, the rate of burning is <br /> [MW]<br />Vertical opening area of walls, Av<br />Height of window, heq<br />Total enclosure area (ceiling, floor, walls along with windows), At<br />Ratio of compartment depth to width, D/W<br />No forced draught<br />
  7. 7. No forced draught<br />Fire compartment temperature<br />Tf = 6000(1 – e - 0.1/o) o1/2 (1- e -0.00286 Ω) + To<br />Flame height<br />This equation may be simplified by taking ρg = 0.45 kg/m3 and g = 9.81 m/s2 as,<br />No forced draught<br />
  8. 8. No forced draught<br />Flame height<br />2h /3<br />2h /3<br />eq<br />eq<br />L<br />L<br />H<br />H<br />No forced draught<br />L<br />L<br />L<br />L<br />w<br />2h /3<br />2h /3<br />t<br />eq<br />L<br />eq<br />L<br />h<br />1<br />h<br />1<br />eq<br />eq<br />Flame dimensions – No through draught<br />
  9. 9. No forced draught<br />Flame width<br />Flame width is taken as window width, wi<br />Flame depth<br /> Taken as 2/3rd of the window height, i.e. 2/3 heq<br />Horizontal projection of flame<br />If a wall is present above window, then<br />LH = heq/3 if heq1.25wt<br />LH = 0.3 heq (heq / wt) 0.54 if heq>1.25wt and distance to any other window > 4wt<br />LH = 0.454 heq (heq / 2wt) 0.54 in other cases<br />In case of a wall not existing above the window, <br />LH = 0.6 heq (LL / heq) 1/3<br />No forced draught<br />
  10. 10. No forced draught<br />Length of flame along axis<br />When LL > 0<br />Lf = LL + heq/2 if wall exist above window or if heq1.25 wt<br />Lf = (LL2 + (LH - heq/3)2)1/2+heq/2 if wall exist above window or if heq>1.25 wt<br />When LL = 0, then Lf = 0<br />Flame temperature at window<br />Tw = 520/(1-0.4725(Lf wt/Q))+To with Lf wt/Q < 1<br />Emissivity of flame at window<br />The flame emissivity at window is taken as εf = 1.0<br />The temperature of flame along the axis<br />Tz = (Tw – To)(1-0.4725(Lx wt/Q))+To with Lx wt/Q < 1<br />No forced draught<br />
  11. 11. No forced draught<br />The emissivity of flames <br />εf = 1 – e-0.3 df<br />Convective heat transfer coefficient<br />αc = 0.00467 (1/ deq)0.4 (Q/Av)0.6<br />Effect of projection above window<br />The flame height LL decreased by Wa(1+√2)<br />The horizontal projection of the flame LH increased by Wa<br />As per EN1991-1-2, <br />αc= (1/ deq)0.4 (Q/Av)0.6<br />But has error in units<br />4.67<br />No forced draught<br />
  12. 12. No forced draught<br />Effect of projection above window<br />c<br />e<br />d<br />b<br />c<br />No forced draught<br />b<br />a<br />a<br /> a-b-c = Lf a-b-c-d-e = Lf and wa = a b<br />Vertical cross section: Deflection of flame due to balcony<br />
  13. 13. No forced draught<br />If the wall does not exist on top of the window or heq>1.25wt<br />- The flame height LL reduced by Wa<br />- The projection of flame in horizontal direction LH with the above LL is increased by Wa<br />No forced draught<br />
  14. 14. Forced draught<br />Forced Draught<br />Burning Rate<br />With ample ventilation known as the free burning condition τF is determined by the burning characteristics of fire load, generally taken as 1200 sec. <br />The rate of burning is given by<br />Q = (Afqf,d)/τF = (Afqf,d)/1200<br />Fire compartment temperature, Tf<br />Tf = 1200((Afqf,d)/ 17.5 – e -0.00228Ω + To<br />Forced draught<br />
  15. 15. Forced draught<br />Flame height<br />For general conditions the equation is simplified with u = 6 m/s as,<br />Forced draught<br />
  16. 16. Forced draught<br />Flame height - Flame dimensions for through draught or forced draught<br />L<br />H<br />H<br />L<br />L<br />h<br />eq<br />Forced draught<br />w<br />w<br />f<br />h<br />t<br />eq<br />L<br />f<br />
  17. 17. Forced draught<br />Horizontal projection of flame<br />Comparing the previous equation, as the speed of wind increases, the horizontal projection increases as the height of flame decreases. It is independent of presence of wall above or not.<br />LH = 0.605 (u2/heq) 0.22 (LL + heq)<br />For simplified calculations, using u = 6 m/s,<br />LH = 1.33 (LL + heq) / heq0.22<br />Forced draught<br />
  18. 18. Forced draught<br />Flame width<br />In forced draught, the flames tend to widen outwards with hot gases away from the opening as shown in the figure above. The width of flame is given by<br />wf = wt + 0.4 LH<br /> <br />Flame length along axis<br />The length of flame along axis, Lf, from tip of flame to the window is found using simple Pythagoras formula<br />Lf = (LL2 + LH2)1/2<br />Temperature of flame at window<br />Tw = 520 / (1- 0.3325 Lf (Av) 1/2 / Q) + To; with LfLf (Av)1/2 / Q < 1<br /> <br />The flame emissivity at the window, εf = 1.0<br />Forced draught<br />
  19. 19. Forced draught<br />The temperature of flame along axis<br />Lx is the length of axis at any point of calculation to the window.<br />Flame emissivity <br />εf = 1 – e<br />Convective heat transfer coefficient<br />αc=0.0098 (1/ deq)0.4 (Q/(17.5Av) + u/1.6)0.6<br />The simplified form of the above formula <br />after the inclusion of wind speed, u = 6 m/s,<br />αc = 0.0098 (1/ deq)0.4 (Q/(17.5Av) + 3.75)0.6<br />Forced draught<br />As per EN1991-1-2, <br />αc= (1/ deq)0.4 .............<br />But has error in units<br />9.80<br />
  20. 20. Forced draught<br />Effect of projection above window - Deflection of flame due to balcony/awning<br />b<br />Forced draught<br />c<br />awning<br />a<br />a<br />b<br />f<br />f<br />
  21. 21. The End<br />