Presentation for Dissertation - Eurocode 1
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Presentation for Dissertation - Eurocode 1 Presentation for Dissertation - Eurocode 1 Presentation Transcript

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