FIRE DESIGN OF STEEL MEMBERS - Civil and Natural Resources ...

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place when higher temperature ‘pockets’ of fluid give up energy to lower temperature pockets with which they make contact as such motion occurs (Tucker, 1999). Usually in fire situations, convective heat transfer involves hot gases from a fire moving past a solid object that is initially cool, and transferring heat or energy to it. The heating rate depends on the velocity of the fluid at the surface of the solid, thermal properties of the fluid and solid and the temperature of the solid. The general formula for heat transfer by convection is given by: q = hc∆T 1.2 h c can be calculated using heat transfer principles, using the properties of the fluid, and geometry of the solid. A typical value for h c is standard fires is 25 W/m 2 K, which is the value used in the SAFIR simulations and the spreadsheet calculations. If a hydrocarbon fire was being simulated, a h c value of 50 W/m 2 K is the recommended value in the Eurocode (Buchanan 1999). Conduction: The conduction form of heat transfer involves interactions between free electrons in a solid material, or between multiple materials. It involves a direct physical contact of the surfaces, and occurs without any appreciable displacement of the matter, excluding displacement due to thermal expansion (Tucker 1999). Unless analysing the temperature profile across a beam at an instant of time, conduction is not as important in heat transfer calculations from fire to unprotected steel as the other modes of heat transfer. For most protected steel beams, however, the predominant mode of heat transfer to and from the steel is through conduction, as the steel is not exposed to radiation from the fire, or the hot gases generated by the fire. Steel beams with box protection do experience radiation and convection, not from the fire, but from the hot inner surface of the insulation board as well as conduction from the edges of the steel in contact with the insulation board. 1.6.2 Thermal Properties of Steel: Various literature searches show that all agree that the thermal and mechanical properties of steel change with varying temperature. The strength of steel, or the load bearing capacity of steel decreases dramatically with an increase in temperature 11
experienced in fire situations, but generally the thermal properties of the steel are assumed to stay constant for simplicity in calculations. The specific heat, density and thermal conductivity do however vary with temperature, and although the difference does not usually effect the temperature of the steel found from analyses significantly, the differences should still be noted. Specific Heat: Of the thermal properties of steel, the specific heat has the largest deviation from a constant value. Stirland (Purkiss, 1996) suggested that the specific heat of steel, c s , be taken as: c s 2 s −4 −2 = 475 + 6.010x10 T + 9.46x10 T 1.3 s This equation is valid for temperatures of the steel, T s, , up to around 750 °C when the specific heat of steel reaches a discontinuity which occurs due to a phase change at the molecular level of steel at this temperature. For the analyses performed here, and in most fire situations, this temperature is too low as the upper limit, so the equations used in this report are found in ENV 1993-1-2, as follows: c s −3 2 −6 3 425 + 0.773T s + 1.69x10 Ts + 2.22x10 Ts = 20 < T s < 600 °C 1.4a c 13002 s = 666 − 600 < T ( T − 739) s < 735 °C 1.4b s c 17820 s = 545 + 735 < T ( T − 731) s < 900 °C 1.4c s c = 650 900 < T s < 1200 °C 1.4d s There are other formulas that are valid up to the discontinuity at around 750 °C, and these are listed in various publications. Vandamme and Janss derived the following relationship (Proe et al 1986): 2 s −4 c = 3.8x10 T + 0.2T + 472 1.5 s s A graph of the specific heat using these equations is shown below in Figure 1.1: 12
Page 1 and 2: Fire Design of Steel Members K R Le
Page 4: ABSTRACT: The New Zealand Steel Cod
Page 8 and 9: TABLE OF CONTENTS: ABSTRACT:.......
Page 10 and 11: 5.4 RESULTS FOR FOUR SIDED EXPOSURE
Page 12 and 13: LIST OF FIGURES: FIGURE 1.1: VARIAT
Page 14: FIGURE 5.11: TEMPERATURE CONTOUR LI
Page 17 and 18: designed with confidence. Full scal
Page 19 and 20: eams and struts when subjected to a
Page 21 and 22: Gilvery and Dexter, (1997), prepare
Page 23 and 24: with conditions specified to ensure
Page 25: 1.6 BACKGROUND INFORMATION: 1.6.1 F
Page 29 and 30: The thermal conductivity is not imp
Page 31 and 32: [ The heated perimeter of the steel
Page 33 and 34: The advantages of board systems are
Page 35 and 36: The values for the thermal properti
Page 37 and 38: insulation has on the rise of the s
Page 39 and 40: ( T − T ) k H p f s ∆t ∆T
Page 41 and 42: spreadsheet method very closely, wi
Page 43 and 44: protection, and for user defined fi
Page 45 and 46: This equation originates from the y
Page 47 and 48: 1400 1200 1000 800 600 400 200 0 0.
Page 49 and 50: time. The value of the wall lining
Page 51 and 52: The Eurocode does not define a meth
Page 53 and 54: f f y y ( T ) (20) = 1.0 0 < T < 21
Page 55 and 56: temperature is chosen because it is
Page 57 and 58: The variation of yield stress data
Page 59 and 60: 15 m -1 < H p /A < 275 m -1 in NZS
Page 61 and 62: = H p A (m -1 ) 7.85 The cons
Page 63 and 64: Connections: NZS 3404 requires that
Page 66 and 67: 4 CALCULATION OF STEEL TEMPERATURES
Page 68 and 69: 4.1.2 Analysis Between Methods of T
Page 70 and 71: Figure 4.2 shows the variation of t
Page 72 and 73: Minimum temperatures Maximum temper
Page 74 and 75: For all three sizes of beam, the sp
Page 76 and 77:
Temperature ( o C) 1200 1000 800 60
Page 78 and 79:
The ECCS equation has a time period
Page 80 and 81:
4.3.1 Results from simulations with
Page 82 and 83:
From Figure 4.5 a-c, the formula ra
Page 84 and 85:
Temperature ( o C) 1000 800 600 400
Page 86 and 87:
average temperature of the beam, be
Page 88 and 89:
Temperature ( o C) 1400 1200 1000 8
Page 90 and 91:
significantly decreased from a cool
Page 92 and 93:
Since both programmes use the same
Page 94 and 95:
Therefore, the ECCS equations provi
Page 96 and 97:
5 CALCULATION OF STEEL TEMPERATURES
Page 98 and 99:
calculations made by the spreadshee
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susceptible to heating, heat up mor
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Figure 5.3 a-c show the results fro
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7850*600*10500 > 2*775*1100*(1869*2
Page 106 and 107:
As can be seen from Figure 5.4 a-c,
Page 108 and 109:
310 UB 40.4 beam H p /A = 241 m -1
Page 110 and 111:
The SAFIR results fit between the t
Page 112 and 113:
constant specific heat being used i
Page 114 and 115:
From Figure 5.7 a-c, it appears tha
Page 116 and 117:
800 Temperature ( o C) 600 400 200
Page 118 and 119:
To examine the limits of the thickn
Page 120 and 121:
1000 Temperature ( o C) 800 600 400
Page 122 and 123:
Thermal conductivity, k i = 0.19 W/
Page 124 and 125:
1000 Temperature ( o C) 800 600 400
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the slab except to reduce the secti
Page 128 and 129:
Figure 5.14 shows the correlation b
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5.8 CONCLUSIONS: The thermal behavi
Page 132 and 133:
6 COMPARISON OF METHODS USING OTHER
Page 134 and 135:
Temperature ( o C) 1200 1000 800 60
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6.2.4 Results for protected steel:
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Figures 6.2 b shows the comparison
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Figure 6.3 a shows that the average
Page 142 and 143:
The fire that the test beams were e
Page 144 and 145:
7 ADDITIONAL MATERIAL IN EUROCODE 3
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For thermal analysis in Eurocode 3,
Page 148 and 149:
To determine the performance of a s
Page 150 and 151:
The plastic neutral axis is located
Page 152 and 153:
It is therefore recommended that th
Page 154 and 155:
These methods may be used to provid
Page 156:
Eurocode has a large annex with gui
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8.3 CHANGES TO NZS 3404: The follow
Page 161 and 162:
H v height of the ventilation (m) I
Page 163 and 164:
EC3, 1995. Eurocode 3: Design of St
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Temperature Conditions. National Re
Page 168 and 169:
APPENDIX A - SPREADSHEET METHOD: Be
Page 170 and 171:
APPENDIX B - SAFIR: In Figure B.2 i
Page 172 and 173:
*.str file developed by the pre pro
Page 174 and 175:
22 FISO 23 FISO FISO 24 FISO FISO 2
Page 176:
APPENDIX C - FIRECALC Below in Figu
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