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

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Figure 3.4: Stress strain curves with varying temperature. Curve 1.24 °C, 2. 99 °C, 3. 204 °C, 4. 316 °C, 5. 427 °C, 6. 482 °C, 7. 535 °C, 8. 593 °C, 9. 649 °C (Harmathy, 1993) Figure 3.4 shows the variance in the stress-strain strain curve with temperature. Eurocode 3 has many curves on separate graphs showing this change in the stress strain relationship. The graph clearly shows the decrease in ultimate and yield stress of steel with temperature. For yield strength (Brokenbrough- Johnson) For yield and ultimate strength (Inst. of Str Eng) Figure 3.5: Variation of ultimate and yield strength of hot rolled steel, (Harmathy, 1993) 41
The variation of yield stress data is shown in Figure 3.5. This graph plots the variation in ultimate and yield strength of steel as taken from experimental data. The spread of data results as seen in this graph explains how difficult it is to compare the equations recommended by different parties, and choose one as being ‘better’ than another. Most data is found for temperatures below 600 °C, which is probably why there is greater spread in this region of the graph. 3.1.3 Determination of Limiting Steel Temperature: The formula to determine the temperature at which the member being analysed will fail is a direct rearrangement of the formula for the variation of the yield stress of steel for temperatures over 215 °C. This implies that the only factor affecting the steel strength is the yield stress with temperature. T = 905 − 690 3.6 l r f where rf is the ratio of the design action on the member under the design load for fire specified in NZS 4203, to the design capacity of the member at room temperature, ie R f /φR u . This formula can be used for three or four-sided exposure to fire, and for steel beams and columns. The design capacity of the steel section is based on the yield stress and the cross sectional area of the beam, so assuming the cross section of the beam remains constant and a uniform temperature is maintained throughout the steel, then this formula is valid. This only occurs with four-sided exposure, as with three-sided exposure to a fire, there will be significant temperature differences across the cross section of the steel. When attempting to use this formula for three-sided exposure a finite element approach is used to obtain a limiting temperature that accounts for the temperature gradient in the steel. Eurocode 3 has a different formula to calculate the limiting or critical temperature of a steel member. This method can be used as an alternative to the strength criteria that must be met as stated in 3.1.1. The critical temperature is based on the degree of utilisation of the element and is given by the following equation: 42
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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 and 26: 1.6 BACKGROUND INFORMATION: 1.6.1 F
Page 27 and 28: experienced in fire situations, but
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: temperature is chosen because it is
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 86 and 87: average temperature of the beam, be
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
Page 100 and 101: susceptible to heating, heat up mor
Page 102 and 103: Figure 5.3 a-c show the results fro
Page 104 and 105: 7850*600*10500 > 2*775*1100*(1869*2
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As can be seen from Figure 5.4 a-c,
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310 UB 40.4 beam H p /A = 241 m -1
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The SAFIR results fit between the t
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constant specific heat being used i
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From Figure 5.7 a-c, it appears tha
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800 Temperature ( o C) 600 400 200
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To examine the limits of the thickn
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1000 Temperature ( o C) 800 600 400
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Thermal conductivity, k i = 0.19 W/
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1000 Temperature ( o C) 800 600 400
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the slab except to reduce the secti
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Figure 5.14 shows the correlation b
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5.8 CONCLUSIONS: The thermal behavi
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6 COMPARISON OF METHODS USING OTHER
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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
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The fire that the test beams were e
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7 ADDITIONAL MATERIAL IN EUROCODE 3
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For thermal analysis in Eurocode 3,
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To determine the performance of a s
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The plastic neutral axis is located
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It is therefore recommended that th
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These methods may be used to provid
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Eurocode has a large annex with gui
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8.3 CHANGES TO NZS 3404: The follow
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H v height of the ventilation (m) I
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EC3, 1995. Eurocode 3: Design of St
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Temperature Conditions. National Re
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APPENDIX A - SPREADSHEET METHOD: Be
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APPENDIX B - SAFIR: In Figure B.2 i
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*.str file developed by the pre pro
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22 FISO 23 FISO FISO 24 FISO FISO 2
Page 176:
APPENDIX C - FIRECALC Below in Figu
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