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

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4 CALCULATION OF STEEL TEMPERATURES FOR UNPROTECTED STEEL – ISO FIRE: 4.1 INTRODUCTION: The temperature rise of unprotected steel can be modelled using a variety of computer programmes and calculation methods. In the New Zealand code, formulas provide an estimate of the time until a limiting temperature is reached, which is sufficient for situations where a designer is confirming the structural stability of a steel member. These formulas however are valid for a limited time period, temperature range and for a limited range of steel member sizes. They also only provide an estimate of the time at which a member will reach a particular temperature, based on test results from experiments performed with unprotected beams exposed to the standard fire. The spreadsheet method is a simple calculation method which estimates the temperature in a one dimensional heat flow or ‘lumped mass’ approach. It was developed using basic heat transfer principles and the amount of temperature rise of the steel is based on the energy being transmitted to the member. The major variable to influence the rate of temperature rise of unprotected steel beams when using the spreadsheet method is the H p /A value, which is a ratio of exposed perimeter to cross sectional area, and the inverse of the average or effective thickness. This is discussed further in Section 1.6.3. In this section, the computer programmes SAFIR, Firecalc, and FIRES-T2 are compared, to examine the repetitivity of these finite element software programmes, and to determine whether the most user friendly, SAFIR, could be used in place of the others with the same accuracy of results. The results from SAFIR are also compared with the results from the spreadsheet method to examine the accuracy of the spreadsheet method. It is also investigated whether the spreadsheet method can be used with the same confidence as with the finite element computer programmes for simple fire related steel member problems such as the average temperature of the steel after a given time. 51
Three beam sizes have been considered here to determine if the methods discussed are applicable for a range of sizes. The geometry of these beams are sized according to Australian universal beams from BHP Steel, (1998). Unprotected beams subjected to four sided exposure have been examined, followed by three sided exposure with and without a concrete slab on top of the beam. 4.1.1 Assumptions The assumptions made when estimating the temperature of the steel beams exposed to fire by the spreadsheet method are that the steel has a uniform temperature distribution across its cross section and that the cross section is uniform along the length of the beam. The beam is exposed to a standard fire and the temperature of the air immediately adjacent to the beam is assumed to be that of the standard fire at the particular time. The spreadsheet method assumes a constant thickness of the steel, which is based on the H p /A value. The emissivity of the flame has been taken as 0.50 as suggested by Purkiss, Drysdale, and the convective heat transfer co-efficient is 25 W/mK in the spreadsheet analysis as well as the in the SAFIR simulations. For the comparison with the FIRE-T2 computer programme the heat transfer coefficient constants are not known. For the purposes of this report the properties of the steel are generally assumed to remain constant with temperature which is slightly inaccurate. Purkiss, (1996) recommends no variation of steel density with temperature and that this value remain at a constant value of 7850 kg/m 3 . Ting, (1999) looked at the discrepancies that arise by assuming constant values for specific heat of steel, and found that for time equivalence results, the variation is less than 10%. A comparison of the results found from SAFIR, with results from the spreadsheet method with varying specific heat has been made to confirm the influence of the variation of the properties of steel. See Section 1.6.2 for details of the variation of steel properties with temperature. 52
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Fire Design of Steel Members K R Le
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ABSTRACT: The New Zealand Steel Cod
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TABLE OF CONTENTS: ABSTRACT:.......
Page 10 and 11:
5.4 RESULTS FOR FOUR SIDED EXPOSURE
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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 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 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
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
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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
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APPENDIX C - FIRECALC Below in Figu
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