1、12Strain(%)0.51.01.52.0Stress(N/mm2)03002502001501005020C200C300C400C500C600C700C800ClSteel softens progressively from 100-200C up.lOnly 23%of ambient-temperature strength remains at 700C.lAt 800C strength reduced to 11%and at 900C to 6%.lMelts at about 1500C.31.00.90.80.70.60.50.40.30.20.1012341000
2、C800C20C200C400C600CStrain(%)Normalised stresslConcrete also loses strength and stiffness from 100C upwards.lDoes not regain strength on cooling.lHigh temperature properties depend mainly on aggregate type used.4Reaction occurs when Oxygen/fuel mixture hot enough5Cooling.ISO834 standard fire curveIg
3、nition-SmoulderingPre-FlashoverHeatingPost-Flashover1000-1200CNatural fire curveTimeTemperatureFlashover630010020004005006007008009001000060012001800240030003600Time(sec)Gas Temperature(C)576675739781842945utesminint)1t 8log(345207200400600800100012000120024003600Time(sec)Gas Temperature(C)Typical E
4、C1 Parametric fire curveExternal FireStandard FireHydrocarbon FirelFire resistance times based on standard furnace tests-NOT on survival in real fires.lEC1 Parametric Fire temperature-time curves.Based on fire load and compartment properties(500m2).Only allowed with calculation models.8CompartmentTe
5、mperatureLoad-bearing resistanceTimeTimeFire severity time equivalentlUsed to rate fire severity or element performance relative to furnace test.lMatches times to given temperature in a natural fire and in Standard Fire.Fire resistance time equivalentStandard fireNatural fireElement9Fire Testing lLo
6、ad kept constant,fire temperature increased using Standard Fire curve.lMaximum deflection criterion for fire resistance of beams.lLoad capacity criterion for fire resistance of columns.ProblemslLimited range of spans feasible,simply supported beams only.lEffects of continuity ignored.Beams fail by“r
7、un-away”.lRestraint to thermal expansion by surrounding structure ignored.101002003000120024003600Time(sec)Deflection(mm)111002003000120024003600Time(sec)Deflection(mm)Span2/400dIf rate tfi.requLoad resistance:Rfi.d.t Efi.d.tTemperature:cr.d d Usually only directly feasible using advanced calculatio
8、n models.Feasible by hand calculation.Find reduced resistance at design temperature.Most usual simple EC3 method.Find critical temperature for loading,compare with design temperature.19Steell Mechanical (effective yield strength,elastic modulus,.)Concretel Thermal (thermal expansion,thermal conducti
9、vity,specific heat)l Mechanical (compressive strength,secant modulus,.)l Thermal (thermal expansion,thermal conductivity,specific heat)20lStrength/stiffness reduction factors for elastic modulus and yield strength(2%strain).Strain(%)0.51.01.52.0Stress(N/mm2)03002502001501005020C200C300C400C500C600C7
10、00C800ClElastic modulus at 600C reduced by about 70%.lYield strength at 600C reduced by over 50%.21Rft0300600900120010080604020%of normal valueTemperature(C)RftEffective yield strength(at 2%strain)SSElastic modulusSS Strength and stiffness reductions very similar for S235,S275,S355 structural steels
11、 and hot-rolled reinforcing bars.(SS)Cold-worked reinforcing bars S500 deteriorate more rapidly.(Rft)221005002004006008001000 1200Temperature(C)654321Strain(%)Strength(%of normal)Strain at maximumstrengthNormal-weight ConcretelAccurate for normal density concrete with siliceous aggregates.lConservat
12、ive for normal density concrete with calcareous aggregates,.Lightweight ConcretelConservative for light-weight concretes.All types treated the same.Strength reduction factors23Stress-strain relationship in cooling from 700C(at 400C)Stress-strain relationship in heating phase(700C)515250,010,020,03St
13、ress-strain relationship at ambient temperatureStress-strain relationship in heating phase(400C)Stress-strain relationship after cooling from 700C(at 20C)2400,51,01,52,02,53,03,54,04,5100 200 300 400 500 600 700 800 900Temperature(C)Expansion Coeff/C(x 10-6)Steel Steel thermal expansion stops during
14、 crystal structrure change in the 700-800C range.Normal-weightconcrete Concrete unlikely to reach 700C in time of a building fire.Lightweight concrete Light-weight concrete treated as having uniform thermal expansion coefficient.25la=45W/mK(EC3 simple calculation model)Thermal conductivity(W/mK)1020
15、304050600200 400 600 800 1000 1200Temperature(C)Steelca=600J/kgK(EC3 simple calculation model)Specific Heat(J/kgK)50000200 400600 800 1000 1200Temperature(C)4000300020001000Steel26NCLCNCLCMay assume constant value for NC:1,60 W/m.KMay assume constant value for NC:1000 J/kg.Kcc*Specific heat cc(J/kg.
16、K)400800100012002006001000 CThermal conductivity lc(W/m.K)2006001000 C12327Thermal analysis:both EC3 Part 1.2 and EC4 Part 1.2 unprotected and protected steel beams Lower and upper flanges Considerably different temperatures proper calculation of temperatures!Temperature 28thVAmaat.aTemperature incr
17、ease in time step t:27327310 x67,5hHeat flux hnet.d has 2 parts:Radiation:mgcc,nethConvection:Steel temperatureSteelFire temperature29perimeterc/s areaexposed perimeterc/s areahb2(b+h)c/s area90%!30Steel temperatureSteelProtectionFire temperaturedp Some heat stored in protection layer.VAdccppaapp He
18、at stored in protection layer relative to heat stored in steelt.g10/t.at.gpaappt.a1et3/11VAcd/l Temperature rise of steel in time increment t31exposed perimeterTotal c/s areaexposed plateTotal c/s areaexposed flangeTotal c/s area32Steel perimetersteel c/s areahb2(b+h)c/s areainner perimeter of boardsteel c/s area90%!谢谢你的阅读v知识就是财富v丰富你的人生
