1Introduction to Eurocode Introduction to Eurocode Structural Fire Structural Fire EngineeringEngineeringStructural Steelwork Eurocodes2Strain (%)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. Steel stress-strain curves at Steel stress-strain curves at high temperatureshigh temperatures31.00.90.80.70.60.50.40.30.20.1012341000C800C20C200C400C600CStrain (%)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. Concrete stress-strain Concrete stress-strain curves at high temperaturescurves at high temperatures4The fire triangleThe fire triangleFuel + Oxidant = Combustion productsFuel + Oxidant = Combustion productsCHCH4 4 + O+ O2 2 = CO= CO2 2 + 2H+ 2H2 20 0Reaction occurs when Oxygen/fuel mixture hot enoughHeatHeatOxygenOxygenFuelFuel5Stages of a natural fire - and Stages of a natural fire - and the standard fire test curvethe standard fire test curveCooling .ISO834 standard fire curveIgnition - SmoulderingPre-FlashoverHeatingPost-Flashover1000-1200CNatural fire curveTimeTemperatureFlashover6The EC1 (ISO834) standard The EC1 (ISO834) standard fire curvefire curve30010020004005006007008009001000060012001800240030003600Time (sec)Gas Temperature (C)5766757397818429457200400600800100012000120024003600Time (sec)Gas Temperature (C)Typical EC1 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.Different EC1 Different EC1 time-temperature curvestime-temperature curves8CompartmentTemperatureLoad-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 fireElementTime-equivalenceTime-equivalence9Furnace tests on structural Furnace tests on structural elementselementsFire Testing lLoad 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 “run-away”.lRestraint to thermal expansion by surrounding structure ignored. 10Standard fire resistance furnace Standard fire resistance furnace testtest1002003000120024003600Time (sec)Deflection (mm)11Standard fire resistance Standard fire resistance furnace testfurnace test1002003000120024003600Time (sec)Deflection (mm)Span2/400dIf rate tfi.requLoad resistance:Rfi.d.t Efi.d.tTemperature: cr.d dUsually only directly feasible using advanced calculation 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.19Material propertiesMaterial propertiesSteell Mechanical (effective yield strength, elastic modulus, . )Concretel Thermal (thermal expansion, thermal conductivity, 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)03002502001501005020C200C300C400C500C600C700C800ClElastic modulus at 600C reduced by about 70%.lYield strength at 600C reduced by over 50%. Steel stress-strain curves at Steel stress-strain curves at high temperatureshigh temperatures21RftDegradation of steel Degradation of steel strength and stiffnessstrength and stiffness0300600900120010080604020% of normal valueTemperature (C)RftEffective yield strength(at 2% strain)SSElastic modulusSSStrength and stiffness reductions very similar for S235, S275, S355 structural steels and hot-rolled reinforcing bars. (SS)Cold-worked reinforcing bars S500 deteriorate more rapidly. (Rft)221005002004006008001000 1200Temperature (C)654321Strain (%)Strength (% of normal)Strain at maximumstrengthDegradation of concrete Degradation of concrete strength and stiffnessstrength and stiffnessNormal-weight ConcretelAccurate for normal density concrete with siliceous aggregates.lConservative for normal density concrete with calcareous aggregates,.Lightweight ConcretelConservative for light-weight concretes. All types treated the same.Strength reduction factors23C Concrete strength oncrete strength in in heating and cooling heating and cooling Stress-strain relationship in cooling from 700C (at 400C)Stress-strain relationship in heating phase (700C)515250,010,020,03Stress-strain relationship at ambient temperatureStress-strain relationship in heating phase (400C)Stress-strain relationship after cooling from 700C (at 20C)24Thermal expansion of steel Thermal expansion of steel and concreteand concrete00,51,01,52,02,53,03,54,04,5100 200 300 400 500 600 700 800 900Temperature (C)Expansion Coeff /C (x 10-6)SteelSteel thermal expansion stops during crystal structrure change in the 700-800C range.Normal-weightconcreteConcrete unlikely to reach 700C in time of a building fire.Lightweight concreteLight-weight concrete treated as having uniform thermal expansion coefficient.25la=45W/mK (EC3 simple calculation model)Thermal conductivity (W/mK)1020304050600200 400 600 800 1000 1200Temperature (C)Steelca=600J/kgK(EC3 simple calculation model)Other steel thermal Other steel thermal propertiespropertiesSpecific Heat (J/kgK)50000200 400600 800 1000 1200Temperature (C)4000300020001000Steel26Other Other concreteconcrete thermal thermal propertiespropertiesNCLCNCLCMay assume constant value for NC: 1,60 W/m.KMay assume constant value for NC: 1000 J/kg.Kcc*Specific heat cc (J/kg.K)400800100012002006001000 CThermal conductivity lc (W/m.K)2006001000 C12327Thermal analysisThermal analysis Thermal 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 28Temperature increase of Temperature increase of unprotected steel unprotected steel Temperature increase in time step Dt:Heat flux hnet.d has 2 parts:Radiation:Convection:Steel temperatureSteelFire temperature29Section factor ASection factor Amm/V - /V - unprotected steel membersunprotected steel membersperimeterc/s areaexposed perimeterc/s areahb2(b+h)c/s area90%!30Temperature increase of Temperature increase of protected steelprotected steelSteel temperatureSteelProtectionFire temperaturedpSome heat stored in protection layer.Heat stored in protection layer relative to heat stored in steelTemperature rise of steel in time increment Dt31exposed perimeterTotal c/s areaexposed plateTotal c/s areaexposed flangeTotal c/s areaSection factor ASection factor Amm/V - /V - inherently protected systemsinherently protected systems32Section factor ASection factor Ap p/V - /V - protected steel membersprotected steel membersSteel perimetersteel c/s areahb2(b+h)c/s areainner perimeter of boardsteel c/s area90%!