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Buchanan 2001 , while the second option is to use speci?chightemperature creep models developed for structural steel. Assuch, it is generally left to the user to choose which creep modelto use. Fig. 8 clearly shows that improper selection of creepmodel can result in incorrect predictions of creep strains at extreme stress and temperature conditions.。 the stress was maintained constantas shown in Fig. 8 b .It can be seen in Fig. 8 a , for the three cases of constant stress49, , and 98 MPa , and constant temperature 550176。 Cand at different but constant stress levels was used. In the second case Fig. 8 b , test data from an experiment reported byHarmathy 1967 and Harmathy and Stanzak 1970 were used,Harmathy tested A36 steel rods under constant temperature of550176。 when the stressrate was held constant d s / dt = 0 and when the stress rate isvariable d s / dt 0 . In the ?rst case Fig. 8 a 。 Dorn 1955 were generally conductedfor steel and metal alloys with variable chemical positions,and very little information is available on the effect of hightemperature creep on the structural response.At present, most ?re resistance analyzes are carried out usingHarmathy’s hightemperature creep model that is mainly based onDorn’s theory Dorn 1955。Huang and Tan 2003 . Creep tests Kirby and Preston 1988。 C. However, it was found experimentally that when thestress level is high, the effect of creep bees signi?cant in steelmembers even at temperatures of 300176。 C see Fig. 7 .Fig. 6. De?nition of yield point and proportionality limit in ASCE,EC3, and Poh 2001Fig. 7. Thermal strain of steel as predicted by different models andas measured in different testsFig. 8. Creep strains predicted using ANSYS and Harmathy models pared to test dataHighTemperature CreepCreep is de?ned as the timedependent plastic strain under constant stress and temperature. At room temperature and under service load levels, creep deformations of steel are insigni?cant,however, at elevated temperatures。 C theASCE model assumes a continuously increasing thermal strainwhile the Eurocode model accounts for the phase change thatoccurs in steel in this temperature range by assuming a constantthermal strain from 750 to 850176。 C, after which pointthermal strain starts to increase again.Variation of thermal strain models as speci?ed in ASCE andEC3 are also plotted in Fig. 7. Minimal differences exist betweenthe Eurocode and ASCE models for thermal strain of steel up to700176。Stirland 1980 . As seen in Fig. 7, thermal strain of steel increaseswith temperature up to nearly 750176。 Cooke 1988。 C, respectively. The yaxis represents normalized stress and the xaxis represents normalizedstrain. Originally, the Eurocode temperaturestressstrain curveswere derived based on transienttests under slow heating ratesTwilt 1991。 C, while ASCE andPoh models assume a loss of 30 and 40%, respectively, at 400176。 Cooke 1988 .These variations in test methods resulted in variations in thereported mechanical properties, which in turn resulted in variations in the constitutive models speci?ed in codes and standards.The following sections present parative study of these variations.Yield Strength and Elastic ModulusAs mentioned earlier, different test regimes were used to obtainyield strength and elastic modulus of steel at elevated temperatures. The variations in test parameters resulted in different testmeasurements, thereby leading to differences in constitutive relationships presented in different codes and standards. Generally,tensile strength tests are conducted to obtain elastic modulus andyield strength of steel. There is a lack of experiments on themodulus of steel under pression. This is because in tensilestrength tests, plications that may arise due to geometric instabilities and con?nement of specimen is eliminated. However, itis generally assumed that the modulus of elasticity for steel, derived based on tensile strength tests, is the same for pressionstate.Fig. 3 and 4 show the yield strength and modulus of elasticityof steel as a function of temperature, respectively. The test dataplotted in the ?gures are piled from various hightemperatureproperty tests as shown on the ?gures. Both the yield strength andelastic modulus decrease as temperature increases. This decreasecan be attributed to the nucleus of the iron atoms in steel movingfarther apart due to rising temperature in steel, leading to decreased bond strength, which in turn reduces the yield strengthand elastic modulus.It can be seen in the ?gures that there is signi?cant variation intest data on yield strength and modulus of elasticity at temperatures above 300176。 Anderbergconstant stress and temperature , can in?uence the resulting1988 . Despite the fact that strain rate has a signi?cant effect onthe test r