Material Properties Of CFS During Full-Range Fires And Simplified Prediction On The Fire Resistance Of CFS Load-Bearing Walls

Cold-formed steel(CFS)structure is one kind of natural industrialization and green livable buildings with the advantages of light weight,high strength,good seismic performance and easy to produce and assemble.It shows good application and development prospects in the field of rapid construction and low & multi-storey residential buildings.Meanwhile,the fire resistance performance has become a key issue for the design of such structure.The present paper conducted experimental and theoretical investigations on CFS and the corresponding load-bearing composite walls,including the material property degradation rules during the whole process of compartment fire,the creep behaviors of CFS at elevated temperatures,the fire-induced failure mechanism for the external insulated CFS composite walls and fast calculation method of fire resistance time.The main contents are as follows:1.Material properties of G550 CFS during the whole process of compartment fireA total of 88 steady-state experiments were designed and conducted on G550 CFS at elevated temperatures,including 44 test conditions during heating and cooling stages.The results showed that:(1)There was an obvious difference of the material properties(elastic modulus,yield strength,and ultimate strength)of G550 CFS between the the heating and cooling stages;(2)The distribution of the yield strength(ultimate strength)reduction factors of G550 CFS during the cooling stage displayed two branches;when the tensile temperature was same,the yield strength(ultimate strength)reduction factor of G550 CFS during the cooling stage with a peak temperature not less than 600 ℃ was significantly lower than that during the cooling stage with a peak temperature no more than 500 ℃;(3)The prediction equations for the reduction factors of material properties and stress-strain curves of G550 CFS were proposed in the whole process of compartment fire and can provide a reasonable prediction accuracy.2.Creep behavior of G550 CFS at elevated temperaturesAn experimental study with the creep duration of 4 h was performed for G550 CFS,including 41 test conditions to quantitatively investigate the creep behaviors under constant stress ratio and different temperatures.The results showed that:(1)The effects of stress ratio on the creep strain of G550 CFS varied significantly at elevated temperatures;specifically,the creep strain of G550 CFS increased notably with the growth of stress ratio when the temperature rose to 450 °C or above;(2)The critical temperatures at which the G550 CFS specimens experienced the onset of tertiary creep under different stress ratios were presented;when the stress ratio was not less than 0.7,the critical temperature was lower than the temperature corresponding to the high-temperature yield strength of G550 CFS,indicating that the creep might lead to early failure of steel members in a fire;(3)A prediction expression for the steady-state creep rate of G550 CFS was proposed and used to predict the creep strain of the transient-state strain of G550 CFS at 500 °C,which showed that the creep strain was an important component of the transient-state testing results of G550 CFS,and also a key factor causing the difference of strain results between the transient-state and steady-state tests.3.Improved fire resistance of CFS composite walls by using SAPSix mid-scale and non-load-bearing composite CFS walls with SAP(super absorbent polymers)insulation materials inside were designed and the corresponding fire experiments were conducted.The results showed that:(1)SAP insulation materials can protect steel structures by heat consumption through water evaporation as a new protection mechanism that was different from traditional fire-resistant coatings and boards,which indicated good potential in the field of fire protection;(2)For the CFS composite walls with the SAP insulation material,increasing the web depth of the rectangle hollow sections stud and applying autoclaved lightweight concrete(ALC)board were two effective methods to improve the fire performance of CFS composite walls;(3)An optimal configuration for CFS composite walls was proposed and rendered its steel studs and ambient-side sheathing without obvious damage when exposed to 2 h ISO 834 standard fire;the proposed configuration not only facilitated post-fire repair,but also significantly improved the fire safety of key components of CFS structures.4.Full-scale experimental investigation of external insulated CFS load-bearing composite walls under fire conditionsFull-scale fire experiments were conducted on 4 new external insulated CFS load-bearing composite walls,and the maximum value of the initial load ratio was0.55 that was nearly 100% of design load level.The results showed that:(1)The present ALC external insulated configuration was more reasonable than rock wool external and cavity insulated configurations,and the fire resistance time of full-scale load-bearing composite walls with ALC external insulated configuration exceeded 3 h which met the maximum fire resistance time of load-bearing walls of residential buildings specified in Chinese codes;(2)The constraint of the sheathing material on the out-of-plane deflection of the wall studs can change the failure mode of wall studs in fire,and the fire-induced structural failure of the present ALC external insulated load-bearing walls was triggered by the local buckling of the stud hot flange;(3)For the CFS composite walls without cavity insulation,the increased cavity depth had an insignificant effect on the temperature rise of wall cavity,wall studs and the ambient surface,if the possible change of heat transfer path caused by the cracking and falling-off of fire-side sheathing was not considered.5.Simplified prediction on the fire resistance time of CFS load-bearing composite wallsFor the non cavity insulated CFS composite walls subjected to one-side fire exposure,theoretical study was applied on its fire performance:(1)Based on the principle of energy conservation,the temperature rise increment equation of stud hot flange was proposed by using finite difference method;(2)The multi-layer fire-resistive assemblies composed of different materials were simplified to a fictitious homogeneous layer with equivalent fire protection effect,and the corresponding calculation method of its equivalent thermal parameters was proposed to realize the rapid calculation of the temperature rise of stud hot flange for such steel members with uniform temperature distribution under fire conditions;(3)A time equivalent approach was proposed for evaluating fire resistance of CFS composite walls based on critical temperature of stud flange;Meanwhile,the fast calculation of the fire resistance time of CFS composite walls was realized under different fire conditions and was in good agreement with the results obtained from the numerical simulation.The innovation points of this paper are as follows:(1)For G550 CFS,the preset paper carried out the steady-state experimental investigation of material properties during cooling stage and the creep behaviour at elevated temperatures,and proved that the yield strength was not equivalent during cooling and heating stages;the constitutive model of G550 CFS during heating and cooling stages and prediction expression of steady-state creep rate were proposed;it was confirmed that the creep strain was a key factor causing the difference of strain results between the transient-state and steady-state tests(2)To overcame the deficiency that the current code was mainly applicable to steel members with uniform temperature distribution in fire,the temperature rise increment equation of stud hot flange and simplified calculation method of the equivalent thermal parameters were proposed for CFS composite walls and realized the goal of fast calculation of the fire resistance time for such steel members with non-uniform temperature distribution under different fire conditions.(3)SAP insulation material was firstly was proved to have great fire protection potential by conducting fire experiments of 6 wall specimens,and its fire protection mechanism was also revealed;A new configuration of load-bearing CFS composite wall was proposed by conducting fire experiments of 4 full-scale wall specimens,which has passed the inspection of the national authority fire department,and met the fire resistance time(3 h)requirement of the load-bearing wall of residential buildings with first-class fire resistance rating specified in Chinese codes.