Fire Experiments Of Cold-formed Steel Walls Under Different Design Fire Conditions

The cold-formed steel(CFS)structure system is a green environmental protection and industrial building,which has good application prospects in China.The CFS wall is the main load-bearing and vertical divider of CFS building,and it can also be used as the non-load-bearing wall in concrete structure.Most previous fire experiments of cold-formed steel(CFS)walls focused on ISO 834 fire.The corresponding results may not represent the actual fire performance of CFS walls in some special circumstances.In view of this,the fire resistance experiments research of CFS wall models of different sizes in different fire conditions is carried out,and the main contents are as follows:1.Mid-scale experiments of non-load-bearing cold-formed steel walls under different fire conditionsDesigned and completed 18 mid-scale CFS wall specimens' fire experiments,there are four types of fire conditions including ISO 834 fire curve,hydrocarbon fire curve,external fire curve and a typical realistic design fire curve,and two kinds of structure including cavity-insulated CFS wall and external-insulated CFS wall.the results show that:(1)The fire severity of external fire condition to non-load-bearing CFS walls is not serious,a quasi-steady heat transfer state was identified at the final stage of external fire exposure;(2)The external insulation become a more reasonable configuration than the cavity insulation to improve the fire resistance of CFS walls.It is recommended to adopt the unexposed surface temperature of 900? as the fall-off criterion of fire-side face-layer GP boards;(3)Under the action of axial thermal expansion load and self-weight,two types of buckling of studs are identified for CFS non-load-bearing walls with cavity or external insulation,and the failure mode generation mechanism is analyzed;(4)It is proved that neither the equal-area method nor the energy-based method is well suited for the prediction of the fire-resistance time equivalency of non-load-bearing CFS walls.The reason is that the CFS wall boards and filling materials will fall off during the fire;(5)The simplified temperature distribution model of the stud section is verified,and a simplified calculation method is proposed to obtain the time-temperature profile of stud cross section of CFS walls in fire.In order to achieve high efficiency of the heat transfer simulation of CFS walls under fire conditions.2.Full-scale experiments of cold-formed steel walls under different fire conditionsDesigned and completed 8 full-scale CFS wall specimens' fire experiments,including four types of fire conditions,two types of plates,and three types of different loading conditions.The results show that:(1)Under the present four typical fire conditions,substituting GP board with CS board as the base-layer sheathing can delay the structural failure of CFS walls and prolong the corresponding FRT;(2)Under four different fire conditions,the failure modes of steel studs of CFS walls subjected to fire are the same,and the critical temperature of the stud hot flange is almost the same when the CFS walls with the same configuration and the same load level reach the fire resistance time.Given this realization,a new time-equivalent method of cavity-insulated load-bearing CFS walls is preliminarily proposed,which takes the same critical temperature of the stud hot flange during fire exposure as the basic principle of equivalency;(3)In the event of identical dimensions and configuration of wall cross-section,the time-temperature profiles of the mid-scale specimens tend to be consistent with that of the full-scale specimens under the same fire condition.Therefore,a simplified experimental method was proposed to determine the fire resistance time for the structural failure of CFS walls with different load ratios through only one time non-load-bearing fire experiment of mid-scale specimen;(4)It has been shown by experiments that the axial load level does not have a significant impact on the heat transfer of CFS walls during fire exposure.This realization has provided a supporting basis for the indirect coupling method adopted in the fire resistance study of CFS walls;(5)A preliminary equivalent explosion time calculation model by setting the reference temperature for correction,which can accurately predict the equivalent explosion time of structural failure of full-scale load-bearing CFS wall;(6)The use of high-temperature camera technology indirectly indicates that the opening in the gypsum board vertical joints of gypsum-sheathed CFS walls during fire exposure is the inherent characteristic of heated gypsum boards;Hence,this phenomenon has no obvious correlation with the load level or out-of-plane deflection of composite walls subjected to fire.The innovation points of this paper are as follows:(1)The first to carry out the load-bearing and non-load-bearing fire experiments of CFS walls in hydrocarbon fire and external fire conditions,confirming the effects of different fire conditions on the failure modes of CFS wall.Besides,An equivalent explosion time calculation model based on the critical temperature of the hot flange was proposed.(2)It has been shown by experiments that the axial load level and plane dimensions does not have a significant impact on the heat transfer of CFS walls during fire exposure.This realization has provided a supporting basis for the indirect coupling method adopted in the fire resistance study of CFS walls,and a simplified fire experiment method for small standard specimens is proposed.(3)It is the first to carry out direct experimental verification of the equivalent explosion time calculation model for CFS walls,and proposes a reference temperature modification method,which effectively improves the prediction accuracy of the equivalent explosion time for structural failure of CFS walls.