Postearthquake Fire Experiments Of Cold-formed Steel Load-bearing Walls

As a new building system,the cold-formed steel(CFS)structures have the advantages of less steel consumption,good seismic performance,green environmental protection,and rapid industrial construction,etc.So it has been increasingly used in China,and its fire resistance research has always been the key point of the CFS structure research.At the same time,multihazard interaction investigation is an important issue for disaster reduction in civil engineering and post-earthquake fires are one of the most frequent secondary disasters facing mankind.As the main load bearing and lateral resistance components,the CFS walls play an important role in the mechanical performance of the whole structure.It is very significant to study the fire resistance of CFS walls with damage for the fire-resistant design of CFS buildings.In this paper,the post-earthquake fire resistance of CFS walls covered with double-layer wall panels is studied.The main contents are as follows:1.Axial compression experiment of three CFS walls covered with double-layer gypsum boards and with different cross-section steel skeleton at ambient temperature were tested.And the check of CFS walls' axial bearing capacity based on AISI as well as the finite element simulation of axial compression test of CFS Walls at ambient temperature were carried out too.The results showed that: a).Lipped channel section studs have stiffeners at the ends and unlipped channel section tracks have flanges that do not significantly increase the ultimate axial bearing capacity of the wall,which within 12%.Therefore,it is recommended that the CFS wall used stiffened studs or lipped channel section track in the actual projects can still be designed in accordance with the traditional wall using unlipped channel section tracks and lipped channel section studs without stiffeners at the end.The actual extra part of the bearing capacity can be used as the safety reserves of the wall.b).The calculation of the axial ultimate bearing capacity of the G550 CFS wall according to the AISI is only 60% of the test results,but the existing calculation of the axial ultimate bearing capacity of the Q345 CFS wall is close to the test results,and the test results is about 90% of the calculated results.This shows that the AISI code does not consider the influence of the steel yield strength on the axial ultimate bearing capacity of CFS walls.The design formula for the axial bearing capacity of CFS walls with different types of steel still needs further improvement.2.The post-earthquake fire resistance tests of four cavity-insulated CFS load-bearing walls lined with double-layer plasterboards were carried out,and thefactors that may effect of fire resistance of CFS walls,such as different earthquake damage,the constraint of floor,and the filling layers of cavity were investigated.The fire failure mechanism of this type wall with damage is described.The results show that: a).When the drift ratio of the specimens within 2% under earthquake actions,the fire resistance time is only 8 min less than that of the wall specimen without earthquake damage,and the fire failure mode of wall studs will not change,which is described as the local buckling of the hot flange and web at the top area,which further results in substantial flexural deflect of the studs towards the ambient side;b).When the drift ratio of the specimens exceed 2.5% under strong earthquake actions,the fire resistance performance of walls drop sharply and the residual FRT is only 8min,which because after the cyclic loading test,an obvious gap is generated between the bottom sheathing board on the fire-facing side and the steel frame,hence,the fire-side GP board cannot provide fire protection for the bottom area of the steel stud during the fire test.As a result,the heat in the furnace can be directly transmitted to the hot flange of the stud through the gap between the sheathing board and steel frame.At the same time,the fire failure mode of the wall studs will also change,which manifested as local crushing on the stud section in the area from the stud bottom,which shows that the risk of collapse of the CFS structure under the strong earthquake secondary fire cannot be ignored;c).The axial thermal expansion constraints provided by the rigid floor will reduce the FRT of load-bearing CFS walls and accelerate the structural failure of CFS wall frame under fire conditions,which results in the commonly used cavity-insulated load-bearing CFS walls lined with double-layer GP sheathings on either side can not meet the fire resistance rating of 60 min listed in the code,and this case is recommended to be considered in the specifications;d).The cavity insulation will hinder the heat transfer from the cavity to the ambient side,which results in a rapid temperature increase in the fire-side sheathing and the hot stud flange,which will not only reduce the fire resistance limit of the bearing wall,but also change the fire failure mode of the wall studs.3.As the cavity-insulated cold-formed steel load-bearing walls lined with double-layer plasterboards do not meet the 1-hour fire resistance limit requirement,the post-earthquake fire performance of four cavity-insulated CFS walls with gypsum and calcium-silicate sheathing were carried out,and the fire failure mechanism of this type wall is described.The results show that: a).When the drift ratio of the specimens is less than 1%,the horizontal earthquake action has no obvious effect on the fireresistance of this type of wall,and the failure mode is described as the local buckling of the hot flange at the top of the stud,leading to local buckling of flanges and web and flexural deflection of the stud towards the ambient side;when the drift ratio of the specimens reaches more than 2%,due to the damage of boards at the horizontal joint of the specimen,which cannot provide lateral restraint for the hot flange of the stud,so that the damage location was shifted to area near the horizontal joints;when the drift ratio of the specimens reaches 3.5%,the residual fire resistance time of the wall specimen is only 40 minutes,and the studs also display local buckling of the cold flange and web at the height of approximately 75 mm from the bottom of the stud;b)The overlapped board joints on the same side of the double-layer sheathing is not an appropriate fire-resistant construction,which should be avoided in actual engineering;c)The fire experiments of CFS shear walls with the same configuration and testing conditions can have good repeatability,especially regarding the fire resistance time,failure mode and time-temperature curve;d)By substituting the GP board with the CS board as the base-layer sheathing,the FRT of the CFS walls lined with double layers on either side extended from 48 min to 66 min,and the shear strength and stiffness are increased by 21.5% and 39%.Meanwhile,the residual FRT of the wall is also significantly improved under post-earthquake fire conditions,so it is recommended to use walls with gypsum and calcium-silicate sheathing in mid-rise CFS buildings.The innovations of this article are as follows:(1)Firstly carry out the fire resistance test study of CFS load-bearing walls that consider the floor constraints,revealed the unfavorable fire resistance mechanism of cavity filling layer,which will hinder the heat transfer from the cavity to the ambient side,and results in a rapid temperature increase in the fire-side sheathing and the hot stud flange;proved that the axial expansion load of the wall studs in the fire environment cannot be ignored,which will accelerate the structural failure of the composite wall and resulting in cavity-insulated CFS load-bearing walls lined with double-layer gypsum boards can not meet the 1-hour fire resistance limit requirement,and provides revised recommendations for related specifications.(2)Firstly carry out the post-earthquake fire resistance test of CFS load-bearing wall,and reveal the failure model of CFS load-bearing wall with earthquake damage lined with double-layer gypsum boards or gypsum and calcium-silicate boards,and explain the mechanism of earthquake damage on the fire resistance of two types of walls.