Experimental Research On Behavior Of Cold-formed Steel-sheathing Board Screw Connections At Elevated Temperatures

Cold-formed steel structures(CFS)are inherent green buildings and industrialized buildings,with numerous advantages such as light weight,good seismic performance and high prefabrication.Self-tapping screw connections have been widely used in CFS structures for steel-steel and steel-sheathing board connections.When it comes to medium-rise CFS structures,fire resistance issue stands out.It is necessary to know the mechanical property of connections subjected to high temperature prior to the study of fire resistance of the whole structure.In this paper,both high temperature thermal conductivity test and material property experiment were firstly conducted upon three categories of sheathing boards.Then,the behavior of steel-sheathing board screw connections at elevated temperatures was studied,and then a simplified loaddisplacement curve model of the high-temperature connections was proposed.Lastly,the finite element method was used to simulate the behavior of screw connections at elevated temperatures.The main contents and conclusions are as follows:Firstly,three categories of sheathing boards,including medium density porter board,low density porter board and OSB board,were selected for high temperature thermal conductivity test and material property experiment.The thermal conductivity,Poisson's ratio,elastic modulus and their variation regulations of each board at different temperatures were studied.The experimental results showed that: the medium density porter board had the largest thermal conductivity,the low density porter board had the second largest thermal conductivity,and the OSB board had the smallest.The thermal conductivities of three categories of boards decreased with the rise of temperature.Under different temperature conditions,the Poisson's ratios of the three boards obtained from one-direction tensile experiment all varied within a certain range.The elastic moduli of OSB board obtained from three-point bending experiment and one-direction tensile experiment were similar.But above 50°C,the elastic moduli of medium and low density porter board,both obtained from the two experiment methods,were quite different.The medium density porter board had the largest elastic modulus,the low density porter board had the second largest elastic modulus,and the OSB board had the smallest.Then,due to the limitation of the experiment device and deficiency of the current research,a new experimental device was designed to conduct mechanical property experiment of steel-sheathing board screw connections at elevated temperatures.The influences of experiment variables such as shear direction,distance of screws to the boundary and the number of screws on the mechanical property were studied.The experimental results showed that: it was brittle failure mode of the medium and low density porter board connections while it was ductile failure of OSB board.The ductile failure of the OSB board was accompanied with the inclination and sinkage of the screws.For the medium density porter board and the OSB board connections with vertical shear direction,the phenomenon of stiffness increment was observed at the end of loading stage,which did not appear in other connections.The peak loads of connections with two screws were obviously increased but not doubled.The shear direction and the distance of screws to the boundary of all specimens exhibited little effects on the peak load and the deformation corresponding to peak load.For medium and low density porter board,the peak load and the deformation corresponding to peak load were the maximum when the shear force was parallel to the sheathing edge and the distance of screws to the boundary was 20 mm.The load-displacement curves of the connections are the basic data which are necessary for the subsequent high-temperature numerical simulation.For convenience,characteristic parameter prediction formulas of peak load,the deformation corresponding to peak load,and elastic stiffness were established firstly based on a large number of experiment results,and then a simplified load-displacement curve model of the high-temperature connections was proposed by considering whether the curves had the phenomenon of stiffness increment.Finally,considering different temperatures,54 three-dimensional solid finite element models of steelsheathing board screw connections were established on the platform of ABAQUS.C3D8 solid elements were used in the models,and the material properties were selected from aforementioned material property experiment.The screws and back-to-back steels were properly simplified in the models,and 5 contact pairs were modeled.The effects of interactions were considered by the definitions of 'hard contact' and 'frictionless'.The loading pattern was controlled by displacement.The failure processes and modes of connections obtained from simulation corresponded well with experimental phenomena and failure modes,respectively.And the errors were basically within 15% between the peak loads obtained from the simulation and experiment results.It is proved that the finite element simulation method proposed in this paper is effective and reasonable.