Experimental Investigation On Mechanical Properties Of Cold-formed Steel And The Steel-to-steel Screwed Connections At Elevated Temperatures

The cold-formed steel structure is derived from the traditional wood structure,and it is a building structure system that connects cold-formed steel components and lightweight building boards through fasteners.It has several advantages,such as light weight,convenient construction,predominant seismic performance and environmental protection.In this paper,the elevated temperature mechanical properties of cold-formed steel and the steel-to-steel screwed connections are studied experimentally,which provides basic data for analyzing mechanical properties and numerical simulation of cold-formed steel structures under fire conditions.The ANSYS finite element software is used for numerical simulation of the steady-state test of the steel-to-steel screwed connections.The main contents and conclusions are as follows:Firstly,this study conducted more than 300 tests at elevated temperature,including steady-state and transient tests.The test materials were the corners and flat parts of Q345(1.0mm,2.0mm)and G550(0.75mm)cold-formed steel.The experimental results show that the material properties of cold-formed steel gradually decrease with the increase of temperature,and the ultimate strength at 700? is only about 10% of value at the ambient temperature.Due to the cold bending effect,the elastic modulus and yield strength of the cold-formed steel at corner part is different with the flat part under the same temperature conditions,and the difference does not change with the increase of temperature.The material properties degradation curve of the steel at elevated temperature conditions given by the specification is not applicable to this test.Based on this research and existing experimental data in China and abroad,this paper proposes mechanical degradation models for plates and corner parts of cold formed steel at elevated temperature.These models provide a safe and reliable prediction of the mechanical properties of cold formed steel at elevated temperature.Besides,the strainstress curve of the transient test is different with the curve of the steady-state test,and the elastic modulus measured in the transient test is smaller than the value of the steadystate test.Therefore,if the steady-state test results are used in the fire resistance calculation of cold-formed steel structures instead of the transient test results,it may lead to inaccurate prediction of fire resistance time.Secondly,more than 120 times elevated temperature steady-state tests and more than 80 times transient-state tests were carried out on four kinds of cold-formed steelto-steel screwed connections.The results show that the failure modes of the screwed connections are mainly composed of three types: tilting and subsequent pull-out of the screw is accompanied by bearing of the joined materials(TB),tilting and subsequent pull-out of the screw is accompanied by edge tearing of the hole plate(TT)and screw shear(SS).The failure mode is affected by the thickness of the steel plate and the strength of the steel.The group effect formula proposed by R.A.Laboube is still applicable to the steel-to-steel screwed connections of two screws under elevated temperature conditions,and it is safety enough.The current North American/Australian specifications and the Chinese specifications for screwed connection design calculation methods are proposed based on the failure mode,and the Chinese specifications are more conservative than the North American/Australian specifications.The formula provided by the specification also has applicability to the calculation of the bearing capacity of the screwed connection at elevated temperatures.For the screws with failure modes of tilted and subsequent pulled out,its strength degradation at elevated temperature is determined by the strength degradation of the steel plate.The shear resistance of the screw determines the bearing capacity of the screwed connection due to shear failure.In this paper,the ultimate strength degradation model at elevated temperature is proposed for the screwed connections of these two failure modes,and the bearing capacity can be safely predicted at elevated temperature.This paper also proposes a prediction model for the failure temperature of the transient test of the screwed connection,which has applicability and safety to other similar connectors.Finally,four kinds of connectors and eight temperature conditions are selected,and three-dimensional finite element models of 32 steel-to-steel screwed connections are established using ANSYS finite element software.The failure mode and forcedisplacement curve of the elevated temperature steady-state test of the screwed connection are simulated.Considering material nonlinearity and contact nonlinearity,the model uses soild185 element.Comparing the simulation results with the experimental results,it is found that the degradation model can safely predict the degradation coefficient of the finite element simulation of the screwed connection.The ratio of the peak load obtained by the simulation to the test is basically in the range of 0.9-1.2.It shows that the finite element simulation method of this paper is correct.It is feasible to simulate the steady-state test of steel-to-steel screwed connections using ANSYS finite element method.