Experimental Research On Behavior Of Self-tapping Screw Connections In CFSS At Elevated Temperatures

Cold-formed Steel Structure (CFSS), which is originated from the traditional wood structure, is a kind of system which consists of cold-formed steel members as load-bearing structure and lightweight boards as maintenance. It has the advantages of light weight, good seismic performance, better economic benefit and easy installation. Self-tapping screw connection is one of the main connection types of this structure system. In this paper, experiments at elevated temperatures on the mechanical behavior of self-tapping screw connections were done to provide basic data for fire resistance research in cold-formed steel structure. FE analysis was also used to simulate the behavior of steel-steel screw connections using steady state test method. The main contents and conclusions are as follows:Firstly, experiments at elevated temperatures on the mechanical behavior of steel-steel screw connections were done using both steady state method and transient state method. The results are shown below:(a) failure modes were observed including tilting and subsequent pull-out of the screw accompanied with bearing, tilting and subsequent pull-out of the screw accompanied with edge tearing, screw shear accompanied with bearing and tilting and purely screw shear failure; (b) the thickness of thin sheet steels and the steel strength had a large effect on the deterioration of the test strengths of the single shear screw connections as the temperature increases, but the effect of size of screws and end distance were not obvious; (c) the ratio between the strengths of the single shear screw connections predicted by the current specification(GB 50018-2002, AISI S100-2007 and AS/NZS 4600:2005) and the experimental strengths generally decreased as the temperature increased; (d) the transient state test results are always conservative compared with the steady state test results, which represents that the critical temperatures determined by the transient state tests are generally higher than those steady state test results.Secondly, experiments at elevated temperatures on the mechanical behavior of steel-board(including Gypsum board and Magnesium oxide board) screw connections were done using both steady state method and transient state method. The results are shown below:(a) the main failure modes were the damage at the edge of the board accompanied with tilting of screw in some specimens; (b) the steel strength, the gypsum board sample position(Magnesium oxide boards regard as homogeneity) and the thickness of thin sheet steels had no effect on the deterioration of the test strengths of the single shear screw connections as the temperature increases, end distances had a large effect on that; (c) the transient state test results of steel-Gypsum board screw connections are consistent with the steady state test results, and the transient state test results of steel-Magnesium oxide board screw connections are always conservative compared with the steady state test results; (d) the width of board in steel-board screw connections restricted the strength of screws in situation of two-screw connections.Finally,90 three-dimensional solid finite element models at 10 different temperatures were established with the help of the finite element program ABAQUS which simulated single shear self-tapping screw connections using the steady state test method. The steel plates and screws were modeled by using the C3D8R solid elements. Material non-linearity and geometric non-linearity had to be included in the non-linear analysis in the FEM. The contact interfaces included 5 contact pairs. The 'hard contact' was used for the normal contact behavior and frictionless was defined in the tangential direction. All the contacts were defined as surface-to-surface contact with a small sliding option. Self-tapping screws were simplified in the model. Material properties of steel and screw obtained from the tensile coupon tests of our project group and other researchers at normal room and elevated temperatures were used. Compared the finite element results with the test results, it turned out that the simulation method was proper and the simplified model was effective.