Behavior Of Screw Connections In CFS-Sheeting Board

Cold-formed steel structure (CFSS), which is originated from the traditional timberwork, is a distinctive new structure. It has been widely used in low-rise residential abroad because of its high strength, lightweight, easy installation, and environmental characteristics. However, this kind of structure system is still in its infancy in our country. Screw connection is one of the main connection types of this structure system. In this paper, experiments on the mechanical behavior of screw connections in CFS-sheeting board was conducted using two different test setups; both the skeleton curve model and hysteretic model were fitted, based on which numerical simulation using the finite element software was performed. The main contents and conclusions are as follow:First of all, elasticity modulus, static bending strength, and poisson’s ratio of four kinds of panels were tested. Experimental study on the shear behavior of 238 screw connections were conducted, from which the mechanical properties and failure mode of connections could be obtained; factors such as test setups, sheathing type (Gypsum board, Magnesium oxide board, Oriented structural board, and Calcium silicate board), edge distance, steel thickness, screw diameter, loading modes (monotonic and cyclic loading) and arrangement direction of panels were considered. The experimental results showed that, for monotonic loading conditions, connections had a same failure mode by two different test setups, with an error for peak load within 15% except the Oriented structural board specimens, and the influence of various parameters presented the same trend; however, the connections’ peak deformation is larger under loading device with several screw connections, which can be reduced through decreasing the difference of the width between the U-type steel and scaleboard. The failure mode of all specimens under monotonic tensile was breaking of sheathing edge without screw being sheared off, while tilting of screws or even screws being sheared off under cyclic reciprocating loading appeared in Oriented structural board specimens and calcium silicate board specimens. Panel type and the edge distance had obvious effects on specimen mechanics performance, with the increase of edge distance, the bearing capacity of connection increases accordingly. The influence of steel thickness and screw diameter on specimens’mechanical performance was small and correlated with panel type. Arrangement direction(refers to the direction of parallel panels long or short when cut out panels) had small effect on Oriented structural board specimens and Magnesium oxide board specimens, while the shear-bearing capacity of Gypsum board specimens and Calcium silicate board specimens can be increased by 10% and 20%～50%, respectively when panels were cut out along the long side direction. Load-deformation curves of connection present obvious "pinching effect" and the shear-bearing capacity fell 4% to 18% under cyclic loading; the bigger edge distance was, the greater decline of connection shear-bearing capacity. Therefore, a reduction coefficient for the shear-bearing capacity of connection under monotonic tensile is needed for in the engineering design.The restoring force model of screw connections was obtained based on the experimental data and several common restoring force model of wooden nail connections, including "Foschi" index type skeleton curve model under monotonic loading and "Pivot" hysteretic model under cyclic loading.In addition, numerical simulation of screw connections in CFS-sheeting board under monotonic loading were performed by finite element program ANSYS. The material nonlinearity and contact nonlinearity were considered in the model establishment. The Self-drilling Screw Connections were simulated by Solid45 element, and contact element was installed in the screw holes. Besides, the pretension force of the screw was ignored. For calcium silicate board (the CSB board) with brittleness properties, Solid45 element was used to simulate the load-deformation curve of connections, while Solid65 element was used to simulate the crack propagation, both of which complement each other in this paper. The failure mode, the load-deformation curve, the bearing capacity of the connection and the effect of various factors (including arrangement direction of panels, screw diameter, steel thickness and the edge distance, etc.) on the performance of the connection were contrasted with the test results, it turned out that the finite element model was accurate and effective.