Experimental Study On Behavior Of Screw Connections In CFS Keel-Sheeting Board

Cold-formed steel(CFS)structure,as a new structure,is characterized by lightweight,environmental protection,energy saving and quick construction.CFS framed composite shear walls,which are the primary lateral load-bearing structural components,are made of CFS keel and sheeting connected by self-drilling screws.The current study shows that the local damage of these self-drilling screw connections is the main reason for the failure of shear walls.Therefore,it is necessary to investigate the mechanical properties of self-drilling screw connections for fully comprehending the forcing features of CFS shear walls.In this thesis,the experimental tests were conducted to study the behavior of four types of screw connections,and then the skeleton curve model and hysteretic model were fitted.Based on the above data,the screw connections were simulated using the finite element software.The main contents and conclusions are summarized as follow:First of all,elastic modulus,static bending strength and poison ratio of two new kinds of sheeting were tested,which provided the basic data for numerical simulation of the connections.Aiming at the limitations of experiment device and concerning the lack of research on new types of plate and screw forcing state,a novel device was designed.Mechanical property experiments were performed on 42 groups of screw connections.Various factors were considered such as sheathing type(Gypsum board,Oriented structural board,Medium-density porter board and Low-density porter board,where,Medium density and Low-density porter board were in key research),edge distance,loading mode,forcing condition of screws(shear parallel to the sheathing edge and shear perpendicular to the sheathing edge)and number of screws.The experimental results showed that: the brittle failure of screw connections occurs when steel keel-LBTB and steel keel-MBTB are uniaxially stretched,whereas the ductile damage happens under monotonic compression.Plate type,screw number and loading mode had significant effects on peak loads of connections,in which,the peak loads of steel keel-MBTB connections were always 48% ~ 95% higher than those of steel keel-LBTB connections and the peak load of single screw in single self-drilling screw connections was about 30% higher than that in multi-screws connections.Only under the case of monotonic tension as well as shear perpendicular to the sheathing edge,the peak loads of screw connections increased by about 30% with every 5mm increase of the screw margin.Under monotonic tension,the forcing condition of screws had an obvious effect on connections,that is,the peak loads and peak deformations of specimens were higher than other cases when shear was parallel to the sheathing edge.Then,concerning that the load-displacement curve of screw connections has direct influence on the accuracy of numerical simulation of CFS shear walls,the restoring model corresponding to screw connections was established base on the test results to improve the reliability and accuracy of numerical simulation.Under monotonic loading case,the skeleton curve was fitted by modified "Forschi" exponential model and the relevant parameters of the model are obtained according to the experimental data.Under cyclic loading case,the skeleton curve and hysteretic curve were obtained using pinch-4 and "Pivot" hysteretic model respectively,and then the restoring model was calculated by SAP2000 which can reflect the characteristics of nonlinearity,strength degradation,stiffness degradation and pinching of specimens.In the end,the numerical simulation of screw connections in CFS keel-sheeting board under monotonic tensile loading case were performed using finite element program ABAQUS.The model was established by C3D8 R solid elements,and the material nonlinearity as well as contact nonlinearity were considered.The self-drilling screw was reasonably simplified,meanwhile the pretension generated by the screw was ignored.The failure mode and the bearing capacity of four kinds of connections obtained by ABAQUS were consistent with the tests,and the error of peak load was less than 20%,which also indicates that the finite element model is accurate and effective.