Study On The In-plane Mechanical Behavior Of Cold-formed Thin-walled Steel-Gypsum Based Self-Leveling Mortar Composite Floor

In order to fully understand the in-plane mechanical performance and design method of CFS composite floor and seismic performance of multi-storey CFS residential structure, this paper makes a deep and systematic research from two aspects of experimental research and theoretical analysis, the main contents of this research are as follows:1. Experimental study on in-plane performances of full scale CFS composite floorsThree CFS composite floor specimens consisting of frame construction as substructure and three different configurations floor slabs as superstructure were designed refer to the real size of practice projects. The C-section and U-section CFS joists made up the frame construction of the specimens. The composite floor slabs included steel deck, gypsum based self-leveling mortar(GBSM) on the steel deck and GBSM with steel mesh on the steel deck,respectively. Through the in-plane low-cycle and reverse horizontal loading test of three specimens, the failure mode, bearing capacity, stiffness degradation, energy dissipation and ductility of the composite floors were achieved. The loading direction of in-plane test paralleled to the length direction of C-section floor joists. The results showed that, all of the composite floors had good in-plane mechanical performance. The failure modes of specimens were shear failure in self-drilling screws connected steel deck and fixed boundary joists, while the floor panel and CFS joists hadn’t occurred damage. The GBSM could increase in-plane bearing capacity, initial stiffness and ductility performance of structural system. Moreover, the steel mesh enhanced the adhesive performance and in-plane stiffness of composite floor but reduced the in-plane bearing capacity and ductility on the contrary.2. To establish accurate and reasonable nonlinear finite element modelThe composite floor finite element models were simulated by ABAQUS software to conduct nonlinear numerical simulation and variable parameters analysis. The results showed that:(1) The screw spacing was the main factor influencing the in-plane mechanical performance of composite floors. It is suggested that self-drilling screw spacing of surroundings should be the minimum between multiples of a single wavelength of steel deckand 150 mm, and the middle self-drilling screw spacing should be the double of the surrounding ones.(2) When 40 mm was selected for the GBSM thickness, it could reduce the self-weight and make the in-plane mechanical indexes of the floors to acquire the greatest economic benefit.(3) The joists dimension and the CFS joists with openings had little impact on the in-plane mechanical performance of composite floors.(4) The joists spacing had great impact on the in-plane stiffness and ductility of the floors, hence 400 mm was suggested for the joists spacing.(5) The increase of length-width ratio of composite floors would reduce the bearing capacity, stiffness and ductility at every loading stage, so the length-width ratio of the floors was suggested to be less than 3.(6) When the span of the joists was beyond 3.6m, single one steel flat was suggested to be set at mid-span of joists, in addition, the rigid support members should be set at each end of the steel flat.3. Research on the in-plane stiffness calculation method and load distribution principle of CFS composite floor(1) This paper investigated the in-plane stiffness calculation method of CFS composite floor, and put forward simplified calculation method of the in-plane deformation and simplified analysis model of composite floors.(2) Based on the simplified analysis model of composite wall with openings, four-layers two-span structural model was established to determine the transitive and distributive relation of the horizontal load among shear walls.4. Research on the seismic performance of multi-storey CFS residential structureBased on practical project, this paper established the mid-rise CFS residential structure finite element model. Referring to the seismic design code, bidirectional dynamic time-history analysis was made to study the applicability of multi-storey CFS residential system in Shaanxi province.5. Applicability analysis on rigid floor hypothesis of multi-storey CFS residential structureIn order to provide reliable theoretical basis for《Technical specification for cold-formed thin-walled steel multi-storey residential buildings 》, the simplify calculation model was established for increasing the convergence speed and enhancing the calculation efficiency.Moreover, the applicability of rigid floor hypothesis was analyzed on the changes of lateral resistance walls spacing, which based on the elastic design method under frequent earthquake action.This paper demonstrated that the CFS composite floors had favorable in-plane mechanical performance, which could provide reliable experimental data and theoretical guidance for practical project applications.