Research On Compressive Behavior Of Intersecting Connection Of Concrete Filled Steel Tubular Diagrid Structures

Diagrid structural system is an innovative structural system, which commonly utilizes intersecting diagonal members instead of the conventional vertical columns as structural supporting system. Compared to conventional frame structures, diagrid structures can provide much larger lateral stiffness and better performance to resist most of the horizontal loads such as wind loads and earthquake effect owing to the triangulated configuration. These advantanges make diagrid structural system widely used in high-rise buildings recently. Previous research indicates that diagonal members carry much larger axial compressive forces than conventional columns, thus circular concrete filled steel tubular (CFST) columns, which can offer high compressive strength and high ductility, are apparently ideal for use in diagrid structural systems. However, connection design is one of the most great challenges of diagrid structural systems because more than six components from various angles are concentrated at one point. In other words, the design and mechanical behavior of the connection are indeed essential for spreading application of diagrid structural system.Based on the conclusion of an amount of references on CFST structures and diagrid structural system, the behavior of CFST diagrid structures was studied, and a kind of intersecting connection with four circular steel tubes, an elliptical plate, a ring reinforcing plate, two ring plates, several stiffening ribs and brackets was proposed. This connection provides high strength, light weight, outstanding structural performance and convienence in construction. In addition, it can effectively connects upper and lower diagonal columns and makes the columns intersecting each other, has a blooming prospect in field construction. By means of experimental study, three dimensional nonlinear finite element analysis (FEA) and mechanical theoretical method, this paper studies the compressive behavior of the intersecting connection, including the following aspects:1) Based on the test results of eight planar intersecting connection specimens with different connection details, in-plane angles and loading types, comprehensive analyses on the test procedure, failure modes and structural behavior are conducted. The details are proved to be reasonably designed and the bearing capacity can fulfilled the requirement stipulated by Chinese codes. The effect of parameters on the behavior of specimens such as bearing capacity, development of the strain and the ductility are analyzed. Test results indicate that the connection with ring reinforcing plate and lining plate provides better performance than the connection with flange plate. Therefore, the confinement effect on the concrete core should be reinforced by increasing the thickness of the steel tubes in the connection zone. 2) A stiffness-based multiple direction loading test method was proposed and applied to a series test of eight spatial intersecting connection specimens with different in-plane and out-of-plane angle, thickness of elliptical plate and loading type. The effect of parameters on the behavior and internal failure modes is analyzed, and the behavior of the intersecting connection is verified to be excellent for practical use.3) An equivalent constitute model for concrete core of intersecting connection is proposed based on evaluation on currenet three-dimensional nonlinear constitute models for concrete. A three-dimensional dual nonlinear FEA is conducted on planar and spatial intersecting connections to investigate the stress distribution. The confinement effect and the bearing capacity of the connection zone are remarkably increased by introduction of elliptical plate, ring reinforcing plate and lining plate. In addition, the effect of in-plane and out-of-plane angles, thickness of elliptical plate, ring reinforcing plate, lining plate and out-of-plane constraint on the behavior of connection are analyzed using the similar model.4) The elastic stiffness and ultimate bearing capacity of the planar intersecting connection is analyzed by mechanical method, the failure cross-section for the connection is assumed. A series test of 12 CFST stub column with a gusset plate under axial compressive loading was conducted on the purpose of investigating the bearing capacity. The effect of different concrete strength, thickness of steel tube and gusset plate on the bearing capacity of the failure cross-section is analyzed by FEA and modified model. As a result, the formulate for calculating the bearing capacity of failure cross-section, planar intersecting connection and spatial intersecting connection is put forward and veried by test and FEA results.5) Based on the structural behavior of tubular structures, a method to estimate the section area of diagonal columns in diagrid structures with rectangular plan is deduced. As a result, the connection proposed in this paper has been applied to an ultra high-rise building in Guanghzou, the details and applied advantages are introduced. Excellent mechanical behavior, convienence in construction and satisfactory commercial results make the intersecting connection ideal for use in the diagrid structures.