# Nie, Qin, and Cai 2008

Fourteen cruciform connection specimens were tested. The strength, deformation, and energy dissipation capacity of these composite connections were analyzed. Interior diaphragms, exterior diaphragms, and anchored studs were tested. Based on the experimental results, 3-D nonlinear finite element models were established to analyze the mechanical propertied of the three types of connection using ANSYS. Analyses were conducted under both monotonic and cyclic loading. Parametric analyses were conducted on the connections with exterior diaphragm under monotonic loading to investigate the influences of axial load ratio, width to thickness ratio, and dimensions of exterior diaphragms on connection behavior.

## Analytical Study

Five kinds of element were adopted in the finite element models: Solid65 element was defined by eight nodes having three translational degrees of freedom at each node; this element was used to model the in-filled concrete and the slab of concrete. Solid45 element was defined by eight nodes having three translational degrees of freedom at each node and was used to model the rollers and supports. Shell181 element was a four node element with six degrees of freedom at each node and was used to model the steel tubes and beams. Link8 was a 3-D spar element and uniaxial tension-compression element with three translational degrees of freedom at each node; this element was used to model the longitudinal rebars in the concrete slabs. Beam4 element was a uniaxial element with tension, compression, torsion, and bending capabilities with six degrees of freedom at each node. Beam4 was used to model the studs anchored in the columns and the studs between the steel beams and the concrete slab. Parametric analyses were conducted based on the numerical model in order to investigate the effects of parameters on the behavior of the connections. The axial load ratio, width to thickness ratio, and dimensions of the exterior diaphragms were varied. Analysis found that the strength and stiffness of connections are not significantly influenced by the axial load ratio and the dimensions of exterior diaphragms under shear failure mode for the connections with exterior diaphragms, while they are significantly influenced by the tube thickness.

## Experimental Study, Results, and Discussions

Three connection specimens with interior diaphragms, seven connection specimens with exterior diaphragms, and four connection specimens with anchored studs were tested under cyclic loading. Beams were welded from steel plates. Vertical stiffeners were welded to the beams in order to prevent torsion of these beams during tests. Studs were welded to the four steel plates first and then the four pieces of steel plates were welded together to form a square tube.

During testing, an axial load was applied at the top of the column through an actuator. Four hydraulic actuators were used to supply the cyclic loading. Testing found that the strength of connections with interior diaphragms was adequate but ductility was low. Deformation capacity of connections with anchored studs was good but their strength was low. Connections with exterior diaphragms had sufficient strength, good ductility, and good energy dissipation capacity; exterior diaphragms are well suited for applications in moment resisting framed in seismic regions. For the connections with exterior diaphragms, while their strength was insignificantly influenced by the axial load ratio and the dimensions of the exterior diaphragms under a shear failure mode, the deformation capacity and energy dissipation capacity were more obviously influenced by these two parameters. With an increase of the axial load ratio or weakening of the exterior diaphragms, the ultimate displacements and the cumulative dissipated energies of the specimens decreased. The strength and stiffness of the connections with a middle interior diaphragm are higher than that of the connections without middle interior diaphragms. Therefore, the contribution of the middle interior diaphragms should be taken into account in the strength and stiffness analysis of the connections.

## References

- Nie, J., Qin, K., and Cai C.S. (2008). "Seismic behavior of connections composed of CFSSTCs and steel-concrete composite beams - finite element analysis," Journal of Constructional Steel Research, 64 (6), pp. 680-688. doi:10.1016/j.jcsr.2007.12.003

- Nie, J., Qin, K., and Cai C.S. (2008). "Seismic behavior of connections composed of CFSSTCs and steel-concrete composite beams - experimental study," Journal of Constructional Steel Research, 64 (10), pp. 1178-1191. doi:10.1016/j.jcsr.2007.12.004