Development of a ductile connection to improve structural robustness in fire

Connection failures which occurred in the Cardington full-scale fire tests and in the collapse of the World Trade Centre indicate that connections are the most vulnerable parts of the structure in fire. Failure of connections can lead to a series of consequences, including the detachment of a connected beam from an adjacent column, the collapse of floors, the spread of fire into other compartments, the buckling of the column, and even the final progressive collapse of the entire building. Therefore, connections play a key role in maintaining structural integrity and stability under exposure to fire. However, conventional connection types lack the ductility to accommodate either the thermal expansion of beams during initial heating by a fire, or the tensile deformation generated by the catenary action of beams at high temperatures.
In order to prevent connection failures and improve structural robustness in fire, a novel connection with high ductility has been proposed in this research project. This novel ductile connection consists of two identical parts, each of which takes the form of a fin-plate, a face-plate and a semi-cylindrical section between these two parts. The latter can provide additional deformability by allowing the fin-plate to move towards and away from the face-plate. Equations have been proposed to quantify the ductility demand of both bare-steel and composite beams under fire conditions, which can be used as an indicator to determine the radius of the semi-cylindrical section of the ductile connection.
The analytical models of the semi-cylindrical section and the face-plate parts have been developed based on simple plastic theory. Experiments and Abaqus simulations have been carried out at both ambient and elevated temperatures to validate the analytical models of the semi-cylindrical section. An initial component-based ductile connection model has been proposed. Analytical models of the FPSC (face-plate-semi-cylindrical) component have been built, in which the face-plate part and the semi-cylindrical section are considered to deform as a whole. A second component-based model has been proposed based on the analytical models of the FPSC component. The two component-based models have been compared and validated against both experiments and Abaqus simulations. Compared with the first model, results from the second component-based model are more consistent with Abaqus simulation results. The component-based model of the composite ductile connection has been established by adding a reinforcement component to the bare-steel connection model, which can consider the pull-out of reinforcing bars the anchorage from the weld points in the mesh.
The component-based models of the ductile connection have been converted into connection elements following the principles of finite element method, and incorporated into the software Vulcan. Single beam models and 2-D bare-steel and composite sub-frame models with ductile connections have been created using both Vulcan and Abaqus to check the performance of the ductile connection elements. The 2-D bare-steel sub-frame models have also been used to compare the performance of the ductile connection with that of other connection types. Comparative results show that, the axial force generated in the beam with ductile connections is significantly reduced compared with those of the beams with other types of connections, indicating that the proposed ductile connection can provide excellent ductility to accommodate the axial deformation of beams in fire. Parametric studies have been carried out on several key parameters, including the connection thickness, inner radius of the semi-cylindrical section, connection temperature, vertical bolt spacings and connection material. The progressive collapse of a three-storey three-bay plane frame with ductile connections has been modelled using the static-dynamic solver in Vulcan.
Parametric studies have also been conducted to test the effects of connection thickness, inner radius of the semi-cylindrical section, and the number of longitudinal reinforcing bars within the effective width of slab, on the performance of the composite ductile connection using the 2-D composite sub-frame models. In order to consider the influence of out-of-plane structure on the composite connection behaviour, 3-D composite frame models have been built to compare the performance of the ductile connection with other connection types within composite structures. Finally, the effects of shear stud spacings and unconnected length between slab and beam at the beam end on the performance of the composite ductile connection have also been investigated.