ABSTRACTSYSTEM LEVEL TRANSIENT FIRE RESPONSE OF DOUBLE ANGLE CONNECTIONS IN STEEL FRAMED STRUCTURESByPurushotham PakalaSteel framed structural systems are frequently used in high-rise buildings due to high structural performance steel provides as compared to other construction materials. In these steel framed structures, connections play a significant role in transferring forces from one member to another member and influence the overall stability of the structural system at ambient and fire conditions. Among different connection configurations available, double angle connections possess superior tying resistance and rotational capacity as compared to other relatively brittle connection types. Performance of connections under fire conditions is much more crucial to the stability of structural system as they experience rapid degradation in load carrying capacity and stiffness, due to deterioration of strength and elastic modulus of steel with temperature. The current approaches for evaluating fire resistance of connections have many limitations and do not take into consideration critical factors governing fire response. For example, connections experience significant fire induced axial restraint forces when exposed to fire and these forces are influenced by factors such as decay phase of fire, restraint to free thermal expansion imposed by the unheated structural elements adjacent to the connection. However, current design provisions do not consider the effect of fire induced restraint forces. Further, current provisions are based on results from scaled/isolated connection tests. However, the response of double angle connection at system-level is completely different from that of scaled/isolated connection behavior.The system-level fire response of double angle connections is simulated by developing finite element models using ANSYS. The finite element models account for material and geometrical nonlinearities, degradation of constitutive material properties with temperature and complex non-linear contact interactions, that have significant influence on the fire response of double angle connections. The validated models are used to carry out parametric studies to quantify the influence of critical parameters. Results from the parametric study show that fire performance of double angle connections is affected by increased load level and fire characteristics, while fire resistance gets enhanced when the presence of concrete slab in the structural frame is taken into account.To study the influence of system level interactions on the behavior of double angle connections and to validate the finite element models, fire resistances tests on two double angle connection assemblies were conducted. The test variables included load level, presence of slab, structural continuity and fire scenario. Results from fire tests show that double angle connections are highly ductile and have inherent rigidity to carry higher fire induced axial forces for which they are not typically designed for. Results from the fire tests, data from parametric studies were utilized to develop a rational methodology for evaluating the fire resistance of connections. The proposed methodology, developed based on equilibrium principles, uses temperature dependent moment-curvature-axial force (M-κ-P) relationships to trace the response of double angle connection assemblies. The proposed methodology accounts for critical factors such as fire induced axial forces, design fire scenarios, material and geometric nonlinearities.
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