ROBUSTNESS IN FIRE OF STEEL FRAMED STRUCTURES WITH REALISTIC CONNECTIONS

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Abstract

Joints are the most critical elements in a steel framed structure. In most design guides or codes, the joints are assumed to a have higher fire resistance than the connected structural members because of the lower temperatures in the joints. However, in severe fire conditions, a connected beam's temperature may be higher than its limiting temperature and the beam may develop catenary action when the beam’s axial shortening from large deflections becomes greater than the beam’s thermal expansion. This beam catenary action force could fracture the joints, increasing the risk of progressive collapse. This research focuses on the interaction between joints and the connected steel beams and columns in steel framed structures in fire, including how the behaviour of a joint-beam assembly may be efficiently analyzed and how the joints may be constructed to achieve high degrees of catenary action. Three methods of simulating the joint behaviour in fire have been developed and implemented in the commercial finite element software ABAQUS. In the first modelling method, all structural members, including the connections, were simulated using detailed solid elements to enable detailed behaviour of the structure to be faithfully represented. In the second method, the columns were represented by conventional line (beam) elements, the joints were represented using springs (Connector Elements) based on the component based method, and the beam was modelled using solid elements. In the third method, the joints were modelled using springs as in the second method and the beam and columns were simulated using line (beam) elements. As expected, the detailed simulation method was extremely time-consuming, but was able to produce detailed and accurate results. The simulation results from the second and third methods contained some inaccuracies, but depending on the simulation objective, their simulation results may be acceptable. In particular, the third simulation method was very efficient, suitable for simulating complete frame structures under very large deflections in fire. The first method (detailed finite element method) was then used to investigate how to change the joint details to increase the survivability of restrained steel beams and beam-column assemblies at high temperatures since it enables detailed behaviour of the structure to be faithfully represented. It is found that by improving joint deformation capacity, in particular, using extended endplate connection with fire resistant bolts, very high temperatures can be resisted. The frame robustness in fire was investigated using the third simulation method to save computation time. The simulation structure was three-bay by three-floor and different scenarios of fire location, fire spread and initial structural damage were considered. The simulation results show that once failure of a column occurs, progressive collapse of the structure could be easily triggered and it would be rather futile to only enhance the joint capacity. Therefore, in addition to the measures of improving joint capacities (both rotation and strength), design of the affected columns should include consideration of the additional catenary forces from the connected beams and the increased effective lengths. Furthermore, the lateral bracing system should be ensured to provide the structure with lateral restraint.

Keyword(s)

Joint, beam, column, frame, detailed finite element method, component based method, catenary action, limiting temperature, fire resistant bolt, fire resistant steel, survivability, robustness.

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