A Component-Based Approach to Modelling Beam-End Buckling Adjacent to Beam-Column Connections in Fire

摘要

The investigation of the collapse of “7 World Trade” as part of the events of 11 September 2001 in New York City (Gann, 2008) indicated that connections were among the most vulnerable elements of steel-framed or composite buildings, and their characteristics can determine whether such buildings survive in extreme scenarios such as fire. In this case total collapse of the building was triggered by the fracture of beam-to-column connections caused largely by thermal expansion of long-span beams. This emphasized the importance of investigating the complex mechanisms through which forces are transferred from the adjacent parts of a structure to the connections under fire conditions. udThe Cardington fire tests in 1995-96 (Newman, 2000) provided ample evidence that both shear buckling of beam webs and beam bottom-flange buckling, near to the ends of steel beams, are very prevalent under fire conditions. Both of these phenomena could affect the force distribution at the adjacent column-face connection bolt rows, and therefore the sequence of fracture of components. However, there is a distinct lack of practical research investigating the post-buckling behaviour of beams of Classes 1 and 2 sections adjacent to connections at elevated temperatures. udIn this PhD thesis, the development of analytical models of pure beam-web shear buckling and a combination of both beam-web shear buckling and bottom-flange buckling of beams of Classes 1 and 2 sections are reported. The analytical models are able to predict the post-buckling behaviour of the beam-end buckling panels in the vicinity of beam-column connections at elevated temperatures. A transition criterion, to distinguish between cases in which pure beam-web shear buckling occurs and those in which the instability is a combination of shear buckling and bottom-flange buckling, has been proposed, including a calculation procedure to detect the transition length between these two buckling modes. A component-based buckling element has been created and implemented in the three-dimensional structural fire analysis software Vulcan. The influence of the buckling elements on the bolt row force redistribution of the adjacent connections has been investigated in isolated beams and a simple two-span two-floor frame. It is expected that the buckling element will be involved in more complex performance-based frame analysis for design, and that it will be used with an explicit dynamic procedure to simulate local and progressive collapse of whole buildings.ud