Abstract
The full-scale fire tests at Cardington in the 1990s, and the collapse of at least one of the WTC buildings in 2001, illustrated that connections are potentially the most vulnerable parts of a structure in fire. Fracture of connections causes structural discontinuities and reduces the robustness provided by alternative load paths. An understanding of connection performance is essential to the assessment of structural robustness, and so to structural design against progressive collapse. The forces and deformations to which connectionscan be subjected during a fire differ significantly from those assumed in general design. The internal forces i generally start with moment and shear at ambient temperature, then superposing compression in the initial stages of a fire, which finally changes to catenary tension at high temperatures. If a connection does not have sufficient resistance or ductility to accommodate simultaneous large rotations and normal forces, then connections may fracture, leading to extensive damage or progressive collapse of the structure. Practical assessment of the robustness of steel connections in fire will inevitably rely largely on numerical modelling, but this is unlikely to include general-purpose finite element modelling, because of the complexity of such models. The most promising alternative is the component method, a practical approach which can be included within global three-dimensional frame analysis. The connection is represented by an assembly of individual components with known mechanical properties. Component characterization must include high-deflection elevated-temperature behaviour, and represent it up to fracture.In reality a connection may either be able to regain its stability after the initial fracture of one (or a few) components, or the first failure may trigger a cascade of failures of other components, leading to complete detachment of the supported member. Numerical modelling must be capable of predicting the sequence of failures of components, rather than considering the first loss of stability as signifying building failure. It is necessary to use a dynamic analysis, so that loss of stability and re-stabilization can be tracked, includingthe movements of disengaging members and the loadsharing mechanisms which maintain integrity and stability within the remaining structure, until total collapse occurs.
Highlights
Structural steel connections have been extensively investigated over the past three decades to determine their moment-rotation characteristics
Thermal expansion of very long steel secondary beams caused the connections of the supporting primary beams to columns to be pushed-off, initiating progressive collapse of the building
In the early stages of heating this will prevent large forces from being transmitted to the surrounding structure. At high temperatures it will allow sufficient movement of the beam ends to reduce the catenary tension in the beam to a level where no fracture occurs at the highest beam temperature
Summary
Structural steel connections have been extensively investigated over the past three decades to determine their moment-rotation characteristics. Even if the connections survive the heating phase of a fire, when beams contract from their distorted state during cooling this imposes higher tensile forces on connections, and many recorded connection failures observed in full-scale testing have occurred during this phase These tying forces, either at high temperature or during cooling, together with the local forces on components of the connections imposed by very high rotations, can clearly fracture components, triggering disproportionate collapse of the structure. This was clearly seen in partial connection failures at Cardington. Thermal expansion of very long steel secondary beams caused the connections of the supporting primary beams to columns to be pushed-off, initiating progressive collapse of the building
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