Key events in the structural response of a composite steel frame structure in fire

摘要

The structural design of composite steel frame buildings in fire has traditionally been based on single element behaviour, under standard fire conditions. This approach is widely recognized as unsatisfactory and there is a general consensus that it leads to excessive fire protection being applied to the steel members based on grossly inaccurate assumptions about whole structure behaviour in fire. A great deal of research has been carried out to understand the mechanics of whole structure behaviour in fire. This paper presents a summary of this research by computational analysis of a small but realistic steel-frame composite structure with mainly unprotected beams and fully fire-protected columns. The main purpose of this paper is to highlight a number of key events that define the response of the steel frame structure in fire. Some of these events can be observed in real fire tests (such as Cardington), however, others have only been discovered by careful analysis of the output data from computational analyses supported by fundamental theoretical analysis. For instance, an event has been discovered entirely from computational modelling of the above-mentioned small structure with no similar events observed in experiments or reported in the literature. This particular event involves a sudden and sharp increase in deflection of the whole floor at particular steel temperatures during the fire. If this event occurred in a real structure, it might cause an overload of the floor because of the dynamic effect of this instability (unaccounted for in the static analysis performed here) leading to compartment breach or collapse. All these events will be presented in order of appearance as the compartment fire progresses and the member temperatures increase. The reason for the occurrence of each event will be discussed supported by simple analysis of the relevant structural mechanics and brief discussion of design implications.