Susan Lamont

Behavior of Structures in Fire and Real Design - A Case Study

A great deal of work on the behavior of composite steel-concrete structures in fire has been developed since the Cardington frame fire tests (UK) conducted in the 1990s. This has now been broadened so that the design of structures to resist fire has a real engineering basis and is not reliant on results from single element testing in the standard furnace. Several projects involving office buildings in the UK and abroad have highlighted the need for developing the understanding of whole frame behavior in fire. Since the collapse of the World Trade Center in New York City in 2001 (9/11), robust engineering solutions incorporating the response of a building to fire are in great demand. The basics of structural mechanics at high temperatures can be used in such designs to understand the fire response of many structures with the aid of computer modeling. This article provides a direct comparison between the structural response of an eleven-story office building in the city of London, when designed in a prescriptive manner with applied fire protection on all load bearing steelwork, and the response of the same structure designed using a performance-based approach leaving the majority of secondary steelwork unprotected. The intent is to demonstrate that structural stability during a fire can be maintained in specific cases without relying on passive fire protection. This study contributes to the field of structural fire engineering by extending the research work previously conducted by the author to a real design case and addresses the issues raised by approving authorities, insurers, and the client when a fire engineered approach is used to calculate structural response to fire. It also demonstrates the use of advanced analysis to understand beam-core connection

Possible ‘panel instability’ in composite deck floor systems under fire

Abstract This paper describes a possible ‘panel instability’ in composite deck floor slabs under fire conditions. The phenomenon is characterised by a rapid increase in deflection of the whole composite floor system (steel beams and concrete deck) forming part of a continuous floor in a compartment fire. This is accompanied by a sudden loss in the thermally induced axial compression in a primary steel beam. The instability occurs when the moment capacity at the end of the composite section (comprising the profiled deck slab and the primary beam) is achieved, and the connection at the column changes from moment resisting (because of the composite action) to pinned. The end moment is the sum of, moment due to imposed load (constant); moment due to increasing gradient over the composite (increasing with temperature); and moment from the increased loading caused by expanding and bowing secondary beams pushing down on the primary beam (increasing with temperature). The moment resisting connections at the ends of the primary beam prevent secondary beam(s) from reaching their natural thermally displaced shape (determined by the temperature distribution in the composite secondary beam–slab section). When the moment capacity at the composite primary beam–column connection is reached, the secondary beams are instantly free to deflect. To the authors’ knowledge, this phenomenon has not been reported in other previous studies. Also the authors’ discovery of this phenomenon is entirely based on computational modelling without any experimental evidence, and much further work is required to ascertain that it would indeed occur under normal fires, loading and restraint conditions.

Modelling of the collapse of large multi-storey steel frame structures in fire

Publisher Summary The chapter presents results from a number of non-linear finite element analyses of simple two-dimensional (2D) models of multi-storey structural frames subjected to fire. One of the analyses was carried out for a large range of fire scenarios. The chapter advances that work to include a number of different structural configurations and presents a few more collapse mechanisms. The results of this work are based on very simple 2D analyses, and further computational and experimental research should be carried out to confirm these initial findings, which nonetheless show a clear failure mechanism in multiple floor fires. The chapter presents a selection of results obtained from the simpler 2D models and describes the most interesting failure mechanism. The understanding developed in this manner is of enormous benefit to designers and consulting engineers to understand the true effect of fire in their structures and to ensure that the mechanisms discovered are prevented from occurring with appropriate design and detailing.