Under fire

The US-based National Institute of Standards and Technology (NIST) has completed a building and fire safety investigation into the factors that contributed to the collapse of the World Trade Centre. Its latest findings confound the belief that one of the main causes of the failure of the towers’ structural steel was the high temperature of the aviation fuel fires.

According to NIST, the fire load was created by the offices’ contents and not the aviation fuel, most of which was dissipated and vaporised in the impact. What happened, it seems, was that the momentum of the aircraft triggered the failure of the structural steel by dislodging the fireproofing. This was followed by the failure of the connections, instability in the external columns and progressive collapse.

Particularly damning for the frame is that, according to NIST, the design of the towers included a high degree of structural redundancy. In other words, this was a robust steel frame that failed to withstand a fire fuelled by office contents. American and European clients are now demanding that high-rise buildings be designed to survive complete burnout.

Initial investigations into the recent inferno at the Windsor Torre, one of Madrid’s most famous skyscrapers, again highlighted the performance of steel frames in a fire. With the Spanish building, failure was limited to the structural steel system whereas the concrete frame survived.

The fire started on the 21st floor of the 32-storey building and quickly spread, helped by the absence of fire stops between the curtain wall facade and the concrete floor slabs. A refurbishment of the traditionally designed and built 1970s tower was under way at the time. Part of the programme was to bring the building’s fire standards up to date with the installation of a range of active fire prevention and resistance measures.

Once the fire had taken hold, the steel perimeter columns, which had no fire protection, gave way, which resulted in the collapse of the floor slabs. The massive concrete transfer slab at the 20th floor prevented further progressive failure. However, as the debris fell, the cladding below was smashed and the fire spread to lower floors.

The extent of the blaze meant that firefighters could only mount a containment operation, and the fire burned for 26 hours. Preliminary investigations have found that, thanks to the concrete slab on the 20th floor and the inherent fire resistance of the central concrete columns and core, the building remained standing.

The Madrid fire comes at a time when the fire performance of reinforced concrete structures has been fully vindicated by the BRE in its report “Fire Safety of Concrete Structures: Background to BS 8110 Fire Design”. The report found that, in many cases, the fire resistance periods presumed for concrete were very conservative. The BRE report investigated the background to methods used for establishing the fire resistance of reinforced concrete structures specified to BS 8110. In particular, it examined and revisited the original research and test results that had been carried out over a number of years, which underpin the tabulated data in the code.

With the passing of time, there was a concern that much of the important work supporting the development of codes and standards could have been lost if it was not published. It was felt that there was a need to collate and assess all the relevant information to ensure that the important lessons from the past were recorded and used to help define the strategy for a new generation of codes and standards to suit modern concrete construction methods.

The research found that the experimental results used as data for developing the tabulated approach to BS 8110 fully supported provisions in the code relating to assumed periods of fire resistance. Furthermore, the research found that these provisions are very conservative in many cases as they are based on the assumption that structural elements are fully stressed at the fire limit state and take into account the spalling characteristics of concrete.

Apart from vindicating concrete’s fire resistance properties, the BRE report suggests that the conservatism of the existing data means further research would potentially result in even greater construction and cost economies for concrete structures.

BS 8110’s prescriptive approach of setting standards for individual building elements will continue to be popular despite increasing adoption of the methods used in the Eurocodes that assess the performance of a building as a whole, not just individual elements. Nonetheless, the research carried out by BRE is important because not only does it validate the fire resistance of concrete by highlighting the conservatism of the prescriptive approach, but also because it proves the relevance of the historic tabulated data for future high-rise and low-rise buildings

The concrete industry is not resting on its laurels. Research continues across Europe, America and Japan to develop bespoke concrete mixes able to withstand the most intense fires, for example those experienced in tunnels where the temperature can reach 1350°C. Design guidance is being developed for the new generation of Eurocodes dealing with fire and research continues to develop a better understanding of the robustness of concrete buildings in fire.

Anna Scothern is head of performance at The Concrete Centre