Structural response

View from the south-west side after the blaze as shown above.

The steel perimeter columns, even if they had been protected, or even concrete columns, would not necessarily be expected to survive the effects of such a 10-storey blaze.

A multiple floor fire can result in lack of restraint of columns over a number of floors due to heating which could trigger a collapse mechanism. However structural fire analysis of this structural form is required before such a conclusion can be drawn.

What is clear however in the case of the Windsor building, is that the concrete perimeter slab could evidently not remain in the place without this support.

The reason the collapse of one internal bay on the North side supported by concrete columns is not immediately apparent but could be attributable to any of a number of factors: less concrete cover, more intense heat exposure, and the increased likelihood of concrete spalling this would create and/or momentum of adjacent bay collapse, amongst others.

The presence of a robust transition floor halfway up the tower is not a standard fire protection measure but undoubtedly played a role in limiting the extent of the collapse.

Using a fire engineered approach for the design of the structure, features such as the transition floor could be taken advantage of in the design. Similarly, weaknesses such as the lack of a tie between the curtain wall and the structure would be apparent and could be addressed at the design stage.

The reduced damage below the 17th Floor can be attributed to a function of the more robust structure below this level, the existence of fire protection measures and a less intense fire due to the more gradual fire spread.

The central concrete core appeared to perform well in the fire and on initial observations seems to have played a major role in ensuring the stability of the building throughout the incident. The role of cores in multiple floor fires is now an immediate area of study required for the industry, and Arup have commenced investigating this issue.

A thermo-mechanical assessment of this structural design, an understanding of why the structure performed as it did and why total collapse did not occur would provide valuable information for future structural fire analysis in design