The Windsor Tower, Madrid — Floors Above the Fireproofing Line Sheared Off and Fell

The Windsor Tower, a 32-storey office high-rise in the Azca financial district of Madrid, partially collapsed during a fire that burned for roughly a full day after igniting around midnight on 12 February 2005, and it did so along a line drawn precisely by its own fireproofing. No one was killed and seven firefighters were injured, but the building’s steel perimeter — the slender mullion columns that carried the outer floor edges — sheared away and fell wherever it had been left unprotected, while the very same columns held wherever fire protection had already been installed. The proximate cause was not the concrete frame, which survived, but bare steel above the 17th floor losing its strength in a sustained, uncompartmented fire.

This was a forensically rare event: a controlled natural experiment in fire protection, conducted at full scale by accident. The Windsor was caught mid-refurbishment, a three-year programme to add sprinklers, board-protect the perimeter steel and spray-protect the internal steel beams. By February 2005 that programme had fireproofed the mullions on every level below the 17th floor except the 9th — and none of those protected mullions failed. Above the 17th, where the steel was still bare, the upper storeys at one end of the tower buckled and pancaked down to the 17th-floor slab, and much of the perimeter above that level later came down with them.

The 17th floor was no ordinary storey. It was a deep, stiff technical floor that functioned as a transfer structure, and when the unprotected steel above it failed, that floor acted as a tray that caught the debris and arrested the collapse before it could run the full height of the building. The concrete core, the internal reinforced-concrete columns and the waffle-slab floors below the strong floor rode out the fire largely intact. The difference between the part of the building that survived and the part that fell was, almost exactly, the difference between protected and unprotected steel.

The Spanish technical investigation, with analysis later corroborated by international fire engineers, concluded that the collapse of the upper storeys would very likely not have occurred had the perimeter fire protection been in place throughout. The Windsor Tower is now the textbook demonstration that fireproofing of structural steel is not a finishing detail but the load path’s survival condition — and that a fire which finds bare steel above a protected line will tear the building apart at exactly that line.

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