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BB-003 fire-weakened steel

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

fire-weakened-steelloss-of-passive-fire-protectionconcrete-core-steel-perimeter-towerhigh-rise-building2000s
Death toll
0 dead (7 firefighters injured)
Structure
Windsor Tower, 32-storey concrete-and-steel high-rise, Madrid
Failed
12 February 2005
Status
Partial collapse

Summary

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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Timeline

1974–1979
Designed and built in Azca
A team of six Spanish architects led by Genaro Alas Rodríguez and Pedro Casariego completes the Windsor Tower, a 106-metre, 32-storey high-rise (29 floors above ground, three below) in Madrid's Azca business district.
1979
A hybrid concrete-and-steel structure
The frame is a reinforced-concrete core and internal RC columns carrying two-way waffle slabs, with the floor edges supported by slender steel perimeter mullion columns and internal steel I-beams — the steel left without fire protection.
1979–2002
A bare-steel perimeter operated for two decades
The unprotected mullions and internal steelwork carry the outer floor loads throughout the building's life, while the tower has no sprinkler system and limited fire compartmentation between floors.
~2002–2005
A refurbishment recognizes the hazard
A multi-year upgrade begins: board fire protection for the perimeter mullions, spray protection for the internal beams, a sprinkler system, and new aluminium cladding — an explicit acknowledgement that the bare steel was a known deficiency.
By Feb 2005
Protection installed below the strong floor — but not above
Fire protection is in place on the mullions on all levels below the 17th floor except the 9th; the storeys above the 17th remain bare steel. The sprinkler system is not yet operable, and fire stops have been removed during the works.
12 Feb 2005, ~23:00
Fire ignites on the 21st floor
A fire, attributed to an electrical fault (or a discarded cigarette), starts on the 21st floor late on 12 February, on a storey of bare perimeter steel above the protection line.
12 Feb 2005, overnight
Fire spreads vertically, unchecked
With no working sprinklers and fire stops removed, the fire climbs and spreads across multiple floors simultaneously through the façade cavity and open compartments, heating the bare upper steel.
13 Feb 2005, early hours
Upper perimeter steel begins to fail
Sustained fire above the 17th floor drives the surface temperature of steel and concrete past critical levels; the unprotected mullions soften, buckle from thermal restraint, and lose their capacity to carry the floor edges.
13 Feb 2005, ~03:00–06:00
Top storeys at one end shear off and fall
The top roughly ten storeys at one end of the building collapse onto the 17th-floor strong floor, which arrests the debris; much of the perimeter above the 17th progressively comes down.
13 Feb 2005, daytime
Fire burns ~24 hours; core holds
The blaze takes about a day to extinguish. The reinforced-concrete core, internal columns and the floors below the 17th survive; seven firefighters are injured, with no deaths.
2005
Technical investigation and demolition
Spanish investigators (Intemac and associated bodies) analyze the failure; the gutted, partially collapsed shell is demolished in the months following the fire.
2006–2011
Code reform and replacement
Spain's Código Técnico de la Edificación (CTE), approved in March 2006, tightens fire-safety requirements for tall buildings; Torre Titania later rises on the cleared site.

The Build: A Concrete Frame Hung on Unprotected Steel Edges

The Windsor Tower was a product of the late 1970s, completed in 1979 to a design by a team of six Spanish architects in the new Azca financial district — a 106-metre tower of 32 storeys, 29 of them above ground, faced in reflective glass. Structurally it was a hybrid. Its spine was a stiff reinforced-concrete core, supplemented by internal reinforced-concrete columns, and its floors were two-way reinforced-concrete waffle slabs. Concrete, by its nature, carries its own fire resistance: it is slow to heat and slow to lose strength, and the core would later prove that by surviving the fire almost untouched.

The vulnerability lived at the edges. The outer perimeter of each floor was carried not on concrete but on slender steel mullion columns — closely spaced vertical members, on the order of sixty per floor — together with internal steel I-beams supplementing the slab. This steel was erected and operated bare. No sprayed coating, no board encasement, no rated cladding stood between the perimeter steelwork and a fire. For a building of concrete construction, the steel mullions were the one element with no inherent fire endurance, and they were precisely the element that held the floor plates' outer span.

There was also a special storey. The 17th floor was a deep, heavily framed technical or service level — a strong floor stiffer and stronger than the typical office storeys, functioning structurally as a transfer floor. Its presence would turn out to be decisive, but for reasons no designer had planned: it would become the tray that caught a collapsing upper building.

By the early 2000s the owners had recognized the perimeter steel for what it was — a latent hazard — and had begun a multi-year refurbishment to correct it: board fire protection to the mullions, spray protection to the internal beams, a sprinkler system, and new aluminium cladding. The fire arrived before the work was finished. The protection had been installed on the mullions on every level below the 17th floor except the 9th, leaving the storeys above the 17th still bare. The sprinklers were not yet operable, and the works had removed fire stops between floors. The building burned in a half-finished state that, by accident, split it cleanly into a protected lower zone and an unprotected upper one.

The Failure: A Collapse Drawn Along the Fireproofing Line

The fire started around midnight on the 21st floor — above the protection line — attributed to an electrical fault. With no working sprinklers to check it and fire stops removed, it spread fast and vertically, running across multiple floors at once and into the façade. For hours the bare upper steel sat in a fully developed compartment fire.

Steel does not melt in a building fire; it weakens. As the fire above the 17th floor sustained temperatures well above 500 degrees Celsius, the unprotected mullions heated through, lost yield strength, and buckled under the combination of gravity load and the thermal expansion they could not relieve — restrained steel pushes against its own supports as it grows, and then collapses as it softens. One by one the perimeter columns at the upper levels surrendered the floor edges they carried. With the outer support gone above the strong floor, the top storeys at one end of the tower — roughly ten of them — sheared off and fell.

What stopped them was the 17th floor. Acting as the transfer structure it was built to be, the strong floor caught the descending mass and arrested the collapse, preventing it from cascading down through the protected lower building. Above that line, much of the perimeter continued to come down; below it, the structure held. The contrast was the forensic evidence in plain sight: the fire-protected mullions on the lower floors did not fail, and even the single bare mullion level at the 9th, though it buckled, did not bring its floors down because the surrounding protected columns redistributed its load. Where steel was protected, the building had redundancy and survived. Where steel was bare, the redundancy evaporated as every member softened together, and the structure fell.

The Reckoning: A Full-Scale Experiment Nobody Meant to Run

The Windsor fire gave fire engineers something they almost never get: a single building that tested protected and unprotected steel side by side under one identical fire. The Spanish technical investigation, carried out by the materials and structures institute Intemac and studied closely by international fire engineers, read the collapse not as a mystery but as a confirmation.

The conclusion was direct. The collapse of the upper storeys would very probably not have happened had the perimeter fire protection been in place throughout the tower. The protected mullions provided alternate load paths and held; the unprotected mullions above the 17th did not, and once enough of them failed, the floors had nothing to hang their edges on. Analysis found that the critical surface temperature was reached to a depth of around 100 millimetres over more than half the floors and columns — but it was the bare steel, not the concrete, that translated that heat into collapse.

Two other failures completed the picture. The absence of effective compartmentation — worsened by fire stops removed during the refurbishment — let a single-floor ignition become a multi-storey fire that heated steel across many levels at once, overwhelming any capacity to redistribute load. And the loss of two internal concrete columns, degraded after roughly an hour and three-quarters of intense exposure, removed interior support and added to the upper failure. The Windsor was not destroyed by an extraordinary fire. It was destroyed by an ordinary fire finding an extraordinary gap: a load-bearing steel perimeter that the building's own owners had already judged unsafe and were halfway through protecting.

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Contributing Factors

01
Unprotected perimeter steel above the 17th floor
The mullion columns carrying the outer floor edges above the strong floor had no fire-resistive coating, board or encasement. Bare steel loses its strength in a developed fire, so sustained heating — not melting — softened and buckled the upper mullions and pulled down the storeys above. Protected mullions on the lower floors, by contrast, did not fail. Fireproofing the perimeter throughout would have changed the outcome.
02
Partial fireproofing created a failure plane
Because the refurbishment had protected the steel below the 17th floor but not above it, the building was effectively two structures: a protected lower tower and an unprotected upper one. The fire tore the building apart precisely along that line. A half-completed protection programme is not half-safe; it concentrates the entire vulnerability into the unprotected zone.
03
No sprinklers and broken compartmentation
The sprinkler system was not yet operable, and fire stops between floors had been removed during the works. A 21st-floor ignition therefore grew unchecked into a fully developed, multi-storey fire that heated bare steel across many levels simultaneously. Suppression controls a fire while small; compartmentation limits the area of heated structure. The absence of both let the fire reach and persist at the temperatures that defeat steel.
04
Loss of redundancy in the heated zone
Where the mullions were protected, surviving columns redistributed load even around a buckled member, as the bare 9th-floor level showed. Where they were bare, every perimeter column softened together, so there was no alternate load path to catch a local failure, and damage propagated into partial collapse. Redundancy survives fire only if the members that provide it survive fire.
05
A known, documented hazard left exposed during works
The owners had recognized the bare steel as a deficiency and begun correcting it — but the building continued in full occupancy through a refurbishment that removed fire stops, left the sprinklers inoperable, and left the upper steel unprotected. The risk was identified on paper before it was tested by fire; the missing element was sequencing the works and protecting the occupied building during them. ---

Aftermath

The Windsor Tower fire killed no one — a margin of luck owed to the late hour and an empty building — but injured seven firefighters and produced one of the most-studied partial collapses in fire-engineering history. The reinforced-concrete core and the protected lower structure survived a roughly 24-hour fire; the unprotected upper perimeter sheared off and fell to the 17th-floor strong floor, and the gutted shell was demolished within months. The case fed directly into Spain's regulatory reform: the Código Técnico de la Edificación (CTE), approved in March 2006, tightened fire-safety requirements for tall buildings, alongside stricter expectations for sprinklers, fire doors and the fire protection of structural materials in high-rises. Torre Titania later rose on the cleared site. In the engineering literature, Windsor became the byword for a single, unforgiving lesson made visible at full scale: structural steel must be protected over its entire height, because a fire will find the line where protection stopped and break the building along it.

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Lessons

  1. Protect structural steel along its entire height, not most of it — a fire will locate the boundary between protected and unprotected steel and shear the structure off precisely there.
  2. Treat a fire-protection refurbishment as a hazardous interim state, not a safe one: a building with removed fire stops, inoperable sprinklers and partially bare steel may be more dangerous mid-works than before they began.
  3. Never rely on redundancy that itself depends on bare steel — alternate load paths survive a fire only if the members carrying them are fireproofed; in a developed fire, unprotected members soften together and no redundancy remains.
  4. Compartmentation and suppression are what keep steel from ever reaching failure temperature — install and maintain both before a tall building is occupied, and especially while protection work is underway.
  5. When an owner has already identified bare steel as a deficiency, the identification is the warning — sequence and stage the remediation so the known hazard is not left exposed in an occupied tower. ---

References