Major fires in modern high rise office buildings are, thankfully, a rare occurrence. What happened on 12/13 February 2005 was even rarer - the 26 year old, 28 storey Torre Windsor was in the process of a major renovation when it caught fire.
Part of the hybrid steel/ concrete structure was close to being brought up to the latest standards in fire protection, and the rest was still in its original 1970s condition. During the blaze some 22 floors burnt out over 24 hours, at least three partial progressive collapses occurred and a total progressive collapse was narrowly averted (NCE 17 February 2005).
Independent research body Intemac, which produced the official report into the fire, carried out a detailed survey of the remaining structure and debris pile, just before it was demolished. Its report is a unique record of how concrete structures, in particular, react to prolonged fires. It promises to have a significant influence on the way future buildings are detailed and constructed.
'Scrupulous upkeep of the systems that limit the propagation of a fire between fire compartments is absolutely imperative, ' Intemac concludes.
The Torre Windsor was seriously deficient in terms of fire doors and fire stopping, and there was no functioning sprinkler system. Holes cut in floor slabs for service ducts meant blazing debris could fall from floor to floor inside the building, igniting new fires as it went. Like New York's Twin Towers, the Torre Windsor had to endure fires of equal intensity on a number of floors at the same time.
Cladding failure worsened the situation, allowing flames to billow up outside the windows of storeys above and ignite new fires via radiation. Fires on higher floors were fed by hot air from burning floors below, increasing temperatures and duration.
Intemac is clear that the partial progressive collapses above the 17th storey were caused by the buckling and failure of steel perimeter columns, which lacked fire protection. It also records that even on the 9th floor a significant number of perimeter columns were unprotected due to the ad hoc nature of the refurbishment programme. These had buckled, but, thanks presumably to the alternative load paths offered by the protected columns at that level, no collapse had occurred.
In fact Intemac concludes: 'Given the performance of these [steel] members on the storeys that had been fireproofed, it is highly plausible, although it can obviously not be asserted with absolute certainty, that if the fire had broken out after the structure on the upper storeys had been fireproofed, they would not have collapsed and the action would have wreaked substantially less damage.' Others might comment that if the retrofit of the sprinklers had been completed on time, the fire would probably never have proceeded beyond its source - or if fire stops and fire doors had been installed the fire would never have progressed down from the 17th floor.
Intemac's investigations into the damage to the concrete frame - which comprised the central core, internal columns and floors with clay pan void formers - produced mixed results. Residual strengths as determined by ultrasonic pulsed velocity and core tests were generally reassuring, but there was significant damage to the floors. In places reinforcement was exposed by spalling cover concrete, and some fractured shear links were discovered.
In a few areas, damage to the concrete extended to a depth of more than 200mm. Intemac was also particularly concerned about the performance of the internal composite floor beams, where differential cooling between the steel beam and its concrete surround had caused dangerous deformations. The steel connections between the steel beams and concrete columns had also suffered due to poor or absent detailing of fire protection.
Some might question Intemac's conclusion that the concrete structure 'performed extraordinarily well', given the building's subsequent demolition.
In reality, however, all the interest groups can take some comfort from the events in Madrid.
There are three main obvious lessons from this disaster.
Effective compartmentalisation is vital - and this includes the cladding and glazing. Sprinklers are equally essential. And the use of strong transfer fl oors at intervals will reduce the risk of major progressive collapse.