AN ABSENCE of fire stops between the curtain wall façade and concrete floor slabs has emerged as a critical factor in the rapid spread of fire that devastated Madrid's Torre Windsor tower block last month.
The 32 storey concrete tower in the Spanish capital's central business district remained in danger of collapse as NCEI went to press, following the blaze that took 19 hours to extinguish. City officials have confirmed that the building is beyond repair and will have to be pulled down.
The re broke out late on Saturday 12 February at 10pm on the 21st fl or, but within two hours had spread to all floors above floor 17, where a massive concrete floor slab is designed to isolate fires.
At 1.15am on Sunday morning the north facade fell off down to floor 17, with the east, west and part of the south facades following by 5pm. It is believed the isolation slab prevented a progressive collapse.
Arup Fire Madrid director George Faller witnessed the blaze: 'I was driving home from the theatre at about 1am when I saw a big fireball in the sky. Debris was flying off the building.
'The rst part to collapse was the north facade. Steel columns supporting the wall would have expanded and buckled under the heat. Huge fl ames engulfed it all, ' he said.
Faller believes that this rapid spread of fire was almost certainly caused by the absence of fire stops.
'The question is how did it spread, and this edge detail needs careful consideration, ' he said.
'It is typical that the floor slab does not touch the curtain wall.
So as the curtain wall gets hot it bulges out, increasing the size of the gap and allowing the fire to spread between floors, ' he said.
Faller added that there should be a connection between the fl oor slab and the facade, and an incombustible fire stop material used to plug the gap. 'In Madrid the architect is responsible for the fire engineering and I have yet to come across an architect who has been aware of how vital this connection is in terms of fire protection, ' he added.
Other local engineers said that the lack of fi e walls between the central concrete core of the building and the floors contributed to the spread of the blaze.
The building, designed in the 1970s, is not believed to have a sprinkler system (see box).
Instead it was fitted with smoke and heat detectors and was built with two 'isolating floors' made of thicker concrete to contain fires. The first lies between the 2nd and 3rd fl oor, the second between the 16th and 17th.
The upper isolating floor contained the fi re until 5.30am when several outer concrete columns and a section of floor slab collapsed. It is believed that this collapse encouraged the fire to spread below floor 17.
The fire eventually spread to all fl oors above the lower isolating fl oor and was not fully extinguished until 5pm on Saturday evening, 17 hours after the blaze began.
Because of the danger of collapse, engineers have not yet been allowed access to the building. A team from the city authorities was expected to make an inspection on Wednesday.
The cause of the fire has yet to be determined, although investigators will focus on three possibilities, the Madrid emergency response team Portavoz Emergencia Madrid said this week.
The building was undergoing refurbishment at the time of the fire.
'The first possibility is that there was an electrical problem on the 21st floor, ' said team member César Gomez. 'The second is that someone was negligent, for example a cigarette in a litter bin.
'The third is that there was a problem with the refurbishment work, ' he said.
Bernadette Redfern in Madrid