RADIANT HEAT from flames leaping up from lower floors was the key factor that triggered last February's partial collapse of Madrid's Torre Windsor, its original structural designer claimed last week.
No-one was killed in the blaze which spread to 30 of the tower's 32 fl oors. An absence of fire stops was initially blamed for the fire's rapid spread (NCE 17 February).
But speaking exclusively to NCE in Madrid last week, original designer Pedro Jaun Blanco said the collapse was speeded by absence of fire protection.
'I believe the fire broke out through the windows very quickly', said Blanco, project team leader for Otep International.
'The low thermal mass of the light section steel perimeter columns caused them to heat up rapidly to around 600°C as the flames burned around them, ' said Blanco.
He onfi rmed that none of the structural steel in the original structure had been fire protected.
'There was very little use of steel as a structural material in Spain at the time, certainly not in high rise, ' he said. 'So there were no requirements for fi e protection in the building codes.' Without protection, enough columns softened and buckled for failure to occur. 'The side that failed fi st and worst was supporting the weight of the external escape stairway, ' Blanco explained (see diagram overleaf).
'There were major service openings alongside the central core. When the perimeter columns failed I believe the entire floor slab on this side ripped away from the central core, toppling the two columns supporting it in the middle and triggering progressive collapse.' The fire is believed to have started on the 21st floor of the 32-storey building while the structure was undergoing refurbishment. It burnt for 19 hours, spreading as far down as the second floor, with the first façade collapsing more than two hours after the first fire reports.
Total progressive collapse was resisted by the massive transfer structure at the 17th floor level. Three sections of the upper floors collapsed in total, leaving the core and the tower crane used in the refurbishment still standing.
The building was midway through a three year refurbishment during which fire protection was being installed.
The first major step was to overclad the entire structure with a new aluminium curtain walling system, leaving the original in situ. Tenants vacated sections of floors on a piecemeal basis, decanting staff into other areas in the building.
As areas became available the original cladding was removed and exposed steelwork both internally and externally protected by a combination of boarding and spraying.
Fire doors were fixed and work began on the installation of a sprinkler system.
These operations proceeded from the lower floors upwards.
By the time the fire broke out on 11 February virtually all the existing steelwork below the 17th floor had been protected.
Above this level some of the internal steelwork had received protection, notably the new steel frame installed above the original 27th floor and around the region where a second external escape stair had been installed.
But the perimeter columns were almost all unprotected, and no fire doors had been fitted, Blanco said.
'And not all the gaps between the cladding and the floor slabs had been sealed with fireproof material, ' he added.
Vertical fire spread via cladding failure and radiant heat ignition of floors above the original source of fire was a feature of the comparable 1991 Basingstoke fire (NCE 25 April 1991).
Blanco said his main lesson from the Madrid fire was to 'use bigger steel sections, and surround them in concrete'.
A formal investigation by independent research body Intemac is to be released later this year.
Dave Parker in Madrid