Overview

Location:	Madrid, Spain
Fire Event:	12 February 2005 Fire started at the 21st Floor, spreading to all floors above the 2nd Floor. Fire duration: 18 ~ 20 hours
Fire Damage:	Extensive slab collapse above the 17th Floor. The building was totally destroyed by the fire.
Construction Type:	Reinforced concrete core with waffle slabs supported by internal RC columns and steel beams, with perimeter steel columns which were unprotected above the 17th Floor level at the time of the fire.
Fire Resistance:	Passive fire protection. No sprinklers.
Building Type:	106 m (32 storeys). Commercial.

The Building

The Windsor Tower or Torre Windsor (officially known as Edificio Windsor) was a 32-storey concrete building with a reinforced concrete central core. A typical floor was two-way spanning 280mm deep waffle slab supported by the concrete core, internal RC columns with additional 360mm deep steel I-beams and steel perimeter columns. Originally, the perimeter columns and internal steel beams were left unprotected in accordance with the Spanish building code at the time of construction

The building featured two heavily reinforced concrete transfer structures (technical floors) between the 2^nd and 3^rd Floors, and between the 16^th and 17^th Floors respectively. The original cladding system was fixed to the steel perimeter columns and the floor slabs. The perimeter columns were supported by the transfer structures at the 17^th and 3^rd Floor levels.

The building was subjected to a three year refurbishment programme of works when the fire broke out. The major works included the installations of:

• Fire protection to the perimeter steel columns using a boarding system
• Fire protection to the internal steel beams using a spray protection
• A sprinkler system
• A new aluminium cladding system

The refurbishment was carried out floor-by-floor from the lower floors upwards. By the time the fire broke out, the fire protection for all steelwork below the 17^th floor had been completed except a proportion of the 9^th and 15^th floors. However, not all the gaps between the cladding and the floor slabs had been sealed with fireproof material (Dave 2005). Also fire stopping to voids and fire doors to vertical shafts were not fully installed.

Fire Protection System

The Windsor Tower's original structural design complied with the Spanish building codes in 1970s. At the time of the construction, the Spanish codes did not require fire protection to steelwork and sprinkler fire protection for the building.

As a result, the original existing steelwork was left unprotected and no sprinkler system was installed in the building. The gap between the original cladding and floor slabs was not firestopped as well. In fact, these weak links in the fire protection of the building was being rectified in the refurbishment project at the time of the fire.

Since the building adopted the "open plan" floor concept, effectively, the fire compartmentation could only be floor-by-floor (about 40 x 25m). However, the vertical compartmentation might not be fully achieved due to the lack of firestop system in floor openings and between the original cladding and the floor slabs.

The original fire protection system and the upgrading works being carried out at the time of the fire are compared as follows (NILIM 2005):

Main Fire Protection System	At time of Construction (1970s Spainish Codes)	At Time of Fire (Refurbishment in Process)
Fire compartmentation	×	Under construction
Fire stopping between cladding & structure	×	Under construction
Fire protection to steelwork	×	17th floor & above: Not yet commencement (18th floor partly completed) 4th ~ 15th floor: Completed (except 9 & 15th floors)
Fire protection to concrete members	×	×
Sprinkler system	×	Under construction
Fire alarm system	√	√
Dry riser system	√	√

The Fire

The fire was believed to have been caused by a short-circuit on the 21^st floor. However, some facts under investigation point that it could be induced by arsonists. The actual cause will be difficult to be found due to the collapse of the break-out floor.

It was reported that the fire started at 23:00 at the 21^st Floor. Within one hour, all floors above the 21^st Floor were on fire. In the following hours, the fire gradually spread downwards to the lower technical floor at the 3^rd Floor. The total fire duration was estimated to be 18 ~ 20 hours.

Based on the various sources of information, Table 1 shows the estimated time frame for the fire development in the Windsor Tower.

Table 1 Estimated time frame of fire development (NILIM 2005)
Time	Fire Development	Cross Section
23:00	Fire started at the 21st Floor
23:05 ~ 23:20	After receiving a fire signal, the security guards went to the 21st floor and attempting to fight the fire before giving up
23:21	Fire brigade was called
23:25	Fire brigade arrived
23:30	Fire brigade started to fight the fire (news report)
00:00	All floors above the 21st floor were in fire (news report)
00:30	Fire brigade retreated and adopted a defensive position, preventing fire spread to adjacent buildings
02:00	Fire spread below the 17th floor
02:15	Chunks of facade started falling off (news report)
03:30	Fire spread below 16th floor, crossing over the upper technical floor
04:00	Floors at upper level collapsed (news report)
05:30	Fire spread below the 12th floor (news report)
08:30	Fire spread below the 4th floor
13:30	Fire was under controlled
17:00	Fire brigade declared the put out of the fire (news report)

The Damage

The Windsor Tower was completely gutted by the fire on 12 February 2005. A large portion of the floor slabs above the 17th Floor progressively collapsed during the fire when the unprotected steel perimeter columns on the upper levels buckled and collapsed (see Figure 1). It was believed that the massive transfer structure at the 17th Floor level resisted further collapse of the building.

The whole building was beyond repair and had to be demolished. The estimated property loss was €72m before the renovation.

Based on the footages of available media filming, Table 2 summarises the estimated time frame for the structural collapses of the Windsor Tower.

Figure 1 Collapse situation of the upper floors

Table 2 Estimated time frame of collapses (NILIM 2005)
Time	Collapse Situation
1:29	East face of the 21st floor collapsed
1:37	South middle section of several floors above the 21st floor gradually collapsed
1:50	Parts of floor slab with curtain walls collapsed
2:02	Parts of floor slab with curtain walls collapsed
2:11	Parts of floor slab with curtain walls collapsed
2:13	Floors above about 25th floor collapsed Large collapse of middle section at about 20th floor
2:17	Parts of floor slab with curtain walls collapsed
2:47	Southwest corner of 1 ~ 2 floors below about 20th floor collapsed
2:51	Southeast corner of about 18th ~ 20th floors collapsed
3:35	South middle section of about 17th ~ 20th floors collapsed Fire broke through the Upper Technical Floor
3:48	Fire flame spurted out below the Upper Technical Floor
4:17	Debris on the Upper Technical Floor fell down

Analysis

The main factors leading to the rapid fire growth and the fire spread to almost all floors included:

• the lack of effective fire fighting measures, such as automotive sprinklers
• the “open plan” floors with a floor area of 1000m²
• the failure of vertical compartmentation measures, in the façade system and the floor openings

It was believed that the multiple floor fire, along with the simultaneous buckling of the unprotected steel perimeter columns at several floors, triggered the collapse of the floor slabs above the 17^th floor. The reduced damage below the 17^th floor might provide a clue.

The fire protection on the existing steelworks below the 17^th floor had been completed at the time of fire except for the 9^th and 15^th floors. When the fire spread below the 17^th floor, those protected perimeter columns survived, except for the unprotected columns at the 9^th and 15^th floors which all buckled in the multiple floor fire (see Figure 2). However, they did not cause any structural collapse. Obviously, the applied loads supported by these buckled columns had been redistributed to the remaining reinforced concrete shear walls. Nevertheless, structural fire analysis should be carried out before such a conclusion can be drawn.

Figure 2 Buckling of unprotected steel perimeter columns at the 9th floor (Photo: Colin Bailey)

On the other hand, the reinforced concrete central core, columns, waffle slabs and transfer structures performed very well in such a severe fire. It is clear that the structural integrity and redundancy of the remaining parts of the building provided the overall stability of the building.

Sources of Information

• BBC News Online / World / Europe - UK Edition (13 Feb 2005)
• Dave, P. (2005). “Madrid tower designer blames missing fire protection for collapse”. New Civil Engineer, 2 June 2005.
• [NILIM 2005] National Institute for Land and Infrastructure Management (NILIM). (1 Jul 2005). “Report on the Windsor Building Fire in Madrid, Spain”, Japan (in Japanese).