Overview

Location:	Los Angeles, USA
Date	4 May 1988
Fire Event	Fire started at the 12th Floor and spread to four floors above. Fire duration: 3 hours 40 minutes
Damage:	Four fours were destroyed and one four was partially damaged.
Construction Type:	Structural steel frame with lightweight concrete slab on profiled steel deck.
Fire Resistance:	Sprayed fire protection to steelwork. Automatic sprinklers system not yet commencement.
Building Type:	262 m (62 storeys). Commercial.

The Building

The 262 m high, 62-story office tower was the tallest building built in Los Angeles, USA, in 1973. The tower was known as the UCB building until 1981, when United California Bank changed its name to First Interstate Bank. The tower was renamed 707 Wilshire Tower in 1996 and became Aon Center in 2003.

The tower has a structural steel frame with lightweight concrete slab on profiled steel deck. A typical floor measures 37.8 m by 56.1 m, providing about 1,626 m² of tenant area per floor around a central service core which contains the lift and staircase shafts. The external cladding system was made of glass and aluminium.

Fire Protection Systems

The fire protection system installed in the WTC towers at the time of terrorist attack was summarised as follows:

Fire Protection system	At time of Construction (1970s )	At Time of Fire (1988)
Fire compartmentation	Applied to the central service core areas only. No fire-rated walls used to separate the tenant places from each other.
Fire stopping between cladding & structure	×	×
Fire protection to steelwork	Spayed fire protection materials
Sprinkler system	Not required by building codes but had sprinkler protection in the basement, garage and underground pedestrian tunnel	Automatic sprinkler system was being installed in the building. 90% of the work had been completed but not yet activated
Fire alarm system	√	√
Riser system	A single zone combination standpipe system with four risers, one in each stairway for each floor, supplied by two water pumps

The Fire

The source of fire was believed to be electrical in an open-plan office area on the 12th floor. However, the precise source of ignition was not determined.

The fire was believed to start before 22:25 at the 12th floor. However, the ignoring of the alarms of smoke detectors delayed the calling of fire brigade and fire fighting. The fire quickly extended over the entire 12th floor except for the passenger lift lobby in the central core and spread upwards until the 16th floor through the gaps between the external cladding and the floors.

The fire was finally put out by fire brigade at 02:19. The estimated fire spreading rate was 45 minutes per floor and burned intensely for approximately 90 minutes on each level. Two floors were heavily involved in fire at any point during the fire.

It was found that smoke and heat travelled vertically above the 12th floor through the floor openings, lift shafts and even the pressurised stairwell. A minor fire occurred in a storeroom on the 27th floor, ignited by fire products escaping from an HVAC shaft that originated on the 12th floor. This fire self-extinguished due to oxygen deficiency.

The estimated timeline for the fire development and fighting is as follows:

Time	Damage and Fire
22:22	Two fire pumps were shut down by sprinkler contractor and the combination standpipe system was drained down to the 58th floor level to facilitate connecting the new sprinkler system to the standpipe at that level.
22:25	Glass failing was heard and light smoke was seen at the ceiling level by the sprinkler contractor. An alarm on the 5th floor was pulled and after sounding for 2 seconds went dead.
22:30	A smoke detector on the 12th floor was activated but reset by security personnel
22:32	3 smoke detectors on the 12th floor were activated and again reset by security personnel
22:34	4 smoke detectors on the 12th floor were activated and reset
22:36	Multiple smoke detector alarms from the 12th to 30th floors activated. A maintenance worker took the service elevator to the 12th floor to investigate the source of the alarms. The worker died when the lift door opened onto a burning lobby on the 12th floor.
22:37	Fire brigade was contacted by persons outside the tower, reporting a fire on the upper floors
22:40	Fire brigade arrived and found that the entire east side and there-fourths of the south side of the 12th floor fully involved with fire
22:41	First report of the fire from inside the building
22:49	Fire had spread upwards to the 13th and 14th floors
23:10	Fire brigade started to fight the fire
01:30	The 15th floor was fully engulfed in fire and the north end of the 16th floor started to burn due to the fire through the space between the external cladding and the floor
02:19	The fire was officially declared as put out

Damage

The total burnout of four and a half floors did not cause damage to the main structural members due to a good application of spayed fire protection on all steelwork. There was only minor damage to one secondary beam and a small number of floor decks.

The non-structural damages included:

• Virtually all external cladding from the 12th to 16th floors was destroyed and fell to the ground.
• The heat of the fire caused some aluminium alloy valves in the occupant hose cabinets to fail, creating water leaks and causing water damage on floors below the fire.

The property loss was estimated at over $200 million, excluding the business interruption loss.

Analysis

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

• the lack of effective fire fighting measures, such as automotive sprinklers
• the delayed reporting of the fire
• the open-plan floors with a floor area of over 1600m²
• the failure of vertical compartmentation measures, in the façade system and the floor openings

The open-plan floors with large quantity of combustible office contents without any internal fire barriers contributed to quick fire growth within a fire floor. In addition, the gaps between the external cladding and the floors were not firestopped and the fire could easily spread to floors above. Without the effective fire fighting on the 16^th floor by the fire brigade, the fire could have spread to all floors above.

In fact, minor fire spreads also occurred through the floor service openings for electricity and communications. This highlights the importance of applying effective fire stopping system to all floor and wall openings to ensure the effectiveness of fire compartmentation.

It was also shown that if fire protection to structural members is adequately designed and applied with quality control, fire damage to fire exposed members will be minimised and structural collapse can be prevented.

Sources of Information

• Los Angeles Fire Department / Historical Archive – Online
• Federal Emergency Management Agency. (1988). Interstate Bank Building Fire – Los Angeles, California (May 4, 1988). United States Fire Administration Technical Report Series.