Case Studies: Historical Fires: World Trade Center Collapse

Overview

Location:	New York, USA
Date	11 September 2001
Damage:	North Tower (WTC 1): A Boeing 767-200ER series aircraft hit between the 94th and 98th floors roughly at the centre of the north face at 08:46. South Tower (WTC 2): A Boeing 767-200ER series aircraft hit between the 78th and 84th floors toward the east side of the south face at 09:03. The plane crashes caused considerable damage to principal structural components and multiple floor fires above the impacted floors.
Construction Type:	WTC 1 collapsed at 10:29, 102 minutes after the crash. WTC 2 collapsed at 09:59, 56 minutes after the crash. Steel perimeter frame-tube system comprising external perimeter columns, central core columns and concrete slabs on steel bar trusses.
Fire Resistance:	Passive fire protection. Automatic sprinklers system.
Building Type:	Commercial. North Tower: 417 m (101 storeys) South Tower: 415 m (101 storeys)

The Building

The twin towers of the World Trade Center (WTC) were among the first skyscrapers built using the framed-tube concept to provide lateral resistance to wind loads.

The steel framed-tube system comprised four major structural components:

External wall	The wall consisted of 236 columns, 59 on each face from the 10th ~ 107th floor. Each column was a welded square box section with external size of 356 mm (14 in.). Adjacent columns, at a spacing of 660 mm (26 in.), were connected at each floor by steel spandrel plates of 1.32 m (52 in.) high. The wall functioned like a square tube, providing resistance to the combined effect of lateral wind and gravity loads.
Central service core	Located in the central area of approximately 41 m by 27 m. The long axis of the core in WTC 1 was oriented in the east-west direction, and the long axis of the core in WTC 2 was oriented in the north-south direction. The 47 columns were made of a combination of wide flange and box sections, and were interconnected by conventional steel beams to support the core floors. The columns mainly carried the gravity load.	(Representative floor plan based on the 95th floor of WTC 1)
Composite floor system	The floor system comprised lightweight steel bar trusses spaced at 2.03 m centres and braced by secondary trusses spaced at 4 m. The secondary trusses supported a profiled steel deck with 102 mm (4 in.) thick cast-in-situ lightweight concrete slab. The top bends (or knuckles) of the diagonal truss bars extended above the top chords and were embedded into the concrete slab to make the whole system composite. The floor system spanned between the external walls and the core. At the external walls, truss top chords were bolted to the seats welded to the spandrels. At the central core, trusses were bolted to seats welded to a girder supported by the core columns. Viscoelastic dampers were installed between bottom chords and spandrels to reduce the sway and vibration of the buildings by winds. The floors supported the gravity loads and provided lateral stability to the external walls.
Hat truss system	Located between the 107th and 110th floors. The system comprised a set of steel braced trusses, designed to support a tall antenna on top of each tower (only WTC 1 had one installed). The system provided additional connections among the core columns and between the core and external columns, providing additional means for load redistribution.

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	Remarks
Fire compartmentation	In the core area, the stairwells and elevator shafts were enclosed by 2 hour fire-rated walls made of gypsum wallboard on steel studs. In the open-plan tenant area, 1 hour fire-rated floor-to-slab partitions were used to separate the tenant spaces from each other and from the common core area. Firestopping materials were used to fill gaps in walls and floors to prevent the spread of smoke and flames. The external wall was connected with the floors without gaps.
Fire protection to steelwork	Most of the core columns were protected by gypsum wallboard. Slab trusses, perimeter columns, spandrels and some faces of core columns were coated with three different sprayed fire protection materials.
Sprinkler system	Automatic sprinklers had been installed on all floors, capable of controlling local fires totalling an aggregate floor area of up to 418 m² (4,500 ft²).
Fire alarm system	Multifunction fire alarm system was installed, capable of alerting staff at the Fire Command Station within the building and providing voice and strobe alerts throughout.
Dry riser system	Standpipes were installed in the stairwells one each floor, gravity fed from 75,708 L (20,000 gal) of stored water and by three large water pumps.

Plane Crash and Damage

On 11 September 2001, two passenger planes were hijacked by terrorists and crashed into the World Trade Center Towers in New York.

When the two aircrafts crashed into the towers, fireballs erupted and jet fuel spread across the impact floors and down internal shafts, igniting multiple floor fires immediately. The resulting fires spread throughout the upper floors above the impact floors of the two towers. The twin towers collapsed shortly afterwards.

Based on the available documents, photographs, video footages, steel data collection and computer modelling, National Institute of Standards and Technology (NIST) of USA produced the reconstruction of the events that accompanied and followed the plane clashes.

The timeline for the collapse of WTC 1 according to FEMA 403 and NIST NCSTAR 1 (Draft) is summarised as follows:

Time	Damage and Fire
08:46:30 EDT (GMT - 4 hours)	A hijacked Boeing 767-200ER airplane crashed into the north face of WTC 1 between the 94th and 98th floors, directly causing Entry slash on the external columns from the 93^rd to 99^th floor Most damage to the 95th and 96th floors 35 external columns cut off, 2 seriously damaged 6 core columns cut off, 3 seriously damaged 43 core columns stripped of insulation on one or more floors Insulation stripped from trusses covering 5,574 m² (60,000 ft²) of floor area
08:47 ~ 09:02	Jet fuel erupted into fireballs and immediately ignited a multiple floor fire Within the first 2 min, fires had burned in the north side windows on the 93rd to 97th floors, the south face of the 96th floor and the east face of the 94th floor Maximum temperature was estimated about 900 �C 1000°C Smoke emerged from the 104th floor
09:02:59	Another hijacked airplane crashed into WTC 2
09:15	Fire intensified on the 94th and 97th floors Large fires erupted on the east sides of the 92nd and 96th floors Fire heated floors had begun to sag
09:30	Vigorous fires on nearly the full perimeter of the 98th floor Almost no fire on the 99th floor and above
09:58:59	WTC 2 collapsed. The pressure purse generated by the collapse of WTC 2 intensified the fires in WTC 1
10:01	Flames came out of the south side of the west face of the 104th floor
10:18	Jets of smoke ejected from the 92nd and 94th to 98th floors Fires raged on the south side of the 96th to 99th floors Floor sagging had increased. The sagging of the south side floors had caused the south perimeter columns bowing inward
10:23	South perimeter columns had bowed inward as much as 1.4 m (55 in.)
10:28	Transmission tower on top of WTC 1 started to move downward and laterally The entire section of the tower above the crashed zone began tilting as a rigid block toward the south The upper section of the tower collapsed onto the floors below Within 12 seconds, the whole WTC 1 collapsed

The timeline for the collapse of WTC 2 according to FEMA 403 and NIST NCSTAR 1 (Draft) is summarised as follows:

Time	Damage and Fire
09:02:59 EDT (GMT +4 hours)	A hijacked Boeing 767-200ER airplane crashed into the east side of the south face of WTC 2 between the 78th and 84th floors, directly causing Entry slash on the perimeter columns from the 77th to 85th floor Most damage to the 80th and 81st floors 33 perimeter columns cut off, 1 seriously damaged 10 core columns cut off, 1 seriously damaged 39 core columns stripped of insulation on one or more floors Insulation stripped from trusses covering 7,432 m² (80,000 ft²) of floor area
09:03 ~ 09:30	Jet fuel erupted into fireballs and ejected from the 79th, 81st and 82nd floors Fireballs burned for 10 sec, extending almost 61 m (200 ft) out from the north, east and south faces Vigorous fires on the east side of the 80th to 83rd floors Within 18 min of the crash, the east perimeter columns between the 79th to 83rd floors had bowed inward due to floors sagging
09:30 ~ 09:58	Fires continued to burn in the east half of the tower
09:58:59	The top of the tower tilted to the east and south, and WTC 2 collapse

Analysis

The impact of the plane crashes directly caused significant structural damages to both WTC towers. The towers resisted this level of damage and did not collapse immediately showing that the redundancy of the tube-frame structures enabled the redistribution of the loads from the damaged zones to the remaining structures.

However, it was generally believed that the impact also extensively striped off the fire protection materials from structural steel at least in the crashed zones. The multiple floors fires ignited by the jet fuel finally weakened the remaining structures and the towers collapsed.

It is worth noting that in the tube-frame system of the WTC towers, the lateral resistance or stability of the perimeter columns were provided by the composite floor truss system. This lateral restraint is reduced as the floor trusses weaken and sag in the heat. In a multiple floors fire, it was expected the effective length of laterally unrestrained perimeter columns would increase at least twice or triple. In addition to the direct thermal effects, the compressive resistance of these columns eventually reduced until a point that they could not sustain the applied load and buckling occurred.

The video footages showed that in WTC 1, the south perimeter columns of the fire floors had buckled before the top section of the tower tilted to the south and collapsed. On the other hand, in WTC 2, the east perimeter columns of the fire floors had also buckled before the top section of the tower tilted to the east and south and collapsed.

Sources of Information

BBC News Online / World / America / America's day of terror - UK Edition (2001)
[FEMA 403] Federal Emergency Management Agency. (2002). World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations, Washington, DC.
[NIST NCSTAR 1 (Draft)] National Institute of Standards and Technology. (2005). Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Final Reports of the National Construction Safety Team on the Collapses of the World Trade Center Towers (Draft), U.S. Government Printing Office, Washington.

Building Fires

World Trade Center Towers Collapse, New York