Dr. James E. Hill

Director, Building and Fire Research Labor

National Institute of Standards and Techno

U.S. Federal Building and Fire Safety Investi

of the 

World Trade Center Disaster

4th Annual Congress on

Infrastructure Security for the Built Environ

October 19, 2005
Investigation Objective

Investigation Objective

• Determine why and how the WTC Towers col• Determine why the numbers of injuries and f

were so low or high depending on location.

• Determine the procedures and practices that 

used in the design, construction, operation, amaintenance of the WTC towers.

• Recommend changes in current national buildi

fire model codes, standards, and practices thwarrant revision


Analysis of Probable Collaps

Analysis of Probable Collaps

Sequences Sequences

• Analyzed the sequence of events

– aircraft impact – spread of jet-fuel-ignited multi-floor fires– thermal weakening of structural compon– progression of local structural failures t

initiated collapse

• Combined

– mathematical modeling– statistical and probability-based analysis 

methods

– laboratory experiments– analysis of visual and physical evidence
Critical Analysis Interdependenc

Critical Analysis Interdependenc

Collapse Seq

Reference 

Structural 

Models 

SAP 2000 

SAP to ANSYS 

Conversion

SAP to LS-DYNA 

Conversion

Compartment Damage

Debris and Fuel 

Distribution

SFRM Damage

Structural 

Damage

Gas Temperat

Time-Historie

(FSI)

Structural

Temperature T

Histories

Resolution1-4 in.10-6 s

Aircraft Impact 

Damage

LS-DYNA

Fire Dynam

(FDS)

Thermal Ana

ANSYS v.8

Structural Res

and Failure An

ANSYS v.8

Time scale: 10 orders of magnitudeLength scale: 5 orders of magnitude

Baseline Performance Analysis
WTC 1
WTC 1 Damage: Composite Summ

WTC 1 Damage: Composite Summ

for Floors 93 to 98 for Floors 93 to 98

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
WTC 1 Damage by Floor WTC 1 Damage by Floor

Floor 94 

Floor 96 Floor 97 

Floors 93 to 98 Floor 93

Cumulative Damage

Column DamageSeveredHeavy DamageModerate DamageLight DamageSevere Floor DamageFireproofing and partitionsFloor system structural damage Floor system removed
Role of Aircraft Impact Role of Aircraft Impact

• Compromised the sprinkler and water supp

systems;

• Dispersed jet fuel and ignited building conte• Created large accumulations of combustible • Increased the air supply into the damaged 

buildings; 

• Damaged and dislodged fireproofing from 

structural components; and

• Damaged ceilings
Reconstruction of the WTC F

Reconstruction of the WTC F
Upper Layer Temperatures (WTC 1, Floo
South Face of WTC1

• Maximum = 55 inches

(uncertainty ~ +/- 6 inches)

• Time: 10:22 AM• Measurements of inward

bowing (inches)

• Floor locations approximate• Blue tinted region digitally

enhanced
Tilting of Building Tilting of Building 

Sections Sections

Initiation of glowas first observtilting of buildinabove the impaboth WTC towe

WTC 1 tilted to the south; WTC 2 tilted to the east and south.

© 2001 Dean Riviere
Evacuation and Emergency Res

Evacuation and Emergency Res

Based on 1,056 interviews of surviving WTC occu116 interviews of emergency responders.

• It is estimated that 17,400 occupants (± 1,200) were pre

WTC towers on the morning of September 11, 2001. 

- About 6 percent of the surviving occupants reporte

pre-existing limitation to their mobility. 

• Approximately 87 percent of the WTC tower occupants

more than 99 percent of those below the floors of impaable to evacuate successfully. 

• 20 percent or more of the 2,567 building occupants and 

emergency responders who were in the WTC towers anlives may have been alive in the buildings just prior to collapse.
Condition of Stairwells Condition of Stairwells

• The stairwells, with partition wall enclosures that provided a 2 h fire-rating but little s

integrity, were damaged in the region of the aircraft impacted floors. 

• One of the stairwells in WTC 2 (Stairwell A on the Northwest side) was passab

of aircraft impact for some period of time after WTC 2 was attacked.

• All three stairwells in WTC 1 and the two other stairwells in WTC 2 were rendered im

the region of aircraft impact.
Key Findings Key Findings

• The buildings would likely not have collapsed due to aircraf

and the subsequent jet-fuel ignited multi-floor fires, if the fireproofing had not been dislodged or had been only minimdislodged by aircraft impact. 

• The existing condition of the fireproofing prior to aircraft im

and the fireproofing thickness on the WTC floor system did a governing role in initiating collapse on September 11, 200

• Approximately 87 percent of the WTC tower occupants, incl

more than 99 percent below the floors of impact, were able evacuate successfully.

• A full capacity evacuation of each WTC tower with 25,000 pe

would have required about 4 hours and as many as 14,000 pmay have lost their lives.

• Documents suggest that the WTC towers generally were de

and maintained consistent with the requirements of the 196York City Building Code.
30 Recommendations 30 Recommendations

• Increased Structural Integrity• Enhanced Fire Resistance of Structures• New Methods for Fire Resistance Design o

Structures

• Improved Active Fire Protection• Improved Building Evacuation• Improved Emergency Response Technolo

and Procedures

• Improved Procedures and Practices• Education and Training
Proposals for Changes to Co

Proposals for Changes to Co

• The National Institute of Building Scie

(NIBS) is organizing a panel of recognexperts to:

- Prioritize the recommendations- Develop proposals for changes to buil

codes

• Panel will prepare proposals for ICC a

NFPA 5000 and coordinate submissiocode update cycles.