S. Shyam Sunder, Lead Investigato

Bill Grosshandler, Associate Lead Inves

Building and Fire Research Labo

National Institute of Standards and Tech

U.S. Department of Commerce

sunder@nist.gov

NIST Response to the World Trade Cente

World Trade Center Investigat

April 29, 2003
94th floor

99th floor

North tower: Collapsed aft

78th floor

84th floor

South tower: Hit at 9:03 AMCollapsed after 56 minutes

South Face

About 0.79 H
North tower

South tower

Point of impact: close to the corner and with an angle

Point of impact: close to the center and nearly normal to the building

An

15-20 

17 mil

space 

Plan Development Process Plan Development Process

• Proposed plan with proje

for public comment

• Public meeting in New Yo

comments on scope of pr

• Due date for public comm

plan

• Refined plan published a

implementation

• Funds transferred to NIST 

emergency appropriation 

June 10, 2002:June 24, 2002:June 30, 2002:August 21, 2002:September 9, 2002:
Goals

• To investigate the building construction, the mate

the technical conditions that contributed to the ouWorld Trade Center disaster

• To serve as the basis for:

• Improvements in the way buildings are designed

maintained, and used

• Improved tools, guidance for industry and safety • Revisions to codes, standards, and practices• Improved public safety, and business and insura

• Estimated Time-to-Completion:

• Final report in 24 months• Interim progress reports and key project reports 
Objectives Objectives

• Determine technically, why and how the buildings 

collapsed following the initial impact of the aircraft

• Determine why the injuries and fatalities were so lo

depending on location, including all technical aspe

protection, occupant behavior, evacuation, and em

• Determine what procedures and practices were us

construction, operation, and maintenance of the W

• Identify, as specifically as possible, building and fi

standards, and practices that warrant revision
Guiding Principles Guiding Principles

• Active, comprehensive, thorough, independent, and objective• Fully informed of concerns and issues of all parties and within limits • Open and inclusive process in planning and conducting investigation 

disseminating findings and recommendations

• Improve practice, standards, and codes and reduce future risks by fo

• Fact-finding and analysis of the facts• Validating and verifying existing knowledge• Creating new technical and/or scientific knowledge

• Non-technical issues outside scope: no findings of fault or negligenc

organization 

• Maintain ongoing liaison with professional community, public, and lo

• Project teams of NIST and external world-class technical experts 
System Design Concept

Technical Services

Technical Services

Skylobby

UndergroundCar Park (total capacity 2000)

Local Elevators Open Plan OfficeExpress Elevators Express Elevators 208 ft

208 ft

35 ft

60 ft 60 ft

35 ft

Technical Servic

Technical ServicTechnical ServicTechnical Servic

Skylobb

Skylobb

Express Elevato

Plaza Leve
Exterior Framed-Tube

Floor Slab

Central CoreBox Columns

59 columns per side

Bar Joist

Concrete Slab

Floor

Coverin

ElectrDuct

Concrete Slab

Joist Steel Truss

Exterior

Wall

Visco-elastic Damper

Floor-Joist to Column Connection

System Des

Structural Steel: WTC 1 North Fac

Structural Steel: WTC 1 North Fac

Simulated distribution of column yield strengths

• Wind loads 

perimeter c

• 14 different g

(36 to 100 k

• Arrangemen

symmetric ntwo towers

• Gravity load

core colum

• 4 grades of s

(99% are 36 strength)

• Conventiona

column & be
NIST WTC Investiga

NIST WTC Investiga

Analysis 

of Steel Structural 

Collapse

Baseline 

Performance

& Impact 

Damage

Analysis of 

Codes and 

Practices

Fire Service 

Response

Active Fire 

Suppression

Thermal and 

Tenability 

Environment

Public InputsDocumentsVideo/ Photographic RecordsOral History DataEmergency Response RecordsRecovered Structural SteelBPAT RecommendationsGovernment, Industry, Professional, Academic InputsPublic Inputs
Public, 

Industry, and TechnicalInputsPublic Inputs, Documentary and MaterialEvidence, Oral History

Analysis 

of Steel

Baseline 

Performance

& Impact 

Damage

Analysis 

of Codes 

and 

Practices

NIST WTC Invest

NIST WTC Invest

Frank Gayle Frank Gayle

Fahim Sadek

H.S. Lew, Dick Bukowski Bukowski
Scope of Project #1 Scope of Project #1

Analysis of Building and Fire Codes and Practices• Review design calculations and project documents; project specifi

drawings; establish design loads and methods used to proportion struc

process for systems not covered by governing code and anticipated ab

• Review building construction (construction logs, change orders, test 

document significant events during construction and over the life of the 

• Review active and passive fire protection features during constructio

(fireproofing, egress, fire stops, compartmentation, enclosure shafts, sp

• Review design, operation, maintenance, and inspection of emergency 

systems; U.S. versus international standards; standards, practices, an

• Compare design requirements versus code provisions (NYC, NYS

during design and now; comparison with provisions of selected other m

• Review structural systems maintenance and modifications from init

11, 2001; review modifications made following the 1993 bombing; focus 

affected by aircraft impact

• Review fire protection systems maintenance and modifications from 

September 11, 2001; focus on portions of building affected by aircraft im
Scope of Project #2 Scope of Project #2

Baseline Structural Performance and Aircraft Impact D! Develop reference structural models of WTC 1 and WTC 2; capture 

structural system, including prefabricated column-spandrel panels, co

hat-truss, and connection details; major modifications during bid, cons

! Analyze baseline performance of WTC towers under design gravity 

estimate safety margins; compare original design wind loads with esti

loads from recent wind tunnel tests

! Analyze damage to structural system due to aircraft impact with W

estimate collateral damage to fireproofing, fire protection system, an

analyze component, subsystem, and system level structural damage; damage states and influence of knowledge and random uncertainties

! Determine role of floor diaphragms and hat truss on structural integ

analyze stability of undamaged towers with missing or damaged floor 

hat truss 

! Determine safety margin of WTC 1 and WTC 2, immediately after ai

service loads
Scope of Project #3 Scope of Project #3

Metallurgical and Mechanical Analysis of Structural • Collect and catalog available structural steel; identify location of s

WTC buildings; document relevant steel specifications and specified 

towers and WTC 7; obtain proprietary data from suppliers as needed

• Document failure mechanisms and damage based on visual obser

steel, especially for available columns, connectors, and floor trusses

• Determine the metallurgical and mechanical properties of steel, w

connections

• Identify grades of steel used for different structural components

• Room temperature properties to analyze baseline performance 

• High strain rate properties for aircraft impact analysis studies

• Creep and high temperature properties to evaluate structural fir

• Analyze the steel and primer paint metallographically to estimate ma

reached by available steel

• Compare steel properties obtained with applicable material spec

• Perimeter column-spandrel panels;• Composite floor truss system• Core columns
NIST WTC Invest

NIST WTC Invest

Public, Industry, and TechnicalInputs

Structural 

Collapse

Baseline 

Performance 

& Impact 

Damage

Analysis 

of Codes 

and 

Practices

Thermal and 

Tenability 

Environment

Analysis 

of Steel

Public Inputs,Documentary, and MaterialEvidence, Oral History

Dick Gann Dick Gann

John G

John G
Scope of Project #5 Scope of Project #5

Reconstruction of Thermal and Tenability Environment• Collect and analyze photographic and video images to guide initial conditio

fire spread through buildings, and floors on which structural collapse may have 

accounts of damage to buildings

• Gather data on internal construction materials/systems, furnishings/contents, a

storage tanks, aviation fuel); characterize types of combustibles and estim

compare data with prior surveys of similar occupancies

• Identify pathways for fire ventilation and compartment-to-compartment f

existing data on fire performance of floor, wall, and ceiling systems, and on the 

shafts, etc); conduct complementary experiments as needed

• Determine thermal properties of structural insulation (fireproofing) system

temperature, and effects of vibration, impact, and shock on thermal performan

fireproofing was in place on September 11, 2001; assess whether chemical int

and steel at elevated temperatures could degrade performance

• Extend capabilities of NIST’s Fire Dynamics Simulator (FDS) to reconstruc

smoke fields (including floor spaces, stairwells); use reduced-scale experimen

• Reconstruct fully involved fires, with and without initial damage from airc

assess probable fire paths and timeline of fire spread, influence of knowledge parameters include ignition mode/location, jet fuel and building contents, ventil

damage, core pressurization, fire protection system, rate and extent of fire spre

toxic conditions
Scope of Project #6 Scope of Project #6

Structural Fire Response and Collapse Analysis• Evaluate thermal-structural response of floor and column subsystems under se

composite floor system, prefabricated column-spandrel panels, and core column systsupports; standard (ASTM E-119, ASTM E-1529) fires, and real fires based on fire dysteel, and steel with different fireproofing thicknesses; assess local failure of componestrength, serviceability, and stability criteria

• Evaluate thermal-structural response of WTC towers under service loads, witho

damage; model structural system to include fire-affected and impact damage regionsE-1529) fires, and real fires based on fire dynamics simulations; unprotected steel, anthicknesses; assess failure of subsystems and system using strength, serviceability, asafety margin of components and subsystems critical to system collapse initiation

• Conduct tests of structural components and systems under fire conditions; use 

and provide response data otherwise unavailable

• Evaluate failure hypotheses for the WTC towers; identify candidate sequence of even

probable collapse initiation sequences, and influence of knowledge and random uncediaphragms, connections, hat truss, and other key elements; identify most probable 

• Document past fire performance of open-web steel bar joist systems; review ava

investigation reports; review available test reports (ASTM E-119) or others; domestic past performance with performance of bar joist floor system in WTC towers

• Analyze thermal-structural response of WTC 7 through collapse initiation; mode

in fire-affected region; standard (ASTM E-119, ASTM E-1529) fires, and real fires bas

determine safety margin of components and subsystems critical to system stability; asystem using strength, serviceability, and stability criteria; assess probable collapse inof knowledge and random uncertainties; evaluate role of transfer trusses and other pi

probable structural collapse sequence
Wind 5 m/s

to south

Simulation Tool for Re-creation of Smoke Movement (Rehm et al.)

FDS 1

Three commercial flights reported data using the Addressing and Reporting System (ACARS)
South Tower Fireball

Seconds after impact. About 3000 kg fuel consumed.
Simulation Tool for Recreation of 

Smoke Movement
NIST WTC Invest

NIST WTC Invest

Public, Industry, and TechnicalInputs

Analysis 

of Steel

Structural 

Collapse

Baseline 

Performance

& Impact 

Damage

Analysis 

of Codes 

and 

Practices

Fire 

Service 

Response

Active Fire 

Suppression

Thermal and 

Tenability 

Environment

Eva

Public Inputs,Documentary, and MaterialEvidence, Oral History

Randy 

Randy 

Dave Evans Dave Evans 
Scope of Project #4 Scope of Project #4

Investigation of Active Fire Protection Systems• Document the design, installation, and normal operation of active f

(sprinkler system, fire water supply, power supply, pre-connected hosescommunication systems, smoke management systems); review availabreview capacities, redundancies, and defense-in-depth

• Document the past performance of active fire protection systems d

• Estimate limits of performance for fully functional sprinkler system

• Review inspection records for active fire protection systems; documen

the fire alarm system after the 1993 bombing incident

• Document performance of active fire protection system on Septem

system performance under similar fires without aircraft damage to the s
Scope of Project #7 Scope of Project #7

Occupant Behavior, Egress, and Emergency Communi• Collect new first-person data from occupants and those with safety responsi

families of victims affected by the WTC disaster via interviews and statistical sbehavior, human factors, egress, and emergency communication

• Compile and analyze first-person media accounts from survivors and famil

data collection efforts

• Collect archival records from prior WTC evacuation incidents and practic

changes made to evacuation procedures, augmented with oral history data fro

• Document pre-event data on egress components (stairs, lighting, back-up p

protection systems, occupancy level and distribution on September 11, 2001, e

systems, and emergency plans and drills

• Analyze data to study movement of people during evacuation, including d

awareness, time-constrained evacuation strategies, emergency communicatiofirefighting, role of fire wardens and fire safety directors, and issues concerning 

develop evacuation timeline

• Evaluate evacuation designs and protocols; compare with code requiremen

observed evacuation data with predictions of alternate egress models and exp

review occupant protection practices for tall buildings, considering different (e.strategies 
Scope of Project #8 Scope of Project #8

Fire Service Technologies and Guidelines• Document emergency response data on first responder fatalities, comman

equipment performance, radio and 911 communications, dispatch logs, and apparatus

• Document operations and functioning of communication systems, on-site 

systems, fire alarm panels, elevator control panels, standpipes and fire hose

emergency equipment

• Collect oral history data from surviving first responders, those in control of 

witnesses to the extent data has not been documented fully

• Analyze data to determine effects of key factors on responder success: 

influence of aircraft impact damage and fuel run-off; occupant egress as rela

ability to fight large fires on upper floors of tall buildings; change in fire servic

bombing; pre-planning, training, and standard operating procedures during iaccountability, location and tracking; fire and emergency response protocols 

available for initial situation assessment and incident management, including collapse; communications and coordination with other authorities

• Identify alternative emergency response practices and technologies and 

advance safety and effectiveness of first responders during massive fires in knowledge/information systems for command and control; elevator use by fir

systems; interoperability of communication systems; fire growth and smoke safety monitoring, assessment, and prediction; simulation tools for training
Lessons to be Derived Lessons to be Derived

• Lessons for Structural Fire Protection

• Dynamics of building fires and collapse vulnerability of buildings 

• General methodologies for fire safety design and retrofit of struct

• Behavior of fireproofing materials and connections used in steel 

• Behavior of open-web steel trussed joists in fires• Collapse mechanisms and role of pivotal components such as tr

diaphragms

• Lessons for Life Safety

• Firefighting technologies and practices for tall buildings• Occupant behavior and evacuation technologies and practices fo• Control of fire spread in buildings with potentially large open floo

• Command, control, and communication systems for fire service r

• Lessons for Engineering Practice

• Evaluation process for innovative systems• Margin of safety and redundancy to accommodate abnormal loa
Comments 

due 6/30/02

Draft Plan 6/10/02

Public Mtg. 6/24/02

Plan Published

Funds Transferred 9/09/02

NCSTAC Charter App. 11/05/02

First Mtg. of NCSTAC 4/29/03

2 Members Appointed 4/25/03

8 Members Appointed 3/20/03

NCSTAC Nominations Closed 11/27/02

Int. Final Rule Pub. 1/31/03

Final Rule/ Prop. Rule Pub. 5/1/03

Final Rule Pub. 6/15/03

NCST Act Signed 10/01/02

Start of Investigation 8/21/02

Draft Invest. ReporNIST & NCSTAC Review 6/01/04 

Draft Proje

Reports 

3/01/04

Review Com6/15/04

Pr8/

Color Key: 

InvestigationInvestigation Plan Development NCST Act Implem

NCST Advisory C

WTC Investigation Timeline
Desired Outcomes Desired Outcomes

Make buildings saferEnhance safety of fire and emergency Better protect occupants and property Better emergency response capabilitieprocedures in future
Web site: Web site: http://wtc.nist.gov http://wtc.nist.go

E-mail: wtc@nist.gov wtc@nist.gov

Facsimile: (301) 975 Facsimile: (301) 975-6122

WTC Technical Information Repo

WTC Technical Information Repo

National Institute of Standards and Tech

National Institute of Standards and Tech

100 Bureau Drive Stop 8610 100 Bureau Drive Stop 8610

Gaithersburg, MD 20899 Gaithersburg, MD 20899-8610

Contact Information Contact Information
Thank You Thank You