Richard Fields

Technical Leader

Materials Science and Engineering L

National Institute of Standards and Tec

U.S. Department of Commerce

frank.gayle@nist.gov

Federal Building and Fire Safety Invest

of the World Trade Center Disaste

Project #3: Analysis of Structu

Update

June 22, 2004
• Task 1 - Collect and catalog physical evi

• Task 2 - Document failure mechanisms a• Task 3 - Determine metallurgical and me

properties (room temperature, htemperature, high strain rate)

• Task 4 - Correlate specified properties w

properties (combined with Task 

• Task 5 - Characterize thermal excursions • Task 6 - Prepare final report

Project 3 Tasks
Task 1 - Collect and catalog physical evidence 

• Structural steel

3⁄4 236 items catalogued; all relevant ste

• Design specifications 

3⁄4 relevant structural documents review

3⁄4 identified upgrades and material sub

• Material specifications (ASTM, foreign spe• Supplier production information

3⁄4 Supplier documents, other 1960’s er

documents used to estimate propertie

3⁄4 Changes identified in test methods

3⁄4 Two reports issued:

• WTC Steel Inventory and Identification within 

• Contemporaneous Structural Steel Specificati
Task 2 - Document failure mechanisms and 

(complete)

• Contractor visual inspection of steel and an

completed; report drafted

• Extensive analysis by NIST of steel

• failure mechanisms analyzed and documen• repeated patterns of fracture/failure analyze• failures mapped on structure

• Photographic evidence enhanced and com

recovered steel.

• many recovered panels not significantly da• allows better understanding of fracture and 

3⁄4 Report being drafted
WTC 2

South Face

(original image)
WTC 2

South Face

(enhanced image)
Broken Bolt Connection

Cut MetalSplit Longitudinal WeldsUnknown / Obscured
Trends

• Collapse did not significantly damage many impacted panels• Impact area: bolt fractures dominate column connection failure

• Columns severed at stiffener plates• Weld unzipping common in impacted panels• Inner webs buckled from fire• Above impact zone: spandrels fail by bolt tear-out • Below impact zone: spandrels fail by tearing off column
Task 3 – Determine metallurgical and mechanic

• Room temperature tensile 

properties (229 tests)

• Perimeter columns and 

spandrels

• Core columns• Trusses• Truss seats (inner and outer)• Bolts, welds

• High strain rate properties

(117 tensile tests) 

• Perimeter columns• Core columns

• High temp

(114 tests

• Perime• Core c• Trusse• Truss s

• Physical p

• Therm• Therm• Heat c
Model Stress Model Stress-Strain Curves: Strain Curves:

Room Temperature Room Temperature

σ p

Fy

(1 exp( )) 0 σ R ε R bε p = + ∞ − −

Voce nonlinear isotropic hardening:

2 Core - 3

20 Truss – 21 Truss – 

8 - 9 Core - 46 - 7 Core - 43 - 5 Core - 4

PerimetPerimetCore - 3

Curve Materia

1

10-1922-29

Tensile pro

for 29 

3⁄4 NIST test 

values esand mill r

3⁄4 NIST test 

required p

3⁄4 ideal stre

develope

3⁄4 material p

29 steels
High Strain Rate Behavior: Room Tempe

High Strain Rate Behavior: Room Tempe

Yield and Ultimate Strength Incre

Increasing Strain Rates

Higher strength of perimeter columns during impact 

reduction of aircraft’s momentum. Such data allow more 

of damage to interior of building.

Perimeter Column (Fy = 65 ksi)

0

20406080

100120140

0.001 0.01 0.1 1 10 100 1000

Strain Rate, 1/s

Yield Strength

UTS

Stress (ksi)

Core Col

0

102030405060708090

0.001 0.01 0.1 

S

Y

UTS

Stress (ksi)
0.00

0.200.400.600.801.001.20

0 200 400 600 800

T (oC)

Fy/Fy(20 oC) Fy = 100 ksi Fy = 60 ksi

FY = 42 ksiFY = 36 ksiModel

0.000.200.400.600.801.001.20

0 200 400 600 800

T (oC)

TS/TS(20 oC) Fy = 100 ksi Fy = 60 ksi

FY = 42 ksiFY = 36 ksiModel

Creep:• modeled for all 

universal equati

• originally devel

steel.

Yield and Tensile • modeled for all 

equation scaled 

• based on literatu

with test data on 

Model Stress Model Stress-Strain Curves: Elevated Te

Strain Curves: Elevated Te
Task 3 – Determine metallurgical and mechanic

room temperature, high temperature, h

(complete)

3⁄4Material properties have been determined 

• all structural steels (29 steels), bolts, and we• Strength and modulus as function of tempera• Strength as function of strain rate for impacte• thermal expansion, heat capacity, and therm

as function of temperature
Task 5 - Characterize thermal excursions of ste

Photographic study• Recovered panels mapped for pre-collapse exPaint study• Paint condition used to map upper limits to 

temperature exposure

• Few perimeter panels (3 of 160 locations map

saw T > 250 °C

3⁄4 Analysis complete, report being drafte
NIST Name:

Panel ID:Panel location:Window lines: 

M-2A130: 96-99WTC 1, North face28-31

Bottom of floor:

Floor:

Floor:

Top of floor:

Column line:

Un-recovered portion of panel

9

98th97th

96t

131 130 129
Area of High Tem

Excursion > 250

WTC 1

92nd Floor Spandrel

92nd floor experienced 9 minutes 

of intermittent internal flaming

93rd Floor Level

93rd floor – no visible flaming
• The collection of recovered steel is adequate for the in

• 236 pieces of steel• Regions of impact and fire damage were emphas• 14 specified steel grades for exterior panels• 2 specified grades that represent 99 percent of co• both specified grades for the steel floor trusses

• Analysis of recovered samples of the many grades of 

based on stampings on the steel and mechanical tests

was supplied to fabricate components as indicated on drawings.

• Metallography and mechanical property tests indicate 

and quality of steel was adequate, typical of the era, aqualifying test requirements. In many instances, the rostrength exceeded the requirements by 5 to 10 percen

• Ten different steel companies fabricated structural ele

supplied by at least eight different suppliers; four fabricmajor structural elements from the 9th to the 107th floo

Project 3 Preliminary Findings Project 3 Preliminary Findings