Frank W. Gayle

Materials Science and Engineering Lab

Richard Fields, Steve Banovic, Tim Foecke, Bill Lue

Dave McColskey, Tom Siewert, Chris McCow

National Institute of Standards and 

Analysis of Structura

in the NIST World Trade Center 
NIST WTC Investigation Pr

Analysis

of Steel Structural Collapse

Baseline

Performance & 

Impact Damage

Analysis of Codes and 

Practice

s

Emergency 

Response

Activ

e

Fire 

Protection

Thermal and 

Tenability 

Environ

ment
Today’s presentat

I. Structure of Buildings & SteeII. Steel Properties and QualityIII. Validated properties to enable IV.

Failure analysis to constrain o

models
Part I 

Structure of Buildin

&

Steel Inventory
WTC Tower Structural Sy

Exterior Framed-Tube

Floor Slab

Central CoreBox Columns

59 columns per side

Bar Joist

Concrete Slab

Floor

Covering

ElectrDuct

• Innov

when many featur
Floor truss assemblies

•

Steels: A 3

(36 

•

Componen

–

Angles

–

Round 

•

Truss Type

–

36’ and 

bridgin

–

Dozen

•

Fabricated 

(fabrication 
Core column assemblies

•

4 strength levels: primarily 36 ksi and 42 ksi

•

Wide-flange (WF) shapes predominant in fire zone 

•

Mixed shapes in WTC 2 (many box 

Æ WF transitio

•

Multiple steel suppliers (!)

Box columnsWide FlangeColumns
Perimeter column 

assemblies

•

Prefab –

3 columns x 3 sto

ries

• 12 strength levels –

Fy= 36 ksi to 100 ksi

(up to six different strengths in panel)

•

Multiple steel suppliers (primarily Yawata)

•

Fire and impact zone mostly 1⁄4” plate
Perimeter columns

• Wind loads controlled • 12 grades of steel use

(36 to 100 ksi yield 

• Arrangement of steel n

nor the same for the tw

Core columns

• Gravity loads primary fact• 4 grades of steel

(99% are 36 and 42 ksi 

• Conventional (albeit mas

beam construction

• Box columns in lower floo• Wide flange columns in up

Simulated distribution of perimeter column yi

WTC 1, North Face

Sept. 11, 2001
93th floor99th floor

North tower: Collapsed af

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

77th floor85th floor

South Face

About 0.75 H

September 11, 2001

All 12 grades of steel involved in im

3642454650556065707580

100

ksi

Columns 

Steel strength in panels damaged by aircraft 
Characterization of th
Initial Task - Collect and catalog physical evi

• Inventory of Structural steel

• Design specifications 

(sections & Fy; derived from 10,000 pag

• Material specifications (ASTM, etc)• Supplier production information

Yawata Steel (now Nippon Steel)Laclede Steel... 
1.5 million tons of debris shipped to NJ 
Steel Search, Collection, Logging and Sh

SEAoNY –

Dave Sharp, many others

NIST –

John Gross (BFRL)

Dave McColskey (Matls Rel.)Steve Banovic (Metallurgy)

~ 1.5 million to1/4-1/3 steelMuch recycled 
Salvaged Steel at NIST
Catalog of Steel – identified perimeter pane

42 panels identified by serial number, 

other markings, or geometry

• Columns – all strengths from 50 to 100 ksi• Spandrels – all strengths from 36 to 70 ksi & 80 ksi

WTC 1

• 26 panels• 22 near impact floors

- 4 hit directly by plane

WTC 2

• 16 panels• 4 near impact floo

3⁄4 samples of all 12 grades available for test
WTC 1

• 8 columns

• 5 wide flange• 3 built-up box columns

• 1 from impact zone

WTC 2

• 5 columns

• 2 wide flange• 3 built-up box columns

• 2 from impact zone

Catalog of Steel – identified core columns

3⁄4 samples of 2 grades (36 and 42 ksi) of both box and wide flange columns, configurations which represent 99% of core columns in the towers.
DOCUMENTATION OF SAMPLE REM

Outside of building looking in at tree

Top

Bottom

C1 C2 C3

Middle

P21-C1M-1

P-21

Missing

CrushedSplitOnly p

art of c

olu

m

n

Only IW left

A 130: 93-96

P21-C1B1

P21-C1B1

P21-C1B-1

P21-C2B-1

P21-C2B-1

P21-C2M-1

P21-C2M-1

P21-C2M-1

P21-C2B-1

1. Generic diagram2. Sample identificatio3. Removal method t

disturbance to t

A 130: 93-96(stencile

d or stamped)
C-40

N-8

N-9

N-10

N-13

N-99

S-9

C-22

N-101

M-26

M-27

M-20

M-30

N-7

C(2n

M-2 WTC 1 
WTC 1 

© 2001 Diane Bondareff/AP
Part II

Steel Properties and Q
Analyze steel properties and qua

• Goals

– (1) assess the quality of steel with respect to desig– (2) assess the quality of steel with respect to 1960’

• Protocols

–

Tensile tests (ASTM E-8) on all relevant strength le

–

Additional tensile tests on steels from fire and impa

• Limitations

–

Not possible t

o identically recreate original mill test 

–

Recovered steel often damaged in collapse and re

–

Natural variability of steel properties (mill test vs pr

• Materials tested

–

Perimeter columns: examples of all strength levels 

–

Core columns both common strengths, both shape

–

Trusses and seats: both strength levels, both shap

of truss seats.
Perimeter column properties

•

Examples o

•

Excess stre

1960’s era p

•

Appearance 

measured/swith natural 

• Differences 

contribute (N

• ASTM A 6: 

are not intenor lower limall possible heat

ChemistryAll plates cons

(presumed) specification 

Plate 3
Core column properties

•

4 wide-f

(36 ksi, 

•

3 box co

•

1 beam 

•

Specim

measurevidenc

•

Chemis

shapes requirem
Core columns: damaged specim

• Low-strength steels are q• YP/YS can be > 1.1•

Mechanical damage can r

•

Most recovered core colu

Work hardening brings strength up to expected val

ue by e=0.03
Truss component properties •

A 36 

Fy=36 k

–Lower cho

•

A 242 

Fy = 5

–Both chor–Both chor–Upper cho

•

Chemistry re

and A242-66 

• Chemistry•

Fy = 50 ksi and 

Fy = 36 ksi steels are 

often similar (microalloyed with V)

•

Fy = 50 ksi steels are consistent with 

requirements of A 242-66, but are similar to modern A 572

Web 
Truss seat properties

•

All trus

with 

Fy

•

Chemi

consis
Part III

Validated propertie

enable modeling
Properties modeled

•

Room-temperature stress-strain behavior

–

Based on tests to assess steel quality

–

Form baseline for high-rate and elevated-tem

•

Room-temperature strain rate sensitivity of streng

–

for airplane impact studies

•

Elevated temperature elastic modulus 

–

for fire studies

•

Elevated-temperature stress-strain behavior

–

for fire studies

•

Elevated-temperature creep behavior

–

for fire studies

• Impact properties

–

U

sing Charpy tests to screen for brittle behav

–

Not covered here
Model properties: stress-strain c

• Goal: provide 

curves for ea

• Methodology

–

1. Elastic 

29.7 Msi

–

2. Plastic 

NIST data

– 3a Perime

evaluated surviving 

– 3b Core c

from histo

–

4 Yield str

strain rate rate sensi

–

5 maximu

–

6 high-rat

these data

(

(

))

p

p

p

σ

=

R

ε

+

R

−

−

b

ε

∞

1 exp

0

Plasticity modeled using Voce equation

Perimeter c

olumn mo

d

el stress-strain curves
Properties: strain rate sensitivity

• Metho

–

H

–

8 

50 

–

5 

– 50 

(a

–

N

–

E

pr

• Res

ul

–

N

ESIS procedure for estimating 

Fy
Properties: strain rate 

sensitivity

m = 0.02 results in 5 % stress increase per decade of strain rate

m

1 0 1 σ

=

σ

ε&
Properties: strain rate 

sensitivity-comparison with literature

•

WTC steels are similar to other lowcarbon steels

•

Data sources: low-carbon, low-alloy 

structural steels

•

No literature data on specific construction 

steels (A 36, A 572, etc)
Properties: high-temperature elas

Wide variation 

UltrNISusinRecindeDetfrom 

Slopes near RT are all similar
Properties: high-temperature stre

behavior

•

Protoc

•

3 perim

Fy=6

•

2 WF 

Fy=3

•

2 truss 

Fy=3

•

Limita

–

St

str

–

Re

str
Properties: high-temperature yie

tensile strength

Yield strength 

•

Model developed on literature data on 1960’s era steels

•

Differences reflect (pres

umed) differences in test protocol

•

Model curve is phenomenological only
Properties: Modeling high-tempera

stress-strain curves

• Methodology

–

Model steels with 

Fy=36 ksi using literature data for A 

–

Model steels with 

Fy >36 ksi using NIST data for A 2

–

Scale stress strain curves for untested steels by ratio

strengths, RTS

• Results

–

Method accounts for change

in work hardening

–

Represents upper limit 

or strength

( ) (

)

n

T

σ

=

RTS

K

T

ε
Properties: Creep

•

Meth

–

C

s

– 0 –

M

lA

–

S

o(

C

B

A

c

(

(

(

ε

Floor 

C

B

A

ε

All o

Dashed lines are global model
Part IV

Failure analysis to constr

of models 
Damage and Failure Analys

Purpose:

•

To provide input material data for the impact m

•

To validate impact model with exterior wall fail

•

To provide insights into collapse mechanisms

•

To assess adhesion of SFRM after impact

•

To determine what temperature was experienc

Evidence available:

•

Recovered steel components (structural and m

•

Photographic and video

Challenges:

•

Ambiguity as to when damage happened 

(impact, collapse or post-collapse)

•

Sample degradation

•

Sample identification
Exterior Steel Dama

Due to Aircraft Impa
Original Image –

North Tower, 

© 2001. 
Processed Image

© 2001. Ro
Photo © 2001
Photo © 2001
Photo © 2001. Roberto Rabanne/CORBIS

Identified failures
Photo © 2001. Roberto Rabanne/CORBIS

Original locations of inventory panels as identified by 
WTC 1, North Face

Photo © 2001. Roberto Rabanne/CORBIS

M-27
C40 hit by tip of tail?

Closer examination shows co

unlikely – damage occ

Direct comparison with state o

Photo © 2001. R

Photo © 2001. Roberto Rabanne/CORBIS
Type of fracture of perimeter 

C

Photo 

© 2001. Roberto Rabanne/CORBI

S
High Rate Failure Mode Obser

•

Measured fracture surface

profile on outer web from M2that broke in the impact

•

Considerable thinning within

an inch of the fracture surface

• Indicates large energy

absorption during failure

•

No need to have a transition

from a low energy to highenergy absorption failuremode in the FEA model ofthe aircraft impact with thebuilding.

Distance from frac

Plate thickness (in.)
Validation of Aircraft Impact 

Prediction With Observations f
Validation of Aircraft Impact 

Prediction With Observations f
Evidence of Collapse

Mech

From Examination o

Recovered Steel
Floor Truss Support: Perimeter Seat Dama

919293949596

97

100

9899

200

157

154

151

148

145

142

139

136

133

130

127

124

121

118

115

112

10

102103104

1

No se

Seat i

Seat iSeat i

Seat a

Seat i

Tab p

Tab p

Tab p

Tab p
Floor Truss Support: Perimeter Seat Dama

95th floor and below:

all attachments bent downwards or missing (components ripped off at welds)

No se

Seat i

Seat iSeat i

Seat a

Seat i

Tab p

Tab p

Tab p

Tab p
Evidence of Maximum Tem

Reached by Compone

From Examination o

Recovered Steel
Methods to Determine High Tem

Excursions of Steel

•

Microstructural changes

•

Calibrated stress relief of welds

•

Thermal analysis of metastable 

•

Annealing of hardened bolts an

•

Degradation of paint

Each approach was examined. Microstruc

and condition of the paint provided useful 
Steel sample: WTC

-Column with Fy = 60 k-No mud cracking of pa

NIST Name:Panel ID:Panel location:Window lines: 

Bottom 101st floor

Floor:

Floor:

Top of 98th floor

Column line:

Un-recovered portion of panel

C-40A136: 98-101WTC 1, north face34-37

100th99th

137 136 135

fire exposure: 

16 minutes minimum

Laboratory expose

- Similar column with F- Furnace exposure at 

0 minutes at 625 

Cementite hafter 15 min 
Summary of metallographic analysis – Perimeter a

For all perimeter column flanges, outer webs, and spa

ksi (controlled rolled)

•136 distinct samples (many from the fire floors) e

spheroidiz

ation observed, and thus no steel temp

for significant time.

Caveats

•

samples represent < 3% of columns in fire zone

•

samples represent < 1% of columns in the build
“Paint” is actually a ceramic 

•

Mostly Fe- and Tioxides with silica sand

•

Applied at fabricator to prevent rust

•

Tnemic 99

As applied •

Suspending liquid -

high vapor pressure

•

Bake cycle to dry out - 200 degrees F

• Left faint, closed drying cracks•

Almost no organics left to “burn”

After 250 de• “Mudcrack•

Roughly sy

•

Caused by 

expansion of 
Important: The Paint Test is a Ne

The absence of cracking of the paint shosteel underneath did not reach 250 degre

The presence of cracking means that one things happened:

•

The steel underneath the paint exceeded 250 de

•

The steel underneath was plastically deformed (

•

The steel underneath corroded
Area of High Te

Excursion > 250°

WTC 1

93rd Floor Spandrel

92nd floor experienced 9 minutes 

of intermittent internal flaming

93rd Floor Level

93rd floor – no visible flaming
Paint Test Results:

•

Paint was examined on all identified 

fire was seen in windows

•

Paint condition was used to map upp

temperature exposure

•

Few perimeter panels (3 of 160 locat

saw T > 250 °C

Caveat

-

samples represent < 3% of co

•

Core columns not characterized due 

paint on identified specimens

• Information provided to fire modelers
Damage to SFRM o

Exterior Columns
Application of SFRM to External 

© Lyle Owe

247 

80th Floor

WTC 2, Nor

• By design, uniform thickness

• As applied, region between flange ends filled

(for example, see column 246 at right)

• Missing SFRM from outer flange indicated by

shadowing and exposed red paint
Direct Impact 

North Face of WTC1 - Left Side of Im

© 2001 Allan Tannenbaum 
Direct Impact 

North Face of WTC1: SFRM Missing f
Internal Impact

North Face of WTC 2

Removed from flanges (red)Intact (green)

Removed from outer 

Covered by weatherpr
Glancing Blow

East Face of WTC2
Photographic Evidence 

Gross Structural Chang

To Towers Prior to Colla
Bowing of Perimeter columns: East Fa

Time: 9:21:29 AM ~18 minutes post impact

• Maximum = 10 inches

(uncertainty ~ +/-

1 inch)

•

Vertical lines establish original

line of vertical columns

•

Small perpendicular bars

show location of inward bowingmeasurements
Bowing of Perimeter columns: South F

• Maximum = 55 inches

(uncertainty ~ +/-

6 inches)

• deflection may be larger 

beneath smoke in center of building face

•

No column deflection 

observed 38 minutes earlier 

•

Measurements of inward

bowing (inches)

Time: 10:22 AM

(6 minutes before collapse) 
East face of WTC 2 

9:21:29 am, ~ 18 minutes post impa

0

324

6

5

037

10

10

0

0

244

0

2

796

0

0245

0

02

0

45

0024

5

0

37

10

86

03

88

048

9

97

03797

6

0

5

6

0

3

5

03

6

035

034

002

0

0

0

4

6

6

0

0

0

0

Pull-in (inches)

Estimated uncertainty: +

/

-

1 inch

Empty regions have no data (smoke, damaged 
East Face of WTC 2 

9:53 am., 7 minutes pre-collapse

159

10

95

07

131613

6

0

6

111412

7

059

1114

10

6

0

1620

17

9

38

14

058

14

0

5

9

12

06

13

6

08

151313

7

07

1213

13

5

0

5

1517

13

7

5

14141316

6

4

12

4

7

12

0

0

15

2

6

1014

1

8

13

5

076

0

86

067

078

0

12

11

0

6

9

70

40

7

0

0

1616

12

840

Obscured i

n Photos

Pull-in (inches)

Estimated uncertainty: +

/

-

1 inch

Empty regions have no data (smoke, damaged 
Summary of Inward Bowing Ob

• Inward column bowing observed on south face 

east face of WTC2

•

Bowing progressed over time, and, in the case 

not exist immediately after aircraft impact

•

Extent and magnitude of deflections for compa

modeling results
Changes?

• Identify, as specifically as possible, areas in 

building and fire model codes, standards, awarrant revision
Current NIST S

Buildings efforts(1) Standard tesfire resistive (FR(2) Evaluated dstructural steel (3) Established assess high temrelevant to struc

Recommendation 11:NIST recommends that the performance and suitability of advanced structural steel, reinforced and pre-stressed concrete, and other high-performance material systems should be evaluated for use under conditions expected in building fires.

A57

2

FR steels
Questions?