Technical Conference on tBuilding and Fire Safety Inof the World Trade Center 

Session VI -

Structural Fire Respo

Collapse Analysis

Fire Protection of Structur

September 15, 2005

John L. Gross

Building and Fire Research Labora

National Institute of Standards and Techno

U.S. Department of Commerce
Fire Protection of Structural Steel Fire Protection of Structural Steel

Thickness m

and variabil

Equivalent u

Specified anthickness of 

Variabl

e thickness

Equivalent Uniform 

thickness

01234567

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4

Count

Ln (Thickness)

(d)
“Specified” Thickness of Fireproo

Specified” Thickness of Fireproo

In October 1969, ... , the Port Authority stated

the fireproofing contractor, that

• “All Tower beams, spandrels, and bar 

requiring spray-on fireproofing are to h

covering of Cafco.

• The above requirements must be adhe

order to maintain the Class 1-A Fire RNew York City Building Code.”[1]

[1] Letter dated October 30, 1969 from Robert J. Linn (M

Planning, The World Trade Center) to Mr. Louis DiBoDiBono Plastering Co., Inc.).
Original Fireproofing Original Fireproofing

Photographs reproduced with permission of U. S. Min

erals Products Co. db

a Isol

atek International and 

Morse Zehnter

Associates.
Upgraded Fireproofing Upgraded Fireproofing

Photographs reproduced with permission of Gilsanz Murray Steficek
Qualitative Data on Condition of F

Qualitative Data on Condition of F

 Sample Area Data Sheets from 1990 contained co

state of the in-place SFRM.

 As an example, the data sheet for floor 29 of WTC

following for the South West quadrant of the floor:

“Fluffy spray-on fireproofing coating the supporand deck above the ceiling. The thickness of tthe beams and joists was consistently about 

the deck it ranged from very sparce [sic] in areother areas. The areas we sampled were coatgreen encapsulant.”

 And, on the 23rd floor,

“truss members located adjacent to the outsid

(within 3 ft.) are devoid of fireproofing material”
Fireproofing Thickness Fireproofing Thickness

Measurements*

Flo

o

r

2

3

0.6

0

0.5

3

0.7

0

0.7

6

0.8

8

0.8

9

0.8

3

1.170.8

8

0.71 0.8

2

0.5

2

0.6

9

0.5

2 

0.6

4

0.5

2

Measurements*

Averages of six measurements (

X

)

• Average thickness = 0.75• Standard deviation of averages = 0.16• Standard deviation of individual 

measurement = ?

* ASTM E 605 -

Standard Test 

Methods for Thickness and Density o

f SFRM Applied to

Structural Members

Source: Port Authority of N
Distribution of Measurements Distribution of Measurements 

- Ori

Ori

Distribution

Normal P

02468

1012

0.48 0.56 0.64 0.72 0.8 0.88 0.96 1.04

Count

Range, in.

02468

1012

0.48 0.56 0.64 0.72 0.8 0.88 0.96 1.04

Count

Range, in.

0.50.60.70.80.9

1

.01 .1 1 5 10 2

y = 0.737 + 0

Thickness, in.

S

=

0.12

6

=

0.29in.

≈ 0.30 in.

S

=

0.12in.

X
Photograph Analysis Photograph Analysis

• Webs of main • Webs of bridg• Diagonal str

ut 
Statistics for Photograph Analyses Statistics for Photograph Analyses

 Main trusses -

original (85 measurem

• Average thickness = 0.6 in.• Standard deviation = 0.3 in. 

 Bridging trusses -

original (52 measu

• Average thickness = 0.4 in.• standard deviation = 0.25 in.

 Diagonal struts -

original (26 measur

• Average thickness = 0.4 in.• Standard deviation = 0.2 in.

 Main trusses - upgrade (52 measure

• Average thickness = 1.7 in.• Standard deviation = 0.4 in.
Equivalent Thickness Equivalent Thickness

• Pseudo random number generator was used to se

a lognormal distribution with central value and dispwith average of 0.75 and standard deviation of 0.3

Generated thickness profile

0.000.200.400.600.801.001.201.401.60

0 10 20 3

0

40 50 6

0

R

ough Texture

Smooth T

e

xture

Thickness, in.

Bar Po

sition, in.

(a)
Equivalent Thickness Equivalent Thickness

Variable thickness

• 3 data se

• Rou

g

• Smo

• average • standard • distributio

X

P

P

60 in

Uniform thickness

X

P

P

60 in

• thicknes

• P = 9800 lb (12,500 psi)• Applied heat flux from a 1100 oC fire
Equivalent Thickness Equivalent Thickness

0.

0

0

0.

2

0

0.

4

0

0.

6

0

0.

8

0

0

5 10 15 20 

R

oug

R

oug

R

oug

Sm

o

o

Sm

o

o

Sm

o

o

0.4" U0.5" U0.6" U

Deformation (in)

Ti

m

e (

min

)

L

oad = 98

0

0 lb

s

(c)

0.6 in uniform 

thickness
Types and Locations of SFRM Types and Locations of SFRM 

– Flo

Flo

B

uildin

g

C

o

m

p

o

n

e

n

t

M

a

t

e

rial S

p

e

cifie

d 

Flo

o

r

S

y

s

t

e

m

O

rigin

al

M

ain tr

u

s

s

e

s

a

n

d

dia

g

o

n

al str

uts

B

L

A

Z

E-S

HIE

L

D

D

C/F

0.5

Brid

gin

g tru

s

s

e

s

–

o

n

e-w

a

y

z

o

n

e

B

L

A

Z

E-S

HIE

L

D

D

C/F

0.5

Brid

gin

g tru

s

s

e

s

– tw

o-w

a

y

z

o

n

e

B

L

A

Z

E-S

HIE

L

D

D

C/F

0.5

U

p

g

r

a

d

e

d

M

ain tru

s

s

e

s

a

n

d

dia

g

o

n

al struts

B

L

A

Z

E-S

HIE

L

D II 1.5

Brid

gin

g tru

s

s

e

s

–

o

n

e-w

a

y

z

o

n

e

B

L

A

Z

E-S

HIE

L

D II 1.5

Brid

gin

g tru

s

s

e

s

– tw

o-w

a

y

z

o

n

e

B

L

A

Z

E-S

HIE

L

D II 1.5
Types and Locations of SFRM Types and Locations of SFRM 

– Ex

Ex

B

uildin

g

C

o

m

p

o

n

e

n

t

M

a

t

e

rial

S

p

e

cifie

d 

E

x

t

e

rio

r

W

all S

y

s

t

e

m

B

o

x

C

olu

m

n

s

E

xterior F

a

c

e

B

L

A

Z

E-S

HIE

L

D

D

C/F 1 3/16

Interior F

a

c

e

V

ermic

ulite

Pla

ster 7/8

S

p

a

n

drels

E

xte

rio

r

F

a

c

e

B

L

A

Z

E-S

HIE

L

D II 1/2

Interior F

a

c

e

V

ermic

ulite

Pla

ster 1/2
Types and Locations of SFRM Types and Locations of SFRM 

– Co

B

uildin

g

C

o

m

p

o

n

e

n

t

M

a

t

e

rial S

p

e

cifie

d 

C

o

r

e

C

olu

m

n

s

Wid

e

Fla

n

g

e

C

olu

m

n

s

Lig

ht B

L

A

Z

E-S

HIE

L

D

D

C/F

2

3/16

H

e

a

v

y

B

L

A

Z

E-S

HIE

L

D

D

C/F 1 3/16

B

o

x

C

olu

m

n

s

Lig

ht B

L

A

Z

E-S

HIE

L

D

D

C/F

N/S

H

e

a

v

y

B

L

A

Z

E-S

HIE

L

D

D

C/F

N/S

C

ore

B

e

a

m

s

BLA

Z

E-S

HIELD

D

C/F 1/2