1

Technical Conference on the Federal Building and Fire Safety Investigation of the World Trade Center Disaster

Session VI - Structural Fire Response andCollapse Analysis

Fire Protection of Structural Steel

September 15, 2005

John L. Gross

Building and Fire Research LaboratoryNational Institute of Standards and Technology

U.S. Department of Commerce

2

Fire Protection of Structural SteelFire Protection of Structural Steel

Thickness measurementsand variability

Equivalent uniform thickness

Specified and “actual” thickness of fireproofing

Variable thickness

Equivalent Uniform thickness

0

1

2

3

4

5

6

7

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4

Cou

nt

Ln (Thickness)

(d)

3

““Specified” Thickness of Fireproofing (SFRM)Specified” Thickness of Fireproofing (SFRM)

In October 1969, … , the Port Authority stated, in a letter to the fireproofing contractor, that

• “All Tower beams, spandrels, and bar joists requiring spray-on fireproofing are to have a ½″ covering of Cafco.

• The above requirements must be adhered to in order to maintain the Class 1-A Fire Rating of the New York City Building Code.”[1]

[1] Letter dated October 30, 1969 from Robert J. Linn (Manager, Project Planning, The World Trade Center) to Mr. Louis DiBono (Mario & DiBono Plastering Co., Inc.).

4

Original FireproofingOriginal Fireproofing

Photographs reproduced with permission of U. S. Minerals Products Co. dba Isolatek International and Morse Zehnter Associates.

5

Upgraded FireproofingUpgraded Fireproofing

Photographs reproduced with permission of Gilsanz Murray Steficek

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Qualitative Data on Condition of FireproofingQualitative Data on Condition of Fireproofing

Sample Area Data Sheets from 1990 contained comments on the state of the in-place SFRM.

As an example, the data sheet for floor 29 of WTC 1 states the following for the South West quadrant of the floor:

“Fluffy spray-on fireproofing coating the support beams, joists, and deck above the ceiling. The thickness of the material on the beams and joists was consistently about 1/2″. Regarding the deck it ranged from very sparce [sic] in areas to 1/4″ in other areas. The areas we sampled were coated with a light green encapsulant.”

And, on the 23rd floor,

“truss members located adjacent to the outside walls (within 3 ft.) are devoid of fireproofing material”.

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Fireproofing ThicknessFireproofing ThicknessMeasurements*

Floor 23 Floor 240.60 0.76

0.53 0.60

0.70 0.90

0.76 0.72

0.88 0.64

0.89 0.80

0.83 0.68

1.17 0.65

0.88 0.67

0.71 0.77

0.82 0.96

0.52 0.66

0.69 0.65

0.52 1.11

0.64 0.95

0.52 0.56

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 of SFRM Applied to Structural Members

Source: Port Authority of New York and New Jersey.

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Distribution of Measurements Distribution of Measurements -- OriginalOriginalDistribution

Normal Probability Plot

0

2

4

6

8

10

12

0.48 0.56 0.64 0.72 0.8 0.88 0.96 1.04

Cou

nt

Range, in.

0

2

4

6

8

10

12

0.48 0.56 0.64 0.72 0.8 0.88 0.96 1.04

Cou

nt

Range, in.

0.5

0.6

0.7

0.8

0.9

1

.01 .1 1 5 10 2030 50 7080 90 95 99 99.999.99

y = 0.737 + 0.118 norm(x) R= 0.98378

Thic

knes

s, in

.

Percent

in.0.30in.29.0612.0 ≈==S

in.12.0=SX

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Photograph AnalysisPhotograph Analysis

• Webs of main trusses• Webs of bridging trusses• Diagonal strut at the exterior wall

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Statistics for Photograph AnalysesStatistics for Photograph Analyses

Main trusses - original (85 measurements):• Average thickness = 0.6 in.• Standard deviation = 0.3 in.

Bridging trusses - original (52 measurements):• Average thickness = 0.4 in.• standard deviation = 0.25 in.

Diagonal struts - original (26 measurements):• Average thickness = 0.4 in.• Standard deviation = 0.2 in.

Main trusses - upgrade (52 measurements):• Average thickness = 1.7 in.• Standard deviation = 0.4 in.

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Equivalent ThicknessEquivalent Thickness

• Pseudo random number generator was used to select values from a lognormal distribution with central value and dispersion consistent with average of 0.75 and standard deviation of 0.3

Generated thickness profile

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

0 10 20 30 40 50 60

Rough TextureSmooth Texture

Thic

knes

s, in

.

Bar Position, in.

(a)

60 in

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Equivalent ThicknessEquivalent Thickness

Variable thickness• 3 data sets each

• Rough• Smooth

• average thickness = 0.75 in.• standard deviation = 0.3 in• distribution = lognormal

X

PP

60 in

Uniform thickness X

PP

60 in

• thickness = 0.4, 0.5, 0.6 in

• P = 9800 lb (12,500 psi)

• Applied heat flux from a 1100 ºC fire

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Equivalent ThicknessEquivalent Thickness

0.00

0.20

0.40

0.60

0.80

0 5 10 15 20 25 30

Rough 1Rough 2Rough 3Smooth 1Smooth 2Smooth 30.4" Uniform FP0.5" Uniform FP0.6" Uniform FP

Def

orm

atio

n (in

)

Time (min)

Load = 9800 lbs

(c)

0.6 in uniform thickness

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Types and Locations of SFRM Types and Locations of SFRM –– Floor SystemFloor System

ThicknessBuilding Component Material

Specified Installed Used in Analysis

Floor System

Original

Main trusses and diagonal struts BLAZE-SHIELD DC/F 0.5 0.75 0.6

Bridging trusses – one-way zone BLAZE-SHIELD DC/F 0.5 0.38 0.3

Bridging trusses – two-way zone BLAZE-SHIELD DC/F 0.5 0.38 0.6

Upgraded

Main trusses and diagonal struts BLAZE-SHIELD II 1.5 2.5 2.2

Bridging trusses – one-way zone BLAZE-SHIELD II 1.5 2.5 2.2

Bridging trusses – two-way zone BLAZE-SHIELD II 1.5 2.5 2.2

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Types and Locations of SFRM Types and Locations of SFRM –– Exterior WallExterior Wall

ThicknessBuilding Component Material

Specified Installed Used inAnalysis

Exterior Wall System

Box Columns

Exterior Face BLAZE-SHIELD DC/F 1 3/16 N/A 1.2

Interior Face Vermiculite Plaster 7/8 N/A 0.8

Spandrels

Exterior Face BLAZE-SHIELD II 1/2 N/A 0.5

Interior Face Vermiculite Plaster 1/2 N/A 0.5

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Types and Locations of SFRM Types and Locations of SFRM –– CoreCore

ThicknessBuilding Component Material

Specified Installed Used inAnalysis

Core Columns

Wide Flange Columns

Light BLAZE-SHIELD DC/F 2 3/16 N/A 2.2

Heavy BLAZE-SHIELD DC/F 1 3/16 N/A 1.2

Box Columns

Light BLAZE-SHIELD DC/F N/S N/A 2.2

Heavy BLAZE-SHIELD DC/F N/S N/A 1.2

Core Beams BLAZE-SHIELD DC/F 1/2 N/A 0.5