John L. Gross, Ph.D., P.E.

Building and Fire Research Labora

National Institute of Standards and Techno

U.S. Department of Commerce

John.Gross@nist.gov

Federal Building and Fire Safety Inves

of the World Trade Center Disast

National Construction Safety T

Advisory Committee Meetin

Project 6 – Standard Fire Te

WTC Tower Typical Floor Cons

October 19, 2004
Purpose of the Standard Fire Tests

Purpose of the Standard Fire Tests

 To establish the baseline performance o

system of the WTC towers as they were built,

 To differentiate the factors that most inf

collapse of the WTC towers as they manormal building and fire safety considerthose unique to the terrorist attacks of S11, 2001, and

 To study the procedures and practices u

accept an innovative structural and firepsystem. 

* ASTM E 119 - Standard Test Methods for Fire 

Building Construction and Materials
Factors Studied Factors Studied

 Fireproofing thickness

• 1⁄2 in. “as specified”• 3⁄4 in. “as applied

”

 Scale of the test

• Full-scale (35 ft span)• Reduced-scale (17 ft span)

 Test restraint conditions

• Restrained• Unrestrained
Test Conditions Test Conditions

3 17 ft Restrained 2 35 ft Unrestrained 

4 17 ft Restrained 1 35 ft Restrained 

Thermal 

Restraint

Test Span
SFRM Thickness SFRM Thickness

Thicknesses of SFRM – original construction:

 “As Specified” = 0.5 in.

• Letter dated October 30, 1969 from Robert J. Linn (Manage

World Trade Center) to Mr. Louis DiBono (Mario & DiBono)

 “As applied” = 0.75 in.

• Determined from thickness measurements of the existing fir

and 24 of WTC 1 - performed by the Port Authority in 1994

Thickness of SFRM - “upgraded” (since 1995)

 “As specified” = 1.5 in.

• PANYNJ white paper titled “Fireproofing Requirements for W

Tenant Floor Joist Construction that Requires Installation D

or Local Removal to Facilitate Construction,” August 1995.

 “As applied” = 2.5 in.

• Determined from thickness measurements – PANYNJ Cons

floors affected by fire
SFRM Application SFRM Application

SFRM was applied as follows:

 “As applied” = 0.75 in. (Tests 1 to 3)

• Applied to main trusses• Applied to bridging trusses at one half • Not applied to metal deck but overspr

 “As specified” = 0.5 in. (Test 4)

• Applied to main trusses• Not applied to bridging trusses• Not applied to metal deck
WTC Tower Floor Truss Fireproofin

WTC Tower Floor Truss Fireproofin

© 1993 Reproduced with permission of U.S. Mineral Products Company

dba Isolatek International and Morse Zehnter Associates

© 1993 Reproduced with permiss

dba Isolatek International and M
Effect of Restraint Against Therma

Unrestr

Co

Restrained Test Condition

Reaction Frame

Reaction Frame

Floor

Assembly

Furnace

Reaction Frame

Floor Slab

Gap
Effect of Restraint Against Therma

If thermal expansion is restrained, forces will develop

Floor assembly is subjected to fire conditions

Floor Assembly 

Furnace

Heated floor assembly wants to expand
Standard Methods of Fire Tests of 

Construction and MaterialsASTM Designation: E 119-61

 The ASTM E 119 - 61 conditions 

acceptance address the followin

• Resistance to passage of flame• Resistance to the transmission 

through the floor

• Structural soundness of the con
Conditions of Acceptance - Restrained Assem

 In obtaining a restrained assembly classific

following conditions shall be met:

• The specimen shall have sustained the 

during the classification period without dunexposed surface conditions which wilwaste.

• Transmission of heat through the specim

classification period shall not have been the average temperature on its unexposthan 250 oF (139 oC) above its initial tem

Standard Methods of Fire Tests of Construction and MaterialsASTM Designation: E 119-00
Conditions of Acceptance – Restrained Assem

 In obtaining an unrestrained assembly clas

following conditions shall be met:

• For specimens employing steel structur

(beams, open-web steel joists, etc.) spaft on centers, the temperature of the steexceeded 1300°F (704°C) at any locatioclassification period

• nor shall the average temperature recor

thermocouples at any section have exce(593°C) during the classification period.

Standard Methods of Fire Tests of Construction and MaterialsASTM Designation: E 119-00
Conditions of Acceptance - Unrestrained Asse

 In obtaining an unrestrained assembly clas

following conditions shall be met:

• The specimen shall have sustained the 

during the classification period without dunexposed surface conditions which wilwaste.

• Transmission of heat through the specim

classification period shall not have been the average temperature on its unexposthan 250 oF (139 oC) above its initial tem

Standard Methods of Fire Tests of Construction and MaterialsASTM Designation: E 119-00
Design of Reduced-Scale Tests

Test conditions of acceptance

 Temperature on unexposed surface of asse

• Thermal properties of concrete• Thickness of concrete

 Temperature of the steel (unrestrained ratin

• Thermal properties of steel and fireproofing• Size of steel sections, thickness of fireproof

 Sustain applied load

• Mechanical properties of steel and concrete• Size of steel sections and thickness of conc• Geometry of assembly• Loading
Preparation of Test Assemblies Preparation of Test Assemblies
UL (Northbrook) Furnace UL (Northbrook) Furnace
UL (Northbrook) Furnace with Test 

UL (Northbrook) Furnace with Test 
Loading of the UL Tests Loading of the UL Tests 

- Northbro

Northbro
Loading of the UL Tests Loading of the UL Tests 

- Toronto Toronto
Post Test Observations Post Test Observations 

- Slab Defle

Slab Defle

Test 1 - before
Post Test Observations Post Test Observations 

- Restraine

Restraine

Unrestrained Test Assembly Unrestrained Test Assembly

Test 1

Test 2
Post-Test Observations Test Observations – Steel Tru

Steel Tru
Post-Test Observations Test Observations – Slab Defl

Slab Defl

Unexposed Surface Deflection

-Center of Assembly0246810121416

0 50 100 150 

Time (minutes)

Vertical Deflection (in)
Unexposed Surface Deflection/Span

-Center of Assembly0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045

0 50 100 150 

Time (minutes)

Deflection/Span

Post-Test Observations Test Observations – Slab Defl

Slab Defl
Results From Standard Fire Tests a

Results From Standard Fire Tests a

___________________________________________________(1) Imminent collapse(2) Vertical displacement exceeded capability to measure accurately(3) Did not occurNote: Colors of the numbers indicate the correlation between the end-point criteria 

and the standard fire test rating.

--- 58 66 58 (3) 120(1

17 ft, restrained, 1⁄2 in. fireproofing

4

210

(2

180 157 86 76 (3) 17 ft, restrained, 3⁄4 in. fireproofing

3

146

(2

--- --- 76 62 (3)

35 ft, unrestrained,

3⁄4 in. fireproofing

2

116 

(1

--- 111 66 62 (3) 35 ft, restrained, 3⁄4 in. fireproofing 1

Maximum

(1300oF)

Average

(1100oF)

Maximum

(Ambient

+325oF)

Average

(Ambient

+250oF)

Failure 

to

Support 

Load

Steel Temperatures Temperature on Unexposed Surface

Test

Termi

ated

(min

Times to Reach End-Point Criteria (min)

Test Description
Observations Observations

 The test assemblies were able to withstand s

conditions for between three quarter hour an

 The restrained WTC floor system obtained a 

rating of 11⁄2 hr while the unrestrained floor sya 2 hr rating. Past experience with the ASTM method would lead investigators to expect thunrestrained floor assembly would not perforthe restrained assembly, and therefore, it wolower fire rating.

 A restrained fire rating of 2 hr was determine

scale test with the “as-applied” (3⁄4 in. or 19 mthickness while a restrained fire rating of 11⁄2 determined from the reduced-scale test with fireproofing thickness.
Observations (Cont’d) Observations (Cont’d)

 The result described above raises the questi

not a fire rating based on the ASTM E 119 p17 ft (5 m) span floor assembly is “scalable” system such as found in the WTC towers whranged from 35 ft (11m) to 60 ft (18 m). 

 A fire rating of 3⁄4 hr was determined from the 

test with the “as-specified” (1⁄2 in. or 13 mm) fthickness. This rating would not have satisfieYork City building code rating requirement foin Construction Class IB buildings—the desigto the WTC towers when they were built.
Observations (Cont’d) Observations (Cont’d)

 An average thickness of fireproofing of 3⁄4 in

variability as determined from measurement

Port Authority (cov=0.4) is approximately eqperformance basis, to 0.6 in. uniform thickne

 Data from two floors in WTC 1are not neces

representative of all fireproofing conditions, thickness may indeed be less than 3⁄4 in.
Issues

Adequacy of the ASTM E 119 Standard to pguidance on:

 Criteria for determining structural limit sta

including failure, and means to measure 

 Scale of test assembly versus prototype  Effect of end restraint conditions (restrain

unrestrained) on test results, including thstiffness,

 Structural connections, Combination of loading and exposure (te

profile) adequately represent expected c
Issues (cont’d) Issues (cont’d)

Adequacy of the ASTM E 119 Standard to pguidance on:

 Procedures to analyze and evaluate data 

resistance tests of other building componassemblies to qualify an untested buildin

 Repeatability and reproducibility of test r

test currently defines rating for system),

 Relationships between prescriptive rating

performance of the construction in realisfires.
Thank you

http://wtc.nist.gov