Federal Building and Fire Safety Inves

of the World Trade Center Disast

National Construction Safety T

Advisory Committee Meetin

Response to Public Comments 

September 12, 2005

John L. Gross, Ph.D., P.E. and Therese McAllis

Building and Fire Research Labora

National Institute of Standards and Techno

U.S. Department of Commerce

Joh

n.Gross@nist.go

v
Statistics Statistics

Comments Submitter

65 Code, Trade, and Professional 

105 Industry

18 Individuals
Statistics Statistics

Comments received - 188 Report Clarification Technical Edito1 31 11 4 1-6 72 7 10 1-6A 9 1 3 1-6B 7 0 5 1-6C 6 0 0 1-6D 6 16 0 

131 35 22 
Nature of Comments Nature of Comments

Clarification

1. Wind measurements to support neglecting of wind 2. Factors other than debris that may have affected 

damage (primer, jet fuel, etc.)

3. Validation of creep models4. Concrete slab role and models in composite floor 5. Fires used for components and subsystem analys6. Basis for 10 min analysis intervals for structural re7. Major subsystem and global modeling approach f

interaction

8. Description of events after collapse initiation 9. Role of thermal expansion and cooling phase in s

fires

10.Standard Fire Test floor assembly –

use of AISC 

strength and weld design

11.Use of heat flux and temperature measurements 
Nature of Comments Nature of Comments

Technical Issues

 Comments provided with technic

al justificatio

• Measured thickness of SFRM before 11 Sept

 Comments that were general in nature

• Extent of SFRM damage• SFRM damage was source of events leading • Extent of floor sagging with SFRM in place• Validity of collapse hypothesis• Alternative hypotheses with regard to blasts o

demolition
Significant Changes to NCSTAR 1 Significant Changes to NCSTAR 1

 Explained/clarified role of thermal expansion 

response

 Explained/clarified the use of SFRM thicknes

in estimating the as installed and upgraded th

 Addressed sequence of events following colla Addressed alternative hypotheses (leading to 

principal finding)
Significant Changes to NCSTAR 1 Significant Changes to NCSTAR 1

-

 Added justification for neglecting wind effects 

wind speed data recorded at surrounding airp

 Expanded discussion of extent of SFRM dam

impact

 Referenced NCST

AR 1-3D for validation of c

 Explained/clarified modeling of concrete slab Referenced NCST

AR 1-5G for basis of temp

min intervals

 Explained/clarified approach for floor-exterior 

for major subsystem and global analys

e

s
Clarification to Findings Clarification to Findings

 NIST found no corroborating evidence f

hypotheses suggesting that the WTC tobrought down by controlled demolition uplanted prior to September 11, 2001. Nfind any evidence that missiles were firetowers. Instead, photos and videos from clearly showed that the collapse initiated impact floors and that the collapse proginitiating floors downward, until the dust obscured the view.
Significant Changes to Technical R

Significant Changes to Technical R

NCSTAR 1-6A

 Explained procedure for determining thickness of  Added new appendix with results of ballistic impac

coated steel members and gypsum board enclosu

NCSTAR 1-6B

 Clarified procedure for designing MIG welds to du

resistance welds used in original fabrication

 Clarified that “furnace temperature measurements

control temperature measurements”

 Expanded discussion of radiation (heat flux) meas

observed differences in heat flux between furnacethis difference on reproducibility (added to list of “further study”)