http://wtc.nist.gov/media/NCSTACmeetingDec18_2007.htm 1/18/2008

Meeting of the National Construction Safety Team Advisory Committee

December 18, 2007

Minutes

Committee Members

Paul Fitzgerald, Chair

Robert Hanson

Charles Thornton

David Collins

Forman Williams

John Barsom (did not participate)

Welcome and Introduction

Mr. Fitzgerald called the meeting to order at 1:05 pm. Five members of the NCST Advisory 

Committee (Fitzgerald, Hanson, Thornton, Collins, and Williams) were present on the 

conference call, which constituted a quorum. Mr. Fitzgerald reminded the committee that the 

meeting was being held in open session and that it was being webcast for the general public. He 

also announced that three individuals had requested the opportunity to speak during the public 

comment session and that those individuals would call in at approximately 2:30 pm to address 

the Committee. 

Shyam Sunder, Director, Building and Fire Research Laboratory, and WTC Lead 

Investigator

http://wtc.nist.gov/media/NCSTAC_December18(Sunder).pdf

Dr. Sunder presented an overview on the progress of the implementation of the recommendations 

that resulted from the investigation of the World Trade Center. He noted that eight 

recommended changes to the International Building Code were accepted for inclusion in the 

supplement to the 2006 International Building Code. He referred to the NIST press release 

issued in June 2007 for details on the code changes that were accepted (available on the NIST 

WTC website, http://wtc.nist.gov). An additional 47 code change proposals have been submitted 

to the International Code Council for the 2007/2008 code cycle. Dr. Sunder also noted that 

other organizations are actively considering changes to standards, codes, and guidance that are 

responsive to the WTC Investigation recommendations. These are positive movements by the 

private sector to respond to the recommendations and improve the safety of high rise buildings in 

the United States.

Dr. Sunder also announced that NIST has released a supplemental list of answers to frequently 

asked questions. This list has been posted to the NIST WTC Investigation website 

(http://wtc.nist.gov).

Dr. Sunder provided the committee with a brief overview of the status of the investigation of 

WTC 7. The overview included a review of the working hypothesis. He stressed that the 
2

working hypothesis is based on scientific/engineering judgment and simple analysis models, but 

has not yet been fully evaluated through rigorous analysis. The working collapse hypothesis has 

not changed since first reported in June 2004. NIST has developed additional detail on the 

initiating event sequence based on fire-induced failures resulting from normal building fires 

occurring in the tenant floors.

Dr. Sunder concluded his remarks with an update on the schedule for completing the 

investigation. He noted that the global analysis is anticipated to be completed by March and that 

NIST anticipates releasing the draft reports for public comment in July of 2008.

Following these remarks, the following questions were posed by Committee members and 

answered by Dr. Sunder. 

Q: What do you mean by normal building fires?

A: These are fires in spaces where the combustibles are normal building contents, ventilation is 

the normal building ventilation, and there are no exceptional combustibles such as diesel fuel in 

day tanks or in large tanks at the base of the building. In the case of the towers, the jet fuel was 

unusual, but even there we talked about normal building fires since the jet fuel burned within a 

matter of a few minutes. What burned over the next hour to hour-and-a-half were normal fires 

where the combustibles were building contents plus the airplane contents.

Q: But they were ventilated fires?

A: In both cases the ventilation was probably somewhat limited. Typically, when flames 

extend out from windows, there is excess fuel looking for air with which to react. 

Q: What will be the magnitude of the final report? Will there be a sizeable amount of 

documentation as with the towers?

A: In the case of the towers, the final reports totaled about 10,000 pages. In the case of WTC 7, 

we anticipate the final reports will total about 1000 pages or less. We anticipate releasing three 

reports, but this has not been finalized.

Therese McAllister, Project 6 Co-Leader

http://wtc.nist.gov/media/WTC7_Approach_Summary_18Dec07-Final.pdf

Dr. McAllister summarized the technical approach being followed in the investigation of WTC 7

and the status of the investigation since the last meeting of the NCST Advisory Committee in 

December 2006. Following her presentation, the following questions were posed by the 

Committee members and answered by members of the Investigation Team.

Q: If fires start in a building and there is no firefighting effort, is the building expected to come 

down? Or would it be expected that the building would remain standing after the fires have 

burned out?
3

A: Buildings are currently designed based upon E119 test results for building components and 

subassemblies. In the E119 test, components of a building are subjected to prescribed standard 

time-temperature exposure and a rating is assigned (e.g., 1 hour, 2 hour, etc.). If the walls, 

floors, and ceilings were intact before the fire and if doors are closed, the fire is presumed to be 

contained in the room of origin for some period of time that is roughly related to the fire ratings 

of these partitions. This would provide sufficient time for people to evacuate and for automatic 

sprinklers or manual firefighting efforts to control the fire. This containment would also limit 

the intensity and duration of the fire such that significant loss of strength of the fire-related 

structural components should not occur. The assumption is that the system as a whole will 

survive that exposure. The implicit assumption is that when there is a situation where the 

sprinklers do not function, there would be burnout of the building contents without collapse. It is 

not surprising that different buildings will react differently when exposed to normal building 

fires. Buildings designed more conservatively, such as those with more compartmentation, 

smaller floor spans, more massive structural elements, and greater redundancy in the structural 

system will perform better than a structure that has large, open floor plans with few 

compartments preventing the spread of the fire. The science has not evolved to the point of 

designing to meet the performance objective of burnout without collapse. This was a 

recommendation made in the final report on the Investigation of the Collapse of the WTC 

Towers. There was also a code change proposal to require providing for burnout without 

collapse that was submitted to the International Code Council (ICC) 2006/2007 code cycle but 

was not approved. Two proposals on burnout without collapse have been submitted for the 

2007/2008 code cycle. If either of these proposals is approved, design for burnout without 

collapse would be built-in to all buildings, not on a case-by-case approach.

Q: The time that the fires will burn is influenced by the fuel loading, so it is not just a question 

of building design, but it is also a question of building contents. If the objective is to design a 

building for burnout without collapse, then there should also be some restrictions on the fuel 

loading that could be put into the building after it is constructed. Has thought been given to that?

A: Yes, decades of thought and research have been devoted to that issue. Fuel (combustible) 

loading by itself does not tell the whole story; the rate of heat release in a fire is the most 

important factor. This is recognized worldwide and is beginning to appear in regulations and fire 

codes. Since the technology exists to manufacture low flammability products, there is the 

potential for additional requirements on families of building contents. 

C: So, for WTC 7, 4 lb/ ft2is an assumption that is closest to the observations, but in fact there 

was considerable uncertainty as to what the fuel loading actually was on the various floors of the 

building.

A: Yes. Remember that this value of 4 lb/ft2is the mass consumed in the fire. The actual fuel 

loading would be higher if much of the combustible mass was contained in file cabinets. The 4 

lb/ft2 was the result of an estimate for the WTC towers, based on the combustible mass of typical 

workstations and other flammable products and the density of these on the tenant floors. An 

estimate for the tenant floors in WTC 7 reached was the same value. There is definitely a 

degree of uncertainty in using these values and applying them to all the fire floors in the 
4

buildings. Our sensitivity analyses indicated that significantly higher fuel loading led to greater

disagreement of the fire simulations with the photographic evidence. 

C: That certainly is true; on the other hand, there could be residual burning.

A: Absolutely.

C: Not all buildings are expected to remain standing after burnout. The building codes allow for 

“frangible buildings”. That is why we limit the heights and areas of certain types of occupancies 

and structures -- so that there is not a catastrophe associated with those kinds of events and so

that we can address the needs of the occupants within a reasonable amount of time should those 

structures eventually fail. Most of the codes today assume that there is going to be some 

measure of intervention for fire protection of a facility if it is going to remain viable. If that does 

not happen, then there is some evidence of structures that have had burnout scenarios but even 

some of those had intervention either by mechanical means or by fire department response. 

Structures are lost on a daily basis. Residences are a primary example of that kind of structure,

and it is not likely that the codes will mandate that there should be a complete burnout of those 

kinds of buildings without failure of the structure.

Q: In your remarks, on page 9, you talk about Case A, B, and C temperatures to be completed

for the 16 story analysis, and then in the next bullet you talk about temperature files for a 47-

story model. Could you describe how the 16-story and 47-story models are interconnected?

A: Yes, there is a four-step sequence of computational simulation, each involving a different 

model. We recreated the fires using the Fire Dynamics Simulator (FDS) combined with our best 

information about the contents and layouts on each of those floors where significant fires were 

observed, which were floors 7, 8, 9, 11, 12, and 13. FDS generated moving fires that gave a 

good match to the observed progression of fire available from photos and videos on the east, 

west, and north faces. This provided our base analysis, Case A. Given that there are 

uncertainties in the exact amount of fuel and in the layouts, that there are gaps in the 

photographic evidence, and that something may have been happening farther inside the building 

that could not be seen from the exterior, we decided to bound this fire by increasing the rise in 

gas temperature by 10% (Case B) and decreasing the gas temperatures by 10% (Case C). These 

changes are within the possible variability of the fires.

In the second step, the Fire-Structure Interface (FSI) was used to superimpose these gas 

temperatures on the structural components for each of the three Cases.

 

In the third step, ANSYS is being used to determine possible initiating events based on the three 

fire cases. The ANSYS model is focused on identifying what local failures occurred within the 

structure. This model includes detailed renditions of the lower 16 floors (which encompass those 

floors that could have been heated by the fires) so that we can account for the thermal and 

structural response. Above sixteen stories, the weight of the rest of the structure is included. 

Nothing is ignored in terms of the forces on that structure. 
5

In the fourth step, we take information from the ANSYS modeling as input to the LS_DYNA 

model, which simulates the full 47 stories of WTC 7. The initiating event is used as an initial 

condition. LS-DYNA is a dynamic analysis that keeps track of mass and impact, such as was 

done for the aircraft impact analysis for the WTC Towers. We give an initial condition and then 

let the analysis run and see if we can simulate what was observed with the vertical progression of 

failure and the horizontal progression of failure.

Q: Were the fires were initiated in WTC 7 when the North Tower came down?

A: This is described in the 59-slide presentation. It is highly likely that the fires were initiated 

when the North Tower collapsed, although there could have been other factors as well. When 

the North Tower came down there were large pieces of debris including structural components 

from the tower that fell on WTC 7. When the South Tower fell there was dust, and possibly that 

went into WTC 7. There was a fire alarm condition recorded but not heard because the fire 

alarm system was on test condition that morning as it had been every morning at about the same 

time. As most in the fire world know, many things can cause ignition, because there are so many 

electrical outlets in a building, and places where a short circuit or other kinds of things can 

happen to ignite a fire. It is not our objective to find out when the fires ignited but to follow the 

fires as they developed and progressed through the building. Perhaps the collapse of the North 

Tower initiated the fires in WTC 7, but there could be other factors as well.

Q: There was a previous mention of the day tanks and pressurized lines for diesel fuel in WTC 

7. A 4 lb/ft2fuel load fire moved every 20 minutes; essentially it started and stopped every 20 

minutes, so if you do not have fuel in WTC 7, how could fires burn for as long as they had and 

taken out this major structure that had good fireproofing?

A: The fires in the towers did not stop after 20 minutes. The fires moved from location to 

location, meaning that at any given location the combustibles needed about 20 minutes to be 

consumed. While the combustibles at a location were being consumed, the fire front would be 

progressing to adjacent combustibles. Also, recall that the 4 lb/ft2 is the fuel load that is actually 

consumed. The total loading of furnishings could have been higher if there were substantial 

numbers of file cabinets, etc. Furthermore, the jet fuel led to widespread ignition of the 

combustibles, but the jet fuel was consumed quickly and was a minimal contributor to the 

sustained fires that weakened the structures in WTC 1 and WTC 2.

 

In the case of WTC 7, the estimated combusted fuel load was similar to the 4 lb/ft2 estimated in 

the towers. The initial fires were small enough that most were not visible at the windows for 

several hours. Most likely, these early fires involved only a small number of workstations at a 

time. The early fires raised the air temperatures, preheating other clusters of cubicles and 

leading to the larger fires seen during the afternoon. 

Once the fires had begun heating the air, the overall air temperatures on a floor continued to rise 

as new combustible material became involved. The local air temperatures began to fall when the 

local fuel supply was depleted and fresh air reached that area. The heating of the structure 

through its protective insulation was a result of both the high air temperatures that were reached 

and the duration of those high air temperatures, not just the duration of local burning.
6

Q: Why would these fires have burned for as long as they did? If beams sagged, buckled, and 

pulled they had to be exposed to the fire for more than 20 minutes.

A: The critical issue is the combination of temperatures to which the beams are exposed and the 

duration of that exposure. The FSI calculations showed that the floor assemblies were sensitive 

to these exposures, reaching 500 °C to 600 °C in some regions. While the metal decking had 

insulation on its lower surface, the concrete floor slabs had no fireproofing on their top surfaces. 

The floor beams were slender elements and had low thermal mass, and the thickness of the 

thermal insulation on the floor beams was less than that on the girders and columns. The 

sensitivity to heat was compounded by the very large spans. In the northeast section of the 

building there were 2000 ft2large span floor areas. Our analysis indicates fairly large 

magnitudes of sag in these areas.

C: It sounds like you are questioning the use of spray-on fireproofing for a 2 hour rating on a 

wide flange beam as effective fireproofing in the absence of sprinkler systems. 

A: What we are beginning to find is that ratings of individual components may not be a 

sufficient indicator of the thermal response of the larger structure.

Q: I am not questioning what you are doing, I am questioning the way that American architects

and structural engineers design buildings with spray-on fireproofing. I think you are basically 

coming out with a conclusion that maybe it does not work.

A: We will not make a recommendation at this point. Your input is very meaningful to us, and 

when all of the analysis is finished we will be in a position to make a statement about design 

practices. 

Q: This question has to do with World Trade Center 4, 5, and 6. My understanding is that the 

portions that collapsed were the portions where the towers came down and the rest of the 

buildings experienced complete burnout with sagging of beams but not collapse. Is that true?

A: World Trade Center 5 had some interior failures which were noticed because the roof was 

intact above these failures. There was a section on the east side of the building where floor 

beams had failed at connections. There were tees sticking out from the columns and the 

connections were made several feet away from the columns. Those connections failed in a 

number of places. You are correct on WTC 4 and 6; the damage there was due to the towers 

falling on top of them.

C: I appreciate all the effort NIST has made, and based on the summaries we have heard I am 

quite pleased. On the other hand, there are some questions that will be coming up, and I look 

forward to reading the documentation that describes and supports what NIST has been doing 

over the last couple of years. It is going to be very important, and the conclusions that come out 

of it could have far reaching effects, so we have to be careful that we do not miss something of 

great import.
7

C: As structural engineers design longer and longer floor spans, we will need to pay attention to 

the fire ratings.

Q: The connections could have had a considerable impact on how this building performed. Can 

you explain what the connections were and how they were fireproofed? 

A: There were two basic types of connections. One was a shear connection where the 

connection was between the web of the beam and the column, and the connection was made 

either with a single plate or double angles and then bolts and welds. How they used the bolts and 

welds either on the column or on the beam web identified three types of shear connections. The 

girders, which often supported the floor beams where they framed into the columns, had what we 

call a bearing seat connection, so there was a fairly substantial plate either constructed to look 

like an angle or supported off the column and that made a seat for the girder to sit on. 

Additionally, they had a clip, a lighter tie either at the web or at the top flange, mostly to provide 

lateral stability when the building was under construction rather than any real transfer of 

structural loads once the building was functioning. Of course there were variations on those 

connections, for example if the girders did not frame orthogonally into the face of the column, 

then we had to consider the angles. The particular geometries might have a change in sizing of 

different elements and that would change the mode of failure or the load at which failures might 

occur.

Q: What was the fireproofing on all those connections?

A: The drawings we received on the connections did not specify any information about 

fireproofing. What we have been using are photographs of the building when it was under 

construction and an understanding of standard practice for applying the fireproofing material that 

was used in WTC 7.

Q: Are you suggesting that there is no concrete information as to whether fireproofing was 

actually applied or not?

A: No, we have photographs that show the nature and extent of the fireproofing. However, in 

general practice the thickness of fireproofing applied to connections is not specified. 

C: I suppose that the average engineer would assume that it is the same as the thickness on the 

beam or the column.

A: Yes, that is how it is done in the field. Prior to this investigation, we did not really know

how connections behave in fire; in terms of the E119 testing that is done. Our recommendation 

in the towers report, which we will probably need to look at in the context of the WTC 7 report, 

is to develop an understanding of how connections perform in fires. 

C: That is where I was going with this. You do have to address that issue.
8

Discussion of the NCST Advisory Committee Report to Congress

Mr. Fitzgerald prepared a draft letter report to Congress which was circulated to the Committee 

members prior to the meeting in order to frame the discussion. The letter format followed the 

format that was used for last year’s report since there were no new investigations and little 

activity on the part of the Committee. The Committee members were in agreement on the format 

and the content of the draft report. Following discussion of a number of editorial changes, the 

Committee voted to accept the report to Congress with the editorial changes discussed. Mr. 

Fitzgerald took the action to make the editorial changes and prepare a final version of the report 

for transmittal to Congress.

Public Comment Period

Mr. Richard Gage

http://wtc.nist.gov/media/AE911Truth-NIST-Written-Submission12-18-07.pdf

http://wtc.nist.gov/media/AppendixC-fema403_apc.pdf

http://wtc.nist.gov/media/JonesWTC911SciMethod.pdf

Mr. James Gourley

Mr. Jerry Leaphart

http://wtc.nist.gov/media/121807CommentNISTJerryLeaphart.pdf