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Banned
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Originally Posted by MSD
How do you get "no diesel fuel involved" from that?
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The disclosure by the chief NIST WTC 7 investigator at the 12/18/07 meeting, that NIST's "working hypothesis" does not currently include diesel oil fueled fires, only fires that are "normal building fires".....
The transcript of the minutes of the 12/18/07 meeting were apparently not available until two days ago. In this post, #180, I posted a link to the recording of the meeting and a log of the time points in the recording, where relevant points, highlighted below, were discussed:
http://www.tfproject.org/tfp/showpos...&postcount=180
Quote:
http://wtc.nist.gov/media/WTC7_Appro...ec07-Final.pdf
Federal Building and Fire Safety Investigation
of the World Trade Center Disaster
WTC 7 Technical Approach and Status Summary
December 18, 2007
Therese McAllister, Ph.D., P.E.Building and Fire Research LaboratoryNational Institute of Standards and Technology U.S. Department of Commerce
....<h3>The working hypothesis is based on an initial local failure caused by normal building fires, not fires from leaking pressurized fuel lines or fuel from day tanks.</h3>
•This hypothesis may be supported or modified, or new hypotheses may be developed, through the course of the continuing investigation.
page 6 of 9
http://wtc.nist.gov/media/NCSTACMeet...utes121807.pdf
1/18/2008
Meeting of the National Construction Safety Team Advisory Committee
December 18, 2007
Minutes
....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......
.....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
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. <h3>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.</h3>
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, <h3>and there are no exceptional combustibles such as diesel fuel in
day tanks or in large tanks at the base of the building.</h3> 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......
page 2
....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/ ft2 is 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/ft2 is 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
page 3
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. <h3>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.</h3>
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.
page 4
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Last edited by host; 01-20-2008 at 04:57 PM..
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