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Garcia concludes his widely applauded primer on the laws of physics
and the myths of conspiracists by explainging the not-so-mysterious
collapse of WTC 7. But
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Now
Bright, windless September morning in
Manhattan, looking south and slightly west across Vesey Street
from the 12th floor of WTC 7. The eight story US Customs House
(WTC 6) lies directly across the way, and beyond it the North
Tower (WTC 1), slightly rightward to the west, with the South
Tower (WTC 2) even further off, left of WTC 1 to the east.
Then a plane, loud, fast, low,
directly overhead flying south; the sun glints off the dimpling
of its shiny aluminum painted skin; its 156 ft wingspan over
three quarters the width of a Tower face -- puff! The lightning
clarity of the moment blinks, the airplane disappears, an orange
fireball erupts out of the north face of WTC 1 engulfing its
ninth decade of stories. Thinking stops.
Hour = 8:46:30
a.m.; Time = 0.
Hurried calls, nervous chatter,
excitement, fear; transfixed, watching the smoke engulf the top
of WTC 1; and then another airplane, flying up from the south
-- puff!
The plane disappears into the
far side of WTC 2, the southern face. A fireball bursts through
the north face, consuming the seventh decade of stories. Rocketing
debris shoots out of the northeast corner -- this way?! -- it
falls short, some whisks past just to the left.
Hour = 9:02:59
a.m.; Time = 16.5 minutes.
In time, the landing gear and
an engine thrown out of WTC 2 would be found two and three blocks
north, and within a block east of WTC 7. A section of the fuselage
from that plane fell atop WTC 5, the nine story North Plaza Building
east of the US Customs House. (1)
Some would wonder, "Am
I shuddering, or did I feel an earthquake?" Seismometers
would record the airplane impacts at local magnitudes 0.9 and
0.7, respectively. (1b), (2)
Phone home, reassure; what
to do, work?, only watching out the window is possible, chatter
is stunned to silence. People are beginning to jump. Time in
front of you is racing, while time for you has stopped. A crack,
the top of WTC 2 twists and drops into a gray opacity of billowing
powder. The South Tower collapses.
Hour = 9:58:59
a.m.; Time = 1 hour, 12.5 minutes
A blast of dust rushes straight
in, the scratching of grit pelting windows and the shattering
of glass can be heard, a veil is drawn over the death of a building
with too many of its occupants. WTC 7 shakes, magnitude 2.1;
over in 10 seconds. (2)
Frantic calls home, some flee,
time to lock up the sensitive files and leave before the trains
are jammed -- are they still running? Should some of the computer
drives be taken along, for security and just in case? Lights
go out, power from Con Edison is cut. The emergency generators
kick in, power and phones work. Another power fault, then some
power returns. Maybe the dust cloud is choking the diesel generator
air intakes along Floor 5, and some units are shut down. Crack!
-- WTC 1 collapses.
Hour = 10:28:22
a.m.; Time = 1 hour, 42 minutes.
The upper block of WTC 1 drops
into the burning impact zone and ejects a cascade of incandescent
metal and heated stone laterally, from near the 97th story (368
m), at between 12 m/s (27 mph) to 15 m/s (34 mph) during the
1.5 seconds it takes to fall down to the original height of the
71st story (269 m). (3)
This hot volley, within the
overall pyroclastic cannonade discharged by WTC 1 during its
collapse, hurtles at 86 m/s (193 mph) at a steep angle down into
the face of WTC 7 from Floors 18 to zero. A solid missile --
a hot section of I-beam? -- punches into Floors 11 and 12, bursting
through the concrete floors and touching off fires. The elevator
shafts at Floors 8 and 9, about 10 to 15 m (33 to 49 ft) into
the building, are ruptured and the elevator cars fall out onto
the floors. The air pressure wave presses on eardrums, stairwells
fill with dust and smoke, and lights go out, the building shakes
for nearly 10 seconds; magnitude 2.3. (2)
Time resumes. Some phones are
still lit, but they make no connection.
Wandering confused, Floor 7,
heavy dust, one cubicle is burning at the west end -- flashlights?,
firemen! They lead people down through the choking haze. The
lobby is layered in white dust, wires hang from the ceiling,
the street is littered with wreckage, a huge cloud rolls all
along Vesey Street blocking out the view south.
Hour = noon;
Time = 3 hours, 13.6 minutes.
WTC 7 was mortally wounded.
In 5 hours and 21 minutes, it would collapse. This article is
a visualization of what probably happened. Only gods and the
dead have certainty; we, the living, have rationality and courage
to guide us through the puzzles and the perils of life.
WTC 7:
By The Numbers
WTC 7 was a 176 m (576 ft)
tall, 47 story building with a trapezoidal cross section (about):
99 m (325 ft) along the north face, 76 m (249 ft) along the south
face, 45 m (148 ft) north-south width, and 47 m (153 ft) along
the east and west sides, (4). WTC 7 was about 107 m (350 ft)
north of WTC 1, across Vesey Street.
A number of engineering reports
have been written about the collapse of WTC 7, because of its
uniqueness. A consistent story emerges through the mass of detail.
The basic model of the WTC 7 collapse was stated in the earliest
report, by FEMA (1c), and increasingly amplified upon by subsequent
investigators at NIST -- the National Institute of Standards
and Technology, a federal agency within the U.S. Commerce Department's
Technology Administration. See footnotes (5), (6), (7) and (8).
WTC 7 was built in 1987 over
an existing Consolidated Edison electrical substation. The Con
Ed substation was three stories high, and took up the northern
half of the footprint of WTC 7. The 1967 construction of the
substation accounted for the eventuality of a building above
it, and a much larger and stronger foundation was built. Also,
a series of columns rose through the area of the substation,
for future use.
The design of WTC 7 was larger
than anticipated by the provisions of 1967, so additional foundation
columns were sunk. Also, the placement of columns in WTC 7 above
Floor 7 did not match all the tops of columns connected to bedrock
and waiting to be used. Thus, a series of trusses were designed
to transfer the vertical loads above Floor 7 and redistribute
them laterally to match the waiting columns below Floor 4. This
transition used triangular assemblies of structural steel joined
into a framework spanning two stories, Floors 5 & 6.
Part of the transition structure
included a Floor 5 made of 11 inches of reinforced concrete on
top of a 3 inch 18 gage composite metal deck (supported on I
beams); Floor 6 was 3 inches of concrete on a 3 inch 20 gage
metal deck; the northern half of Floor 7 was 5 inches of reinforced
concrete on a 3 inch 18 gage metal deck, and the southern half
of Floor 7 was 8 inches of concrete with two layers of reinforcement
(no metal deck). Floors 8 and up (except 21, 22, 23) had 2.5
inches of concrete over 3 inch 20 gage metal decks. These metal
decks were sheets of metal with corrugations (metal thickness
listed by gage number).
The combination of three massive
floors and interconnected triangular supports made the framework
of Floor 5 to Floor 7 a diaphragm locking WTC 7 together laterally,
core columns and walls (encasing elevator shafts and stairwells)
to perimeter columns. The construction of WTC 7 above Floor 7
was similar to that of the WTC Towers (9). The irregular framing
between Floors 5 and 7 made for less desirable tenant space,
but it was well protected by the robust construction, an ideal
location for the building's machinery and the emergency power
systems.
Machine
Space and Emergency Power Systems
Only machinery resided on Floors
5 and 6. Floor 6 had two large cut-outs, one along the east side,
another in the southwest corner, to allow for two-story mechanical
spaces. A set of louvers spanned the height of Floors 5 and 6
along the eastern face of the building. Table 1 lists the equipment
that resided on Floors 5 through 9 (ground level is floor 0).
Table 1, Machinery on Floors
5 to 9, WTC 7
Floor Items
9 1 generator (1 tank) for
(tenant) U. S. Secret Service
8 1 generator (1 tank) for
(tenant) American Express
7 3 generators (1 tank) for
the Mayor's Office of Emergency Management
The "tank" noted
in the table would be a 275 gallon diesel fuel tank, the maximum
size allowed on any given floor by the NYC Building Code.
There were five emergency power
systems in WTC 7. Three of them (American Express, OEM, U.S.
Secret Service) drew fuel from the other two and larger systems
(Salomon Smith Barney, Silverstein Properties). (1c), (8)
The emergency power for the
building (Silverstein Properties) was provided by two 900 kW
generators on the southwest corner of Floor 5. They drew fuel
from a 275 gallon tank nearby, and this was replenished by pumps
drawing from two 12,000 gallon tanks at ground level under the
loading dock, at the southwest corner of the building.
The SSB emergency power system
used nine 1,725 kW generators on Floor 5: three in the southwest
corner, two near the west end of the north face, four at the
east end of the north face. Louvers for air intake and exhaust
were situated on the building faces near the generators. Because
there was already a 275 gallon "day tank" on this floor,
the SSB system pumped on demand from their own pair of 6,000
gallon storage tanks, also situated under the loading dock, under
the southwestern part of the building.
The fuel supplier was contracted
to keep the tanks full, and they were full that day.
Fuel pipes for all systems
except SSB ran up the western side of the core of the building,
along elevator shafts. The SSB pipes ran up a shaft through mechanical
spaces near the southwest corner of the building.
Kindling
After 1 p.m. on September 11,
2001, WTC 7 was an evacuated, stricken building. The southwest
corner and central third of the south face had been ripped open
by the cascading debris from the collapse of WTC 1. Fires burned
in sections of Floors 6 through 30 at different times, and they
migrated along their floors independently, seeking new sources
of fuel. From the street the fires on Floors 11 and 12 appeared
most intense. Many fires in the area went unchecked because utility
power for electrical pumps, and water pressure for fire engines
had either diminished or been lost.
This is what happened.
A Pumped
Oil Spill
The debris fall ripping into
the southwest corner ruptured the oil pipes of the SSB pressurized
fuel distribution system. Operating as intended -- the lack of
utility power triggering the "need", and the lack of
pressure due to a severed pipe signaling the "demand",
the SSB system pumped oil up from its 12,000 gallon basement
reservoir, maximally with a pressure of 50 psi (pounds per square
inch) and flow rate of 75 gpm (gallons per minute), onto Floor
5.
Pumping would have started
at 9:59 a.m., when Con Ed cut utility power to WTC 7; and the
spilling would have started a half hour later when the pressurized
pipe was cut. The SSB pumps could have drained the two 6,000
gallon tanks in 2 hours and 40 minutes. Engineers from the New
York State Department of Environmental Conservation found that
"there was a maximum loss of 12,000 gallons of diesel from
two underground storage tanks registered as 7WTC." (10)
Additionally, "Both tanks
were found to be damaged by debris and empty several months after
the collapse. Some fuel contamination was found in the gravel
below the tanks and the sand below the slab on which the tanks
were mounted, but no contamination was found in the organic marine
silt/clay layer underneath." (7)
By contrast, 20,000 gallons
of oil was recovered from the two 12,000 gallon tanks of Silverstein
Properties. (10)
Pulled up by the emergency
pumps, the SSB diesel fuel went , from the 6,000 gallon storage
tanks, under the loading dock, under the southwestern part of
the building, to floor 5.
It may all have been pumped
out by 1 p.m., or it may have been pumped out at a rate as low
as 29 gpm for 7 hours. Since this fuel was absent from the wreckage,
it was burned. You can see it as the huge plume of black smoke
rising from the World Trade Center, in panoramic photographs
of that day. Diesel fuel can supply 2.13 MW of power per gpm
given an air supply of 1333 cfm (cubic feet per minute). (11)
Thus, a diesel fuel gusher
of 75 gpm burning with excess air would produce 160 MW of heat;
a total energy of 1536 GJ for the 12,000 gallons. This energy
is equivalent to that released by an explosion of 367 tons of
TNT. If the pumping rate is lower, or the air supply is throttled,
then the burning would occur at a lower rate. Since the louver
system along Floor 5 was designed to supply each of the nine
SSB engines with 80,000 cfm, it seems likely that a fuel oil
fire there would find sufficient air for combustion. For a discussion
of heat at 9/11, and energy units, CounterPunchers will soon
be able to have my study, "the Thermodynamics of 9/11",
to be published shortly on the CounterPunch website as part of
our final package on the actual physics and engineering realities
of the collapse of the WTC buildings.
The diesel fuel spill spread
out along Floor 5, which had been partly shielded from damage
by the sturdiness of Floor 7, in addition to its own robustness.
The fuel spilled down elevator shafts and breaks near the center
of the south face. Floor 4 has a great deal of open space along
its eastern two thirds near the south face. Fuel spilling from
above would find an easy route to the eastern side of the middle
of the building down to Floor 3.
Truss 1,
Truss 2 and Truss 3
A bicycle frame is a truss
supporting your weight on the axles of the wheels.
A truss is a rigid framework
of beams used to support a bridge, roof or floor. The beams in
a truss are usually joined so the empty spaces they enclose have
triangular and rectangular shapes. A truss transfers the weight
it supports along its span, laterally out to its ends where this
weight is then carried by columns or foundations into the ground.
A truss is how structural engineers shift vertical loads laterally
to distant supports. Many railroad bridges are trusses, hollow
rectangular space defined by a network of beams joined in a triangulated
fashion, and through which trains move.
A folding ladder opened into
an "A frame" is a truss. It supports your weight and,
say, a can of paint near the top, by transferring the downward
force out to the feet of the A. It relies on a horizontal bar
connecting the sloping legs to resist the lateral force pushing
the A to open and the top to drop.
WTC 7 had three major trusses,
two at the eastern end of the building, and one at the western
end. These trusses transferred gravity loads carried by columns
above Floor 7, laterally to the positions of columns below Floor
4. These trusses supported relatively large floor areas above
Floor 7 for the number of columns below Floor 4.
Truss 1 and Truss 2 were aligned
roughly along the east-west direction, roughly parallel to each
other, and roughly alongside each other when viewed along the
north-south direction.
Truss 1 was recessed from the
north face by over 1/3 of the width of the building, Truss 2
was recessed from the south face by over 1/3 of the width of
the building.
Truss 1 was roughly aligned
with the northern edge of the building core as it existed above
Floor 7, and Truss 2 was roughly aligned with the east-west centerline
of this core.
The northern edge of the mechanical
bay on Floors 5 and 6 aligned roughly with Truss 1. Truss 2 aligned
with the east-west line bisecting the area of this mechanical
bay, but it only extended from the core region to half the distance
to the east face.
The eastern end of Truss 2
was a column at the center of the nearly-rectangular space mapped
out by the eastern mechanical bay of Floors 5 and 6. This particular
column was Column 80. The columns rising out of Truss 2 were
Columns 80, 77 and 74, from east to west.
Recall, the pattern of columns
below Floor 4 did not match the pattern above Floor 7; for example
there was no continuation of Column 77 (middle of Truss 2) below
Floor 4.
Truss 3 was another formidable
structure, and it occupied a similar zone at the western end
of the building; it was aligned in the north-south direction.
Diagrams of these trusses may help to visualize the distribution
of gravity loads in WTC 7. (13)
Dark Fire
Oil pooled in the vicinity
of Truss 2. It was ignited by local office fires, and this burning
heated the volumes occupied by the spill, further volatilizing
combustible hydrocarbon materials. Air entered the fire through
the louver system of Floors 5 and 6, as well as through the opening
gouged out of the south face by the debris fall from WTC 1. Air
probably entered the south face along Floors 0 to 5, and smoke
exhausted up through the south face above Floor 5.
The observation of dense smoke
rising out of the south face while the other sides remain clear
is consistent with an airflow entry through the east face louvers
at Floor 5; combustion of vapors in Floors 3 to 6; heating of
the eastern interior of the building; and smoke billowing up
through the open atriums and lobbies of Floors 3 and 4, and the
breached mid-third of the south face up to about Floor 14. See
photos, (14)
Most of the heat generated
would be trapped within the heat capacity of the building's structure.
The maximum heat we expect here is 1536 GJ, which is half (51%)
of that released in the WTC 2 fire (3000 GJ) and one-fifth (19%)
of the WTC 1 fire (8000 GJ). (12)
However, the fires in the Towers
occurred within larger volumes. Continuing the "ironcrete"
example from (12), let us assume that heat is stored in the structure
of WTC 7, which is estimated to fill 5.4% of the building volume,
and be made of a fictitious homogenization of 72% iron and 28%
concrete -- ironcrete -- that has a volumetric heat capacity
of Cv = 2.8*10^6 joules/(Centigrade*m^3).
Trapping heat in an ironcrete
matrix can be thought of as the charging of a thermal battery.
If the WTC 1 fire was concentrated
in 6 stories, with a total volume of 96,480 m^3, then the volume
of ironcrete would be 5210 m^3, and its average temperature rise
would be 549 C.
If the WTC 2 fire was concentrated
in 4 stories, with a total volume of 64,320 m^3, then the volume
of ironcrete would be 3473 m^3, and its average temperature rise
would be 309 C.
If we assume the oil fire in
WTC 7 occurs on the eastern third of the floor space of three
floors, then the volume of the oil fire equals that of one floor,
which is roughly 15,000 m^3. In this case, the ironcrete volume
is 810 m^3, and its temperature rise is 677 C. This estimate
assumes all the energy of combustion contained in the fuel oil
is released and captured by the structure; clearly, an overestimate.
If we assume that up to half
of the combustion energy is lost, because the air is throttled,
and because fuel vapors are lost to the atmosphere (as hot, smoky
pollution), then we arrive at 768 GJ released within 3 to 7 hours,
producing an ironcrete temperature rise of 339 C. Since the flame
temperature is about 1100 C (2000 F), we can expect metal supports
within continuing fires to heat up by much more than the average
amount estimated here.
Table 2, WTC Ironcrete
Examples
Item Units WTC 1 WTC 2
WTC 7
energy GJ 8000 3000 768
solid volume m^3 5210 3473
810
energy density MJ/m^3 1536
864 948
temperature rise C 549 309
339
duration minutes 102 56
180-420
The thermal energy density
is near 1000 MJ/m^3 in all three examples, which correspond (as
highly simplified idealizations) to three buildings that collapsed
because of thermal weakening of their frames. Perhaps this observation
can help to estimate the risks posed by potential fires elsewhere.
Heat Exhaustion
All of the structural analysis
done by FEMA and NIST points to a failure of Truss 1 or Truss
2 -- Truss 2 seems more likely to me -- as the initiating failure
in WTC 7. The sequence is as follows:
-> thermal weakening of
Truss 2 leads to its failure,
-> the loss of support low
in the eastern interior propagates to the roof,
-> the weight (and dynamic
force) of material falling onto the diaphragm based on Floor
5 tips this rigid layer of the building,
-> this causes failure of
column joints to the diaphragm,
-> lack of vertical support
through the diaphragm progresses up the interior of the building
west of Truss 2 (and/or Truss 1),
-> the difference in collapse
timing east and west of Truss 2 creates a vertical crack/crease/kink/fold/break
through the building above Truss 2 (Column 80),
-> a progressive collapse
propagates up and material falls freely,
-> since the building implodes,
exterior walls falls in.
To sum up:
The blast of hot debris from
WTC 1 kindled fires in WTC 7 and caused an emergency power system
to feed the burning to the point of building collapse.
One of the building's major
bridging supports was heated to the point of exhaustion by the
burning of an abundant store of hydrocarbon fuel.
Manuel Garcia a native New Yorker who works as a
physicist at the Lawrence Livermore National Laboratory in California
with a PhD Aerospace & Mechanical Engineering, from Princeton
His technical interests are generally in fluid flow and energy,
specifically in gas dynamics and plasma physics; and his working
experience includes measurements on nuclear bomb tests, devising
mathematical models of energetic physical effects, and trying
to enlarge a union of weapons scientists. He can be reached
at mango@idiom.com
Endnotes
(web sites active on dates
noted)
[1] FEMA 403, "World Trade
Center Building Performance Study: Data Collection, Preliminary
Observations, and Recommendations, (second printing)," September
2002, (9 October 2006):
[a] Cover
& Table of Contents, ,
[b] Chapter 1, "Introduction,"
(a recommended summary of events; 21 pages of text),
[c] Chapter 5, "WTC
7,(32 pages of text)
[6] "Interim Report on
WTC 7," June 2004, NIST, http://wtc.nist.gov/progress_report_june04/appendixl.pdf,
(8 October 2006), 56 pages (text), (I think Figures L-6 and L-7
have captions switched; Figure L-8 repeats "Floor 2 plan"
and Floor 3 plan is not shown.)
[7] "Part
IIC - WTC 7 Collapse," NIST presentation April 5,
2005, 42 pages, ( October 9, 2006); shows correct Floor 3 plan,
compare to (6)
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