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When hijacked airliners crashed into
the tall Towers of the World Trade Center, in New York City,
each injected a burning cloud of aviation fuel throughout the
6 levels (WTC 2) to 8 levels (WTC 1) in the impact zone. The
burning fuel ignited the office furnishings: desks, chairs, shelving,
carpeting, work-space partitions, wall and ceiling panels; as
well as paper and plastic of various kinds.
How did these fires progress?
How much heat could they produce? Was this heat enough to seriously
weaken the steel framework? How did this heat affect the metal
in the rubble piles in the weeks and months after the collapse?
This report is motivated by these questions, and it will draw
ideas from thermal physics and chemistry. My previous report
on the collapses of the WTC Towers described the role of mechanical
forces (1).
Summary
of National Institute of Technology and Standards (NIST)
Basic facts about the WTC fires
of 9/11 are abstracted by the numerical quantities tabulated
here.
Table 1, Time and Energy
of WTC Fires
Item WTC 1 WTC 2
impact time (a.m.) 8:46:30
9:02:59
collapse (a.m.) 10:28:22
9:58:59
time difference 1:41:52 0:56:00
impact zone levels 92-99
78-83
levels in upper block 11
27
heat rate (40 minutes) 2 GW
1 GW
total heat energy 8000 GJ
3000 GJ
Tower 1 stood for one hour
and forty-two minutes after being struck between levels 92 and
99 by an airplane; the block above the impact zone had 11 levels.
During the first 40 minutes of this time, fires raged with an
average heat release rate of 2 GW (GW = giga-watts = 10^9 watts),
and the total heat energy released during the interval between
airplane impact and building collapse was 8000 GJ (GJ = giga-joules
= 10^9 joules).
A joule is a unit of energy;
a watt is a unit of power; and one watt equals an energy delivery
rate of one joule per second.
Tower 2 stood for fifty-six
minutes after being struck between levels 78 and 83, isolating
an upper block of 27 levels. The fires burned at a rate near
1 GW for forty minutes, diminishing later; and a total of 3000
GJ of heat energy was released by the time of collapse.
WTC 2 received half as much
thermal energy during the first 40 minutes after impact, had
just over twice the upper block mass, and fell within half the
time than was observed for WTC 1. It would seem that WTC 1 stood
longer despite receiving more thermal energy because its upper
block was less massive.
The data in Table 1 are taken
from the executive summary of the fire safety investigation by
NIST (2).
The NIST work combined materials
and heat transfer lab experiments, full-scale tests (wouldn't
you like to burn up office cubicles?), and computer simulations
to arrive at the history and spatial distribution of the burning.
From this, the thermal histories of all the metal supports in
the impact zone were calculated (NIST is very thorough), which
in turn were used as inputs to the calculations of stress history
for each support. Parts of the structure that were damaged or
missing because of the airplane collision were accounted for,
as was the introduction of combustible mass by the airplane.
Steel loses strength with heat.
For the types of steel used in the WTC Towers (plain carbon,
and vanadium steels) the trend is as follows, relative to 100%
strength at habitable temperatures.
Table 2, Fractional
Strength of Steel at Temperature
temperature, degrees C fractional
strength, %
200 86
400 73
500 66
600 43
700 20
750 15
800 10
I use C for Centigrade, F for
Fahrenheit, and do not use the degree symbol in this report.
The fires heated the atmosphere
in the impact zone (a mixture of gases and smoke) to temperatures
as high as 1100 C (2000 F). However, there was a wide variation
of gas temperature with location and over time because of the
migration of the fires toward new sources of fuel, a complicated
and irregular interior geometry, and changes of ventilation over
time (e.g., more windows breaking). Early after the impact, a
floor might have some areas at habitable temperatures, and other
areas as hot as the burning jet fuel, 1100 C. Later on, after
the structure had absorbed heat, the gas temperature would vary
over a narrower range, approximately 200 C to 700 C away from
centers of active burning.
As can be seen from Table 2,
steel loses half its strength when heated to about 570 C (1060
F), and nearly all once past 700 C (1300 F). Thus, the structure
of the impact zone, with a temperature that varies between 200
C and 700 C near the time of collapse, will only have between
20% to 86% of its original strength at any location.
The steel frames of the WTC
Towers were coated with "sprayed fire resistant materials"
(SFRMs, or simply "thermal insulation"). A key finding
of the NIST Investigation was that the thermal insulation coatings
were applied unevenly -- even missing in spots -- during the
construction of the buildings, and -- fatally -- that parts of
the coatings were knocked off by the jolt of the airplane collisions.
Spraying the lumpy gummy insulation
mixture evenly onto a web of structural steel, assuming it all
dries properly and none is banged off while work proceeds at
a gigantic construction site over the course of several years,
is an unrealistic expectation. Perhaps this will change, as a
"lesson learned" from the disaster. The fatal element
in the WTC Towers story is that enough of the thermal insulation
was banged off the steel frames by the airplane jolts to allow
parts of frames to heat up to 700 C. I estimate the jolts at
136 times the force of gravity at WTC 1, and 204 at WTC 2.
The pivotal conclusion of the
NIST fire safety investigation is perhaps best shown on page
32, in Chapter 3 of Volume 5G of the Final Report (NIST NCSTAR
1-5G WTC Investigation), which includes a graph from which I
extracted the data in Table 2, and states the following two paragraphs.
(The NIST authors use the phrase "critical temperature"
for any value above about 570 C, when steel is below half strength.)
"As the insulation thickness
decreases from 1 1/8 in. to 1/2 in., the columns heat up quicker
when subjected to a constant radiative flux. At 1/2 in. the column
takes approximately 7,250 s (2 hours) to reach a critical temperature
of 700 C with a gas temperature of 1,100 C. If the column is
completely bare (no fireproofing) then its temperature increases
very rapidly, and the critical temperature is reached within
350 s. For a bare column, the time to reach a critical temperature
of 700 C ranges between 350 to 2,000 s.
"It is noted that the
time to reach critical temperature for bare columns is less than
the one hour period during which the buildings withstood intense
fires. Core columnsthat have their fireproofing intact cannot
reach a critical temperature of 600 C during the 1 or 1 1/2 hour
period. (Note that WTC 1 collapsed in approximately 1 1/2 hour,
while WTC 2 collapsed in approximately 1 hour). This implies
that if the core columns played a role in the final collapse,
some fireproofing damage would be required to result in thermal
degradation of its strength." (3)
Collapse
Airplane impact sheared columns
along one face and at the building's core. Within minutes, the
upper block had transferred a portion of its weight from central
columns in the impact zone, across a lateral support at the building
crown called the "hat truss," and down onto the three
intact outer faces. Over the course of the next 56 minutes (WTC
2) and 102 minutes (WTC 1) the fires in the impact zone would
weaken the remaining central columns, and this steadily increased
the downward force exerted on the intact faces. The heat-weakened
frames of the floors sagged, and this bowed the exterior columns
inward at the levels of the impact zone. Because of the asymmetry
of the damage, one of the three intact faces took up much of
the mounting load. Eventually, it buckled inward and the upper
block fell. (1)
Now, let's explore heat further.
How Big
Were These Fires?
I will approximate the size
of a level (1 story) in each of the WTC Towers as a volume of
16,080 m^3 with an area of 4020 m^2 and a height of 4 m (4).
Table 3 shows several ways of describing the total thermal energy
released by the fires.
Table 3, Magnitude of Thermal
Energy in Equivalent Weight of TNT
Item WTC 1 WTC 2
energy (Q) 8000 GJ 3000
GJ
# levels 8 6
tons of TNT 1912 717
tons/level 239 120
lb/level 478,000 239,000
kg/m^2 (impact floors) 54
27
lb/ft^2 (impact floors) 11
6
The fires in WTC 1 released
an energy equal to that of an explosion of 1.9 kilotons of TNT;
the energy equivalent for WTC 2 is 717 tons. Obviously, an explosion
occurs in a fraction of a second while the fires lasted an hour
or more, so the rates of energy release were vastly different.
Even so, this comparison may sharpen the realization that these
fires could weaken the framework of the buildings significantly.
How Hot
Did The Buildings Become?
Let us pretend that the framework
of the building is made of "ironcrete," a fictitious
mixture of 72% iron and 28% concrete. This framework takes up
5.4% of the volume of the building, the other 94.6% being air.
We assume that everything else in the building is combustible
or an inert material, and the combined mass and volume of these
are insignificant compared to the mass and volume of ironcrete.
I arrived at these numbers by estimating volumes and cross-sectional
areas of metal and concrete in walls and floors in the WTC Towers.
The space between floors is
under 4 meters; and the floors include a layer of concrete about
1/10 meter thick. The building's horizontal cross-section was
a 63.4 meter square. Thus, the gap between floors was nearly
1/10 of the distance from the center of the building to its periphery.
Heat radiated by fires was more likely to become trapped between
floors, and stored within the concrete floor pans, than it was
to radiate through the windows or be carried out through broken
windows by the flow of heated air. We can estimate a temperature
of the framework, assuming that all the heat became stored in
it.
The amount of heat that can
be stored in a given amount of matter is a property specific
to each material, and is called heat capacity. The ironcrete
mixture would have a volumetric heat capacity of Cv = 2.8*10^6
joules/(Centigrade*m^3); (* = multiply). In the real buildings,
the large area of the concrete pads would absorb the heat from
the fires and hold it, since concrete conducts heat very poorly.
The effect is to bath the metal frame with heat as if it were
in an oven or kiln. Ironcrete is my homogenization of materials
to simplify this numerical example.
The quantity of heat energy
Q absorbed within a volume V of material with a volumetric heat
capacity Cv, whose temperature is raised by an amount dT (for
"delta-T," a temperature difference) is Q = Cv*V*dT.
We can solve for dT. Here, V = (870 m^3)*(# levels); also dT(1)
corresponds to WTC 1, and dT(2) corresponds to WTC 2.
dT(1) = (8 x 10^12)/[(2.8
x 10^6)*(870)*8] = 410 C,
dT(2) = (3 x 10^12)/[(2.8
x 10^6)*(870)*6] = 205 C.
Our simple model gives a reasonable
estimate of an average frame temperature in the impact zone.
The key parameter is Q (for each building). NIST spent considerable
effort to arrive at the Q values shown in Table 3 (3). Our model
gives a dT comparable to the NIST results because both calculations
deposit the same energy into about the same amount of matter.
Obviously, the NIST work accounts for all the details, which
is necessary to arrive at temperatures and stresses that are
specific to every location over the course of time. Our equation
of heat balance Q = Cv*V*dT is an example of the conservation
of energy, a fundamental principle of physics.
Well, Can
The Heat Weaken The Steel Enough?
On this, one either believes
or one doesn't believe. Our simple example shows that the fires
could heat the frames into the temperature range NIST calculates.
It seems entirely reasonable that steel in areas of active and
frequent burning would experience greater heating than the averages
estimated here, so hotspots of 600 C to 700 C seem completely
believable. Also, the data for WTC Towers steel strength at elevated
temperatures is not in dispute. I believe NIST; answer: yes.
Let us follow time through
a sequence of thermal events.
Fireball
The airplanes hurtling into
the buildings with speeds of at least 200 m/s (450 mph) fragmented
into exploding torrents of burning fuel, aluminum and plastic.
Sparks generated from the airframe by metal fracture and impact
friction ignited the mixture of fuel vapor and air. This explosion
blew out windows and billowed burning fuel vapor and spray throughout
the floors of the impact zone, and along the stairwells and elevator
shafts at the center of the building; burning liquid fuel poured
down the central shafts. Burning vapor, bulk liquid and droplets
ignited most of what they splattered upon. The intense infrared
radiation given off by the 1100 C (2000 F) flames quickly ignited
nearby combustibles, such as paper and vinyl folders. Within
a fraction of a second, the high pressure of the detonation wave
had passed, and a rush of fresh air was sucked in through window
openings and the impact gash, sliding along the tops of the floors
toward the centers of intense burning.
Hot exhaust gases: carbon monoxide
(CO), carbon dioxide (CO2), water vapor (H2O), soot (carbon particles),
unburned hydrocarbons (combinations with C and H), oxides of
nitrogen (NOx), and particles of pulverized solids vented up
stairwells and elevator shafts, and formed thick hot layers underneath
floors, heating them while slowly edging toward the openings
along the building faces. Within minutes, the aviation fuel was
largely burned off, and the oxygen in the impact zone depleted.
Thermal
Storage
Fires raged throughout the
impact zone in an irregular pattern dictated by the interplay
of the blast wave with the distribution of matter. Some areas
had intense heating (1100 C), while others might still be habitable
(20 C). The pace of burning was regulated by the area available
for venting the hot exhaust gases, and the area available for
the entry of fresh air. Smoke was cleared from the impact gash
by air entering as the cycle of flow was established. The fires
were now fueled by the contents of the buildings.
Geometrically, the cement floors
had large areas and were closely spaced. They intercepted most
of the infrared radiation emitted in the voids between them,
and they absorbed heat (by conduction) from the slowly moving
("ventilation limited") layer of hot gases underneath
each of them. Concrete conducts heat poorly, but can hold a great
deal of it. The metal reinforcing bars within concrete, as well
as the metal plate underneath the concrete pad of each WTC Towers
floor structure, would tend to even out the temperature distribution
gradually.
This process of "preheating
the oven" would slowly raise the average temperature in
the impact zone while narrowing the range of extremes in temperature.
Within half an hour, heat had penetrated to the interior of the
concrete, and the temperature everywhere in the impact zone was
between 200 C and 700 C, away from sites of active burning.
Thermal
Decomposition -- "Cracking"
Fire moved through the impact
zone by finding new sources of fuel, and burning at a rate limited
by the ventilation, which changed over time.
Heat within the impact zone
"cracks" plastic into a sequence of decreasingly volatile
hydrocarbons, similar to the way heat separates out an array
of hydrocarbon fuels in the refining of crude oil. As plastic
absorbs heat and begins to decompose, it emits hydrocarbon vapors.
These may flare if oxygen is available and their ignition temperatures
are reached. Also, plumes of mixed hydrocarbon vapor and oxygen
may detonate. So, a random series of small explosions might occur
during the course of a large fire.
Plastics not designed for use
in high temperature may resemble soft oily tar when heated to
400 C. The oil in turn might release vapors of ethane, ethylene,
benzene and methane (there are many hydrocarbons) as the temperature
climbs further. All these products might begin to burn as the
cracking progresses, because oxygen is present and sources of
ignition (hotspots, burning embers, infrared radiation) are nearby.
Soot is the solid end result of the sequential volatilization
and burning of hydrocarbons from plastic. Well over 90% of the
thermal energy released in the WTC Towers came from burning the
normal contents of the impact zones.
Hot Aluminum
Aluminum alloys melt at temperatures
between 475 C and 640 C, and molten aluminum was observed pouring
out of WTC 2 (5). Most of the aluminum in the impact zone was
from the fragmented airframe; but many office machines and furniture
items can have aluminum parts, as can moldings, fixtures, tubing
and window frames. The temperatures in the WTC Towers fires were
too low to vaporize aluminum; however, the forces of impact and
explosion could have broken some of the aluminum into small granules
and powder. Chemical reactions with hydrocarbon or water vapors
might have occurred on the surfaces of freshly granulated hot
aluminum.
The most likely product of
aluminum burning is aluminum oxide (Al2O3, "alumina").
Because of the tight chemical bonding between the two aluminum
atoms and three oxygen atoms in alumina, the compound is very
stable and quite heat resistant, melting at 2054 C and boiling
at about 3000 C. The affinity of aluminum for oxygen is such
that with enough heat it can "burn" to alumina when
combined with water, releasing hydrogen gas from the water, 2*Al
+ 3*H2O + heat -> Al2O3 + 3*H2. Water is introduced into the
impact zone through the severed plumbing at the building core,
moisture from the outside air, and it is "cracked"
out of the gypsum wall panels and to a lesser extent from concrete
(the last two are both hydrated solids). Water poured on an aluminum
fire can be "fuel to the flame."
When a mixture of aluminum
powder and iron oxide powder is ignited, it burns to iron and
aluminum oxide, Al + Fe2O3 + ignition -> Al2O3 + Fe. This
is thermite. The reaction produces a temperature that can melt
steel (above 1500 C, 2800 F). The rate of burning is governed
by the pace of heat diffusion from the hot reaction zone into
the unheated powder mixture. Granules must absorb sufficient
heat to arrive at the ignition temperature of the process. The
ignition temperature of a quiescent powder of aluminum is 585
C. The ignition temperatures of a variety of dusts were found
to be between 315 C and 900 C, by scientists developing solid
rocket motors. Burning thermite is not an accelerating chain
reaction ("explosion"), it is a "sparkler."
My favorite reference to thermite is in the early 1950s motion
picture, "The Thing."
Did patches of thermite form
naturally, by chance, in the WTC Towers fires? Could there really
have been small bits of melted steel in the debris as a result?
Could there have been "thermite residues" on pieces
of steel dug out of the debris months later? Maybe, but none
of this leads to a conspiracy. If the post-mortem "thermite
signature" suggested that a mass of thermite comparable
to the quantities shown in Table 3 was involved, then further
investigation would be reasonable. The first task of such an
investigation would be to produce a "chemical kinetics"
model of the oxidation of the fragmented aluminum airframe, in
some degree of contact to the steel framing, in the hot atmosphere
of hydrocarbon fires in the impact zone. Once Nature had been
eliminated as a suspect, one could proceed to consider Human
Malevolence.
Smoldering
Rubble
Nature is endlessly creative.
The deeper we explore, the more questions we come to realize.
Steel columns along a building
face, heated to between 200 C and 700 C, were increasingly compressed
and twisted into a sharpening bend. With increasing load and
decreasing strength over the course of an hour or more, the material
became unable to rebound elastically, had the load been released.
The steel entered the range of plastic deformation, it could
still be stretched through a bend, but like taffy it would take
on a permanent set. Eventually, it snapped.
Months later, when this section
of steel would be dug out of the rubble pile, would the breaks
have the fluid look of a drawn out taffy, or perhaps "melted"
steel now frozen in time? Or, would these be clean breaks, as
edge glass fragments; or perhaps rough, granular breaks as through
concrete?
The basements of the WTC Towers
included car parks. After the buildings collapsed, it is possible
that gasoline fires broke out, adding to the heat of the rubble.
We can imagine many of the effects already described, to have
occurred in hot pockets within the rubble pile. Water percolating
down from that sprayed by the Fire Department might carry air
down also, and act as an oxidizing agent.
The tight packing of the debris
from the building, and the randomization of its materials would
produce a haphazard and porous form of ironcrete aggregate: chunks
of steel mixed with broken and pulverized concrete, with dust-,
moisture-, and fume-filled gaps. Like a pyramid of barbecue briquettes,
the high heat capacity and low thermal conductivity of the rubble
pile would efficiently retain its heat.
Did small hunks of steel melt
in rubble hot spots that had just the right mix of chemicals
and heat? Probably unlikely, but certainly possible.
Pulverized concrete would include
that from the impact zone, which may have had part of its water
driven off by the heat. If so, such dust would be a desiccating
substance (as is Portland cement prior to use; concrete is mixed
sand, cement and water). Part of the chronic breathing disorders
experienced by many people exposed to the atmosphere at the World
Trade Center during and after 9/11 may be due to the inhalation
of desiccating dust, now lodged in lung tissue.
Did the lingering hydrocarbon
vapors and fumes from burning dissolve in water and create acid
pools? Did the calcium-, silicon-, aluminum-, and magnesium-oxides
of pulverized concrete form salts in pools of water? Did the
sulfate from the gypsum wall panels also acidify standing water?
Did acids work on metal surfaces over months, to alter their
appearance?
In the enormity of each rubble
pile, with its massive quantity of stored heat, many effects
were possible in small quantities, given time to incubate. It
is even possible that in some little puddle buried deep in the
rubble, warmed for months in an oven-like enclosure of concrete
rocks, bathed in an atmosphere of methane, carbon monoxide, carbon
dioxide, and perhaps a touch of oxygen, that DNA was formed.
In part one of this report
I discuss the physics of 9/11. In part 3, I address the collapse
of WTC 7.
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
[3] "Fire Structure Interface
and Thermal Response of the World Trade Center Towers,"
NIST NCSTAR1-5G, (draft supporting technical report G), http://wtc.nist.gov/pubs/NISTNCSTAR1-5GDraft.pdf,
(28 September 2006), Chapter 3, page 32 (page 74 of 334 of the
electronic PDF file).
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