What failed, when and how?
Outline:
*Initial theories: NOVA, Eagar, truss failure and pancaking
*Problems with truss theories: the floor system was a composite
unit, trusses tightly bound to floor pans and cross-trusses
*The Weidlinger Associates study: no pancaking, blamed core
collapse with no attempt to model actual core temperatures
*Still no sequential model backed by data
A Few Words About Zippers
The "zipper" analogy
presumes a rapid sequential failure of the floor trusses' attachments to the
outer and core columns, due to the shifting of loads after a single truss fails
completely. This is claimed to happen fast enough that the affected floor
falls at essentially free-fall velocity, and therefore impacts the floor below
at a high enough speed to dislodge it and begin a serial collapse. But the
actual floor structure was much more rugged and interconnected than what is
represented in this model, with the tops of the trusses rigidly attached to the
bottoms of the steel floor pans, and with both ends bolted to hanger brackets
welded to the columns.
In addition, transverse
trusses at right angles to the main trusses were connected to them at the
intersection points, creating a very strong and highly interconnected egg crate-like
structure. Under these conditions no amount of heating short of the
melting point could have weakened a single truss enough to cause it to sag into
a catenary curve. The trusses were also thermally connected to the floor
pan, turning the concrete and steel of the floor into a heat sink, along with
the mechanical support it provided. Without the complete failure of a
single truss we are left with no mechanism to begin
unzipping the zipper, and the the many interconnections between trusses and
floors would not allow the failure to propagate.
Several early explanations of
the Twin Tower collapses, including those proposed in the FEMA
report and on PBS in a NOVA
documentary, suggested that the initiating event for the collapses was the
failure of the floor trusses as shown in the animation below:
"As the temperature of floor slabs
and support framing increases, these elements can lose rigidity and sag
into catenary action. As catenary action progresses, horizontal framing elements
and floor slabs
become tensile elements, which can cause failure of end connections (Figure
2-21) and allow
supported floors to collapse onto the floors below." (FEMA report, section
2.2.1.4)
The most glaring problem with this visualization is that it
ignores the existence of the floors - and this is much more than just a
convenience to make the trusses more visible. The floors did not merely
sit on the trusses, but were rigidly attached to the tops of the trusses
for their entire lengths. The floors consisted of a corrugated steel pan
with 4" thick reinforced concrete on top. Those corrugations were
riveted and spot-welded to the top member of the truss, which means that the
kind of catenary sagging depicted in the animations was simply impossible.
The animation also omits the cross-trusses that
interconnected with the main trusses to create create a more rigid composite
structure, one that could not crumble away piece by piece as a result of local
heating.
Eagar Beavers and
Faulty Towers
Thomas W. Eagar, Sc.D. P.E.
Thomas Eagar espouses
a bizarre but well-publicized (thanks largely to NOVA) theory that "like most buildings, the World Trade Center was mostly air" and therefore was incapable of
toppling over. Any building bigger than a certain (unspecified) absolute
size, he claims, simply cannot topple over, no matter how it is damaged.
If more than some (unspecified) amount of damage is inflicted on such a
building, it will simply collapse straight down:
"In our normal experience, we deal with small things, say, a glass of
water, that might tip over, and we don't realize how far something has to tip
proportional to its base. The base of the World Trade Center was 208 feet on a
side, and that means it would have had to have tipped at least 100 feet to one
side in order to move its center of gravity from the center of the building out
beyond its base. That would have been a tremendous amount of bending. In a
building that is mostly air, as the World Trade Center was, there would have
been buckling columns, and it would have come straight down before it ever
tipped over.
Have you ever seen the demolition of buildings? They blow them up, and they
implode. Well, I once asked demolition experts, "How do you get it to
implode and not fall outward?" They said, "Oh, it's really how you
time and place the explosives." I always accepted that answer, until the
World Trade Center, when I thought about it myself. And that's not the correct
answer. The correct answer is, there's no other way for them to go but down.
They're too big. With anything that massive -- each of the World Trade Center
towers weighed half a million tons -- there's nothing that can exert a big
enough force to push it sideways."
(http://www.pbs.org/wgbh/nova/wtc/collapse.html
)
It is important to note that that
although Eagar is a Professor in the MIT Department of Materials Science,
his specific concentration is not in
structural analysis or failure analysis, subjects which would give him true expertise in
collapse analysis, but in the field of metallurgy and specifically the
properties of exotic welding alloys. His novel
theories of the
properties of tall buildings being dependent not on their proportions but on absolute
size, i.e. that a tall slender object greater than a certain size will lose
the ability to topple over and can only fail by telescoping into itself, have
never been expressed by any actual structural engineer to my knowledge and are
provably
false. His assertion that the top of the building cannot be pushed far enough
to move the center of gravity outside of the building's footprint is irrelevant to the realities of an actual collapse, since it
imagines a situation in which the top of an intact tower is pushed to one side
to initiate the collapse. This is an odd hypothetical, since it imagines a
tower that is not attached to the ground being tipped over by a lateral
force. But even so he gets the distance wrong, since
the actual center of gravity was near the middle floors of the tower - the top would have to
have moved at least twice as far for the middle floors to move the required 104
feet.
In reality, tall slender objects tend to topple in an identical manner
regardless of size*, and the "percentage of air" in a structure has
nothing to do with its mechanical properties or structural integrity. Any
failure beginning low enough in a tall structure will initiate a toppling event,
and even a progressive collapse beginning near the top will tend to wander off
to the side, causing it to terminate in an incomplete collapse. Eagar's
blithe assumption of the superiority of his notions to the practical experience
of demolition experts, and his implicit assumption of the ignorance and
simplicity of the experts in that field is quite remarkable, and says a great
deal about how he sees technical issues outside his area of expertise.
*At least over a very broad range
between the quantum and cosmological scales of size.
A
Point-by-Point Deconstruction of the Eagar/NOVA theory
The Silverstein/Weidlinger Associates Study
Matthys Levy, P.E.
Weidlinger
Associates
Los Altos, Calif. 94022
4410 El Camino Real, Suite 110
(650) 949-3010; fax (650) 949-5735
FACS has secured Weidlinger engineers as a resource for journalists. The firm is
a leader in defense-related blast effects research and design programs for more
than four decades. Weidlinger is one of the only US firms in the security
engineering field with decades of experience in both commercial structural
engineering and blast resistant design.
(http://www.facsnet.org/issues/specials/terrorism/engineerindex.php3)
"There is no doubt left about the sequence of failure," says
Matthys P. Levy, chairman of Weidlinger Associates Inc., the New York City-based
engineer that led the study.
"Failure of the floors...was shown not to have had any significant role in
the initiation of the collapses," says the report. Levy describes the floor
truss system as "not unsubstantial," acting more like a membrane than
a one-way system. "There was nothing wrong with it," he says. If the
floor trusses had collapsed first, there would have been a mass of smoke as
opposed to differentiated smoke, floor by floor, he adds.
(http://www.construction.com/NewsCenter/Headlines/ENR/20021104d.asp)
The report exonerates the floor trusses for the
collapses. "Failure of the floors...was shown not to have had any
significant role in the initiation of the collapses," it says. Studies
by Hughes Associates and ARUPFire led the team to conclude that tower floors
survived the initial impact of the planes, suffering only localized damage. On
the basis of a review of smoke plumes and fire spread, for each tower, the
engineers concluded that the fires did not lead to the collapses of the floors
affected before the towers fell.
The findings are intended to build on the study
initiated by the American Society of Civil Engineers and sponsored by the
Federal Emergency Management Agency, called World Trade Center Building
Performance Study. Released last May, it suggested subjecting the floor truss
system to "more detailed evaluation." But the [Weidlinger] study
also stated the truss systems "should not be regarded" as design
deficiencies, says the Silverstein report.
The Silverstein report also concludes that fire
temperatures were lower than typical "fully developed" office fires.
The fires were fueled by office furniture and floor contents initially ignited
by the jet fuel, which burned out quickly. Dust and debris distributed by the
crashes inhibited the fires, which at the impact floors were between 750°F and
1,300°F.
The engineering team is comprised of: Weidlinger
Associates Inc., led by Matthys Levy and Najib Abboud; LZA Technology/Thornton-Tomasetti
Group, led by Daniel Cuoco and Gary Panariello; ARUPFire, led by Richard Custer;
Hughes Associates Inc., led by Craig Beyler; SafirRosetti, led by Howard Safir;
Hillman Environmental Group, led by Christopher Hillmann and John B. Glass Jr.;
RWDI, led by Peter Irwin; Dr. W. Gene Corley, who led the ASCE-FEMA study;
Professor Sean Ahearn; and Z-Axis Corp., led by Gary Freed and Alan Treibitz.
Silverstein commissioned the reports for its insurance
claim on the World Trade Center. The organization has already given the reports
to the National Institute of Standards and Technology, which is studying the
collapses of the towers and Seven World Trade as part of a two-year study.
(http://www.construction.com/NewsCenter/Headlines/ENR/20021025b.asp)
One frame from the computer model of the initiation of the collapse of Tower 2
performed by Weidlinger Associates
After this promising debunking of the
truss failure/pancake theory, and some astute observations about fire
temperature, the report (at least what has been revealed of it publicly) goes on
to offer a very unsatisfying alternative. As Matthys Levy says above,
there is no doubt about the sequence of failure: the core columns had to fail
completely from the very first instant of the collapse. And as he
has pointed out elsewhere, to
create the vertical collapses that we saw in the Twin Towers all of the
47 very large columns that comprised the core had to fail at the same
instant.
This means that the initiating
event was a complete and simultaneous failure
of the core. And since the only thing around that can be blamed is the
fires, the report simply states (in the face of its own comments about fire
temperature) that the fires somehow weakened the core to the point of collapse:
"What happens to steel as a result of fire is that
it loses its strength," explained Matthys Levy, a founding partner of
Weidlinger Associates. "By the time the temperature inside the buildings
reached 400 degrees, the steel would have lost approximately 50% of its
strength. Eventually, gravity took over and the towers began to fall."
(Real Estate Weekly- Engineering firm investigates WTC collapse. ...)
So how much does temperature really
weaken steel? This diagram from "German
Engineers Help The US" gives a more realistic appraisal:
Collage
of images of the South Tower Collapse - (avi video)
With interesting comments from eye-witnesses.
Truss Animation: "...like a clothesline"
- (mpeg) More drooping trusses and disappearing floors,
and the miraculous catenary curve that appears as hardened steel turns into wet
noodles. Again the floors, the attachments of the trusses to the floor
pans and the transverse trusses are all ignored for clarity.
But wait, there's the core... - Although
Weidlinger and NIST (most recently in June 2004) now say definitively that the
floors did not initiate the collapse, and that total core failure is the
earliest identifiable event, this early video analysis from the FEMA/ASCE team
shows what certainly looks like the dusty outline of the core as the floors fall
away. And then just as mysteriously, after being relieved of the weight of
the floors, the core itself disintegrates and falls straight down, like a larger
version of the spire.
Part 4: The Ongoing NIST
Investigation
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