...through my friend Henry Barkhorn. ---Christopher Gray
>Subject: WTC attacks, perspective of an engineer
>Date: Sun, 30 Sep 2001 17:27:48 -0400
>From: "Cosema Crawford" <[log in to unmask]>
>To: "princeton-engineers"
><[log in to unmask]>
Folks -
Many news reports have reported on the design and performance of the
towers, with their remarkable pancaking collapse. My own involvement has
been more below ground.
I am frequently down at "ground zero" as deputy chief engineer at the
Transit Authority (a recent change from my previous career as a
suspension bridge engineer). I have been in the subway tunnels that run
under and adjacent to the WTC complex, have seen the destruction that
was caused by the collapsed buildings, and have been amazed to see large
areas of the basements that are OK. (See Friday's NY Times for a
interesting story on subway damage at
http://www.nytimes.com/2001/09/28/nyregion/28SUBW.html?searchpv=past7days
) One of the things that amazed me to see was the steel columns from
upper storeys of the WTC towers which speared down through the roadway
and 10 feet of overburden to pierce through our tunnels. The N&R station
on Church Street is a block away from the south tower. Steel columns
from the 80th floor of that tower pierced down over 30 feet into the
earth next to our subway box, and were sticking up out of the ground at
least 50 feet. Removal of those spears is difficult to do without
damaging our tunnel.
I am serving on a special committee advising on methods to secure the
70' high slurry basement wall which is now in danger of being breached.
Collapse would let the Hudson River flow into the box. The wall was
built with temporary tie-backs, or steel cables anchored in the outlying
soil, but those were cut after the basement floors were in place (if not
cut, they can snap over time and become missiles in the building). With
the basement floors collapsed, the walls are being held in place by
non-engineered rubble. Engineers (called "cave dwellers") are being
lowered by cable through holes in the rubble to ascertain what parts of
the basement flooring are intact and therefore presumably lending
strength to the basement wall. A map is being developed for each
basement level (7 levels in all) identifying good and bad areas.
Removal of debris will take a long time just based on volume of
material, but it will take even longer due to the need to protect the
basement wall. For example, cranes cannot be placed within 30-50 feet of
the wall for fear of adding to the outside pressure on it. This longer
reach on the crane booms limits the load they can pick. Plans are
underway to re-install the tie-backs so that the box can be emptied.
Initially there were over 1000 tie-backs. Now they have to be installed
hanging down on a cable from the top of the wall, a decidedly more
difficult operation. Just to protect ourselves in case the wall is
breached, we have installed concrete walls in the 1/9 tubes north and
south of the WTC site to serve as plugs. The 1/9 tunnels are
interconnected with other subway lines downtown. The Port Authority is
doing the same thing in the Path train tubes over in New Jersey.
The change in the site over time has been amazing. Now it is much more
of a construction site than a recovery site, although firefighters and
FBI are still all over the place. Security requirements to get into the
sight are changing every other day, as a new entity takes over which is
confusing (Fire Dept, Police Dept, Mayor's Office of Emergency
Management, FEMA). They are initiating a new pass system this weekend,
this time with photos I understand, which means another wait in line. I
am also involved in a private sector initiative by the NYC Partnership
(a group of industry titans with political and financial pull) to come
up with short- and long-term action plans to restore New York City to
glory. I am part of a group pushing for a redistribution of NYC's
financial district to a larger area. It doesn't make sense to have the
financial might of the city (the country, the world?) in a single
condensed neighborhood. I'm in favor of development along the lower west
side of Manhattan, some of which is now a wasteland, along with improved
transit in that area.
I feel very lucky that in the aftermath of the destruction that I was
able to work productively, that I didn't feel helpless, as did many of
my friends. I am also very much impressed by the efforts of the Transit
Authority, where I have been employed only 4 months, to restore train
service so quickly. The 2/3 tubes in the area were completely full of
water on Thursday and Friday after the attack (the water main feeding
collapsed WTC was severed and pumped water into our subway, Con Ed cut
off the power feeding our pump stations), yet they re-opened for service
on Monday. To me, this is simply incredible.
Although our office is several blocks from the site, we did not loose
anyone in the collapse. We did have a lot of people get close to the
site who saw things that no one should have to see. Some employees lost
loved ones. The emotional damage from this event touches just about
everyone in some way.
Some lessons learned for disaster response:
- The as-built drawings for the WTC were closely held at the WTC by the
Port Authority due to terrorist threats, and hence were destroyed in the
attack. The only complete and accurate set that wasn't destroyed was in
the office of Les Robertson, the original designer and a recent ENR
Newsmaker of the Year. Unfortunately, his office was a couple blocks
from Ground Zero, and couldn't be accessed for 2 days. Rescuers needed
the drawings to identify potential pockets of survivers and means to get
to them. We had similar experience trying to get a hold of drawings for
other buildings in the area. A centralized, secure database for all
buildings and infrastructure elements (water mains, etc.) is essential
in times of disaster.
- Similarly, home and cell phone numbers for key people need to be
readily available. This includes staff, consultants and contractors. We
were displaced from our office, and it was difficult to locate and
communicate with our own staff much less private sector folks, many of
whom were also displaced. Thank goodness for my Palm Pilot, although I
didn't have all the numbers I needed.
- Evacuation of people from lower Manhattan was accomplished mostly on
foot, walking north on the perimeter highways. Tens of thousands walked
over the Brooklyn Bridge, which was not a great idea considering it is a
likely target. It took a long time to clear everyone out. A more secure
means of quickly transporting lots of people needs to be considered.
- Our old subway tunnels were designed with a steel frame every 5 feet.
An interesting byproduct is that the limits of destruction are abrupt
and clearly defined. The tunnels are fine up to the first frame
impacted, and then they are collapsed. Other types of construction would
have had damage extend far beyond the zone of compression.
- The value of engineers has been greatly elevated during this disaster.
No decisions, large or small, are made at Ground Zero without first
consulting with an engineer. Engineers are quoted all over the NY Times
and other newspapers and magazines. Engineers are even being referred to
as "heros" which is not often heard.
Connie Crawford '78 *81
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