Engineers at Ground Zero

[Music] 911 was a Tuesday morning my wife has a name for it she calls it uh September 11th Sky because it was so blue that day there was a major incident in lower manhatt a twin engine not quite an airliner it could in fact be a commuter plane people rushed by my desk saying a plane hit the World Trade Center now hold hold on just a moment we got expion exping right now got I come out of the subway and a whole crowd of people are looking South with their heads raised to the sky a woman standing next to me says do you think it's coming down I go no the Lords have already redistributed it's achieved equilibrium which to me is a lifelong lesson not to draw conclusions instantaneously I understood that Engineers had a role when something like this occurs we all work together we just marched all the way down to the uh to the site when the towers collapsed the the ground shaking was felt and measured at the Lamont DH Observatory up in along the Palisades as like a 3 point something on the RoR scale which is how we measure earthquakes right the only thing that we could think of that was somewhat similar were how the structural engineers in California had developed a process for evaluating buildings after an earthquake you know the the city was shut down from 14th Street down so who decides when is it safe for people to go back to their homes so in order to to assess the 400 buildings immediately surrounding the Trade Center site we we used aerial photographs we sent people to the roofs of buildings we know materials and we know physics so and we know gravity like if something looks like it's going to fall we're pretty good uh candidates to make that assessment so the difference between doing a rapid assessment of a building that's collapsed already and doing a rapid assessment of a building that's standing and just is old and in need of Maintenance uh they're they're two very different things we do try to send everyone two construction sites as much as we can just so they can see how things get put together because as you see things getting put together it's easier to understand how things can fall apart there were so many buildings that were damaged and some of them you know very severely had the Winter Garden uh which is a glass structure that is between the AMX building and World Financial Center 1 and 2 there was actually a piece of the facade that was stuck in the corner of the American Express building that was kind of dangling they W up taking some you know cabling aircraft cabling what have you and tying it around the piece and anchoring it to one of the Interior columns we're structal engineers in the areas that were rebuilt specifically after the 93 bombing included the chiller plants we had the knowledge the ability to get to those same thing with the refrigerant from the tanks right if those are heated or they catch fire they create mustard gas that could kill people that are in you know working and you know spending all that time there we initially had 8 hour shifts all right but that wasn't working because there was so much going on so we had to change that to two um two 12-hour shifts I was surveying buildings at some point I was in the subway system looking at stuff and we were all over the place there was tremendous grief there were people that would stand along the west highway to thank the people that were going down and that was very emotional because we were going down there to help and these people that were standing there also wanted to help but the only way that they could help is to try to thank us and support us there were people who would stop their lives and and restaurants sent food and water for for everyone involved in the rescue effort so um I will never forget New Yorkers and and the nation the way the nation came to support New York and and the other areas attacked at that time that was pretty powerful someone came up to me and said that they're digging there with a back hole trying to clear out debris but the subway tunnels immediately below that if they puncture that tunnel groundwater will rush in and it will fill the entire New York City subway system slurry wall is dug in all the way around the the site and then as the basement floors are are built they put pressure against the slurry wall they hold back the Hudson River all right one morning it was a Sunday I get a call from someone from DDC and they said you better get down here so we run down there and we see this crack in the ground so it was very dangerous situation and I remember walking down behind the slurry wall which George Tamaro with our hard hats on and I said to him what are we doing here I was always aware that the Hudson was the threat the Hudson was the threat when we did the original wall we began to recognize that there was a Valley North of the Liberty Street slurry wall where the debris was not the full depth of the excavation which meant that the slurry walls did not have lateral support and we one morning we detected there was something like 14 in of movement we began pumping water outside to diminish the load on the wall and we began putting soil on the inside to support the wall so it wouldn't collapse that was that was the most critical part the water elevation was only about 10 or so feet below street level that if the Hudson River flooded into the site just due to the elev elevation differences between the Train the World Trade Center site New Jersey was that you would have flooded the World Trade Center site which in turn would have flooded the path tubes which in turn would have then flooded all the way up to 14th Street and flooded the entire New York City subway system the pieces of the tower were being uh shipped over to uh the New Jersey uh salvage yards where the pieces were being cut up and thrown on uh boats to go be uh recycled my first day on the site was September 13th and I was the last engineer to leave on July 15th 2002 we had a fence around the site locked it up and handed the keys back to the the city and to the port authority and you know it's it's like okay our job is done I was at the World Trade Center site from um that first week in September um through the the the last column being taken out of the out of the site so it was about 9 months a huge lesson learned for me and something that I will never forget is that despite the number of super smart people in the design and construction community that I had access to that were volunteering their time the group where this the sum of all their opinions is far better than the opinion of a few people the original Trade Center was designed to resist an airplane impact but airplanes have gotten so much bigger the the plane hit but because of the redundancy in the system it was able to redistribute the loads around the hole and so the gravity wasn't the problem the problem was the fire after 911 within a few weeks started what ultimately proved out to be the most extensive investigation of any Towers collapses or structural collapses ever this is actually the physics BAS B modeling of that impact looking from the top you can see the penetration of the airplane the severance of the columns in the front facade but also in the middle of the core of the building it's thin aluminum sheets uh uh that get shredded but they're still flying at about 500 mil an hour so essentially you have a power wash of metal impacting the structure that's keeping the building up the steel columns that were not severed are getting their fireproofing Stripped by that power wash of materials that is the primary cause for ultimately the collapses the structure of the tower had what is known as a three-dimensional Outrigger truss at the RO at the top of the building it was there to carry the TV antennas the roof truss on that day on 911 played a role for which it wasn't designed as you can see from this graphic it is connecting the columns in the core to some of the columns on the perimeter of the building that steel Grillage now will be able to act as a stabilizing and load R distribution mechanism since this building has excess strength it remains standing thanks to the role of the roof trust that roof trust saved lives on that day it allowed the full evacuation of occupants below the Impact Zone after 102 minutes which is the time the North Tower lasted it's no longer able to carry as much load as steel gets is heated it softens we recreated through physics-based computer modeling the collapse of wtc1 and you can see it's largely symmetric but the first thing you saw is the roof trust come down from the top that is because the core columns had been attacked by fire at the Impact Zone and they are less resilient than the perimeter tube they start sinking in first but after a while even the perimeter tube is being dragged down and you get what we call a pancaking event after that gravity is simply the strongest force you can ever imagine 3,000 people were lost the emotional toll of participating um in the entire post 9911 uh era was taxing uh on people people who would come back from their rotation on the site clearly exhausted with masks full of dust having seen things that are difficult to unsee I don't know that I really understood the health implications until much later you know a lot of us got sick also and years later uh obviously a lot of firefighters they were there you know every day uh but for those of us that were there four five 10 times uh exposed to the dust uh we started getting sick you know I mean things have changed since the 9/11 attack you know uh there's a there's a much more organized way that we're able to evaluate buildings that do [Music] collapse so many people thought downtown was dead it would never revive but it was the developers the real estate people The Architects also I don't know something about human nature that when something breaks they want to fix it okay that you don't just accept this as a loss the initial reaction was no one's ever going to go on a building again okay that was the initial reaction which turned out not to be correct okay we have more tall buildings now than than we ever had I think it's important to keep the memory of all these events alive because there's so much to learn from them I think the exact reason to to tell this story now is because there were generations of people who have no idea that this occurred they know about the firefighters they know about the eer emergency responders they know that the Red Cross was here but that there was a group of Structural Engineers who stood up and put their health on the line in many cases and contributed to this Collective effort to help New York City when when the city needed us the way everybody came together was an amazing thing and I don't think it is a rarity I think that this is human nature we want to take care of each other I I I I have much more faith in mankind after this [Music]