Accounts of Tower Structural Instability and Expected Collapse

Unless otherwise noted, FDNY accounts are from here and PAPD accounts are from here. Further reference: NIST Structural and Fire Protection Damage Due to Aircraft Impact.

The North Tower

"Who cannot (sic) forget that eerie creaking sound that emanated throughout the city right before the North Tower fell?" –Marc Morial, President and CEO, National Urban League Source


• At 9:30 am, a FDNY Chief Officer inside WTC 1 feels the building move and makes the decision that the building is no longer safe.
• At 9:49 am, NYPD helicopters provide a radio report stating that “large pieces” are falling from WTC 2.
• At 10:07 am, NYPD aviation units warn that WTC 1 may collapse.
• At 10:20 am, NYPD aviation unit reports that WTC 1 is leaning to the south. Source

FDNY Assistant Chief Joseph Callan: "Approximately 40 minutes after I arrived in the lobby, I made a decision that the building was no longer safe. And that was based on the conditions in the lobby, large pieces of plaster falling, all the 20 foot high glass panels on the exterior of the lobby were breaking. There was obvious movement of the building, and that was the reason on the handy talky I gave the order for all Fire Department units to leave the north tower." Source

Callan: "For me to make the decision to take our firefighters out of the building with civilians still in it, that was very tough for me, but I did that because I did not think the building was safe any longer, and that was just prior to 9:30."Source

EMS Division Chief John Peruggia: "I was in a discussion with Mr. Rotanz and I believe it was a representative from the Department of Buildings, but I'm not sure. Some engineer type person, and several of us were huddled talking in the lobby and it was brought to my attention, it was believed that the structural damage that was suffered to the towers was quite significant and they were very confident that the building's stability was compromised and they felt that the north tower was in danger of a near imminent collapse.

I grabbed EMT Zarrillo, I advised him of that information. I told him he was to proceed immediately to the command post where Chief Ganci was located. Told him where it was across the street from number 1 World Trade Center. I told him "You see Chief Ganci and Chief Ganci only. Provide him with the information that the building integrity is severely compromised and they believe the building is in danger of imminent collapse." So, he left off in that direction."

NYPD Aviation Reports

FDNY firefighter Kevin Gorman:"Guys were giving us water, wet rags to put on our head, and we were standing there, and there was a cop I knew who came by and gave me a drink of water, and then as he was standing there, he said, "Aviation just reported that the north tower is leaning." I said, "Which way is it leaning?" He said, "This way." So we started to turn around walking. John Malley, who was right behind me, I turned around for him, because he was doing something, either putting his coat on or something, and as I was looking at him I heard the explosion, looked up, and saw like three floors explode, saw the antenna coming down, and turned around and ran north.
Q. About how long would you say it was from when the police officer told you it was leaning?
A. Within 30 seconds."

FDNY Firefighter Brendan Lowrey: "We started walking south to the command center when a Police Officer stopped us and said, "hold up, guys. I have helicopters --" he was on the cell phone "--on the cell phone here." And he says, "when this one comes down, it's coming right for us." Meaning coming up West Street.

NYPD Aviation Units: Minutes after the south tower collapsed at the World Trade Center, police helicopters hovered near the remaining tower to check its condition. "About 15 floors down from the top, it looks like it's glowing red," the pilot of one helicopter, Aviation 14, radioed at 10:07 a.m. "It's inevitable."

Seconds later, another pilot reported: "I don't think this has too much longer to go. I would evacuate all people within the area of that second building." Source

10:20 NYPD – Aviation 14 states the WTC 1 is leaning. (NYPD, McKinsey & Company) NIST NCSTAR 1-8, p. 227

Federal engineering investigators studying the destruction of the World Trade Center's twin towers on Sept. 11 said New York Police Department aviation units reported an inward bowing of the buildings' columns in the minutes before they collapsed, a signal they were about to fall.

"The NYPD aviation unit reported critical information about the pending collapse of the building,'' said Sivaraj Shyam- Sunder, who heads the institute, at a press briefing in New York. "Any time that information could have been communicated faster to the emergency responders in the buildings, it would have helped save lives."

According to Shyam-Sunder, the concave bowing of the steel was seen on the sides of the towers opposite where the planes hit them. At 10:06 a.m. that morning, an officer in a police helicopter reported that ``it's not going to take long before the north tower comes down.'' This was 20 minutes before it collapsed. In another radio transmission at 10:21 a.m., the officer said he saw buckling in the north tower's southern face, Shyam-Sunder said. Source

Summary of NYPD SOD radio channel communications about the north tower (NIST NCSTAR1-8 p.37)

10:06: NYPD officer advises that it isn't going to take much longer before the north tower comes down and to pull emergency vehicles back from the building.

10:20: NYPD aviation unit reports that the tip of the tower might be leaning.

10:21: NYPD aviation unit reports that the north tower is buckling on the southwest corner and leaning to the south. NYPD officer advises that all personnel close to the building pull back three blocks in every direction.

10:27: NYPD aviation unit reports that the roof is going to come down very shortly.

Inside the North Tower

Firefighter Mike Cancel, Ladder 10:We could feel the building starting to twist above us. I called Ladder 10 three times, Ladder 10 roof to Ladder 10. There was no answer. I said we have to evacuate, the building's coming down. Again, there was no response.Source

On the 56th floor, an architect believes the building was failing structurally.
Architect Bob Shelton had his foot in a cast; he'd broken it falling off a curb two weeks ago. He heard the explosion of the first plane hitting the north tower from his 56th-floor office in the south tower. As he made his way down the stairwell, his building came under attack as well. "You could hear the building cracking. It sounded like when you have a bunch of spaghetti, and you break it in half to boil it." Shelton knew that what he was hearing was bad. "It was structural failure," Shelton says. "Once a building like that is off center, that's it."Source

Structural Engineer Al Masetti: At some point, perhaps when I was down around the 20th floor (north tower), there was a very clear and distinct radio message: "...structural instability...." It seemed obvious to me that some lightly dressed and unencumbered fireman had reached the scene of the impact, was able to evaluate what was there, and was able to report what he saw. Source

An engineer from the Department of Buildings reported that the structural damage appeared to be immense. The stability of both buildings was compromised. In particular, the engineer was worried about how long the north tower would stand.
Jim Dwyer and Kevin Flynn. "102 Minutes: The Untold Story of the Fight to Survive Inside the Twin Towers" New York: Times Books, 2004

Roy Bell: "They said they had stretchers and wheelchairs down there, but I just wanted to get the hell out of the building," he said. "I ran into a building engineer, who told me there was only one safe exit out and that the building wasn't stable." Source

Around 9:15, Drohan heard (Port Authority WTC Construction Manager Frank) DeMartini over the walkie-talkie.
"Any construction inspector at ground level."
Drohan acknowledged that he was on the street.
"Can you escort a couple of structural inspectors to the 78th floor?" DeMartini asked.
DeMartini had seen something in the steel–Drohan was not sure what–that he did not like. The drywall had been knocked off parts of the sky lobby, exposing the elevator shafts, and revealing the core of the building. That had prompted his first radio alert, warning that the elevators might collapse. Now DeMartini wanted inspectors from a structural engineering firm to come up to the 78th-floor sky lobby and take a look. (102 Minutes, p. 147)

William Rodriguez, who led firefighters partway up the stairwells (from an account of a speech of his): The firemen made it up to the 27th floor but were exhausted from the burden of their equipment. As William had ascended the stairwell he, as well as the firemen, had heard explosions from the 20th through the 30th floor. Chunks of the building were falling down all around them and they could literally hear the creaking in the building.Source
William Rodriguez:The stairs were cracking. The sheet rock, when I went up opening the doors, was falling on top of me and on top of the firemen constantly. And the swaying of the building made it easier for that to come off. Source

PAPD Sergeant David Lim: On the way down, we were losing our lights & could feel the bldg falling apart. ...You could feel the building starting to collapse internally. I got down to about the twenty-first floor, when I met Chief Romito, Captain Mazza and Lieutenant Cirri, and they had come from the Police Academy to assist. And I remember, they were trying to make a stretcher for this fellow that had difficulty breathing.

So I told the Chief, I didn’t know if he knew that the other Tower was down. And I remember everybody starred at their blackberries. They had these special pagers I guess that told them when things were happening.And I remember showing the Chief the inside of the floor, because we were still in the staircase so you could see part of it: part of the building is already collapsed in, internally. So the Chief said, “Okay, forget about this, would you just carry him down?”(Lim later became trapped in the north tower B stairway. Source: 9/11 Commission Public Hearing testimony, day 1)

B.J.B.: Water was pouring down the stairwell, and all the while the building was creaking and cracking, and it felt like it was coming apart.Source

Erik O. Ronningen : I remember how calm and orderly the descent in the stairwells was… and how smoky… accompanied occasionally with the snapping sounds of tortured pipes and walls stressed beyond endurance. Source

Sandra Gonzales: "All the way down it felt like the ground was falling out from under you. I knew the building had been severely damaged, and all the way down you could feel that it was about to collapse." Source

FDNY Firefighter Hugh Mettham: We reached the sixth or seventh floor and are met by many firefighters coming down the stairs, informing us that the upper floors are collapsing and that there’s a heavy odor of gas and fuel.


PAPD Chief Joseph Morris: "And also met with Chief Tony Whitaker, who is the commanding officer of the World Trade Center. At that point he told me he felt that Building 1 was going to collapse, very soon, based on what he had seen. At that point we were able to get the command bus, which had stalled because of all of the dust, were able to move it back two blocks. While we were conferring over our engine's down and what we were going to do, Building 1 collapsed. At that point, it was again like the blizzard, people again knocking to come in for cover, any place you would go to protect yourself and hope that you could get through that cloud." –9/11 Commission 11th Public Hearing, May 18, 2004

PAPD Sergeant Quentin DeMarco: While forming teams and assembling equipment to make entries into the WTC, Captain Whitaker informed the undersigned that no one is to enter the buildings, they were structurally compromised and could collapse. A short time later I observed tower 2 collapse into West St.

FDNY Firefighter Richard Banaciski: So I was kind of looking around over there, up and down West Street and looking on Vesey and I just remember there was a police officer standing there and he just started saying, it's starting to lean, it's starting to lean. I remember looking up, looking at the second building and just seeing it starting to move. I just started running back down Vesey towards the water again to where I had come from. That's -- the second building came down there.

FDNY Lieutenant Robert Bohack: With that as soon as I said that the building [north tower] made a groan like steel twisting. I didn’t have to tell those guys twice. We just started making line for West Street or the western side, the entrance we came in.

With that we ran out the front. There was, I think, a Chief’s aide sort of as a lookout saying “come on come on come on.” we stopped at the entrance as soon as he waved us on we go. We get to him. He was maybe 50 yards ahead of us, in front of us. On West Street I get to him and he says, “look at the building, Lou. The other one collapsed and this one is collapsing.” He showed me, about 20 stories up you see crack in the building. I look, “holy shit, the other buildings gone.”

FDNY Lieutenant George DeSimone: After that, I got out of there as quick as I could because the building was decaying. I mean, there was fire coming out of it, fire dropping down, and at that point I think we started to notice bodies dropping from the buildings.

FDNY Firefighter Dennis Fischer: Guys, get the fuck out of here that building is going to come down. It's coming down. That building is coming down. We looked at each other. We kind of looked around, we didn't really think it was going to happen. Maybe 15, 20 seconds after he said that, we heard the rumbling. We looked up, that I remember as plain as day. I looked up and I saw from the top, I actually watched it with my own eyes, I saw the top start to pancake down.

FDNY Firefighter Paul Bessler: Just at that point, my radio came clear as day, "Imminent collapse. This was a terrorist attack. Evacuate." That's exactly what I heard. I think that was Chief Picciotto that was giving the order. We relayed that again, hoping that the brothers would hear it above us, and I remember the look on Andy's face, like apprehension that we were going to leave this building.

FDNY Firefighter Frank Sweeney: I remember when we heard abandon the site, I said, wow, this would be really good to keep with us. So I started pushing this cart, and I got stuck in the doorway with it, when we started hearing this rumble. I can remember -- I specifically remember this like twisting sound of metal. We were probably about half a block away from the complex at this point.

You heard a big boom, it was quiet for about ten seconds. Then you could hear another one. Now I realize it was the floors starting to stack on top of each other as they were falling . It was spaced apart in the beginning, but then it got to just a tremendous roar and a rumble that I will never forget.

Thomas Bendick, FDNY EMT: "At that point I could actually visually see the top floors of the north tower starting to give way and that began to collapse. At that point we all began to run north."

FDNY Supervising Fire Marshal Brian Grogan: "At that point I looked up, and it was Joe Pasquarello grabbed me, and he gave me a pull on my arm. He said, "Get the fuck out of there." I looked up –"

Q: "Were you still on West Street at this time?"

A: "Still standing there. The building just started coming down. It buckled." Source

FDNY Firefighter Brian Fitzpatrick: "Then we exited out by the marina, the North Cove Yacht Harbor, where we all basically just took a knee and we waited a couple of minutes. Everybody was in shock. We didn't know what happened. We just thought it was debris or an explosion or a secondary explosion or another bomb inside the building or another plane.

We got up and we made our way around through what turned out to be the North End Avenue and we hit Vesey. I'd say probably 25 minutes had elapsed by now. We were walking up Vesey and we got to Vesey and the West Side Highway and we were making the turn. I remember seeing the bridge as we turned and somebody came running by us saying the north tower was leaning. I didn't even know the south tower fell yet. I looked up and I actually saw the antenna coming down." Source

PAPD Detective Edward Rapp: While I was on the phone with Stacy at the Police Desk we all of a sudden heard metal creaking. I looked up and saw the North Tower buckling from the top. It looked like the north and west sides of the building were twisting and then separating like a banana peel.

FDNY Firefighter Fernando Camacho: What happened was that as I was standing there and getting bandaged, somebody said the tower is leaning. So me and Gorman -- he had the irons. We turned around and looked, and we could see the tower leaning. As it started to lean, it just came straight down. Now we're running again.

Q. Which way was it leaning? Towards West Street?

A. The tower was leaning not towards -- it leaned somewhat northwest but not -- it came down pretty straight after it leaned. It didn't really continue to lean. It just leaned a little bit and then came straight down.

Q. Fine. Let me get one thing straight. From the time you noticed the leaning to the time of it coming down, are we talking about seconds here?

A. No more than three, four seconds.

PAPD Police Officer Middleton: Pedestrians began to form a crow[d] in the streets and I began to tell them to move back as far as the next block before the other building falls. As I continued to wave them back periodically you would hear a loud boom go off at the top of tower one. ...After approximately 15 minutes suddenly there was another loud boom at the upper floors, then there was a series of smaller explosions which appeared to go completely around the building at the upper floors. And another loud earth shattering blast with a large fire ball which blew out more debris and at that point everyone began to run north on West Broad Street.

PAPD Inspector Timothy Norris: Just at this time, another firefighter began to yell to us from across the street. He was looking up at the Towers and yelled for us to hurry up since he thought the second Tower was about to fall. The two firefighters and myself again picked up the injured man and managed to walk three of four steps when we felt extreme vibration and an incredible noise “like a thousand freight trains.” I knew instantly that the Tower was falling down.

Robert Jones, ACE Elevator mechanic: You could see A Tower, the outside, the columns were glowing red by that time, because that had been on fire for at least a good 25 minutes by that time. B Tower, I could see tremendous structural damage to the outside of the building. We stood on the corner across the street from the towers. Source

PAPD P.O. Barry Pikaard: I was standing there about 15-20 seconds when Inspector Fields ran up to me and said the building was going to come down.

PAPD P.O. E. Finnegan: At this time an NYPD officer entered the church and ordered us to evacuate. He stated that the other tower was about to collapse.

FDNY Chief Joseph Dunne: Another ten or 15 minutes or so later, one of my guys said to me, "listen, the north tower is making noise, we're not safe here, that building is going to come down too.”
Dennis Smith. Report From Ground Zero. New York: Viking Penguin, 2002

South Tower

There are similar, but fewer, descriptions of structural failures and warning that the south tower would collapse:

FDNY Battalion Chief Steve Grabher: "There was - somebody was insinuating that there had been a structural collapse and this is probably 15 to 30, 15 to 20 seconds before the building came down. I was hearing this. There was some collapse."

Q. "It might have been an interior collapse happening prior to the exterior?"

A. "Yes."

Q. "Nobody knew exactly what was going on. But you are saying you are pretty sure it was Ladder 4, the roof man."

A. "I'm almost positive it was Ladder 4 roof. I remember him saying his name. This guy was calm as a cucumber. I'm thinking this guy is about to die and he is as calm as a cucumber."

Q. "He didn't know it, I don't think."

A. "Then he said I'm losing consciousness."

Claiborne Johnston: "It seemed we were walking down very calm, very orderly . . . and all of a sudden you felt like the ground was falling out from under you," said Claiborne Johnston, who escaped from the 64th floor of the south tower. "You knew the structure had been altered severely, and the rest of the way down you could feel that."Source

FDNY EMS EMT Peter Constantine: We parked the ambulance with a lot of other ambulances on Vesey Street and West. We basically started grabbing equipment and helping out people who were running out from the building originally. At that time it was just chaos, everybody going everywhere. We were helping any way we could. A lot of people wanted to help. At that time, I don’t know what time it was, we heard across the radio the tower’s gonna fall, the tower’s gonna fall.

Q: You heard that on the radio?

A: Oh yeah, it came across. And then all of a sudden, when you looked up, you froze for a couple of seconds, you saw these little pieces falling off. Then all of a sudden, everyone started to run. Then, you started to run, your helping people, helping them run. You saw it, it was out of a movie, you know, the cloud’s just chasing you. As you look back, you see it engulf people. After that was over, we all went back in.

And then, it felt like ten minutes, the other building fell. I read in the paper the next day that it was actually something like 30 minutes later that it fell. When that one fell everybody ran again. Source

FDNY EMS EMT Mary McMillan: As I was gathering it, I was saying to Mala, that I'm looking up at the building and I said to her, this is going to fall. I said we should get out of here. This is going to fall. She says I know. I think we are in the wrong place. I tried to tell Chief Grant and Chief McCracken I think it's going to fall, but they were so busy talking among themselves, I didn't have a chance to tell them what I was feeling. I think I should have shared with everyone, but when I felt that way, I looked around me. I gathered the resources, but I'm thinking what do I do when this thing falls. I looked around me and I saw this building on my right. It had the glass bottom doors and I just made reference to it in case god forbid, anything happened, my plan would be to run behind that building, grab my jacket over my head and just stay there for a while.

Sure enough, after my thoughts traveled through my head, all I heard was run and I heard the sound, a sound -- I can't really describe it. It was an unusual sound. Then they are saying run, so I started to run and I looked back and I saw this white smoke just moving in this boiling motion towards us and I was like oh, my god, what's going on. I was scared, I was frightened and I'm running and everybody's running. Source

"At 9:37, a civilian on the 106th floor of the South Tower reported to a 911 operator that a lower floor – the '90-something floor' – was collapsing." (9/11 Commission Report, pg. 304)

Stanley Praimnath & Brian Clark: It was like steel bending and creaking. It made this -- I can't explain the sound, but it's like -- it was an eerie sound.

CLARK: And we heard this boom, boom, boom.

OKWU: As the tower collapsed, Stanley Praimnath and Brian Clark were watching from less than a 100 yards away, just before they ran, two of the very last people to get out alive.Source

Tommy Castaldi: "Police and firefighters were coming in to the building when we reached the lobby," Castaldi said. "The building was swaying, burning and shaking. But they went up, not back to the street...." Source

Jaede Barg: The lights in the staircase went out. There were cracks in the stairwell walls with exposed pipes breaking through the plaster. The building was forcefully swaying, enough to require significant balancing. I recall the incredible sound of twisting metal with each sway of the building.Source

When the tower started to cave, it began with a low rumble. Slowly, amid a dark cloud of smoke, the debris rained down. “My God, it’s falling,” someone shouted. Mesmerized, no one moved. Source

"From the lobby, they were directed by police through an entrance to the mall under the Trade Center. After following their convoy inside, Finegold and Borst heard the screeching, cracking sound of Two World Trade Center collapsing. The force of it hurled them to the ground. Finegold fell to his knees. Borst lost his radio and the pinky ring off his finger. Source

Some knowledgeable onlookers who expected the towers to fall

Mark Loizeaux of Controlled Demolitions, Inc. (Excerpt from
The Tower Builder: Why did the World Trade Center buildings fall down when they did? The New Yorker, 11/19/01)

Among the dozens of people I have spoken to recently who are experts in the construction of tall buildings (and many of whom witnessed the events of September 11th as they unfolded), only one said that he knew immediately, upon learning, from TV, of the planes' hitting the buildings, that the towers were going to fall. This was Mark Loizeaux, the president of Controlled Demolition Incorporated, a Maryland-based family business that specializes in reducing tall buildings to manageable pieces of rubble. "Within a nanosecond," he told me. "I said, 'It's coming down. And the second tower will fall first, because it was hit lower down.' "

...They are structural undertakers, which may explain whyMark, when confronted with the spectacle of the crippled buildings, lacked the sentiment that builders feel for their creations—that innate sympathy which helped blind the engineers of the World Trade towers to the reality of what was about to occur. "I thought, Somebody's got to tell the Fire Department to get out of there," Loizeaux told me. "I picked up the phone, dialled 411, got the number, and tried it—busy. So I called the Mayor's Office of Emergency Management"—which was in 7 World Trade. "All circuits were busy. I couldn't get through."

Loizeaux said he had an enhanced video of the collapses, and he talked about them in a way that indicated he had watched the video more than once. "First of all, you've got the obvious damage to the exterior frame from the airplane—if you count the number of external columns missing from the sides the planes hit, there are about two-thirds of the total. And the buildings are still standing, which is amazing—even with all those columns missing, the gravity loads have found alternate pathways. O.K., but you've got fires—jet-fuel fires, which the building is not designed for, and you've also got lots of paper in there. Now, paper cooks. A paper fire is like a coal-mine fire: it keeps burning as long as oxygen gets to it. And you're high in the building, up in the wind, plenty of oxygen. So you've got a hot fire. And you've got these floor trusses, made of fairly thin metal, and fire protection has been knocked off most of them by the impact. And you have all this open space—clear span from perimeter to core—with no columns or partition walls, so the airplane is going to skid right through that space to the core, which doesn't have any reinforced concrete in it, just sheetrock covering steel, and the fire is going to spread everywhere immediately, and no fire-protection systems are working—the sprinkler heads shorn off by the airplanes, the water pipes in the core are likely cut. So what's going to happen? Floor A is going to fall onto floor B, which falls onto floor C; the unsupported columns will buckle; and the weight of everything above the crash site falls onto what remains below—bringing loads of two thousand pounds per square foot, plus the force of the impact, onto floors designed to bear one hundred pounds per square foot. It has to fall." [Note that many of Loizeaux's comments about the conditions of the towers were accurate, but that the assumed cause of collapse – floors pancaking – was not. Determining the actual cause of collapse would take much research.]

Ray Dougherty
My Thayer School engineering training came back, and I realized that with that intensity of heat in a building in which the steel girders were insulated with asbestos, it had to collapse within one hour. I called the fire department, police, etc. and told them the building was guaranteed to collapse. I was told that 911 was only for emergencies, and I should callsomewhere else.

After about 40 minutes, as I saw (I have telescopes, binoculars, etc.) the top segment of the building listing about 3 degrees, I left my apartment and went out to walk in the street. Buidlings collapse if they list more than 3 degrees. As I walked down Bleecker Street, people gasped as the building collapsed. Like Lord Jim, my imagination surpasses any reality. I should have stayed and watched. I did for the second tower.

...We watched the second building, and I noticed it was more than 3 degrees, but as the telescope revealed, that was because the beams were buckling on both sides. A building like the WTC does not 'break off in the middle' and fall like a tree.

JREF forum member and mechanical engineer rwguinn
In fact, the 4 engineers, 6 designers, 3 tech writers, and 17 fabicators/welders/mechanics at the place I was working all said, pretty much in unison- "Those buildings are going down!" about 10 minutes before the first collapse.

North Tower bowing that led to collapse

South Tower bowing that led to collapse. This bowing was visible within 18 minutes of flight 175's impact.

See video of inward buckling of south tower's east wall, which leads to collapse.