When the World Trade Centre towers collapsed on 11 September 2001, the world was devastated – and then demanded to know how it had happened. So Chicago-based skyscraper expert William Baker was sent on a reconnaissance mission to ground zero, to assist in the clear-up operation and report back. Here he recounts his experiences
William Baker is a partner and senior structural engineer at architect Skidmore Owings & Merrill's Chicago office, and is an expert on the structure of tall buildings. In the days following 11 September, Baker was one of the engineers to visit the site to advise on the clear-up operation. His experience of the design of tall buildings led to his involvement in the team of experts that put together the report for the Federal Emergency Management Agency on the collapse of the World Trade Centre towers. He spoke to Building's technical editor Andy Pearson about his role at the ground zero site.

I wasn't in New York the day the twin towers collapsed, but some of my engineers were. They watched it happen. Sadly, Skidmore Owings & Merrill lost one of the architects from our interiors group in the disaster. He was going to a meeting in one of the towers, got trapped in an elevator and couldn't get out.

The attack on the towers happened on Tuesday. All of us involved in high-rise building design had expected the buildings to stand once they had survived the initial impact – I couldn't imagine that the buildings would collapse.

On Wednesday and Thursday, engineers from SOM's Chicago office were speaking to structural engineers in New York. By Wednesday evening you could tell these guys were haggard. And by late Thursday they were fried. They weren't sleeping and they were getting really stressed out. SOM had calls from engineers in the City asking for help with the rescue. So on Friday, me, John Zils – project engineer on the 110-storey Sears Tower – and another engineer drove the 12 hours from Chicago to New York.

The first thing we did on Saturday was to inspect SOM's New York office, which was very close to the World Trade Centre site. On Sunday, we went to assist at the site itself. A group of engineers met up in Greenwich Village at 5.30 in the morning. It took us until eight o'clock to get to the site – we had to walk all the way, and pass through numerous checkpoints.

When we first got to the site, it looked like a big rubbish heap. It was burning, and there was a lot of smoke and a lot of dust. You had the contents of two 110-storey buildings in a pile. It was a pretty amazing sight, because it was not that big a pile – the twin towers had big basements and a lot of the debris was there. On top of the pile were bits of furniture, sections of steel columns, pieces of floor deck and conduit.

As soon as we arrived on site, the questions started. We were up in the north of the site where this 47-storey building – World Trade Centre 7 – had collapsed. The fire fighters were on the top of the rubble . The first thing the fire chief said was: "Is this pile safe to be on?" The pile looked reasonably stable but it could have shifted at any moment. We advised them to "get the hell off there".

At this end of the site there were two adjacent buildings we had to inspect – 30 West Broadway and 101 Barclay Street. One of them had serious structural damage, whereas the other had a lot of broken glass. It was incredibly difficult work. You have no drawings, and go in with a flashlight. You wander around to try and find the fire stairs. When you've found them you can take a look at the other floors. In one building we were accompanied by an FBI agent because both the FBI and CIA had offices in the building that had collapsed across the street.

On another part of the site, a contractor uncovered a big hole. We put a ladder down and found ourselves in the basement of the World Trade Centre next to the truck dock. There were vehicles there that were burned out. But, further down the truck ramps, there were other parts of the basement where the automobiles were fine. There was a lot of concern about the amount of water in the basement because the Hudson River is close by. However, it turned out that it was from water mains that had been speared when the towers collapsed.

The next problem we had to deal with was to find a route along which to bring in the heavy lifting equipment. We didn't have a clue what was underground, but we knew we had to bring the equipment across a subway line. There was nothing for it but to get into the subway tunnel and inspect its condition.

Amazingly, during the recovery nobody was killed. This was the most dangerous construction site I'd ever been on in my life. And you were making real seat-of-the-pants engineering judgments. Things you would never consider doing on a normal job site, you had to do – but cautiously. The most dangerous thing was all the broken glass from thousands of broken windows.

At the beginning of the rescue, we worked an eight-hour shift every day. But that quickly went up to 12 hours. But a 12-hour shift was more like 14 hours because you had to brief the team that was relieving you and then you'd walk them up onto the site to show them what was going on.

That first week, the American Institute of Steel Construction called me. Because I'm a high-rise building designer, they wanted me to be on the American Society of Civil Engineers' investigation team looking at the building's performance after the attacks. Other members were from the American Institute of Steel Construction, the American Concrete Institute, the National Fire Protection Association and the Society of Fire Protection Engineers. The whole group was sponsored by the Federal Emergency Management Agency.

If this had been in the UK, there would have been a royal commission. Over there it was primarily voluntary labour – and not very well funded. We didn't have subpoena powers. We could only cajole. People were co-operative because they felt they had to be.

A lot of the debris was pretty mangled and you were trying to figure what was this like before it came down. There was no concrete left, it had all been pulverised. You could go for a long time seeing zero concrete. Occasionally you would find a piece of concrete, but it would be the size of a child's fist. Underneath the debris nothing remained. You would pull off the surface rubble only to find that underneath everything had been pulverised – just a big pile of grey dirt.

Occasionally, you would find a piece of heavy steel. Some of the steel still had the erectors' marks still on it. We did not crack the erectors code for a long time. If we'd cracked the code earlier, we would have been able to understand a lot better what we had seen. And we could have salvaged some steel that was scrapped.

We think there was a different process involved in the collapse of each tower. It appears the collapse started in the core of the north tower – probably because the fireproofing had been knocked off some columns in the core – because, on video, the rooftop antenna moves about 0.3 seconds before you can detect any other movement. We think the collapse of the south tower started once the floor plates fell away leaving the external columns without any lateral support so that they eventually gave way.

This initial study is mostly a reconnaissance report to gather data and suggest likely scenarios leading to the towers' collapse. The twin towers were not a standard construction, so a detailed study of them will not be that useful. In the report there are recommendations for further study, especially of the 10 or so damaged buildings adjacent to the towers, but there are no recommendations for changes to building codes. The National Institute of Standards and Technology, funded by US government, will undertake a detailed, three- to five-year study of all the buildings on the basis of recommendations of the FEMA report.

Key findings of the investigation

The structural framing system must be robust
One of the big issues is robustness – the ability of the structure to withstand the unexpected. If you look at all the buildings that have collapsed at the site – not only the twin towers – the failures are in the joints. Steelwork connections need to have to have a little extra gas to take something other than the intended load and also to withstand a load from different directions. I can see more emphasis being put on the design and fire-testing of connections. Passive fire protection systems must be able to withstand damage
Before 11 September, there had never been a collapse of a fireproofed building because of a fire. Fire protection systems need to be tested to understand how well they can withstand mechanical damage. Egress systems need to be re-evaluated and evacuation procedures should be reviewed
The way you would fight a tall-building fire in the USA is that you’d evacuate the incident floor and the floor above the fire and that’s it. Everyone else is supposed to stay put. But following 11 September, that is no longer the case. It is a big issue because you need to work out the number of people that need to exit a building, which will affect the number and width of escape staircases. We are also looking at a time limit to empty a building. One of the problems for evacuation of a tower is that you cannot pressurise the fire stairs to keep them clear of smoke if all the exit doors are open. I also think the whole lift thing needs to be rethought for much larger buildings. In the UK you have the fireman’s lift but these are pretty small. We need to look if there is a way to use the service lift or even a passenger lift for the fire crews and also for bringing disabled people out. Overall building performance under fire must be evaluated
The detailed modelling of fires in tall buildings and the resulting stress in the structure should be conducted to understand how fires progress and where a structure may fail. In my opinion all buildings should be sprinklered. This is the reason tall buildings are historically so safe. It is also very important for the fire fighters because the likelihood that they’ll ever have to get up there and put a hose on a big fire is remote. Fire loads need to be assessed
In the future, I can see a need to look more closely at the fire loads. There is no reason not to have file storage but you’ve got to be sure you address the fire issues relating to storage of combustible material.
  • Copies of the report into the World Trade Centre collapse are available on www.fema.gov/library/wtcstudy