TOM BEARDEN: Until the Twin Towers of New York's World Trade Center disintegrated, no modern steel skyscraper had ever collapsed.
David Childs is a prominent New York architect who is working on the redevelopment of ground zero. He was an eyewitness to the disaster.
DAVID CHILDS, Architect: Well, I was shocked when they fell down, quite frankly. I was standing at the window when I saw this young man who works with me with this... this look of horror on his face, a very young man, and he said, "Will they fall down?" I said, "Absolutely not."
TOM BEARDEN: In fact, the entire architectural and engineering communities were stunned.
At a congressional hearing today, the Federal Emergency Management Agency and the American Society of Civil Engineers released the conclusions of an investigation into why it happened.
The document points out that the buildings absorbed the enormous impact of the fully loaded 767s without collapsing immediately, but couldn't withstand the fires that followed.
Jonathan Barnett is a professor of fire engineering at Worcester Polytechnic Institute in Massachusetts, and one of the authors of the report.
JONATHAN BARNETT, Fire Protection Engineer: When the aircraft entered the building, the aircraft broke up. They hit columns, they hit things... the floor slabs. And it fractured the fuel tanks.
The jet fuel poured out of the tanks in a spray of liquid droplets. A hot engine part or some other ignition source ignited this cloud of droplets, and flames progressed through the cloud, reaching the edge of the building and then coming outside in the huge fireballs that we saw.
Our calculations suggest that anywhere from 1,000 to 3,000 gallons of fuel were consumed outside the building in these fireballs. Other fuel, a large percentage, poured down the shafts, the elevator shafts and the stair shafts.
The jet fuel acted like charcoal lighter fluid on your barbecue, all right? It ignited all the other fuels in the impact area. And in fact, that was the problem. You had the furniture in the offices, on multiple floors. We had the seats and the other solid fuels in the planes, and all of a sudden, these were ignited by the jet fuel.
TOM BEARDEN: Each floor of the World Trade Center was nearly an acre, and multiple floors were on fire. The report says the heat was incredible, generating as much energy as a large commercial power station.
JONATHAN BARNETT: When the steel gets too hot, it weakens. It also as it gets hot, it expands.
So as it's expanding and the rest of the structure is trying to keep it from expanding, because it's not being heated by a fire, there are stresses that are induced.
So two things happen with a fire: One, often the stresses increase; and two, the steel gets weaker. And if one or the other becomes too great, well, then you have a failure.
TOM BEARDEN: The World Trade Center had state-of-the-art sprinkler systems and standpipes to bring water to burning floors, but the report says the aircraft probably destroyed them both.
JONATHAN BARNETT: So it's very likely that there was no water above the impact floors. And in fact, with the standpipes being severed, if that did occur, then there was no water in the building to fight the fire.
TOM BEARDEN: The Twin Towers were what engineers call "tube" structures. They had a central steel core, which was connected to the outer load-bearing wall by steel trusses. During construction, all of the steel was sprayed with a fire- resistant material, which was supposed to protect it from the heat.
JONATHAN BARNETT: Normally, we don't design our buildings to be hit by aircraft. And so the fireproofing that was used in the World Trade Center wasn't designed to stick to the steel if it was hit by an aircraft.
We think much of the fireproofing in the impact area fell off the steel it was meant to protect.
TOM BEARDEN: The report also pointed to the fact that the stairways were clustered in the center core of the tube, making them all vulnerable to a single impact. The collapse of the Towers has led some to question the tube design itself.
Critics point to the Empire State building, which was struck by a World War II era B-25 bomber in 1945. Damage was relatively minor, and the building was never in danger of collapse. That building has a heavy steel frame, and the beams are encased in concrete, itself an excellent fireproofing material.
But today's report points out that the 767s that struck the Trade Center were much, much larger, and carried a great deal more fuel -- less than 1,000 gallons in the B-25; 10,000 gallons in the 767s.
Architect Childs agrees with the report and insists the basic tube design is sound.
DAVID CHILDS: The question will be, is what is that spanning material between the solid core and that outside structure? What are the moment connections, the strength of that connection between them?
And in fact, these kinds of designs have performed extraordinarily well over history. In fact, until this occurrence, no building had fallen down because of fire. There have been major fires, and a couple of floors had fallen down or whatever, but the steel structure is a very efficient and safe structure, particularly this kind of core design.
TOM BEARDEN: The report actually focuses on the welded and bolted connections between structural members. They are not specifically tested for fire resistance.
One building in the World Trade Center complex suffered a partial collapse because the connectors failed due to the extreme heat.
The leader of the team that wrote the report for FEMA and the engineering society, Dr. Gene Corley, thinks the connector issue is important.
W. GENE CORLEY, Structural Engineer: The fire test requires you to test just the span of the beam, not the locations where it's connected into the rest of the building.
TOM BEARDEN: That would seem to be a major oversight.
W. GENE CORLEY: Well, it's not necessarily an oversight. It doesn't mean that you don't provide fire protection for the connections.
What people do normally is they provide the same fire protection for the connection that they do for any other part of the system. The appearance there is that the connections did not have the same resistance as the rest of the structure.
And this is one of the primaries that we point out needs some attention. We first need to see if it really is a problem. And then, once we know that, building codes would have to be changed to remedy that problem.
TOM BEARDEN: The release of the report today is just the first step in a process that the American Society of Civil Engineers hopes will lead to additional research into the behavior of buildings in catastrophic events.
Dr. Corley says those studies will take about three years and cost more than $40 million.
W. GENE CORLEY: That will get us to the point of knowing what research might need to be done in some areas, particularly this area of connections. We first have to find out how serious the problem is, and what needs to be tested. But then, after that, the testing has to be funded.
TOM BEARDEN: Dr. Corley says the team saw no need to call for immediate changes to building codes.
W. GENE CORLEY: If you're going to do something to minimize this type of effect, the right thing to do is invest it in security so that the terrorists don't get the tools that they need to cause this type of an attack.
TOM BEARDEN: But the horrendous loss of life and the indelible images of the collapsing towers have caused some to question whether skyscrapers have outlived their usefulness. They point to press reports of companies moving out of New York since 9/11, of increasing vacancy rates in skyscrapers because people and companies are afraid to work in them anymore.
Carol Willis is a skyscraper historian and curator for the Skyscraper Museum in New York.
CAROL WILLIS, Director, Skyscraper Museum: I'm sure they have a future because high-rise buildings really provide a kind of efficiency for urban life. The problems of bringing people together in great density need to be solved efficiently, and skyscrapers are very efficient ways to use energy, as a matter of fact -- and to have many people concentrated in one place so that they can take advantage of the economies of scale.
TOM BEARDEN: David Childs thinks we'll still build skyscrapers, they just won't be 110 stories tall.
DAVID CHILDS: I think that we won't have those types of buildings anymore, at least for the immediate future, because people just don't feel comfortable going to work every day on the 70th floor, especially if they're exposed to the rest of this world in a way that sort of makes you feel like you're a target.
So I think we won't have the excesses of height as before. We'll be probably in the maximum, 60, 65, perhaps even 70 stories, but... which is considerably lower than the World Trade Center was, for example.
TOM BEARDEN: The World Trade Center site today looks very much like it did in the '70s, leaving architects and engineers with a blank slate for a new development.
The hope is that further study will lead to a complete understanding of what happened on September 11, and perhaps lead to changes in building codes that might mitigate loss of life in buildings subjected to damage far beyond their design limits.