INTERVIEW WITH PROF. GRAUBNER (re: WTC and the building of skyscrapers)

skyscrapers dot com ^ | Sept. 2001 | Interview by Michael Wutzke

[doug from upland]

Professor Dr.-Ing. Carl-Alexander Graubner
Darmstadt University of Technology, Germany

Interview by Michael Wutzke.
Translated from German into English by John W. Cahill, AIA.
New York City Photos by Nate Lindsey.

What should architects learn from the terrorist attack on the World Trade Center? We must seriously re-examine the topic of evacuation, especially in buildings which were built between the 1960's and 1970's. There are two points in which we have to differentiate between the situation in the USA and that in Germany. In Germany, skyscrapers must have two separate exitways in different corners of the building. In the USA, these separate exitways are typically located directly next to each other in the core, as was the case in the twin towers of the World Trade Center. This is done for economic reasons. As well, the exitways in steel-built American skyscrapers are designed to resist fire, but not explosion. They are designed to resist fire for 90 to 120 minutes, assuming, however, that the structural steel elements are in their normal condition. Apparently, the explosions destroyed the walls and stairs, making evacuation impossible from the floors above the point of impact. Buildings in the United States rely to a large degree on sprinkler systems, an active form of fire protection. In the case of this disastrous attack, one could well assume that the sprinkler system was cut, making fire extinction in the upper stories impossible. It is important, therefore, to equip existing buildings with duplicate fire protection systems.

According to media reports, the World Trade Center was designed to withstand an airplane crash. Why, then, did the two towers collapse? The buildings indeed did withstand the airplane crashes. That is not a problem for skyscrapers, since the impact of an airplane is relatively small in comparison with strong wind loads. The situation becomes much more serious, however, when energy in the form of jet fuel is introduced into the building and released. Buildings cannot be made to resist that intensity of fire, heat, and burning. The World Trade Center did not collapse because of the crash, but rather as a result of the catastrophic effects of fire on this particular type of steel-tube construction, in which the outer walls carry the building loads while the core contains only service functions. It is the unanimous opinion of experts that the exterior supports eventually failed because of these effects. Because of the fire, the floor trusses, connecting the exterior structure to the larger columns in the core, failed. Without this connection, both the trusses and the exterior columns became more susceptible to buckling. Another contributing factor was the thin three inch depth of the concrete floor plates. Each floor collapsed into the next in a sort of domino effect, caused by the additional weight of the floors above. This chain reaction was set off by the buckling of the upper floors. It should be noted that the building performed completely according to plan in the case of catastrophe, standing long enough to allow a large number of people to escape.

WTC is collapsing

Would a structure of reinforced concrete better resisted these massive effects? A reinforced concrete structure would have had massive reinforced concrete stairwells. This would have enhanced evacuation and conceivably resisted the fire longer. One must remember, however, that reinforced concrete construction would not have been possible with the given site conditions, foundation, and height. Only with steel can one build to such heights.

The two twin towers of the World Trade Center were erected between 1969 and 1973. What kind of fire protection would be used today to safeguard the structural steel? Fire protection in the World Trade Center was provided by insulation sprayed onto the steel. The structure was designed to resist the effects of fire and remain standing for between 120 to 180 minutes. This worst case scenario was calculated with a factor of safety of 1.0, corresponding to the total weight carried by the building. That total failure occurred after just 60 minutes indicates that the structure was indeed seriously damaged by the impact of the airplane. The affected floors in particular were not able to withstand the effects of the fires and explosions.

Following the collapse, a layer of dust one centimeter thick covered Lower Manhattan. Could you say something as to the chemical content of the sprayed-on fire protection? The sprayed insulation on the structural steel apparently consisted of the fire-resistant material asbestos. One cannot rule out that there is a potential public health hazard for those on site. I assume, however, that the asbestos was fused with concrete, and thereby poses no direct threat. I cannot give any details at this time

Could such a disaster be repeated elsewhere? If a terrorist were once again to take control of an airplane, then of course there would be the threat of this kind of danger. This potential danger is not limited to skyscrapers, however, but extends also to other types of structures. One instance where the risk of this type of danger is relatively small is atomic energy plants, since they are planned to withstand the crash of a large airplane. They are also provided with a thick layer of concrete, which would cause jet fuel simply to flow off and not allow it to penetrate inside. Atomic energy plants therefore are not vulnerable to such attacks. Such security is not realizable in the case of skyscrapers. If we tried to prevent any and every possibility of skyscrapers being attacked by airplanes, then these buildings would become impossibly expensive and visually unattractive.

What lessons can be drawn by architects and engineers from the tragic events in the USA? We will perhaps have to reconsider how buildings are to be evacuated, and possibly also reconsider the feasibility of light-weight glass and steel construction in skyscraper construction. Architects have to ask themselves whether steel-tube construction with load-bearing exterior walls can still be considered a state-of-the-art concept. Perhaps we will have to design more robust construction at the expense of aesthetic quality. In these matters, the recent tragic events affect all people in such buildings and their sense of security. Strictly speaking, the whole of society is affected, since architects in the end design that which the general public wants.

Is it technically possible to guarantee adequate protection of the structural integrity of a skyscraper against terrorist attack? Presently we are working on making tall buildings more resistant to such occurrences. Redundant systems are designed into the skyscraper projects with which I have been involved here in Frankfurt. For example, should a column be blasted away or heavily damaged by an explosion the structure would not collapse. Beyond this, however, there is not a great deal technology can do to protect buildings from terrorist attacks with airplanes.

The rapid evacuation of the New York towers played a critical roll. Are there any efforts afoot to do this more effectively? Separate escape routes in different parts of the building are required. We must also make comprehensive computer simulations of the evacuations of tall buildings. Naturally, we must then make necessary changes in the spacial arrangements within the buildings themselves. I know that many people did not leave the World Trade Center after the first hit. Apparently these people were convinced that nothing could happen to them on the lower floors. The problem thus lies in the recognition of the danger. But such safety measures engender high costs. A skyscraper is rather uneconomical in comparison with low rise buildings. They are often favored for reasons of prestige, and thereby are able to justify high rents. Should higher security costs continue to be generated, investors will be forced to ask whether tall buildings are worth the expense. The problem of security is not concerned just with skyscrapers. Possible targets could also be the Olympic Stadium in Munich or the new Arena in Gelsenkirchen. The potential danger here also involves tens of thousands of people. The question, then, is not how to make skyscrapers more secure, but rather how terrorist attacks are effectively hindered. There are also potential targets that architects or engineers cannot secure - think of the Oktoberfest in Munich. In this case, governments must undertake the necessary security measures. On the whole, I don't believe that there will be a repeat of this attack. In my opinion, these terrorists will seek another way to bring pain and sorrow to mankind. It is conceivable that the perpetrators themselves had no idea of the dramatic proportions their terror would assume.

Can one expect that in cities such as Hong Kong, Chicago, or Frankfurt fewer tall buildings will be erected or that their construction will be changed? I don't believe that our cities will change because of these direct attacks. It is conceivable that governmental oversight will be stepped up. Basically it is a purely economic decision. There can be as much security as people are willing to pay for.

What kind of a role does the building density in New York play? Numerous other buildings were destroyed or made partially uninhabitable because of the tremendous explosions and the collapse of the twin towers. Technically, these neighboring skyscrapers performed as they were supposed to, collapsing only after they had been evacuated. Preventing their collapse entirely was unfeasible, since in the end it would be too expensive to make the building usable again. Many professionals have not been able to imagine what might have happened had the airplanes flown lower into the buildings. The entire corner of the South Tower was cut off; it stood for awhile. Because of the way it was designed and constructed, it then fell in on itself. What would have happened if the airplane had hit 40 stories down, say on the building's side instead of at the corner? With a reinforced concrete building, these massive buildings could have crashed down sideways into Lower Manhattan. A much greater number of other buildings would have been involved, and all in all the damage would have been much worse. The scenario of a tall building falling sideways has up to now not been considered or discussed in the professional world.

WTC Plaza WTC Lobby

Have other parts of the city been affected besides the World Trade Center? I have no information to indicate that the subway under the World Trade Center has remained undamaged. It is imaginable that the tunnel systems could not withstand the massive force from above. They, too, were a part of the evacuation plan. If they were blocked, people had little or no chance to flee. This raises the question as to what kinds of loads subway tunnels should be designed to withstand.

Do you believe that the World Trade Center will be rebuilt? Americans are very confident and patriotic. The buildings will be rebuilt. Whether or not they will be rebuilt in their original form remains to be seen. The new symbol will conceivably arise in a new external form, but its meaning, however, will be no less than that of the destroyed World Trade Center. In my opinion, the planning will be started within the next two years.

Thank you very much for this conversation.

Interview by Michael Wutzke
Translated by John W. Cahill, AIA.

September 2001

[nothingnew]

Interesting post dfu....."I see dead people and many lawsuits filed by unscrupulous lawyers".

FMCDH

[boris]

"Americans are very confident and patriotic. The buildings will be rebuilt."

They will not.

Again I ask:

Who would lease space in rebuilt towers?
Who would agree to work there--especially in upper floors?
Who would insure the buildings--and at what cost?

Answer: nobody, nada, zip. Perhaps Lloyds of London would insure them for $1,000 per square foot.

--Boris

[all4one]

Great points. I just saw a news report this evening that was listing the stadiums, shopping malls, office towers, airports etc. that may be paying 300% more on their insurance.

This is a significant cost factor that I had not thought about previously. Of course, the insurance companies had their adjusters working on new figures by September 12, 2001.