American Structural Engineers Defend ‘Heroic Performance’ of WTC Towers
The profound shock felt in the United States and its construction community at the enormous destruction wrought by the attack on the World Trade Center was vividly conveyed by Ron Klemencic at the recent Building for the 21st Century ‘summit conference’ in London.
Introducing a graphic presentation on the fall of New York’s two highest buildings, Mr. Klemencic, Chairman of the Council on Tall Buildings and Urban Habitat, and President of the structural consultancy Skilling Ward Magnusson Barkshire in Seattle, said:
“September 11th 2001 will forever be etched in our minds – a date when the world witnessed an horrific act, challenging the innocence or naivety of ‘freedom’ to the very core. How will we respond? What price are we willing to pay to protect ourselves?”
Mr. Klemencic said the destruction of the World Trade Center presented an enormous problem to the design and construction industry. “The fruits of our labor have been the target of an attack and we must prove to the world that we can overcome this challenge.”
Some had gone so far as to propose radical solutions such as blast absorbing concrete and steel composite panels that would stop an in-bound aircraft; placing super-strong refuges every ten floors; foam-based fire suppression systems to combat aviation fuel fires, even spiral slides in lieu of exit stairs from tall buildings. Worse yet, others had attempted to make this an issue of concrete against steel, or American codes vs. international standards.
Before answering any of these claims, Mr. Klemencic asked people to consider the facts about the structural form of the towers and how they stood up to these unprecedented assaults. He pointed out that they possessed two unique qualities present in hardly any other of the world’s buildings.
First, the sheer size of the floor plate, 209 ft. across (64m approx) giving floor space of 43,000 sq. ft (4000 sq. m) on each level, compared with the majority of tall buildings in the United States which are between 120 and 140 ft. across (36-43m). Second, the redundancy of the exterior wall. This was of perimeter tube design with steel columns spaced at 40 in. centers (1m approx), 60 columns on each face of each tower. These two qualities, and these alone, said Mr. Klemencic, allowed the towers to stand for nearly one to one and a half hours after the initial impacts. Most other buildings in the world, he said, would have collapsed immediately.
Next he asked people to look at the facts about the type of aircraft that struck the towers. The Boeing 767-300 has a wing span of 156ft. (47.5m); it is 180ft. (55m) in length and weighs around 300,000 pounds. The aircraft were each carrying some 20,000 gallons of aviation spirit and travelling at between 300 and 350 mph. Bearing in mind that the force of impact is related to the square of the velocity, to protect a building from that kind of force it would be needful to arrest the plane within 5ft, requiring resistive force equivalent to 400 million pounds applied in one tenth of a second. Compared with that, the wind load the World Trade Center towers were designed to withstand was 15 million pounds. Once this was understood, it should be clear that buildings could not be designed to stand against this kind of impact.
Engineers from around the world have concurred in acknowledging the heroic nature of the towers’ performance. The Press have been searching for the ‘smoking gun’ which would explain why so many people died. But, said Mr. Klemencic, there is no smoking gun. Nothing from the events of September 11th shows that American building codes are inappropriate.
Apart from the structures themselves whose integrity had been breached by openings up to 140ft (43m) wide, five safety systems in the towers were compromised simultaneously. These were the fireproofing, the fire stairs, the fire suppression system, the stairwells and compartmentalisation over the 3 – 5 floors involved in the immediate impacts.
“Let’s assume”, said Mr. Klemencic, “that we are able to develop some miracle technology to address this situation. In the United States each year, the commercial construction industry spends about $500 billion on new construction. If we are to increase the cost of construction by one or two per cent, that would mean $5 to $10 billion of added costs, to fight an unknown hazard at an unspecified time and place.
“We must not over-react. We must not jump to the conclusion that our buildings must somehow be changed to protect them against the threat of an in-bound aircraft. Rather, our response must be thoughtful and deliberate. We must educate the community at large as to appropriate expectations and assist in striking a balance in the costs so that our limited resources are spent wisely.”
Lightweight Steel Floor Trusses Said to be the ‘Weakest Link
Shortly after Ron Klemencic’s presentation in London, a television documentary program on the United Kingdom’s Channel 4 gave another explanation of why the WTC towers collapsed. This pointed to the use of lightweight steel floor trusses which had never been used in any tall building previously to span as much as 60 ft. (18m). In a commentary published on the Channel 4 web-site after the program, Paula Hawkins said that the core and elevator system of the buildings were also unusual. “Because it was feared the pressure created by the buildings’ high speed elevators might cause conventional elevator shafts to buckle, engineers used a drywall system fixed to a steel core to house the elevators.”
When the planes hit the towers, as many as 40 vertical columns at the perimeter of the building were knocked out. However, the towers were initially able to withstand this damage. “Indeed, the force with which the planes hit the buildings was 95 per cent of the wind load which they were designed to withstand.”
Lightweight steel trusses were alleged to be the weakest link in the structure. The commentary went on to quote Charles Clifton, a structural engineer at New Zealand’s Heavy Engineering Research Association as saying that having penetrated the perimeter frames, the planes would have done much more than just stripping the fire protection off the columns…..The effect would have been to completely shatter and eliminate large areas of the floor slabs and many of the internal supporting columns…..leaving the rest vulnerable to fire attack.
The Channel 4 web-site statement also said: “The structure of the WTC towers is crucial when considering the impact of the fire. The towers, being lightweight and devoid of concrete, were relatively difficult to protect from fire. The weakest link was the floor trusses, which spanned considerable distances. Made of such thin steel (about 4 cm thick), compared to the area they spanned meant they would have heated up very quickly…..Once the floors had succumbed to the heat of the fire, the integrity of the building was threatened. Core columns were not only bearing extra loads, but were subject to intensely high temperatures. Once they began to buckle, the crash site floors collapsed onto the floor underneath…..Once this domino effect had begun, it took seconds for the towers to be reduced to rubble.”
Insurers Found Structures Beaten by Temperatures Well Beyond Design
Munich Re, the German reinsurance company, in its own assessment of the disaster that struck the World Trade Center on September 11th, said that the outer facades and core of the towers were interconnected by horizontal steel trusses approx. 20 m long, which were covered with a layer of concrete roughly 10 cm thick to form the floor of each storey. This made it possible to create large office areas without additional supports. All the steel trusses were coated with the usual mixture of fireproof material used at that time.
Shortly after the aircraft struck, the fire reached temperatures of over 800 deg. C and as much as 1400 deg. according to some experts.
“The fireproof coating of the steel trusses in the core area was designed to withstand at best a local fire, such as burning archives. At temperatures of only 600 deg. C steel loses around 75 per cent of its strength. Despite their coating, the columns consequently gave way or melted completely.
“Although the North Tower had been struck first, the aircraft hit the building higher up and the fire raged longer there before the weakened steel columns in the floors finally caved in abruptly. Due to the dynamic force of this sudden failure of the loadbearing structure, the upper storeys hit the undamaged floors below with their full weight. The lower floors were not designed to withstand such loads and likewise collapsed. As a result, the North Tower caved in like a telescope at 10.28, almost an hour and three-quarters after the collision.
“In the case of the South Tower, the aircraft struck the building lower down and also severed the columns of the outer façade near one of the edges. Due to the higher load of the 35 or so floors above…..the upper half of the tower initially buckled. Then, at 10.02, almost exactly an hour after the collision, the tower completely collapsed in a huge cloud of dust. “The third building to succumb was the 47-storey 7 WTC on Vesey Street. Severely damaged by flying debris from the twin towers it collapsed floor by floor, almost in slow motion, at 17.40. Subsequently the other four buildings of the WTC collapsed one after the other too.”
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