“A lot of people have told me, ‘You should have used more concrete in the structure,’” said Robertson. However, his chart plotting the strength of steel vs. concrete at various temperatures showed that at the incendiary levels that raged in the towers, the two materials become similarly weak.
http://www.berkeley.edu/news/berkeleyan/2002/04/17_alum.html
The towers were believed to have been the first to rely on "shaft-wall" interior cores, made of gypsum-based wallboard instead of harder materials, masonry or reinforced concrete. The shaft-wall design was considered a breakthrough at the time, favored for its fire resistance and air-tight qualities. A question today is whether abandoning shaft-wall construction is worth the additional weight and cost.
Leslie E. Robertson, who directed the structural design of the Trade Center, said he would be "astonished" if codes are changed to require harder interior cores. The proper response, he says, "is not making buildings resistant to the airplane, but to keep the airplane from running into it."
http://www.absconsulting.com/news/wsj-oct10-01.pdf
For example, in the Delaware talk, Dr. Thornton contrasted the lightly protected, wallboard-encased stairwells of the World Trade Center — which were severed by flying debris, trapping hundreds of people above — with the thick concrete walls enclosing the Petronas stairwells. In the talk, Dr. Thornton said "concrete-encased stairwells probably would have survived that and allowed people from above to get down," Dr. Chajes recalled.
He describes Mr. Robertson's design as visionary and points out that the advanced, high-strength concrete holding up all of the Petronas Towers — not just the core — had not been developed when the trade center went up in the 1960's and 1970's.
In that respect, the Petronas Towers, built in 1998, were made possible by rapid improvements in the most mundane of materials, said Franz Ulm, a professor of civil and environmental engineering at the Massachusetts Institute of Technology. "In the '80's and '90's, there occurred a real revolution in concrete materials that is almost not known to the public," he said.
He said that high-strength concrete generally resisted both blasts and fires better than steel like that in the trade center, where the plane impacts probably knocked loose a lightweight form of fireproofing that had been sprayed onto columns and beams, which then buckled in the heat.
Advocates for steel construction, which has long battled concrete in the marketplace, dispute the claim that there is any real difference in properly constructed buildings that use either of the materials — especially since even the high-strength concrete is reinforced with embedded steel bars. "There's this perception that just because we wrap something in concrete, that it's protected from the fire," said Charlie Carter, chief structural engineer for the American Institute of Steel Construction in Chicago. "That's not true."
For a combination of historical, cultural and economic reasons, tall, concrete-core buildings dedicated to office use are unusual in New York, where builders prefer the wallboard-enclosed cores with steel frames that Mr. Robertson pioneered in the trade center.
But Patricia J. Lancaster, an architect who is the city's building commissioner, said that despite the possibility of higher costs, the city should look at revising its building codes. "Certainly, the hardening of core areas including elevator shafts and stairway enclosures is something that needs to be looked at," she said. "Comparing 2 Sets of Twin Towers" by James Glanz. New York Times, October 23, 2002
The gypsum panels were used to form fire-resistant enclosures around steel core columns, stairwells, mechanical shafts, and the core area in the towers. The core column fireproofing varied according to the column location and exposure to occupied spaces. The primary function of the core columns was to carry the building gravity loads. The exterior columns
resisted wind loads and, in addition, carried approximately half of the gravity loads.
Preliminary analysis of the core and exterior columns considered their individual buckling behavior and how it varied for uniform elevated temperatures. The columns were found to have sufficient capacity for
tower gravity loads, even under elevated temperatures and a loss of lateral support at several floors. This was also found in more detailed finite element models of the columns.
The core columns were studied to determine the most efficient way to reduce the complexity of the model while still capturing buckling behavior at room and elevated temperatures. Four classifications of core column structural damage were established: severed, heavy damage, moderate damage, and light damage. Classification criteria included plastic strain levels and lateral deformation from the column centerline. Columns that were severed or heavily damaged were removed to simulate impact damage in the global analysis of each tower. Two types of floor structural damage were identifiedfrom the impact analysis results: (1) missing floor areas and (2) severely damaged floor areas incapable of supporting loads.
Fireproofing was assumed to be dislodged from core columns only if the columns were subject to direct debris impact that failed wall partitions in the immediate vicinity of the column1.
Case D predicted more damage to core columns than Case C, but the extent of the fireproofing damage was similar, as shown in Fig. E–4.
Thermal Weakening of the Core:
• The undamaged core columns developed high plastic and creep strains over building stood, since both temperatures and stresses were high in the core and creep strains exceeded thermal expansion in the core columns.
• The shortening of the core columns (due to plasticity and creep) was resisted truss which unloaded the core over time and redistributed loads to exterior
• As a result of the thermal weakening (and subsequent to impact and prior of the south wall), the north and south walls each carried about 10 percent loads, and the east and west walls each carried about 25 percent more loads. Core columns carried about 20 percent less gravity loads after thermal weakening.
http://wtc.nist.gov/pubs/NCSTAR1-6ExecutiveSummary.pdf
Photos:
http://www.nist.gov/public_affairs/Gayle-Proj3 Mech Props NCST Final.pdf
NARRATOR: To support most of the downward weight of the building Robertson created a separate inner core made of steel girders. The core also housed the lifts and emergency stairwells, but neither the outer skeleton nor the inner core could stand alone, so Robertson used steel floor trusses to knit the whole thing together.
LESLIE ROBERTSON: The World Trade Center is a very large project. In essence it still boils down to a series of small pieces and this is an example of a top part assembly of a typical floor truss.
NARRATOR: The floor trusses had a vital structural role. They held the towers firm bracing the outer skeleton against the inner core. Without the trusses the towers could not stand. Their performance is now at the heart of the investigation into what happened. Another area of innovation was in fire protection. To save weight the trusses were coated not in concrete but in the latest, lightweight, heat-resistant foam and instead of protecting the inner core with concrete the architects used both the spray and a lightweight fire resistant plasterboard called drywall. Drywall is very effective at keeping out fire, but it has one problem: it's not very strong.
BRIAN CLARK: Drywall had been blown off the wall and was lying on, you know propped up against the railing here and, and we had to move it, shovel it aside.
http://www.bbc.co.uk/science/horizon/2001/worldtradecentertrans.shtml
Most of the load was supported by several fourteen-inch columns, forty inches on center, surrounding the building; giving it the appearance of a pinstriped suit.* The only other main structural component was the core of steel columns that supported the elevator shafts and stairwells in the center of the building.
The columns in the core were substantial and capable of bearing huge gravity loads.* However, they depended on the floor truss system to provide lateral support.* As the flooring system was destroyed by fire, greater lengths of core columns were exposed, which were already overloaded because of the destruction of the exterior columns.* Taking away the lateral support of the flooring system caused the core columns to have a larger effective length factor (K), and the columns buckled, causing the floors to crash straight down on one another.* The figure above shows that with a larger K-value, the allowable load on a column is significantly less.
The core structure consisted of steel beams with this spray on fire protection material.* It is suspected that the material was stripped off the steel, directly exposing the columns to the intense fire.*
From "A Secure Skyline: A comprehensive analysis of the World Trade Center collapse and recommendations for revisions to current fire resistance building codes"*
http://wps.ablongman.com/wps/media/objects/697/714416/ModelsTemplates/ReportTemplate.doc&e=9797
NARRATOR: In Robertson's design, the downward weight of the building was also supported by large steel columns around the building's inner core, which is where he placed elevator shafts, emergency stairs and other building services. But the tall vertical columns of the inner core and outer walls were like freestanding stilts until Robertson tied them together with floor trusses.
MATTHYS LEVY: The core in concrete might have actually stood for a much longer period of time, allowing many, many more occupants to leave the building. It would certainly have allowed the occupants on the upper floors to have a safe passage through at least one of the vertical stairwells. The core in concrete might have actually stood through the fire and survived.
NARRATOR: Long and thin, these horizontal steel assemblies were connected by bolts to the columns at each end and then welded to the exterior columns for extra support. The trusses were critical for holding the buildings together, and their performance is now at the heart of the investigation into what happened.
Robertson tried to save weight and costs wherever he could. He fireproofed all steel members, including the trusses, with the latest lightweight heat-resistant foam. And he kept the core area light by walling it off with drywall or Sheetrock(TM) rather than concrete.
JONATHAN BARNETT (Professor, Fire Protection Engineering): This is very typical. We often build buildings this way, two layers of Sheetrock on either side of a steel framework. It's just like you might build a wall, except we use special Sheetrock that's particularly fire-resistant.
http://www.pbs.org/wgbh/nova/transcripts/2907_wtc.html
Mr. Robertson's groundbreaking structural designs that have influenced the design and construction of tall buildings include: ...The creation of the shaftwall system now almost universally used for fire-resistive walls in high-rise buildings.
http://www.tc.umn.edu/~eeriumn/leslie.htm
The swaying of the cables in the elevator shafts has been known to dislodge the fire protection from the columns in the cores of these buildings... The Twin Towers would be perforated steel boxes surrounding a hollow steel core.
http://www.newyorker.com/fact/content/articles/011119fa_FACT
The buildings were architecturally interesting in many ways. Each structure is based on a central steel core, which is surrounded by the outside wall, a 209-foot by 209-foot cube of 18-inch tubular steel columns, set 22 inches apart. The cores and "tube walls" share the enormous physical weight of the structures and protect them against the extraordinary wind forces of buildings that tall. There are trusses that support each floor, but no other columns between the cores and outside walls.
http://archive.salon.com/news/feature/2001/09/11/collapse_background/index.html
One of the major issues of concern during the design was that this building did not include the masonry infill that had been included in the skyscrapers of the past.* Although thought on paper not to contribute much to the overall stiffness of a building, comparative analysis of the as-built stiffness of this and other skeletal buildings was substantially less than the masonry-infill predecessors.
http://www.engr.psu.edu/ae/WTC/LERPresentation.htm
While the exterior provided protection against the winds, the interior served as the main support for the building. The internal columns formed a core that took care of the weight. Second, Yamasaki had to make sure the air pressure generated by the express elevators would not buckle the elevator shafts. The engineers of Otis Elevators came up with a solution to this problem. By using a drywall system fixed to the reinforced steel core, the shafts were strengthened enough that air pressure was not an issue.
http://www.unc.edu/courses/2001fall/plan/006e/001/engineering/index.html
