CEE and industry panelists discuss World Trade Center collapse at huge campus lecture

On Oct. 3, over 450 people crammed into Rm. 54-100 to hear "Structural Engineers' Perspective on the World Trade Center Collapse." An extensive summary by Steve Ashley, "When the Twin Towers Fell," appears on the Scientific American website http://www.siam.com for Oct. 10. Since much of the structural information discussed at the lecture is covered in the articles printed on pp. 1-5 of this newsletter, this summary emphasizes material not contained in those works.

Ashley wrote, "After first describing the highly redundant structural system that kept the 110-story twin towers standing for decades despite hurricane-force winds and a terrorist truck bomb, the engineers then delineated how that system was breached and finally overcome.... The main culprits...they concluded, were the two intensely hot infernos" fed by tens of thousands of gallons of aviation fuel.

"Once high temperatures weakened the towers' supporting steel structures, it was only a matter of time until the mass of the stories above initiated a rapid sequence 'pancaking' phenomena in which floor after floor was instantly crushed and then sent into near free fall to the ground below. Significantly, the panel stated that any mitigating reinforcements and redundancies could have only delayed the inevitable failure, though they would have bought more time" for occupants to flee.

"'The World Trade Center rose during the late 1960s, a new era of construction characterized by rapidly erected, lightweight steel structures rather than heavy masonry walls,' explained Robert Fowler [senior engineer of the structural engineering firm McNamara and Salvia]." The Trade Center was much lighter compared to earlier skyscraper designs.

To handle the immense wind shear forces for such tall structures, the engineers "'designed the World Trade Center essentially as a large beam section,'" explained panel member Robert McNamara, president of McNamara and Salvia. Each twin tower was strongly framed in structural steel with inner and outer rectangular box tubes consisting of closely spaced steel box columns connected by steel spandrel members or truss beams that supported 40,000 square-ft. cross-braced floors, each nearly an acre in area.

"The huge inner and outer rectangular tubes 'needed to be protected to maintain their structural integrity, so the floors acted as reinforcing diaphragms or bulkheads,' said panel member Jerome Connor of MIT CEE. 'The office floors, which each comprised a 35- to 60-foot clear span from the core to the exterior grid, were panelized structural members supported by open web joists with steel decks above them.' With almost an acre of area for each floor and figuring about 100 pounds per square foot, he estimated that each floor system weighed about 3,200,000 pounds.

"With all of its structural redundancies, 'the World Trade Center was probably one of the more resistant tall building structures,' McNamara said. The support structures of both towers withstood the initial hits of the two kamikaze airliners despite the breaching of many levels of framing. 'The buildings displayed a tremendous capacity to stand there despite the damage to a major portion of the gravity system. The lateral truss systems redistributed the load when other critical members were lost. It's a testament to the system that they lasted so long.'"

Although the towers had been designed to withstand a collision with a Boeing 707, "'the World Trade Center was never designed for the massive explosions nor the intense jet fuel fires that came next-a key design omission,' stated Eduardo Kausel" of MIT CEE and panel member.... 'It was designed for the type of fire you'd expect in an office building-paper, desks, drapes,' McNamara said," not the much hotter temperature of burning aviation fuel.

In general, the panalists agreed that as the structure warped and weakened at the top of each tower, the frame, along with the concrete slabs, furniture, file cabinets and other materials, became an enormous consolidated weight that eventually crushed the lower portions of the structure below.

Connor's collapse theory focused on weaknesses in how the vertical and horizontal structural members were tied together. "The floor trusses sat on beams and were tied down so the core was locked to the exterior. If a damaged floor system were to fall, it would break the end connections in the lower floors," and they would tumble down on top of each other. He theorized that the fire weakened the supporting joint connection. "When it broke, one end of a floor fell, damaging the floor system underneath, while simultaneously tugging the vertical members to which it was still attached toward the center of the building and down," a process that accelerated until the structure fell in on itself.

"Eduardo Kausel proposed an alternative failure explanation that he acknowledged was independently developed by Zdenek Bazart at Northwestern Univ. 'I believe that the intense heat softened or melted the floor trusses and columns so that they became like chewing gum and that was enough to trigger the collapse. The floor trusses are likely to have been the first to sag and fail. As soon as the upper floors became unsupported, debris from the failed floor systems rained down onto the floors below, which eventually gave way. The dynamic forces were so large that the downward motion became unstoppable.'" Using two simple models, Kausel determined that the fall of the upper building portion down onto a single floor must have caused dynamic forces exceeding the buildings' design loads by at least an order of magnitude.

Probably all these failure mechanisms occurred and interacted, said panelist Oral Buyukozturk of MIT CEE. "'The prolonged effect of high heat is likely to have led to the buckling of the columns, collapse of the floors, as well as to the shearing of the floors upon the failure the joints.' He noted that videotapes showed some tilting of the top portion of the south tower before it collapsed. 'This indicates the buckling of one building face while the adjacent face was bending.' After that, the upper portions of the tower are shown disintegrating, with 'a dynamic effect and amplification process' following that led to a progressive collapse-'a kind of pancaking or deck of cards effect,' down to ground zero.

Robert McNamara's failure theory "'focuses on the connections that hold the structure together,' but he cautioned that 'we really need to wait for a detailed investigation, before we decide if we have to up the code ratings for these connections in signature structures.'"

The expert panel then turned its attention to changes for future tall structures, realizing that the design scenarios for possible disasters have been irrevokably altered. "'Retrofitting is very expensive and is therefore usually done only for monumental buildings,' Connor said. 'There will never be a building that won't fall,' Kausel noted. 'The best we can do is to ensure that it will stand long enough for all the people to escape.' Panel members discussed providing improved fire protection for the structural elements, alternative load paths to stand in for damaged structures and fixing diaphragm floor beams more strongly to vertical members." They mentioned the idea of installing blast-resistant, energy-absorbing materials such as concrete-encased steel exterior columns and/or cavities (reinforced concrete cores) in future large structures that could help them survive or at least promote failure in certain slower, less-deleterious sequences.

"The panel also considered the need to improve the effectiveness of building safety systems. Kausel pointed out problems with the twin towers' emergency communications systems, the emergency illumination system and protection against smoke." He also suggested providing alternative escape routes, so evacuees don't run into a wall of smoke if one explosion blocks two adjacent stairwells. Other ideas ranged from using aqueous film-forming foams employed in aviation fires, and creating protected access ways for firefighters, all the way to exotic evacuation systems such as escape tubes deployed out windows, exterior people-lowering machines, or even parachutes.

"A lively discussion ensued about whether the terrorist pilots knew where to hit the buildings for maximum effect. McNamara opined...'They hit at just the right place--about two-thirds to three-quarters of the way up. ...If they had hit the very top of the building, the fire damage wouldn't have had such a catastrophic effect. At the bottom, the columns are much heavier and stronger and so they would have taken a much larger load.' Connor offered that one would 'need graduate-level engineering training to choose the prime target location.'

Prof. CC Mei reports, "This was the largest technical lecture staged by the CEE Dept. that I can remember." He extends congratulations to "our brilliant student organizers Marc Washington, Lisa Sandoval, Sean McCone and Aurora Kagawa, and panelists and/or organizers: Robert McNamara and Robert Fowler of McNamara & Salvia; and CEE profs. Oral Buyukozturk, Jerry Connor, Eduardo Kausel and Franz Ulm.