COVER STORY STRUCTURES (9/24/2001) Most Surrounding Buildings Safe By Nadine M. Post

IMPACT ZONE About 10 nearby buildings have structural damage that is severe. (Photo by Michael Goodman for ENR) I n the mountain of wreckage that was the 12-million-sq-ft World Trade Center, hundreds from the construction industry have been working in shifts around the clock since the Sept. 11 plane attacks on the 110-story twin towers. They are fighting a multiheaded monster never before encountered. Contractors have been mobilizing to move an estimated 1.2 million tons of rubble and debris, which could take six months to a year to untangle and remove. Engineers have been "on-call" to help them design supports for heavy equipment. Other engineers have been designing a tieback system for the complex's slurry wall foundation, now partly packed with twisted steel from the towers. And even more have been combing surrounding buildings to assess damage done by the collapse of the towers. While an army of construction industry forces was mobilizing around "Ground Zero" in lower Manhattan, experts around the country were also mobilizing to learn lessons about the disaster and possibly to reconsider codes and standards relating to the design and construction of tall buildings. Beyond that, the attack on the WTC has triggered a national debate on whether the twin towers should be replaced in kind, replaced with something taller or whether the site should be left as a memorial to the deada number which could top 5,000. Additionally, there is much heated discussion over the future of the skyscraper as a building form. As of Sept. 18, most of the 50 or so buildings closest to the impact zone immediately surrounding the complex have cosmetic damage, but only 10 or so have structural damage. "Some of that is severe, but none of the buildings damaged is currently in danger of collapse," says Daniel Cuoco, a managing principal of LZA-Thornton Tomasetti, the New York City engineer leading the structural engineering operations, including damage assessment surveys, for the New York City Dept. of Design and Construction. Cuoco adds that "no building outside the WTC has collapsed," contrary to news reports. This includes One Liberty Plaza and the Hotel Millennium Hilton, both across the street from the WTC. MAPPED OUT Efforts focused on damage assessments in and around complex. The roster of collapsed or partly collapsed buildings includes the south tower, called Two WTC, which came down at 10 a.m. on Sept. 11; the north tower, One WTC, which crashed down at 10:29 a.m.; the 47-story Seven WTC, which collapsed in the evening, presumably as a result of fire caused by the towers' collapsing. Three WTC, most recently a Marriott Hotel, is mostly down and burned. Four, Five and Six WTC are standing but ruined. A structural damage assessment survey of 406 buildings several blocks in each direction outside the immediate impact zone has found 382 buildings are fine, 20 have some damage and need a closer look and five are unsafe. The effort was completed Sept. 18 by some 60 engineers organized by the Structural Engineers Association of New York (SEAoNY) at the request of Mayor Rudolph W. Giuliani (R). Plans are forming for a relief commission to oversee reconstruction, and city officials clearly intend it to be a major vehicle for recovery. Reports indicate the City Council might amend the City Charter to give the commission extraordinary powers to redo and rebuild. Outside the 16-acre complex, there have also been no reports of foundation damage, says George Tamaro, an engineer with Mueser Rutledge Consulting Engineers, which is leading the city's foundation engineering operation. Tamaro was involved in the original construction of the foundation while working for the WTC's original owner-developer, the Port Authority of New York & New Jersey. Tamaro says there are no foundations in imminent danger of collapse even within the WTC complex, though he cautions, "We have to go through a verification program." With that said, all eyes are on the 7.5-acre reinforced concrete slurry wall foundationa giant bathtub in which the towers sat. The fear is that superstructure steel, which compressed as it collapsed into the basement levels, undermined the bathtub's reinforced concrete floor diaphragms that hold up the slurry walls. The approximately 1,000 x 500-ft basement has six levels of slabs to support 65-ft-deep perimeter walls socketed a minimum of 2 ft into rock. The integrity of the excavation depends on all four slurry walls being intact, says Tamaro. Currently, those could be supported by remaining structure or by the debris from the towers. "Where the debris is standing and there is no sound structure," it is necessary to resupport the wall with tieback structures, he says. The first priority is the west wall, closest to the Hudson River. Engineers are developing three ways of resupporting the foundation walls, if necessary. Rock anchors figure into the picture in all three cases. "We probably should replicate the original design, which worked then," says Tamaro, referring to the temporary rock anchors that supported the wall until the floor slabs were in place, during the original construction. The system had six levels of rock anchors, each using up to twenty-one 1Ú2-in. strands of steel, which were installed at an angle in a 6-in.-dia hole drilled through the wall. At the top, the anchors were 100 ft long. 90 WEST STREET Some buildings nearby do have serious damage. ( Photo by Michael Goodman for ENR) Tamaro's first choice is to find a "good platform" inside the bathtuban intact floor slab or stable debrisfor a tieback machine to install new tiebacks. Another choice would be to locate a crane on the street outside the perimeter of the foundation box with a set of flexible leads hanging over the wall to install the anchors from the inside. Engineers suspect there are some areas of the foundation floor slabs that are in very good shape. But all have probably been breached in the tower areas. There may be small holes in the diaphragms that can be bridged. At press time, engineers had only been able to enter the concourse level, just below grade, under the northeast corner of the site, and walk in about 50 ft to assess the condition inside and under Five WTC. "The floors are in place and we believe intact," says engineer David Cacoilo, also of Mueser Rutledge. The foundations of the burned-out Five WTC are still there, he adds. "The northeast corner of the complex is more stable than any place on site," says Cacoilo. Still, "huge beams and columns [from the towers], much like projectiles, penetrated the concourse, which is the roof of the subway tunnel." Mueser Rutledge has organized itself into task teams to provide support to the contractors and authorities during the rescue and recovery effort, says Tamaro. Each task force is assigned to a groupone for the transit authority, one for Battery Park City, one for the contractors and one for the port authority. Above ground at and around Ground Zero, LZA/Thornton-Tomasetti's team, expanded by SEAoNY engineers, is doing "engineering on the fly," as Cuoco describes it. The engineers have switched from three, eight-hour shifts to two, 12-hour shifts. There are about 26 engineers during each shift. "There are lots of grillage designs" to support the up-to-300-ton cranes that contractors are positioning for debris removal. All things considered, things have been going "pretty well," says Cuoco. But there have been moments. One came on Sept. 17, when the engineers got a report that a 60-ft-tall section of tower cladding and structural tube that had fallen from the 80th floor of the south tower, was moving. It was soon pulled out of the ground with a cable attached to a backhoe. HOTEL MILLENNIUM HILTON Collapse reports on news casts were unfounded. (Photo by Richard Rodriguez) Cuoco says that the top portion of the south tower, above the floor of impact, toppled to the east during the Sept. 11 debacle. The balance of the tower fell to the northwest, he says. The north tower came down "pretty much vertically," says Cuoco (ENR 9/17 p. 10). Reports that the plaza is bouncing somewhat are true, he says, but "one would expect that considering the large pieces of steel and other debris over that area of the plaza." For the 406 damage assessments, a SEAoNY team used a database provided by the city's Dept. of Finance, which had been compiled for use in an earthquake preparedness study. Each engineer received a checklist for each building, which contained an address, lot and block number. Engineers made a rapid engineering evaluation of each building to determine those that required further attention. "It was a collective effort," says Guy Nordenson, a professor of architecture and engineering at Princeton University. With so many authorities involved, it is taking time to get organized. Leslie E. Robertson Associates, the consultant to the port authority on the World Trade Center, is working directly under the agency as well as participating in the SEAoNY effort. "We have pointed out concerns" about the nine-story Five WTC, says Saw-Teen See, LERA's managing partner. Its columns are supported on transfer girders that span the subway. "There's a lot of rubble and added weight," she says. "The girders may be overloaded at and below street level." LERA has turned over hundreds of drawings of the complex to the Federal Emergency Management Agency and the city's Office of Emergency Management. "We even gave them plumbing drawings," says See. Though the pace is expected to slow down somewhat, Cuoco, who himself has been working 16-hour days, says the structural engineers' involvement will likely last for a while: "This will be going on for months." CLICK HERE FOR PAST COVER STORIES AND FEATURES © 2001 Engineering News-Record Site Sponsors