Standing Tall

THE TASK OF REBUILDING

Within hours of the collapse, the debate began over what -- if anything -- should be built to replace the World Trade Center. Some suggested rebuilding exact replicas of the original towers -- a signal to terrorists that the city's spirit is intact. Others suggested creating a park as a memorial to the attack's victims. The World Trade Center's current manager, Larry Silverstein, has suggested erecting something new: a cluster of four 50-story buildings. Such a plan would confirm the old adage that there is safety in numbers. Grouping the buildings around a central courtyard would protect the sides of the buildings that face inward from an airplane attack -- and cut the cost of facade -- strengthening measures in half.

But before any plan can be put into action, the painstaking and precarious process of clearing rubble from the site must be completed. Like players in a life-or-death Jenga game, the cleanup workers are carefully removing twisted metal and debris from the six floors below the towers, hoping their actions won't cause the entire foundation to collapse.




The six floors beneath the Twin Towers -- which contained shops and restaurants, a parking garage, the subway, and PATH trains -- were surrounded by an underground retaining wall. Known to engineers as the bathtub, the wall kept out the surrounding soil as well as water from the Hudson River, which flows just a few blocks away. When the World Trade Center was intact, steel beams in the floors of the subterranean levels acted as struts, propping the bathtub from within so it would stay upright. The question now being asked by forensic engineers is whether the role of those struts has been usurped by the rubble that fills the bathtub. If that's the case, then the cleanup effort could end up causing the underground retaining wall to collapse -- flooding the site and endangering workers.




But Dan Hahn of Mueser Rutledge Consulting Engineers is confident it's basically intact. "We check the retaining walls for water constantly," says Hahn, whose firm is a member of the city-appointed task force charged with ascertaining the stability of the World Trade Center's foundations. "As long as the walls are dry, we're doing fine."




Another challenge is the damage to the train tunnels that run beneath the World Trade Center complex. The Cortlandt Street station, a stop on the IRT subway line, must be rebuilt, as must 1,000 feet of track -- a process that could take years. Meanwhile, the PATH train, which connects lower Manhattan with New Jersey, was also significantly damaged. "Between the broken water pipes and the water from the firemen's hoses, there was water in the PATH tubes as far as New Jersey," says Hahn. Mueser Rutledge created two concrete plugs, 16 feet in diameter, to seal the PATH tunnels and prevent further flooding. Once leaks have been investigated and the excess water pumped out, the plugs will be removed and tunnel rebuilding will begin.




Given the nature of the attacks, the threat of bioterrorism has been revived. In response, the Federal Transit Authority is developing so-called Urban Chemical Release Detectors, devices that can detect fire, smoke, biological agents, and other hazards within underground tunnels. As many as 10 of the devices could be placed in camouflaged locations around a subway station to alert station personnel of potential peril.




But even if all these technological advancements in building and transportation are implemented, will we be safe? Would concrete cores, steel-plated facades and bomb-sniffing detectors have prevented the tragedy that occurred on September 11th?




Ideally we'll never know, because there will never again be such an attack. That hope, however, is no reason to skimp on safety measures, says Cote, the chief engineer at the National Fire Protection Association. According to Cote, the public's attention span for safety concerns is painfully limited. A flurry of improvements often takes place after a tragedy, but all too soon the event recedes in people's memories and builders begin chipping away at safety measures, complaining that they cost too much. "Sadly," he says, "almost all of our advances in things like mandatory fire code come about after disasters."