5 years after 9/11, are we building as safely as possible?

In August, British authorities foiled a reported plot by fanatics to blow up ten passenger jets over the middle of the Atlantic Ocean. Civilian airliners have long been choice targets for terrorists. In fact, al Qaeda in Asia had planned an almost identical attack for 1995--except that a dozen jetliners were to be blown up over the Pacific Ocean. That scheme was also foiled.

Make no mistake, high-profile commercial and government office buildings, along with mass-transit systems, are other perennial terrorist priorities. New Yorkers know this only too well. Before the 1995 bombing of the federal office building in Oklahoma City, demented terrorists in 1993 triggered a bomb in the underground garage of the World Trade Center. Six people were killed and more than 1,500 injured, but the building stood--until the atrocities of September 11, 2001.

So there's no reason to doubt that Islamist zealots will try again, sooner or later, to bring down more landmark towers in U.S. cities. Unfortunately, many New York skyscrapers are especially vulnerable in terms of occupant safety. Although some portions of the city's building code were upgraded after 9/11, it is still permissible to build stairwells protected only by a fire barrier of l/2-inch-thick sheetrock (a euphemistic name for drywall).

Everywhere else, not only in America but also worldwide, building codes for high rise buildings typically utilize cast-in-place-reinforced concrete for emergency stairwells--and encourage its use for the entire structural framework. Study after study has shown that reinforced concrete provides superior blast and fire resistance compared to fireproofed spray on steel flaming. And the new technique of cast-in-place reinforced concrete puts the economics of concrete flaming on a par with steel.

Even though stairwells safeguarded within concrete cores are not yet mandated in New York City, progressive developers and builders must recognize the increasing value of tenant safety for maximum egress. As a result, the world's oldest man-made building material is claiming a growing share of New York City's commercial high-rise market. Here are the major structures destined to expand the legacy of safety established by the venerable Wang Building, a 50-story concrete office building dating back to 1984: 505 Fifth Avenue (completed April 2005), 28 stories; 7 World Trade Center (April 2006), 52 stories; One Bryan Park (2008), 54 stories; U.S. Mission to the United Nations (2009), 24 stories; Freedom Tower (2011), 82 stories.

Near Freedom Tower, three more new office skyscrapers on Greenwich Street will stand guard over the September 11th memorial. Heights and completion dates for this trio may be announced soon. Tentatively, at least two could be finished by 2012, perhaps climbing to around 68 and 71 stories, while the third might poke up 75 stories.

Also, three mixed-use, office-and-residential concrete towers have been punctuating the Manhattan skyline for more than a decade: Carnegie Tower (1991), 60 stories; CitiSpire Center (1987), 72 stories; Trump Tower (1983), 58 stories

Among the projects now under construction, the U.S. Mission to the United Nations is particularly intriguing: Its structure will be framed entirely in cast-in-place reinforced concrete. Don't get too curious, though. Design details are shrouded in secrecy.

What's no secret is why reinforced concrete was specified by the U.S. General Services Administration (GSA), the agency that acts as contractor and manager of federal buildings. Last summer, GSA project manager Sek Eng told Construction Equipment Guide that "steel doesn't meet our security needs."

In the design by Gwathmey Siegel & Associates Architects, the building has a windowless, six-story base with fortress-thick walls that can withstand a car bomb. The solid reinforced-concrete exterior will be attractively disguised to please the eyes of visiting dignitaries and passers-by. The bunker base will hold the heating, ventilation, and other mechanical systems that typically get located near the middle or top of a high-rise.

This basic approach can already be found at the recently completed 7 World Trade Center--and will be used to protect Freedom Tower. Both have a reinforced-concrete pedestal topped by offices with glass curtain walls. Presumably the nearby trio of skyscrapers lining Greenwich Street will get a similar treatment as well. That's because developer Larry A. Silverstein has become a staunch advocate of cast-in-place-reinforced concrete--for safety's sake--and his company as you know, controls all construction at the World Trade Center.

Silverstein Properties Inc. owned the first 7 World Trade Center (7 WTC) building. Situated just outside the north boundary of the WTC, that steel-framed structure was brought down--without loss of life--when burning debris from the Twin Towers turned 7 WTC into an inferno. Its Phoenix-like successor relies on a cast-in-place-reinforced concrete core two-feet thick to soar beyond the original 47 floors, to 52 floors. The $700 million building rolled out the welcome mat on May 23rd.

Silverstein hails the reborn 7 WTC as America's safest office building. But that status will be shared with Freedom Tower. "Everything we do at 7 World Trade," he said last March, "we will do at the Freedom Tower [its construction began in late April] and at all subsequent World Trade Center buildings."

There's even a chance that one or more of the Greenwich Street trio could surpass the safety enhancements offered by 7 WTC. Jacob Grossman of Rosenwasser/ Grossman Consulting Engineers seems optimistic about that prospect. "In the past few years," he told Architectural Record in late 2004, "the system [post-tensioning] has improved a great deal," and he hopes that continued development will erase the last of his lingering reservations.

Still, Grossman doesn't think that new technology is a prerequisite for the rapid growth of concrete construction in New York. For the past 20 years, he notes, the rest of the world has built the tallest buildings with cast-in-place reinforced concrete. So he predicts that concrete's share of the structural members in New York commercial buildings will also edge past steel's in the near future.

Rosenwasser/Grossman did its bit to fulfill that prediction. It engineered 505 Fifth Avenue, which is built around a cast-in-place reinforced-concrete superstructure. Thanks to concrete's strength, the 28-story building has thinner floor plates than would be feasible with steel. That gave developer KippStawski Management Group two extra floors of rentable space.

Both 7 WTC and Freedom Tower were designed by Skidmore, Owings & Merrill (SOM), so their safety similarities are hardly surprising. At 7 WTC, the cast-in-place-reinforced concrete base, sheathed in ornamental metalwork, climbs 11 stories and houses a Consolidated Edison electrical substation as well as the building's mechanical systems. From its foundation up to 80 feet, 7 WTC is 100% cast-in-place reinforced concrete. Above that, it has a cast-in-place concrete core and steel framing around the perimeter.

Pretty much the same goes for Freedom Tower, except that it will sit on a 200-foot cube of extra-dense reinforced concrete. This base is the same size as the footprints of the Twin Towers, which will be enshrined as solemn voids in a lush park. Also, of course, Freedom Tower will be taller than 7 WTC. Tacking on its mast, Freedom Tower will reach the symbolic height of 1776 feet.

To justify the claim that 7 WTC is the "safest office building" in America, Silverstein and SOM architects rattle off a long list of safety features that exceed current building codes. In particular, there's the vertical core. It is made of two-foot-thick, reinforced concrete with a rating of 12,000 psi. It encases the elevators and their vestibules (protected behind smoke-stopping doors), electrical conduits, sprinkler-system feed pipes, and dual emergency exit stairs.

"We learned on 9/11 that you don't build a core out of plasterboard," Silverstein told New York magazine last year. "Plasterboard doesn't burn-terrific--but you can take a penknife and carve through it. It's permitted by code, but it has no strength." In contrast, the two-foot-thick walls of 7 WTC's cast-in-place-reinforced concrete core are impregnated on every floor with 70 tons of steel reinforcing bars--"every floor!" Silverstein exclaimed. "Nothing is going to destroy that core."

The stairs are 66 inches wide, or 20% more than code requires, to provide room for two-way traffic. Silverstein wants firefighters to be able to climb up relatively unimpeded while tenants rush down. Freedom Tower will go further. It will have an extra stairway exclusively for rescue workers.

In the event that one of 7 WTC's stairwells is blocked by some catastrophe, SOM's designers pulled the stairs as far apart as possible, to diagonal corners of the core. Under New York code, exit stairs can be back-to-back if there's a fire wall in between. The Twin Towers showed that can be a recipe for disaster. In the North Tower, where the stairs were bunched together and simultaneously destroyed, no one escaped from above the plane's point of impact. In the South Tower, however, one stairwell remained passable because it was separated from the others, and at least 18 people used it to flee from higher floors.

SOM partner Carl Galioto cites 7 WTC's "robust structure" as another important code-exceeding safety provision. In the Twin Towers, when fire weakened vertical support members, that overburdened adjoining members and led to the collapse of each tower. But if any of 7 WTC's exterior steel columns are compromised, the structure is engineered with redundant steel beams that can absorb the load.

Silverstein boasts that the safety measures implemented in both the 7 WTC and the Freedom Tower anticipated many of the recommendations in last summer's massive, 10,000-page report by the National Institute of Standards & Technology. NIST's three-year study dissected the causes of the collapse of the Twin Towers and outlined 30 improvements to increase tall-building safety. The recommendations include hardening elevators so they can continue to function in a fire, placing stairwells apart from each other, and designing structures to ward off progressive collapse.

Reacting to September 11th, New York City's Department of Buildings quickly formed a building code task force and began gathering facts and insights from industry and government experts. The task force also studied a report by the Federal Emergency Management Agency, then issued 21 recommendations in February 2003. In June 2004, 13 of those proposals became part of New York's building code.

"Building egress issues are too important to fall victim to the competition between concrete and steel," declares Alfred G. Gerosa, president of the Concrete Alliance.

That's why he urges real-estate developers, architects, and government officials to keep up the pressure to reevaluate traditional approaches to public safety.

BY HERBERT MARGRILL

VICE PRESIDENT OF COMMUNICATIONS

CONCRETE ALLIANCE, INC.