(continued)

So resilient was the newly strengthened section of the Pentagon that a glass display case only 40 feet (12 meters) from where the plane entered the building survived without a crack.

Countless additional injuries were prevented because new windows in the renovated section were "blast-resistant" and did not explode into flying glass splinters, because new fire sprinklers operated as designed, and because alert personnel turned off power and utilities to the affected areas before evacuating.

Fortress of the Forties

The Pentagon was built during World War II, in an amazing 16 months, to accommodate a consolidation of diverse branches of the U.S. military establishment. The five-story building of 6 million square feet (560,000 million square meters) is actually five concentric building rings separated by open-air courtyards, and connected by ten radial corridors.

From inside to outside, the five rings of buildings are labeled A through E. The airliner entered the ground floor of the west face of the E ring and penetrated through the C ring. Where the airplane struck, the impact, explosion, and fire brought down all five stories and created a hole about 100 feet (30 meters) wide. In the surrounding area, the newly stiffened walls remained only partly damaged or not at all.

The primary structure of the Pentagon is 42,420 steel-reinforced concrete columns. Of these, only 32 were destroyed and 15 seriously damaged. As recovery efforts continue, the structure is being shored up with pressure-treated wood posts to protect against further collapses.

Thus, in relative safety, workers have been able to recover the remains of victims and to examine and clear away debris. Within a week the airplane's voice and data recorders had been found.

Ironically, the fortress-like appearance provided by the exterior Indiana limestone is misleading. The limestone is not structural but a veneer supported by steel hangers. As the facade is repaired in the near future, matching replacement stone will come from the original quarry in deference to the building's historic status.

The building was originally constructed as five chevron-shaped wedges, and the fact that each of these has its own independent mechanical and electrical systems further contributed to minimizing the damage on September 11.

The Renovation Plan

The renovation plan, which began in 1993 with the construction of a power plant, was inspired in part by other terrorist attacks in Oklahoma City and abroad. It was also made necessary because the facility was woefully dysfunctional with leaking pipes, a 60-year-old HVAC system including 17.5 miles (28 kilometers) of ducts made from asbestos, a basement floor that had settled up to 12 inches (30 centimeters) in some areas, and electrical and communications systems that had been incrementally jury-rigged to bring them up from 1940s standards.

With Wedge One work nearly complete, a new design/build team of architects, engineers, and builders had just begun work on renovating Wedge Two. The total project was scheduled to conclude in 2012. Although the repairs from the recent attack will probably cost several hundred million dollars above the original renovation budget, Pentagon officials expect the original renovation schedule to be met.

In addition to major overhauls of the mechanical and electrical systems, the Wedge One renovation included the fire sprinklers, automatic fire doors, and the steel which saved many lives on the day of the attack.

The blast-resistant windows were nearly two inches (5 centimeters) thick. Some of them remain remarkably intact and in place adjacent the point of impact. Some were popped out of their frames by the force of the exploding jet fuel, but they fell without breaking or splintering.

Also on the exterior walls, between the steel columns, the renovation crew had placed Kevlar cloth, similar to the material used for bullet-proof vests. This had the effect of holding together building materials so they wouldn't become deadly projectiles in an explosion.

As in the original building, the renovated Wedge One kept independent mechanical and electrical systems which were shut down shortly after the attack without affecting ongoing operations of two-thirds of the building.

Already repair and clean-up work has begun. Aside from the structural collapse, there was considerable soot damage from fire, especially on the upper floors.

There are varying degrees of water damage in about one third of the building. Some mold-infected carpets have been quarantined until they can be cleaned or removed, and water has destroyed valuable computers and documents.

So far, air quality levels have been judged acceptable so as not to endanger clean-up workers. The new air control system pressurizes air and forces the contaminates to remain in the area of damage.

Learning from the Disaster

While Pentagon work continues, architects and engineers are doing what they can to improve the security of their projects elsewhere. Indeed, for DoD projects, any new construction or major modifications is required to mitigate threats of both natural hazards and attacks by explosives or chemical or biological weapons.

The Society of American Military Engineers (SAME) will be holding a previously planned conference November 1-2, at The Citadel in Charleston, South Carolina to educate professionals about such issues.

The object of the Symposium on Comprehensive Force Protection is to "promote the development and dissemination of comprehensive force protection technology and methodology for the built environment."

No doubt symposium attendees will be listening with a renewed appreciation for the reality of the threats and the importance of architectural protection. As Pentagon renovation manager Evey grimly concluded, "that the [terrorists] happened to hit an area that we had built so sturdily was a wonderful gift."

B.J. Novitski is managing editor for ArchitectureWeek.

Some of the Pentagon wall took a half hour to collapse, enabling hundreds of workers to escape. Photo: U.S. Department of Defense The pattern of structural columns destroyed shows the Boeing 757's diagonal entry path into the Pentagon. Image: U.S. Department of Defense Newly renovated offices suffered extensive fire and water damage. Photo: U.S. Department of Defense When the Pentagon collapsed, the structure was held together by the "web" of steel columns and beams, show in maroon, which are bolted together at the floors. Photo: U.S. Department of Defense Structural steel, running through all five floors, strengthened the walls around the blast-resistant windows. A Kevlar cloth stretched between the steel tubes prevented debris from becoming shrapnel during the explosion. Photo: U.S. Department of Defense Aerial view of the Pentagon roof damage, with the area of impact in the E ring shown upper left. Photo: U.S. Department of Defense Damage visible in one of the open-air courtyards between building rings. Photo: U.S. Department of Defense Two days after the attack, Urban Search and Rescue crews from Montgomery County, Virginia worked to clear debris and strengthen temporary shoring at the disaster site. Photo: Jocelyn Augustino/ Federal Emergency Management Agency News Photo   Click on thumbnail images to view full-size pictures.