7 World Trade Center First to be Rebuilt

In June of 2005, the National Institute of Standards and Technology (NIST) issued a 10,000 page report on the World Trade Center collapse. The report focused largely on what could be done to help prevent the collapse of buildings in the future.

The report consisted of thirty recommendations that were divided into eight separate groups, which were “increased structural integrity, enhanced fire resistance of structures, new methods for fire-resistance design of structures, active fire protection, improved building evacuation, improved emergency response, improved procedures and practices, and education and training”.

Even though the report was not available when 7 WTC was redesigned and built, the architects (Skidmore, Owings and Merrill) employed similar concepts later found in the report into the new building. Their attention to design focused on fire suppression, protection from fire, and building evacuation. These concepts were further enhanced by “redundancy and diversity”, which will to the greatest extent possible allow the building to continue performing even if similar circumstances as occurred on 9/11 happen again.

One of the things that the NIST report brought out was that had the sprinkler systems in the World Trade Centers remained functional after the attack, the collapse may not have occurred. Anecdotal evidence for this is demonstrated by the fire at One Meridian Plaza in Philadelphia in 1991. That building was being retrofitted with a sprinkler system when a fire broke out. The fire continued to burn for 18 hours at the top of the skyscraper until it reached a floor that had active sprinklers installed. When the sprinklers went off, the fire stopped spreading. The building did not collapse.

At the WTC, since the water supply was cut off from the floors above the airplane’s impact point, sprinklers were not functioning. At the new 7 WTC, separate water supply lines, with each serving every other floor and connecting at the top to form a loop, would allow one line to be cut and still continue to supply sprinkler heads at every floor.

Because of their height, the water supply for the sprinkler system in a skyscraper is normally kept in a single tank at the top. When the buildings were hit, the single water supply line was severed. In the new 7 WTC, multiple tanks are dispersed around the building at various levels.

It has been widely accepted that one of the reasons the twin towers collapsed was that the explosive impact of the airplanes blew off the fireproofing from the steel columns, beams, and floor joists. The type of material that had been used was a very soft material that would perform well in ordinary fires. However, it was not strong enough to stay adhered to the structure when the impact occurred.

The structural members of the new 7 WTC is coated with a newer fireproofing material that is much denser and has much better bonding capability.

When the twin towers were struck, the stairwells were cut, cutting off any means from escape for those on floors above the impact point. To enhance the evacuation capability of the new 7 WTC, the stairways and elevator shafts were constructed wider than was required by code, and of reinforced, high strength concrete that varied in thickness from 12 inches to 28 inches. This affords more protection from an impact than the shafts in the twin towers had, where they were constructed of metal studs and drywall.

To protect people who are evacuating the building through the stairwells, the shafts are equipped with an exhaust system that will draw smoke out and bring fresh air in. It is also pressurized to prevent smoke from entering from the floors that are on fire.

Local New York Codes require that stairways be painted with photo-luminescent paint so that they will glow if the lights go out. In addition to that, the lighting in the stairwells of 7 WTC is equipped with an emergency generator, as well as a set of batteries in case the generator fails.

Many of these safety features are not required by code, but the owner of 7 WTC was willing to go the extra expense to give its occupants the best chance to survive another attack. It is a great example of conscientious design and construction.