Thoughts on World Trade Center - Thoughts from a life safety engineer

Saturday, 01 December 2007

Thoughts on World Trade Center - Thoughts from a life safety engineer

So I have evolved into something of a fire and life safety engineer (can't seal blueprints or anything - especially no t in nyc) but my birth into the industry came through my personal trial by fire at three floor Spa located in NYC. Through a ambitious relative and a shifty Puerto Rican contractor, I was given birth into an industry that is known to be the last contractor on the job and the last to leave, due to its comprehensive nature. My years of studying and training have still not exacted my complete transition into this industry nor into the best business knowledge as of yet but it has revolutionized the way I see things.
The issues behind the fire and security industry, I am involved in, are closely tied to the issues underlying what occurred on 9/11 ... Understanding what happened at the World Trade Center is not only like understanding a little bit more about our collective American conscience, but for me personally, it was like learning a little bit more about myself... in many different senses.
The thoughts below are from an engineer..

World Trade Center Tragedy

The World Trade Center was constructed differently from other buildings.

	*The towers were* constructed to withstand the impact of the largest commercial airliner of the time**, a Boeing 707. It was thought that it would only occur if a pilot lost his/her way in a fog, thus moving slowly and not fully laden with fuel. Since then, planes became much larger, and they were loaded with fuel and moving at high velocity.
	The exterior walls actually held up the towers. Most high-rise construction is "curtain-wall", i.e. the exterior walls don't hold up anything except themselves. This can be observed when such buildings are built, first the steel beams go up, then the floors, the exterior walls go up only to enclose the structure, but don't support anything. The WTC exterior walls were bearing walls, and actually held up the building (along with the center core), allowing each floor to have nearly an acre of open space without columns. The four exterior walls of each building functioned as a huge box beam, giving it great strength. A building of conventional construction could well have collapsed immediately when hit by a plane, but this didn't happen because of the design. The weight above the impact area was transferred to adjacent areas, allowing the building to remain standing long enough for occupants below the impact floors to evacuate (occupants above were doomed, since the stairs and elevators were destroyed or blocked by debris, and the roof could not be used by helicopters because of the TV antenna on the north tower). The exterior walls also served to contain the collapse of the north tower, confined the material within the area of the building; everything went straight down and pancaked the floors. This 'pancaking' also suggests why there were so few survivors of the collapses — out of about 3,000 people dead or missing, there were less than 20 survivors saved from the collapsed buildings, no later than the day after the collapses.
	Each floor was held up by trusses (zig-zag steel pieces welded to angle iron) connected to the exterior walls. This type of construction is not as resistant to high temperatures than I-beams would have been because of the thinner materials used. It seems likely that the floor-wall connectors failed at the fire floors because of the heat of the fires, significantly weakening the building structure and causing catastrophic failure. The TV transmitting antenna on the top of Building One wound up standing vertically on top of the rubble, showing that everything went straight down (see photo). The collapses were probably caused by the heat of the fires which weakened the core in the north building on the floors which had been impacted, any sprinkler systems and standpipes were most likely severely damaged by the planes' impacts, as well as not being designed to handle a fire of other than what was in the occupied spaces (as you know, a flammable liquid fire cannot be extinguished with water). The core of the north tower collapsed, and the floors above came down on them, all this was too much for the next floors down, etc. The south tower collapsed because the fire weakened the connection between the trusses and the exterior walls. Although the south tower was hit last, it collapsed first because there was more weight above the impact floors. Another site explains this in greater detail - read it here.
	*The towers were not* constructed to NYC building or fire standards.** Since they were built by the Port Authority of NY and NJ, they didn't have to comply. Consequently they weren't built with roof tanks or sprinkler systems! As a compromise they were built with double gypsum board walls to slow any fire, which explains the large amount of dust generated when they collapsed.
	Each of the towers, whose floors were one acre in size, had only three stairways, and each was wide enough to accommodate only two people. (FDNY Battalion chief John Salka, at the Fire Department Instructors Conference in Indianapolis; as reported by Newsday on 4/11/02). There weren't enough stairways to accommodate all the evacuees on their way down and the firefighters on their way up to fight the fires. On the other hand, no tall building is designed to evacuate all floor simultaneously because the stairways would take up so much room on the lower floors that there wouldn't be room for anything else.
	City Code didn't address the potential problems of the towers. Even though each tower was essentially three buildings stacked above one another (sky lobbies at the 44^th and 78^th floors), deputy fire safety directors were not required at either sky lobby after the installation of an advanced fire alarm system in 1997 (which was reportedly beyond that required by city code). There was no one in authority to direct occupants above the ground floor (other than through the PA system), nor was there any way for the fire safety director on the ground floor to view the situations at either sky lobby (since TV cameras at each location had been removed). (Newsday 4/11/02). An authoritative estimate of time required by firefighters to climb stairways with appropriate equipment and hoses is 1 minute per floor. At that rate the firefighters would have to climb for more than an hour before reaching the fire floors, and probably be exhausted by the time they got there.

Preliminary reports show different causes of collapse for the two buildings:

	The north tower collapsed from the center out. When the fires sufficiently weakened the remaining core steel, the building collapsed from the center, and the exterior walls followed.
	The south tower collapsed from the outside in. The fires there weakened the floor trusses holding the exterior walls together, and when their connections to the exterior walls failed the building collapsed from the outside in. Additionally because the plane hit at a lower floor there was more load on the damaged area.
	The primary cause of the collapses was the burning of the contents of the buildings. Structurally the buildings probably would have been repairable, but the burning contents (carpets, furniture, paper, etc.) weakened the structures and caused the collapse. The aviation fuel started the fires but burned off before causing any structural damage. More complete explanations: FEMA Report PBS Report;

A February, 2004 PBS Innovations telecast on engineering extremely tall skyscrapers provided a slightly different scenario of the WTC collapses:

While the primary cause was the fires, roof truss failures finally allowed the towers to collapse. Each plane removed many structural supports on the impact floors, and the loads transferred to the roof trusses. When the fires weakened the remaining beams on the fire floors, the towers fell.

This program also provided recommendations for changes in building codes worldwide in light of the WTC experiences:

	Provide different load paths in case part of the building structure is removed. The WTC did so with the roof trusses which, while inadequate, delayed collapse long enough for many thousands of people to escape.
	Change evacuation strategies so more occupants can safely escape, and protect them along the way. This includes hardened stairways and elevators, refuge floors and providing paths to transfer from one stair to another if one is blocked from below. Many WTC occupants were trapped above the fire floors with no way to escape
	Besides sprinkler systems, construct using fire-resistant methods which will remain effective even if the building is damaged. The WTC steel beams had their insulation blown away by the plane impacts, and the sprinkler system was deactivated as well.
	Provide better communication to authorities about conditions in all areas of the building. The FDNY commanders had no reliable communication with their firefighting forces, nor any idea of where building occupants had gathered in the buildings. PBS Innovations program summary

Items which should be addressed in future standards of building construction:

	Cores should be protected from impacts and fires by concrete. Both towers' cores were only shielded by double gypsum board walls, which were destroyed by the planes and fires. If they had been better protected, more people could have escaped from above the fire floors. The sprinkler system in the core was also destroyed in the core, leaving no water to fight the fires.
	Spray-on insulation is not sufficient to protect trusses. The impacts blew off the insulation, leading to structural failure much earlier than if stronger insulation had been installed.
	Stronger connection from exterior to truss would have allowed the buildings to stand longer. Because the exterior walls were dependent on the trusses, when the connections failed the buildings collapsed.
	Public Address systems must be available to advise occupants what to do. After the planes hit, people were left on their own without knowing whether to evacuate or remain.
	There should be some redundancy in the sprinkler/standpipe system. Because the standpipes in each tower were destroyed by the planes' impacts, no water was available to extinguish the fires.
	Perhaps the building contents should be more fire-resistant. If the contents hadn't burned with such ferocity, perhaps the fires could have been extinguished.

Changes were proposed at a conference in June, 2002

	Floor trusses would not be permitted.
	All buildings would be required to have evacuation plans.
	Stairways would be required to be wider, better protected, and further apart.
	Elevator shafts would have to be smoke-proof.
	Ventilation systems would have to be designed to limit the spread of smoke.
	Structures around lobbies and loading docks would have to be strengthened.
	All building fire safety staff would be FDNY certified.

Whether any of these proposals will ever go into effect is unknown.

Evacuating the fire floor and the floor above are correct under ordinary circumstances.

You were taught that in the event of a fire you evacuate the fire floor and the floor above; other building occupants remain where they are. This is to prevent clogging the fire stairs with individuals who should not be at risk, and allow firefighters to use the stairs to gain access to the fire floors. This could not take into account the unique situation at the WTC, since suicidal peacetime air attacks on buildings had never happened before. Newsday (Sept. 13, 2001 page W11) reported that after the first plane hit building one, an announcement was made that "Building One is in a state of emergency, Building Two is secure. You're fine, you can return to your work stations." In retrospect it was a terrible announcement, because people remained in Building Two and were killed or trapped when the second plane hit that building, but it was the right instruction to make at that time. Evacuating Building Two at that time would have exposed evacuees to debris falling from Building One, and clogged the surrounding area, making it difficult for firefighters to get to the scene. They were safer in the building than outside, and would have been fine if the second plane had not hit that building. Of course if you're the FSD, it's your choice on whether or not to evacuate. In a building which has been attacked in some fashion (airplane, bomb, etc.), you can certainly order a complete evacuation, especially if done in a manner which won't cause panic.

Take-aways:

Unless the FDNY changes their FSD test to cover such extraordinary circumstances, the answer is "evacuate only the fire floor and the floor above."

Six fire safety directors and deputy fire safety directors lost their lives. 343 members firefighters were lost (including numbers 2, 3, and 4 of the top brass); they died when the buildings collapsed while they were attempting to rescue victims and extinguish the fires. 147 promotions were required to fill the command ranks to replace those who died.

Keep looking forward and not back New york... hand clap for knowledge and wisdom of the original author.