It Was the Fire That Caused the Twin Towers’ Collapse
Scheuerman on WTC7 Collapse. (The Collapse of Building 7 by Arthur Scheuerman December 8, ‘06)
By Arthur Scheuerman Battalion Chief FDNY Retired, Former Deputy Chief Instructor Nassau County Fire Training Academy.
An analysis of the cause of the collapse of the WTC Towers. Possible lessons to be learned to improve high-rise building life safety. Feb. 13, 2002
The following is an essay on the possible causes of the World Trade Center collapse and possible means to prevent similar occurrences. It is based on my experience and knowledge gained in the NYC fire dept in all ranks up to and including Battalion Chief and Nassau County Fire Departments in 25 years fire fighting and training and my work as a Safety Director in numerous high-rise buildings in NYC. The details are my opinions and are written to stimulate debate and possibly improve the Fire and Building codes and their enforcement.
To enter the debate as to whether the plane crashes or the resultant fires caused the collapse of World Trade Center Towers 1 and 2, I would like to weigh in on the side of the fires. These buildings were designed to take the impacts of a large plane crashes, and I doubt whether either building would have collapsed and whether multitudes of people would have been trapped above the crash floors except for the fire, smoke and heat. Apparently the effects of the inevitable explosion and fire after the simulated plane crashes were not considered in the design of the building. The point is; these buildings didn’t immediately collapse, they took almost an hour for Tower 2 and well over an hour for Tower 1 the North Tower to collapse. According to Ronald Hamburger a structural engineer investigating the disaster, "We have reason to believe that, without the fire, the buildings could have stood indefinitely and been repaired." The fire caused most of the life loss and building damage and the buildings were evidently deficient in fire protection.
All of the ‘plane caused’ collapse theories depend on the destruction of numbers of core columns by the plane crash impact and the subsequent failure of the remaining core columns by the heat of the fire. The core is the interior rectangular section of the building containing the stairways, elevators and bathrooms etc. An outer ring containing the large open office space surrounds the core. The outer ring, floor assembles consisted of long-span, open-web steel bar joists spanning the distance between the outer perimeter columns and the interior core columns. These bar joists supported a steel pan and concrete floor. Each façade had 59, high strength, steel box columns 40 inches on center. These exterior steel box columns were very strong and being 36 feet long, each was backed up by at least two of the 4-inch concrete floors on edge, built at 12-foot intervals vertically. Each box column was bolted to the column above and below and welded to spandrel girders ringing the perimeter at each story. The shiny aluminum skin covering each column would, of course be stripped by the planes and melted away by any fuel fire.
I have not as yet seen the enhanced videos but, I maintain that since, in the pictures I have seen, we can not really see the remaining columns because of the heavy black smoke issuing from the impact area; we cannot tell how many, if any, were severed. Using CAD simulations Tony Fitzpatric of Arup America determined that it took a direct hit by the engine’s shaft at 200 mph to punch through one steel H column and box columns are stronger than H columns and the interior core columns were stronger than the exterior perimeter columns. The planes would have been shredded passing through the perimeter columns, possibly taking out a few, and the number of interior core columns destroyed would have been much less.
I believe the intensity of the fire (as it relates to building collapse) was comparable to a heavy ordinary combustible fire after the explosion dissipated much of the jet fuel. According to Francis Brannigan author of Building Construction for the Fire Service, "…temperatures in excess of 2000 degrees are the rule in severe fires. The average person has no idea of the temperatures which can be reached in a quite ordinary fire."(Brannigan 1971, p245). The heat output of an interior fire is limited, by the amount of air reaching the combustibles and the smoke produced. In the standard furnace tests used to determine the collapse-resistance of building components, authorities switched from oil fires to natural gas since; "The smoke emitted by the fire at times seriously interferes with the transfer of heat by radiant energy within the fire building. Test fires use smokeless natural gas, so radiant heat transfer is important in tests."(Brannigan p206). A jet fuel fire would produce great quantities of smoke, which would reduce the radiant heat energy entering structural components. According to G. Charles Clifton HERA structural engineer, speaking of the fires in the Towers; "In my opinion, based on available evidence, there appears no indication that the fires were as severe as a fully developed multi-story fire in an initially undamaged building would typically be."(Elaboration..., p5) My point is that given the inadequate partial sprinkler system, deficient ‘fireproofing’ on the steel, the use of lightweight long-span steel bar joists, and large open areas undivided by fire walls, any uncontrolled large area fire would have eventually produced the same total collapse. The importance of early fire control, in most ordinary constructed buildings, to save lives by prevention of collapse in generally not appreciated even by engineers. It has not been that important, until now, in fire resistive buildings.
Instead of the columns failing first, I believe the weakest link was the long-span, open web, steel bar joists. The position of these joists, over the fire and the small-diameter steel elements of these joists would allow them to heat up to the failure temperature, (approximately 1100 degrees F.), much more rapidly than the massive columns which would act as a heat sink and conduct some heat away.
According to Deputy Chief, (Ret.) Vincent Dunn, FDNY writing in his book Collapse of Burning Buildings, "A large steel I-beam can absorb heat and take a relatively long time to reach its failure temperature, while a lightweight steel beam, such as an open web bar joist, can be heated to its failure temperature much faster." (Dunn, 1988, p142)
It has been shown that, at times, at the WTC, the fire resistance of both bar joists and columns were deficient, due to flaking off of sprayed on coverings in certain places. (NY Times, Science Sec. Dec 13, 2001). Removal of a small area of protective insulation from a bar joist would seem more detrimental than removal of a small area from a large column, since temperature would build up faster in the small element. According to Francis Brannigan, "the failure of any one element of the truss can cause the failure of the entire truss." A bar joist is a truss, and the failure of one bar joist can lead to successive failure of adjacent joists due to load transfer.
"In fact, successive failure of trusses appears to be the rule rather than the exception." (Brannigan p46). . As in a truss, a fire resistive building without built in redundancy depends on all the critical elements and their connections retaining their fire resistance and thus their integrity during a fire. The WTC exterior box column walls were shear-walls transmitting lateral loads through the floors to each other and to the ground. These exterior walls "together with the floors, formed a torsionally rigid framed tube fixed to the foundations."(Clifton p3). Removal of the floor rigidity by the heat caused sagging, break up of the concrete or collapse of these bar-joist floors removed much of the buildings horizontal bracing. Any and every critical element and its fire protection may be important in maintaining the integrity of the entire building at a serious fire. Since this situation is compounded as a building’s height and weight is increased, redundancy and extra protection should be increased through out a building as the height and areas are increased.
I surmise that as those floor sections, which were intact after the crash and fuel explosion, were weakened by the heat and let down their concrete loads and live loads onto the floors below, a progressive mechanical collapse began in the floors. In conventional fire resistive construction the spans are shorter and beams and columns rigidly restrained vertically, horizontally and diagonally, by strong connections and masonry walls built between columns. A progressive collapse due to impact loads is less likely since the masonry walls and strong connections between columns and girders can redistribute the loads. A floor collapse in a conventional steel-framed building would have been localized since the area between girders would be small. On the other hand "when huge spans are achieved by…trusses or space frames, collapse can be sudden, general and tragic." (Brannigan p215).
A pancake, V- shaped collapse or a lean-to collapse of a long span bar joist floor would impart a concentrated impact load on the floor below. I doubt weather the long span bar joists in the floor below could sustain such an impact, as well as steel I beams and reinforced concrete floors could. The connections of the joist ends to the columns, at the WTC, seem a likely spot for impact load failure. An impact causing a depression anywhere in the top chord of a truss could also cause collapse of the truss since such top chord is in compression and could buckle. The floors were providing lateral support to the exterior columns and core columns and in effect were integral to the stability of the whole structure. Removal of lateral support for enough of these columns, (by floor collapse) would allow the weight of the building above to buckle both the outer perimeter and interior core columns, letting down the entire upper portion of the building.
More likely, as the architect Mr. Malott points out in "Why the World Trade Center Collapsed", Nov./Dec issue of Designer Builder magazine, the bar joists themselves pulling with them the exterior walls" and the core columns started the collapse. "Steel members which sag due to fire will try to carry their loads as suspension members. This causes large horizontal forces; if they are transmitted to the fire wall, it can be destroyed."(Brannigan p253) It seems likely that such floor sagging in the Southeast corner of Tower 2 affected the corner core columns and/or corner perimeter columns causing the initial list to the Southeast just before the rapid, avalanche collapse of the 110 story structure.
The "bulging ripple going down the outside of the skin in advance of the collapsing floors" (Malott p12) in Tower 1 would have, in fact, been caused by floors collapsing ahead of the column failure. If it was a flat pancake collapse of the floors, the increasing dynamic weight of the concrete laden floors along with their live loads, hitting each level could easily break the connections to the columns or spandrels on each floor. The tips of the joist ends sliding down the interior face of the columns could have caused the "bulging ripple"; or this moving bulge could have been caused by the air pressure from the bellows effect as the collapsing floors compressed the air which pushed on and bowed out the windows and the aluminum skin.
In Tower 1 it appears the top floor or floors began failing first possibly because the top floors were receiving most of the super heated gasses rising up the damaged elevator shafts and other vertical openings. These fire gasses could have accumulated and heated the entire upper ceiling area of one floor, initiating a rapid sequential collapse of the joists. Or, more likely, after filling the upper floors (mushrooming) these heated gasses could have exploded, as happens at times in unventilated void spaces at serious fires, to start the initial pancake floor collapse. A third possibility as to the initial trigger for the Tower 1 collapse is sprinkler system water overloading one floor. For instance if the restaurant on the 107th floor were sprinklered and the heated smoke set off some or all of the heads, after a time, the water buildup over a large floor area could initiate the sequential bar-joist failure. "From the video footage this collapse appeared to occur (begin) uniformly around the building ("at or near the top of the building") and spread rapidly down to the floor above the impact region. That region than pancaked…" (Clifton, p8).
This type of flat floor collapse reminds me of bathroom floor failures in old six story apartment buildings. These localized, progressive collapses were so common, in the Bronx that we would try to stay out of bathrooms during apartment fires. One firefighter reported riding down such a bathroom floor collapse and said it felt like being in an elevator which momentarily stopped at each floor, as each bathroom floor hit the one below and broke the joists. Amazingly he stepped out unhurt at the ground floor. The reason for these failures was the weight of the heavy fixtures, mortar bed and tile floors, and fire attacking the dry rot in the wood joist ends. This wood rot was caused by constant water spills wetting the joists. (For more details on this type of collapse see Dunn p86).
The fact that the collapse began, apparently simultaneously, around the entire upper floor outer ring and possibly the inner core of Tower 1 rather suggests an explosion or rapid combustion of gasses such as carbon monoxide or other flammable vapor residue from the jet fuel, over-pressuring the area. "A room or area requires only 25 percent of its space to contain the explosive mixture for the entire area to explode."(Dunn, WNYF p9) This may have been another reason the fire temperatures in general not being any greater than an average fire- incomplete combustion due to lack of oxygen in the main body of fire. "The observed fire behavior points to temperatures in the building not being particularly severe — say no more than about 600 to 700 Deg. C. Possible reasons for this may involve the coating of combustible material in dust from pulverized concrete and wall linings (gypsum) and the volatility of the aviation fuel leading to large amounts of fuel being pyrolised but not burnt in the interior of the building."(Clifton, "Elaboration…" p6). Pyrolysis involves thermal degradation in the absence of oxygen. In a large area fire the high heat is distilling off more flammable gasses from combustibles than can be burned, since the available air is being quickly used up in combustion. These flammable vapors and gasses, produced by heat but unburned, can migrate to remote spaces due to rising convection currents, where if they attain the right mixture with air and are hot enough, will explode. This is one of the reasons fire-buildings are ventilated form upper areas by the Fire Dept. The overpressure produced by rapid combustion can vary from low pressure as in a flashover to severe as in a backdraft. The overpressure in Tower 1’s upper floors may have been strong enough to start the collapse but not strong enough to be noticed on the outside of the building. If you carefully watch the film of the collapse you can notice a sudden small loom up of black smoke from the top floor areas on the left and possibly right sides just before the avalanche collapse began.
Lightweight trusses are affected sooner by fire than heavy beams and since they can span such large distances, any failure becomes more serious than a short span element. Trusses were commonly used in supermarkets to eliminate columns and provide unobstructed views, so that people could easily see the food items. For a post fire analysis of a supermarket truss roof collapse, that killed six firefighters in 1978, see Ch.20 of Chief Dunn’s book. Until now the fire service has had little experience with ‘fire resistive’ floor truss construction, but its experience with common exposed roof trusses has been disastrous. According to Chief Dunn, "Truss construction is the most dangerous roof system that a firefighter will encounter." "A [unprotected] steel bar joist system may collapse after only ten minutes of exposure to fire."(Dunn p125).
The size of a fire is also a major factor that affects steel failure. "A large area fire in which flames involve much of the steel beam in a short period of time will heat the steel beam to its critical temperature more quickly. A so called "flash fire," suddenly involving a large area with flame, can heat steel rapidly to its failure temperature."(Dunn p142). Because truss construction is often used to provide this wide-open space within buildings another hazard is produced compounding the problem.
Large open areas, containing combustibles, within buildings, are a nightmare for firefighters because of the possibility of spread of fire, throughout the space and the resultant large volume of fire. This situation is exacerbated when elevators and standpipes must be used by responding firefighters, delaying the operation of hose streams and rescue. The difficulty in extinguishing such large, open area fires when extend throughout an interior space, arises because, as the fire in one section is extinguished and the hose streams are repositioned to attack another area, the fire re-ignites in the previous section by convected and radiated heat from the freely burning section. This hazardous situation occurs even in well-ventilated areas and fire-suppression in such large open spaces, within buildings, often requires the cooling of all areas at once, an effect, which sometimes can only be accomplished by sprinkler systems. According to Chief Dunn "The best kept secret in America’s fire service is that firefighters cannot extinguish a fire in a 20- or 30-thousand-square-foot open floor area in a high rise building." (From an excellent article on the operational problems related to design and construction, at high rise fires; see Chief Vincent Dunn’s article in Fire Engineering magazine December ‘95).
According to Chief Dunn, "The only real fire protection for a commercial or residential high-rise building is an automatic sprinkler and smoke-removal system to vent the smoke after the sprinkler extinguishes the fire." Mr. Brannigan comes to the same "…inescapable conclusion that full automatic sprinkler protection is vital to the safety of occupants of high-rise structures."(Brannigan p370) In my opinion, total sprinkler protection, if it had been installed and remained intact, would have provided enough cooling of the protected steel to at least slow down total collapse at the WTC. It certainly would have reduced the smoke and heat output to a more manageable level thereby saving many more lives.
Full automatic sprinkler protection means every area and room on every floor is covered by the discharge pattern of a sprinkler head. While a partial water spray system is often recommended and necessary for certain special hazard areas, it is not generally known that partial water spray systems can sometimes cause difficult problems if they are only installed in hallways or exit-ways, or only on certain floors.
If a fire starts in an unsprinklered area the fire may rage out of control in this area and the superheated gasses can flow across the ceiling to a sprinklered exit-way setting off the spray heads in this area. These sprinkler heads cannot control the fire since they are not over the fire, but will create expanding quantities of steam, at times, making line advancement down a hall or exit-way difficult or may even trap people if the exit-hall becomes untenable. Full coverage with a sprinkler system will solve the problem.
I found this out the hard way at a training exercise I was giving at the Nassau County Fire Academy. Several firemen were scalded by boiling hot water created by sprinkler heads, which opened in the hall well behind the nozzle as we advanced a hose line into the fire training room. A quantity of heated fire gasses rolled across the ceiling over our heads and set off the spray heads behind us producing this boiling cloud of steam and smoke. While this occurrence cools the ceiling gasses from superheated levels; as the water spray is converted into steam in expanding 1600 times, the process will turbulently redistribute these reduced but still scalding temperature gasses and water droplets from the ceiling level, pushing them to lower levels. Spray heads were not installed in the fire training rooms, of course, since we could not have had the training fires. We simply extinguished the fire and solved the problem, a solution that may not be that simple in a large area fire.
The other way partial sprinkler systems can be troublesome is if the fire in the unprotected area gets out of control and cannot be cooled quickly, the heated gasses can be forced up shafts or other openings to a remote floor above the fire and set off the heads there. Since the areas above an uncontrolled fire may be dangerous to enter the sprinkler water may not be able to be shut off in time, consequently accumulating on the floor or in the contents, overloading the floors, leading to collapse from overloading. Again full coverage by sprinklers will mitigate this problem by reducing or eliminating the production of these super heated gasses. On-off sprinkler heads and floor drains or scuppers to drain the water may also help. In spite of all these problems, "Sprinklers are the core of fire safety for the occupants of high-rise buildings" (Brannigan 1992, p502).
Conclusion & Recommendations
For some arcane legal reason the Port Authority of NY State and NJ did not have to comply with the New York City Building Code, and Fire Codes. If the Port Authority had to submit plans and get approval before starting construction and have been subject to inspection during construction by experienced Building inspectors and Fire inspectors and had to receive a certificate of occupancy before opening the building, many more lives could have been saved. All buildings built in the City should, at least, have to follow the City Codes, which also need some updating and revisions, but are still the most comprehensive in the world. The Port authority had a ‘policy’ to comply with City Codes, but still there were serious deficiencies in sprinkler protection, steel protection from heat, exit-ways & enclosures and building design, which would have been detrimental in any serious fires in these buildings. Sprinkler systems were not even built into the original buildings.
Long span, steel bar joists should be prohibited for floor construction in any new public building due to their early failure in fires.
I believe a survey and re-assessment of all existing high-rise buildings which use long-span, steel bar joists should be conducted in order to consider rebuilding them, using conventional methods.
I support Mr. Malott’s and Chief Dunn’s suggestions about encasing columns and beams in concrete or masonry for protection rather than using current ineffective spray-on fire retarding material.
Full-scale furnace tests for ‘long-span’, I beam floor assemblies with sprayed-on "fireproofing" should be conducted to determine their actual fire rating (endurance time), and what the effect of removal of sections of fire insulation would have on the collapse resistance of such ‘long span’ steel girders or beams.
Since long span floors are inherently weaker than short span floors, impact load tests to determine their progressive collapse potential would also be productive. Since it is impossible to evacuate a high-rise building rapidly, each floor in a high rise building should be able to support the impact weight of several floors collapsing from above; this in order to prevent a progressive collapse. Also the effect of an ordinary natural gas or smoke explosion on such long span floors should be determined. As building height and areas increase, columns girders and beams and walls and floors should be strengthened and redundancy increased accordingly with an increased factor of safety to take care of unexpected emergencies.
Evidently the crashing plane parts or the fuel air explosion destroyed some of the wall enclosures of the stairways and elevator shafts and cut off escape from above by filling the stairways with debris and heated toxic smoke. The elevators were also disabled due to shaft destruction and flaming jet fuel, pouring down the shafts. Tests should be developed to determine whether the impact load of a fuel air explosion alone or of a hose stream could affect the integrity of the "shaftwall" gypsum board, enclosing the stairways and elevator shafts. If an ordinary natural gas or smoke explosion, or the impact of an interior or exterior hose stream could affect the integrity of stairways or elevator shafts than then this type of "shaftwall"gypsum board construction should not be allowed for such use in any public building. As building heights increase more effective enclosures such as reinforced concrete should be required throughout.
The plane impacts apparently moved the buildings several feet wracking the walls thereby binding some exit doors in their frames. This suggests inadequate diagonal bracing throughout the buildings.
The ‘shaftwall’ and other drywall gypsum were dislodged in numerous places by the impact. This suggests the means of attachment was possibly inadequate.
Scissor stairs should be re-evaluated because of the possibility of both stairways being affected by a disruption of the enclosure.
Egress pathways leading between stairways or to the outside should be hardened to preserve their integrity.
Automatic fail safe door latches should be installed throughout the stairways to unlock all exit-way doors in the event of fire. This was to have been accomplished after the first bomb exploded in the WTC cellar in 1993. This suggests inadequate enforcement. Compliance with recommendations should be determined by actual re-inspections and not by reliance on written documentation.
Since elevators frequently fail to provide adequate Fire Department response to the floors of high-rise buildings, provisions for fire and smoke resistive, impact protected, elevator shaft enclosures should be developed for Fire Dept. access to upper floors and handicapped rescue from upper floors. Fire proof, ventilated vestibules as presently used in the old ‘fire tower’ stairways could be used. Ventilated ‘areas of refuge’ as elevator landing areas on each floor, could be used in conjunction with fire rated elevator shaft doors. If they can ring the entire, 16-acre, foundation area with 3-foot thick reinforced concrete 7 stories high to keep the Hudson River out, they can ring the areas of stairs, elevators and lobbies on each floor with fire walls and ventilation gaps to keep fire and smoke out.
Full sprinkler protection should be mandatory in all buildings over 6 stories or 75 feet in height, no matter what the building occupancy. We cannot always control the amount and type of combustibles entering the buildings.
Since hose stream coverage is limited and sprinklers are sometimes inactivated open area spaces should be limited to 2500 sq. ft. between fire containment walls and automatic fire doors.
Chief Dunn’s recommendation that air conditioning systems should cover only one or two floors should be implemented. Air conditioning systems should be designed to be able to safely exhaust fire gases directly to the outside after sprinkler extinguishment. Supply fans feeding fire area should be shut down.
I am sure there will be many additional recommendations for Building Code improvements, which will be gleaned, from the WTC catastrophe. The New York City Building code has, of necessity, evolved out of the many historic NYC disasters. We are naturally loath to imagine possible hazardous situations and disasters that can happen; and unless we actually experience them we apparently have difficulty developing preventative or precautionary measures. Effective regulations have been and are obtained from analysis of actually experienced disasters and through well-conducted experimental tests. History has proven that a good Fire Prevention and Building Codes, knowledgeable people and strong enforcement capabilities are absolutely necessary to build and maintain safe buildings. Critical code sections should be protected from special interest
changes. Code changes allowing smoke detectors to substitute for full sprinkler coverage in high rise buildings is a good example. The actual fire is the ultimate test of construction practices and the World Trade Center Towers failed the test twice.