IT WAS THE FIRE,
CAUSED THE TWIN TOWER COLLAPSE
An analysis of the cause of the collapse of the WTC Towers. & Possible lessons learned to improve high-rise building life safety.
March 12, 2002,
By: Arthur Scheuerman, Battalion Chief FDNY (Retired), Former Deputy Chief Instructor Nassau County Fire Training Academy and high-rise Fire Safety Director NYC.
The following is an essay on the possible causes of the World Trade Center collapse and possible means to prevent similar occurrences. The essay is based on my experience and knowledge gained in the NYC Fire Dept as I rose through the ranks to Battalion Chief and the Nassau County Fire Departments, to Deputy Chief Instructor, Nassau County Fire Training Academy, and my work as a building Safety Director in numerous high-rise buildings in NYC. It is written for builders, architects, and civil engineers and attempts to give some perspective on fire dynamics relating to building construction. The details are my opinions and are written to stimulate debate and 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 I and 2, I would like to weigh in on the side of the fires. These buildings were designed to take the impacts of 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 didnt 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 air conditioning supply and return shafts, utility shafts and restrooms etc. An outer-ring containing the large open area 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. Apparently the 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 girder 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 engines 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. When the B-25 bomber hit the Empire State Building in 1945 the fire damaged several steel beams but the impact did not take out one steel column.
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, the use of lightweight long-span steel bar joists, deficient fireproofing on the steel, 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, to save lives by prevention of collapse in most ordinary constructed buildings is generally not appreciated even by engineers. Many years ago hard experience had taught the Fire Dept. that when the fire was beyond their control in the old brick and joist, first generation high-rise buildings the fire forces were withdrawn and the fire fought from the outside in anticipation of collapse and preparations begun to limit fire spread to exposures. The whole concept of the second generation fireproof buildings was that fires could be fought from the inside without the danger of collapse until all the occupants could be removed and the fire extinguished. Heavy reliance (was placed) on the integrity of the building, its design and its systems. (OHagan p 145, 149). Consideration of collapse in fire proof resistive buildings has not been of critical importance, until now.
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. 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 Dunns 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). This is not enough time for the Fire Dept. to get water on the fire even in a low rise building.
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 bes9ides being the bearing walls for the floors were shear-walls transmitting lateral wind loads through the membrane 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-joists removed much of the buildings lateral bracing.
I surmise that as those floor sections, which were intact after the plane 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, steel frame 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, have been shown to be 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 (floor) 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.
In Tower 1 the bulging ripple going down the outside of the skin in advance of the collapsing floors (Malott p12) could 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 covering the columns.
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).
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 and stair shafts and other vertical openings such as un-fire-stopped pipe or wire runs, or air conditioning shafts. These fire gasses could have accumulated and heated the entire upper ceiling area or truss voids of one floor, or several floors, starting a softening and sagging of the joists. Or, more likely, after filling the upper floors (mushrooming) these heated gasses could have exploded, and triggered the initial floor collapse. This happens at times in unventilated void spaces at serious fires. A third possibility as to a contributing cause for the Tower 1 collapse is sprinkler system water overloading one or more floors. For instance, if the restaurant on the 107^th floor were sprinklered and the heated smoke set off some of the spray heads, after a time, the water weight buildup over a large floor area could exacerbate the sequential bar-joist failure. Water would accumulate in the depression in the sagging floors caused by the bar joists softening, thus hastening the collapse. 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).
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 flammable gasses, such as carbon monoxide or vaporized jet fuel, over-pressuring the area.
Incomplete combustion, due to lack of oxygen, in the main body of fire in addition to producing these flammable gasses, may have been may have been another reason the fire temperatures in general not being any greater than an average fire. According to Charles G. Clifton 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 decomposition in the absence of oxygen. In a large area fire the high heat is distilling off (generating) 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. 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 is one of the reasons fire-buildings are promptly 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 postulated overpressure in Tower 1s upper floors and/or bar joist voids 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 observe the film of the collapse you can, however, notice a sudden small loom up of black smoke from the top floor areas just before the avalanche collapse began.
Air Conditioning
Central heating, ventilation and air conditioning (HVAC) systems could have affected the building stability in several ways. According to Former Fire Commissioner John T. OHagan; The air-conditioning system increases the flow of air and of oxygen to the seat of the fire, thereby increasing its rate of development and its ultimate severity. The return portion of the system recirculates smoke and contaminated air on the floors above and below the fire increasing the life hazard, complicating the evacuation and rescue problem, increasing the difficulty of locating the seat of the fire and delaying the actual extinguishing operation. This allows the fire to increase in severity and extent. The undivided ceiling space which is utilized as a plenum for the collection and direction of the recirculated air to the return shaft is also an effective medium for the transfer of heat to the remainder of the floor area.(p132)
In writing about a third alarm World Trade Center fire which occurred on Feb.13, 1975, Commissioner OHagan speaks of the common ceiling plenum as the means of fire spread between two areas. (p37). This plenum (the 3 foot deep ceiling void formed by the bar joist truss system) became a route for the fire gasses since it covered the entire outer ring floor area and served as the primary avenue for return air to the air conditioners. Even after the air conditioning fans were shut down this truss-produced void could accumulate and spread super-heated gasses since these gasses would continue entering through the ceiling grills. This configuration, using the truss voids as a return space for air movement back to the mechanical equipment fan rooms, would negate any fire protection that the sheetrock ceilings afforded the steel trusses thereby hastening their softening, sagging and eventual collapse.
In tests done by Fire Commissioner OHagan, combustible gasses were shown to spread to other areas of buildings through the ventilation systems. It was noted that locations which were somewhat remote from the fire achieved a relatively high concentration of carbon monoxide, moreover, these did not necessarily correspond to regions wherein either the temperature or the smoke concentration was extremely high.(p90). Carbon monoxide is a highly toxic gas with a wide explosive range and is lighter than air. It could have easily spread along with other explosive products of incomplete combustion to remote areas within the truss voids or other spaces on other floors and upon ignition, caused explosions which contributed to the collapse.
Large Open Areas
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. 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 this additional hazard is produced compounding the collapse problem. As pointed out these lightweight steel trusses are affected much sooner by fire than heavy beams and since they span such large distances, any failure becomes more serious than a short span element.
This rapid fire growth situation is exacerbated in high-rise structures 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. The convected and radiated heat becomes an impossible barrier to hose line advancement. This hazardous situation occurs even in well-ventilated areas and fire-suppression in such large open areas, within buildings, often requires the cooling of all spaces at once, an effect, which in high-rise buildings sometimes, can only be accomplished by sprinkler systems. The truss voids used as return plenums for HVAC systems adds to the problem by allowing fire to spread in these concealed voids possibly over the heads of firefighters. According to Chief Dunn The best kept secret in Americas 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. (For an article on the operational problems related to design and construction, at high rise fires; see Chief Vincent Dunns excellent article in Fire Engineering magazine December 95. According to Commissioner OHagan writing in his book /High Rise / Fire and Life Safety;/
The main problem associated with the protection of life from fires in high-rise buildings is the limitation of the size of the fire. Public fire protection services can usually contain a fire in an elevated portion of the building if the fire area is limited to 5000 square feet of less. If the horizontal spread of the fire exceeds this limit, the heat developed will also increase the risk of vertical spread. If the building contains vertical arteries, then a tragedy can be expected.(p245, 246)
Sprinklers
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).
However, if the fire originates in or penetrates the (truss) void, the sprinklers will not be in a position to control the fire (Brannigan.p548, 01) In my opinion, total sprinkler protection including the truss voids, if it had been installed and remained intact, would have provided enough cooling of the protected steel to slow down total collapse at the WTC fuel fire. 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 and every void space 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 or up open shafts 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.
*Note:* 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 available 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 elevator shafts or other openings such as unfire-stopped pipe and wire shafts or poke through openings, to remote floors above the fire and set off the heads there.
Since the areas above an uncontrolled fire may be dangerous to enter because of developing smoke and heat conditions, we may not be able to quickly shut off sprinkler water which consequently accumulates on the floor or in the contents, overloading the floors, leading to floor collapse from the weight of the water. If the steel bar joist floors began to sag from the heat, water would tend to accumulate in the depression in the floors and not drain off down stairways or other shafts, further hastening collapse. 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
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. Since New York City has been the premiere skyscraper capitol of the world, the City Building Code regulations have evolved out of the many historic fire disasters over the past 200 years. These codes, although recently compromised by a myriad of special interest changes and in need of extensive revision and simplification for clarity, in my opinion, are still the most comprehensive in the world. Since the NYC high rise fire experience has been fairly good recently many builders, architects and civil engineers have grown complacent in their knowledge of fire dynamics relating to building design and construction. 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 apparently can only be obtained from analysis of actually experienced disasters and through well-conducted experimental tests.
Building and Fire Code regulations and procedures are of critical importance for life safety and property protection. According to Fire Commissioner OHagan, The time and place to ensure life safety in high-rise buildings is during the period that the building is being designed.(p243). If the Port Authority had to submit plans and get plan approval from the City before starting construction, it would have been subject to plan review by experienced Code experts and inspection during construction by experienced Building inspectors and Fire inspectors who had the power to stop the job until construction violations were corrected. The Port Authority would have had to receive a final inspection and certificate of occupancy before opening the building. In my opinion the design and construction would have been radically changed and many more lives could have been saved.
All buildings built in the City should, at least, have to follow the City Codes. 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, and many areas were never fully covered by subsequent retrofits.
Recommendations for Possible Code Changes
Long span, steel bar joists should be prohibited for floor construction in any new public building of any size, due to their early failure under fire conditions. Since this early failure problem is endemic in any lightweight steel joist system such as steel C joists, all lightweight steel joists should be banned from use in floor construction.
I believe a survey and re-assessment of all existing buildings which use long-span, steel bar joists should be conducted in order to consider rebuilding them, using conventional methods.
I support Mr. Malotts and Chief Dunns 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.
These tests should include tests using dropped ceilings as HVAC return plenums for the heated gasses, to determine the effect this configuration has on I beam and Q deck supported, concrete floor failure.
Since long span floors (including I beams supported floors) are inherently weaker than short span floors, impact load tests to determine their progressive collapse potential should be productive.
The effect of an ordinary natural gas or smoke explosion on such long span floors should be determined.
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.
Columns should be designed to support their loads even with the collapse of several floors.
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 buildings height and weight is increased, columns, girders and beams and walls and floors should be strengthened and redundancy of protective systems 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 vapor 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 shaftwallgypsum board construction should not be allowed for such use in any public building. As building heights increase more effective protection for exit way enclosures such as reinforced masonry or concrete should be required throughout.
Egress pathways leading between stairways or to the outside should also be hardened to preserve their integrity and continuity.
Scissor stairs should be re-evaluated because of the possibility of both stairways being affected by a disruption of the enclosure.
The plane impacts apparently moved the buildings several feet, wracking the walls in the central core thereby binding some exit doors in their frames. This suggests inadequate diagonal bracing throughout the core areas of these buildings.
The shaftwall and other drywall gypsum were dislodged in numerous places by the impact loads or the building shifting. This suggests the means of attachment was possibly inadequate.
Escaping occupants had to reverse direction and go back up stairways in several instances due to locked exit doors from the stairwells. I understand the need for security but this suggests a total lack of understanding on managements part of the function of and critical need for availability of fire exit stairways in high rise buildings. Automatic fail-safe door latches should be installed and maintained throughout the stairways to unlock all exit-way doors in the event of fire. If a stairway suddenly fills with smoke occupants should be able to exit these stairways immediately and find other stairs or areas of refuge.
There seems to be a natural tendency for people to flee up the stairways if the fire is below them; this has to be discouraged since it is a most dangerous practice. Smoke and heated gasses expand, produce pressure and become buoyant and rise up any available open shafts, including stairways if their enclosures are breached, however roof doors in stairways are required to be easily open able from the inside, in the City Codes, recognizing that some people will attempt it.
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 we can ring an entire, 16-acre, foundation area with 3-foot thick reinforced concrete 7 stories high to keep the Hudson River out, we can ring the areas of stairs, elevators and lobbies on each floor with reinforced masonry fire walls and provide 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.
Sprinkler systems should be separate from Standpipe systems since with combination systems failure of either systems piping or supply will affect water supply to the other system possibly leaving areas deficient or devoid of extinguishment capabilities.
Since hose stream effective coverage is limited at large area fires and sprinklers are sometimes inactivated, the area of open floors should be limited by fire containment walls which extend through any ceiling plenums to the floor above, in order to keep fires to controllable size.
The Fire Department should not have to carry hose up the stairs. Sufficient F.D. specification hose should be available on each floor at the standpipe hose outlets.
Central air conditioning systems servicing many floors have shown time and again to accelerate the fire and spread deadly smoke through out many floors in high-rise fires. Chief Dunns recommendation that air conditioning systems should cover only one or two floors should be implemented. Presently HVAC systems using common open ceiling voids (plenums) to return air for reconditioning can hamper fire control and rescue problems and aggravate collapse problems by spreading fire, heat and smoke to remote areas and heating structural steel within the plenum. Perhaps air return systems could be designed using separate ducts to be able to safely exhaust fire gases directly to the outside after sprinkler extinguishment. Supply fans feeding air to fire area should be shut down automatically to avoid accelerating the fire, and return fans also shut and dampers closed until extinguishment to avoid lateral and vertical fire and smoke spread.
I am sure there will be many additional recommendations for Building Code improvements, which will be gleaned, from the WTC catastrophe. 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.
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