A major portion of the uncertainty in these estimates is due to the scarcity of data regarding the initial conditions within the building and how the aircraft impact changed the geometry and fuel loading. Temperatures may have been as high as 900-1,100 degrees Centigrade (1,700-2,000 degrees Fahrenheit) in some areas and 400-800 degrees Centigrade (800-1,500 degrees Fahrenheit) in others.
The viability of a 3-5 trillion Btu/hr (1-1.15 GW) fire depends on the fuel and air supply. The surface area of office contents needed to support such a fire ranges from about 30,000-50,000 square feet, depending on the composition and final arrangement of the contents and the fuel loading present. Given the typical occupied area of a floor as approximately 30,000 square feet, it can be seen that simultaneous fire involvement of an area equal to 1-2 entire floors can produce such a fire. Fuel loads are typically described in terms of the equivalent weight of wood. Fuel loads in office-type occupancies typically range from about 4-12 psf, with the mean slightly less than 8 psf (Culver 1977). File rooms, libraries, and similar concentrations of paper materials have significantly higher concentrations of fuel. At the burning rate necessary to yield these fires, a fuel load of about 5 psf would be required to provide sufficient fuel to maintain the fire at full force for an hour, and twice that quantity to maintain it for 2 hours. The air needed to support combustion would be on the order of 600,000-1,000,000 cubic feet per minute.
Air supply to support the fires was primarily provided by openings in the exterior walls that were created by the aircraft impacts and fireballs, as well as by additional window breakage from the ensuing heat of the fires. Table 2.1 lists the estimated exterior wall openings used in these calculations. Although the table shows the openings on a floor-by-floor basis, several of the openings, particularly in the area of impact, actually spanned several floors (see Figure 2-17).
Sometimes, interior shafts in burning high-rise buildings also deliver significant quantities of air to a fire, through a phenomenon known as "stack effect," which is created when differences between the ambient exterior air temperatures and the air temperatures inside the building result in differential air pressures, drawing air up through the shafts to the fire area. Because outside and inside temperatures appear to have been virtually the same on September 11, this stack effect was not expected to be strong in this case.
The exterior and interior pressures may have been similar, but they were not the driving features of the stack effect on this day. The impacts had opened holes in the exterior envelope of the building and started fires on the impacted floors. These wall openings allowed the wind to draw cool air up through the interior shafts. The fires created large volumes of very hot gases which escaped through the wall openings, in doing this, they also drew large quantities of cool air up the shafts.
Based on photographic evidence, the fire burned as a distributed collection of large but separate fires with significant temperature variations from space to space, depending on the type and arrangement of combustible material present and the available air for combustion in each particular space. Consequently, the temperature and related incident heat flux to the structural elements varied with both time and location. This information is not currently available, but could be modeled with advanced CFD fire models.
Damage caused by the aircraft impacts is believed to have disrupted the sprinkler and fire standpipe systems, preventing effective operation of either the manual or automatic suppression systems. Even if these systems had not been compromised by the impacts, they would likely have been ineffective. It is believed that the initial flash fires of jet fuel would have opened so many sprinkler heads that the systems would have quickly depressurized and been unable to effectively deliver water to the large area of fire involvement. This is garbage. Who would design a sprinkler system that could not deliver sufficient water to deal with a floor wide fire. Floor wide fires are a common occurrence in serious office fires. Hence, sprinkler systems are designed to handle such. Also, your typical office fire involves an event called flashover, which means that after ten or twenty minutes, the hot gases of a localized fire have heated the office contents to a point where the fire spreads to the remainder of the office "in a flash".
Further, the initial spread of fires was so extensive as to make occupant use of small hose streams ineffective.
Table 2.1 Estimated Openings in Exterior Walls of WTC 1
Some further thoughts concerning the World Trade Center Tower fires (from various sources).
(1) Most of the jet fuel burnt outside the buildings. This was particularly evident in the case of the south tower. After the impact nearly all of the jet fuel would have been spread throughout the area as a flammable mist. When this mist ignited it would have emptied the building of almost the entire fuel load, which then "exploded" outside the building. This is exactly what was seen in the videos of the impacts.
(2) If any quantity of liquid jet fuel did manage to accumulate in the building, then its volatility would lead to large amounts of it being evaporated and not burnt (pyrolysed) in the interior of the building. This evaporated fuel would burn on exiting the building, when it finally found sufficient oxygen.
(3) The jet fuel fires were brief. Most of the jet fuel would have burnt off or evaporated within 30 seconds, and all of it within 2-3 minutes (if all 10,000 gallons of fuel were evenly spread across a single building floor as a pool, it would be consumed by fire in less than 5 minutes). The energy, from the jet fuel, not absorbed by the concrete and steel within this brief period, would have been vented to the outside world.
This means that the jet fuel fire did not heat the concrete slabs or fire protected steel appreciably. Large columns such as the core columns would also not heat appreciably, even if they had lost all their fire-protection. Unprotected trusses may have experienced a more sizeable temperature increase. The jet fuel fire was so brief that the concrete and steel simply could not absorb the heat fast enough, and consequently, most of the heat was lost to the atmosphere through the smoke plume.
(4) Even if the fire-rated suspended ceilings and spray on fire-protection from the trusses was removed by the impacts and the trusses were heated till they had lost most of their room temperature strength, we know from the Cardington tests and real fires like Broadgate, that the relatively cold concrete slab will supply strength to the structural system, and collapse will not occur. Remember, that at Broadgate and Cardington, the beams/trusses were not fire-protected. Consider this quote: After the Broadgate Phase 8 fire and the Cardington frame tests there were benchmarks to test composite frame models. Research intensified because almost all the tests had unprotected steel beams (no fire rated suspended ceiling and no spray-on fire retardant) but collapse was not seen .
(5) Since the jet fuel fire was brief, and the building still stood, we know that the composite floor slab survived and continued to function as designed (until the buildings were demolished one or two hours later). After the jet fuel fire was over, burning desks, books, plastic, carpets, etc, contributed to the fire. So now we have a typical office fire. The fact that the trusses received some advanced heating will be of little consequence. After some minutes the fires would have been indistinguishable from a typical office fire, and we know that the truss-slab combination will survive such fires, because they did so in the 1975.
(6) Of course, most of the weight of the building was supported by the central core columns. There is no indication as to how these 47 massive columns might have failed (at least in the case of the north tower, some of these columns, perhaps two or three, might have been displaced by the impacts). We know that the jet fuel fire was too brief to heat them appreciably. Since the central core area contained only lift shafts and stairwells, it contained very little flammable material. This meant that the core columns could only have been heated by the office fire burning in the adjacent region. Consequently, the core columns would have never got hot enough to fail. But we already know this because they did not fail in the 1975 WTC office fire.
(7) Also, the building engineers placed the ventilation system in "purge mode." This forced fresh (cool) air into the core area keeping it free of smoke and hot gases.
(8) You should consider that it has been calculated that if the entire 10,000 gallons of jet fuel from the aircraft was injected into just one floor of the World Trade Center, that the jet fuel burnt with the perfect efficency, that no hot gases left this floor and that no heat escaped this floor by conduction, then the jet fuel could have only raised the temperature of this floor to, at the very most, 536°F (280°C). You can find the calculation here.
(9) Another reason that we know the fires were not serious enough to cause structural failure, is that witnesses tell us this. The impact floors of the south tower were 78-84. Here are a few words from some of the witnesses:
Stanley Praimnath was on the 81st floor of the south tower: The plane impacts. I try to get up and then I realize that I'm covered up to my shoulder in debris. And when I'm digging through under all this rubble, I can see the bottom wing starting to burn, and that wing is wedged 20 feet in my office doorway.
Donovan Cowan was in an open elevator at the 78th floor sky-lobby: We went into the elevator. As soon as I hit the button, that's when there was a big boom. We both got knocked down. I remember feeling this intense heat. The doors were still open. The heat lasted for maybe 15 to 20 seconds I guess. Then it stopped.
Ling Young was in her 78th floor office: Only in my area were people alive, and the people alive were from my office. I figured that out later because I sat around in there for 10 or 15 minutes. That's how I got so burned.
It is claimed that temperatures in the south tower were hot enough to cause the trusses to fail, but here we have eye-witnesses stating that temperatures were cool enough for them to walk away.
Interestingly, a tape of radio conversations between firefighters exists (but only relatives of the dead men have been allowed to hear it). Kevin Flynn, of the New York Times, reported:
Chief Orio Palmer says from an upper floor of the badly damaged south tower at the World Trade Center. Just two hose lines to attack two isolated pockets of fire. "We should be able to knock it down with two lines," he tells the firefighters of Ladder Co. 15 who were following him up the stairs of the doomed tower. Lt. Joseph G. Leavey is heard responding: "Orio, we're on 78 but we're in the B stairway. Trapped in here. We got to put some fire out to get to you." The time was 9:56 a.m.
So now we know that, just a few minutes before the collapse of the south tower, firefighters did not consider the fires to be that serious, and were in fact able to get right into the impact region without being killed by the heat that was (according to Eagar) so intense that the trusses glowed red-hot and failed.
(10) When fully developed fire conditions (temperatures of over 700°C) are reached, this results in the breaking of window glass. For example, the 1988 First Interstate Bank fire in Los Angeles, which showed greater heating effects over larger regions than those observed in either tower, rained broken window glass down on the streets below, presenting a considerable hazard to those on the ground. The First Interstate Bank did not collapse.
Photo: Region glowing red hot. From the large compartment test at Cardington, towards the end of the fire (fire load of 40 kg/m2, maximum average atmosphere temperature of about 675°C, with a maximum recorded temperature of 746°C, maximum steel temperature of 691°C (recorded at the centre of the compartment)).
(11) If the temperatures inside large regions of the towers were of the order of 700°C, then these regions would have been glowing red hot and there would have been visible signs of this from the outside. Even pictures taken from the air looking horizontally into the impact region show little sign of this.
(12) Another reason the fire would not have been as hot as your typical office fire (at least on the impact floors) is that cross ventilation would have cooled it somewhat. Consider the quote: Cross ventilation resulting from (broken) windows present in opposite walls causes a high intake of air and cooling effects .
(13) If there had been severe fires burning in the core region this would have made the stairwells impassible. However the stairwells below the impact region on the North Tower were sufficiently clear to allow some occupants close to the impacted floors to escape and to allow firemen to reach at least the floors around the 70th level. In the South Tower, at least one stairwell remained operable as there were survivors from above the impact region.
Some occupants of WTC 1 and WTC 2 began to voluntarily evacuate the buildings soon after the first aircraft struck WTC 1. Full evacuation of all occupants below the impact floors in WTC 1 was ordered soon after the second plane hit the south tower (Smith 2002). As indicated by Cauchon (2001a), the overall evacuation of the towers was as much of a success as thought possible, given the overall incident. Cauchon indicates that, between both towers, 99 percent of the people below the floors of impact survived (2001a) and by the time WTC 2 collapsed, the stairways in WTC 1 were observed to be virtually clear of building occupants (Smith 2002). In part this was possible because conditions in the stairways below the impact levels largely remained tenable. However, this may also be a result of physical changes and training programs put into place following the 1993 WTC bombing. Important modifications to building egress made following the 1993 WTC bombing included the placement of photo-luminescent paint on the egress paths to assist in wayfinding (particularly at the stair transfer corridors) and provision of emergency lighting for the stairways. In addition, an evacuation training program was instituted (Masetti 2001).
Shortly before the times of collapse, the stairways were reported as being relatively clear, indicating that occupants who were physically capable and had access to egress routes were able to evacuate from the buildings (Mayblum 2001). People within and above the impact area could not evacuate, simply because the stairways in the impact area had been destroyed.
Some survivors reported that, at about the same time that WTC 2 collapsed, lighting in the stairways of WTC 1 was lost (Mayblum 2001). Also, there were several accounts of water flowing down the stairways and of stairwells becoming slippery beginning at the 10th floor (Labriola 2001).
Anecdotes indicate altruistic behavior was commonly displayed. Some mobility-impaired occupants were carried down many flights of stairs by other occupants. There were also reports of people frequently stepping aside and temporarily stopping their evacuation to let burned and badly injured occupants pass by (Dateline NBC 2001, Hearst 2001). Occupants evacuating from the 91st floor noted that, as they descended to lower levels of the building, traffic was considerably impaired and formed into a slowly moving single-file progression, as evacuees worked their way around firefighters and other emergency responders, who were working their way up the stairways or who were resting from the exertion of the climb (Shark and McIntyre 2001).
220.127.116.11 Structural Response to Fire Loading
As previously indicated, the impact of the aircraft into WTC 1 substantially degraded the strength of the structure to withstand additional loading and also made the building more susceptible to fire-induced failure. Among the most significant factors:
1). The force of the impact and the resulting debris field and fireballs probably compromised spray applied fire protection of some steel members in the immediate area of impact. The exact extent of this damage will probably never be known, but this likely resulted in greater susceptibility of the structure to fire-related failure.
2). Some of the columns were under elevated states of stress following the impact, due to the transfer of load from the destroyed and damaged elements.
3). Some portions of floor framing directly beneath the partially collapsed areas were carrying substantial additional weight from the resulting debris and, in some cases, were likely carrying greater loads than they were designed to resist. As fire spread and raised the temperature of structural members, the structure was further stressed and weakened, until it eventually was unable to support its immense weight. Although the specific chain of events that led to the eventual collapse will probably never be identified (so they hope)
the following effects of fire on structures may each have contributed to the collapse in some way. Appendix A presents a more detailed discussion of the structural effects of fire.
4). As floor framing and supported slabs above and in a fire area are heated, they expand. The people who designed the towers were not fools and knew all this. They designed the towers to survive much more serious fires than those that occurred on September 11. Their design was actually put to the test on February 23 1975 in the WTC North Tower fire (mentioned above). The North Tower suffered no serious structural damage from this intense fire.
As a structure expands, it can develop additional, potentially large, stresses in some elements. If the resulting stress state exceeds the capacity of some members or their connections, this can initiate a series of failures (Figure 2-20).
It should be noted that concrete takes a long time to heat up, and usually remains relatively cool until the fire has burnt through an area. So, even in intense fires of long duration, the concrete slabs maximum average temperature is usually a few hundred degrees less than that of the steel.
The reasons that the authors give only a very cursory explanation (if it can even be called an explanation) is that they are selling you two contradictory features, as part of their "theory" and hoping that you buy both without giving it much thought. In figure 2.20 you are told that the fire caused the steel to expand and push the exterior walls out, however in figure 2.23, you are told that the fire caused the steel to sag and pull the exterior walls inward. Notice that this is exactly how things have been illustrated. In figure 2.20 the wall has been pushed out, in figure 2.23 the wall has been pulled in. So, which is correct? Is the thermal expansion of the beams/trusses accommodated by (axial) expansion, or by sagging?
At relatively low temperatures the beams/trusses expand axially until they buckle. Once they buckle the thermal expansion is accommodated by sagging. This buckling of the beams/trusses is beneficial as it allows the thermal expansion to be accommodated by sagging. The large axial restraint due to the trusses composite action with the concrete and the restraint due to the end columns, means that sagging is the predominant feature. At 500°C (a temperature the slab probably never reached) the 60 foot sections of concrete floor slab between the core and perimeter wall would expand by about 3 inches, however, this extra length was easily accommodated by the sagging of the slab.
5). As the temperature of floor slabs and support framing increases, these elements can lose rigidity and sag into catenary action. As catenary action progresses, horizontal framing elements and floor slabs become tensile elements, which can cause failure of end connections (Figure 2-21) and allow supported floors to collapse onto the floors below. The presence of large amounts of debris on some floors of WTC 1 would have made them even more susceptible to this behavior.
The above photo was taken after the office demonstration test fire at Cardington. It demonstrates that the thermal expansion of the beams/trusses was accommodated by downward deflection, not by the forcing of the exterior walls away from the core (axial expansion) as claimed above. There was also no failure of the end connections. Even though the beams could only contribute as catenary tension members (the beams were reduced to 3 or 4% of their room temperature strength), the concrete floors supplied strength to the structural system by membrane action and no collapse occurred. The beams/trusses were not fire protected. Here is a summary of features of the office demonstration test fire at Cardington.
Test 6: The office demonstration test fire at Cardington:
A compartment 18m wide and up to 10m deep with a floor area of 135m2, was constructed on the second floor, using concrete blockwork. The compartment represented an open plan office and contained a series of work-stations consisting of modern day furnishings, computers and filing systems. The test conditions were set to create a very severe fire by incorporating additional wood/plastic cribs to create a total fire load of 9.4 pounds per square foot (46kg per square meter). Less than 5% of offices would exceed this level (mainly office libraries). The fire load was made up of 69% wood, 20% plastic and 11% paper.
The steel columns were fire protected but the primary and secondary beams (and their connections) were not. The maximum atmosphere temperature was 2215°F (1213°C) and the maximum average temperature was approximately 1650°F (900°C). The maximum temperature of the unprotected steel was 2100°F (1150°C) with a maximum average temperature of about 1750°F (950°C). The steel beams would have only have had 3% of their strength at 2000°F (1100°C), with such little remaining strength left in the steel, the beams could only contribute as catenary tension members. It is also clear that the concrete floors were supplying strength to the structural system by membrane action.
The structure showed no signs of collapse.
One of the conclusions derived from the Cardington tests, was that fire protection for the beams (trusses) was not necessary (in a composite steel structure).
In addition to overloading the floors below, and potentially resulting in a pancake-type collapse of successive floors, local floor collapse would also immediately increase the laterally unsupported length of columns, permitting buckling to begin. As indicated in Appendix B, the propensity of exterior columns to buckle would have been governed by the relatively weak bolted column splices between the vertically stacked prefabricated exterior wall units. This effect would be even more likely to occur in a fire that involves several adjacent floor levels simultaneously, because the columns could effectively lose lateral support over several stories (Figure 2-22).
6). As the temperature of column steel increases, the yield strength and modulus of elasticity degrade and the critical buckling strength of the columns will decrease, potentially initiating buckling, even if lateral support is maintained. This effect is most likely to have been significant in the failure of the interior core columns.
To believe the silly little tale you are being told here, you must believe that the designers were fools and did not follow the law and design a building that could resist a serious multi-floor office fire. But, of course, we know that the designers were not fools and did follow the law, as their design (the North Tower) survived the serious multi-floor office fire of February 23, 1975. Also, note, that if the above scenario is correct then the towers would collapse in the event of any such fire. The aircraft impact plays no significant role in the sad little tale told here, only the fire.
18.104.22.168 Progression of Collapse
The fact that the towers collapsed in a little over 10 seconds (essentially free-fall) is massive evidence that they were deliberately demolished. This is all that one needs to know, to be able to conclusively prove that the Twin Towers were demolished.
Anyone with a little common sense will realize that the top of a building does not pass through the concrete and steel that comprises the lower portion of the building at the same rate that it falls through air. This just doesn't happen, unless, of course, the lower part of the building has lost its structural integrity (and this is usually due to the detonation of a multitude of small explosive charges as seen in controlled demolitions).
The fact that the towers collapsed in about 10 seconds is a statement that the upper portion of each of the towers passed through the lower portion at about the same rate that it would have fallen through air. The fact that the towers fell this quickly (essentially at the rate of free-fall) is conclusive evidence that they were deliberately demolished.
Believing that there is nothing wrong with the towers collapsing so quickly, is roughly analogous to believing that people pass through closed doors as quickly as they pass through open doors.
The fact that they fell at such a rate means that they encountered essentially no resistance from the supposedly undamaged parts of the structure. That is, no resistance was encountered from any of the immensely strong parts of the structure that had held the building up for the last 30 years. From this, one can conclude that the lower undamaged parts were actually very damaged (probably by the detonation of a multitude of small explosive charges as is usual in a controlled demolition).
Construction of WTC 1 resulted in the storage of more than 4 x 1011 joules of potential energy over the 1,368-foot height of the structure. Of this, approximately 8 x 109 joules of potential energy were stored in the upper part of the structure, above the impact floors, relative to the lowest point of impact. Once collapse initiated, much of this potential energy was rapidly converted into kinetic energy. As the large mass of the collapsing floors above accelerated and impacted on the floors below, it caused an immediate progressive series of floor failures, punching each in turn onto the floor below, accelerating as the sequence progressed.
This is saying that the WTC towers were designed and built like a house of cards. Real buildings do not exhibit this type of behavior (if they did the designers and/or builders would be hung).
As the floors collapsed, this left tall freestanding portions of the exterior wall and possibly central core columns. As the unsupported height of these freestanding exterior wall elements increased, they buckled at the bolted column splice connections, and also collapsed. Perimeter walls of the building seem to have peeled off and fallen directly away from the building face, while portions of the core fell in a somewhat random manner. The perimeter walls broke apart at the bolted connections, allowing individual prefabricated units that formed the wall or, in some cases, large assemblies of these units to fall to the street and onto neighboring buildings below.
Review of videotape recordings of the collapse taken from various angles indicates that the transmission tower on top of the structure began to move downward and laterally slightly before movement was evident at the exterior wall. This suggests that collapse began with one or more failures in the central core area of the building. This is not correct. See the article,
Evidence of Explosives in the World Trade Center Towers Collapse.
This is consistent with the observations of debris patterns from the 91st floor, previously discussed. This is also supported by preliminary evaluation of the load carrying capacity of these columns, discussed in more detail in Section 22.214.171.124. The core columns were not designed to resist wind loads and, therefore, had less reserve capacity than perimeter columns. As some exterior and core columns were damaged by the aircraft impact, the outrigger trusses at the top of the building shifted additional loads to the remaining core columns, further eroding the available factor of safety. This would have been particularly significant in the upper portion of the damaged building. In this region, the original design load for the core columns was less than at lower floors, and the column sections were relatively light. The increased stresses caused by the aircraft impact could easily have brought several of these columns close to their ultimate capacity, so that relatively little additional effects due to fire would have been required to initiate the collapse. Once movement began, the entire portion of the building above the area of impact fell in a unit, pushing a cushion of air below it. As this cushion of air pushed through the impact area, the fires were fed by new oxygen and pushed outward, creating the illusion (no illusion)
of a secondary explosion. This is absolute garbage, if one floor falls on another as just described, then the "cushion of air" is comprised of the smoke and oxygen depleted air trapped between the floors. Also, the fires were nearly 2 hours old and everything that could burn, had. There was no fuel left to create the illusion of a secondary explosion. These were the hot gases of a primary explosion.
Figure 2-23. Aerial photograph of the WTC site after September 11 attack showing adjacent buildings damaged by debris from the collapse of WTC 1.
Although the building appeared to collapse within its own footprint, a review of aerial photographs of the site following the collapse, as well as damage to adjacent structures, suggests that debris impacted the Marriott Hotel (WTC 3), the Customs House (WTC 6), the Morgan Stanley building (WTC 5), WTC 7, and the American Express and Winter Garden buildings located across West Street (Figure 2-23). The debris field extended as far as 400-500 feet from the tower base.
2.2.2 WTC 2
126.96.36.199 Initial Damage From Aircraft Impact
United Airlines Flight 175 struck the south face of WTC 2 approximately between the 78th and 84th floors. The zone of impact extended from near the southeast corner of the building across much of the building face (Figures 2-24 and 2-25). The aircraft caused massive damage to the south face of the building in the zone of impact (Figures 2-26 and 2-27). At the central zone of impact corresponding to the airplane fuselage and engines, six of the prefabricated, three-column sections that formed the exterior walls were broken loose from the structure, with some of the elements apparently pushed inside the building envelope. Locally, as was the case in WTC 1, floors supported by these exterior wall sections appear to have partially collapsed. Away from this central zone, in the areas impacted by the outer wing structures, the exterior steel columns
Figure 2-24. Southeast corner of WTC 2 shortly after aircraft impact.
Figure 2-26. Impact damage to the south and east faces of WTC 2.
were fractured by the impact. Photographic evidence suggests that from 27 to 32 columns along the south building face were destroyed over a five-story range. Partial collapse of floors in this zone appears to have occurred over a horizontal length of approximately 70 feet, while floors in other portions of the building appeared to remain intact. It is probable that the columns in the southeast corner of the core also experienced some damage because they would have been in the direct travel path of the fuselage and port engine (Figure 2-25).
It is known that debris from the aircraft traveled completely through the structure. For example, a landing gear from the aircraft that impacted WTC 2 was found to have crashed through the roof of a building located six blocks to the north, and one of the jet engines was found at the corner of Murray and Church Streets. The extent to which debris scattered throughout the impact floors is also evidenced by photographs of the fireballs that occurred as the aircraft struck the building (Figure 2-28). Figure 2-29 shows a portion of the fuselage of the aircraft, lying on the roof of WTC 5.
Figure 2-29. A portion of the fuselage of United Airlines Flight 175 on the roof of WTC 5.
As described for WTC 1, this debris doubtless caused some level of damage to the structure across the floor plates, including interior framing; core columns at the southeast corner of the core; framing at the north, east, and west walls; and the floor plates themselves. Figure 2-30, showing the eastern side of the north face of the WTC 2 partially hidden behind WTC 1, suggests that damage to the exterior walls was not severe except at the zone of impact. The exact extent of this damage will likely never be known with certainty. It is evident that the structure retained sufficient integrity and strength to remain globally stable for a period of approximately 56 minutes.
Figure 2-30. North face of WTC 2 opposite the zone of impact on the south face, behind WTC 1.
There are some important differences between the impact of the aircraft into WTC 2 and the impact into WTC 1. First, United Airlines Flight 175 was flying much faster, with an estimated speed of 590 mph, while American Airlines Flight 11 was flying at approximately 470 mph. The additional speed would have
Figure 2-28. Conflagration and debris exiting the north wall of WTC 2, behind WTC 1.
given the aircraft a greater ability to destroy portions of the structure. The zone of aircraft impact was skewed toward the southeast corner of WTC 2, while the zone of impact on WTC 1 was approximately centered on the building's north face. The orientation of the core in WTC 2 was such that the aircraft debris would only have to travel 35 feet across the floor before it began to impact and damage elements of the core structure. Finally, the zone of impact in WTC 2 was nearly 20 stories lower than that in WTC 1, so columns in this area were carrying substantially larger loads. It is possible, therefore, that structural damage to WTC 2 was more severe than that to WTC 1, partly explaining why WTC 2 collapsed more quickly than WTC 1.
188.8.131.52 Preliminary Structural Analysis
An approximate linear structural analysis of WTC-2 was performed using SAP-2000 software (CSI 2000) to provide an understanding of the likely stress state in the building following the aircraft impact. The upper 55 stories of the building's exterior-wall frame were explicitly modeled using beam and column elements. This encompassed the entire structure above the zone of impact and about 20 stories below. The lower 55 stories of the exterior were modeled as a "boundary condition" consisting of a perimeter super-beam that was 52 inches deep and about 50 inches wide, supported on a series of springs. A base spring was provided at each column location to represent the axial stiffness of the columns from the 55th floor down to grade. The outrigger trusses at the top of the building were explicitly modeled, using truss-type elements. The interior core columns were modeled as spring elements.
An initial analysis of the building was conducted to simulate the pre-impact condition. In addition to the weight of the floor itself (approximately 54 psf at the building edges and 58 psf at the building sides), a uniform floor loading of 12 psf was assumed for partitions and an additional 20 psf was conservatively assumed to represent furnishings and contents. At the 80th floor level, exterior columns were found to be approximately uniformly loaded with an average utilization ratio (ratio of actual applied stress to ultimate stress) of under 20 percent. This low utilization ratio is due in part to the unusually close spacing of the columns in this building, which resulted in a very small tributary area for each column. It reflects the fact that wind and deflection considerations were dominant factors in the design. Core columns were more heavily loaded with average calculated utilization ratios of 60 percent, which would be anticipated for these columns, which were designed to resist only gravity loads.
A second analysis was conducted to estimate the demands on columns immediately following aircraft impact and before fire effects occurred. Exterior columns were removed from the model to match the damage pattern illustrated in Figure 2-27. Although some core columns were probably damaged by the aircraft impact, the exact extent of this damage is not known and therefore was not considered in the model. As a result, this analysis is thought to underestimate the true stress state in the columns immediately after impact. The analysis indicates that most of the loads initially carried by the damaged exterior columns were transferred by Vierendeel truss action to the remaining exterior columns immediately adjacent to the impact area. If the floors at this level are assumed to remain intact and capable of providing lateral support to the columns, this raised the utilization ratio for the most heavily loaded column immediately adjacent to the damage area to approximately a value of 1.0. At a value of 1.0, columns would lose stiffness and shift load to adjacent columns. Based on this analysis, it appears that the structure had significant remaining margin against collapse. However, this analysis does not consider damage to the building core, which was likely significant. Columns located further from the damage area are less severely impacted, and columns located only 20 feet away from the damaged area experience almost no increase in demand at all. These data are plotted in Figure 2-31.
The columns immediately above the damage area are predicted to act as tension members, transferring approximately 10 percent of the load initially carried by the damaged columns upward to the outrigger trusses, which, in turn, transfer this load back to the core columns. Not considering any damage to the core columns, utilization ratios on these elements are predicted to increase by about 20 percent at the 80th floor level. In upper stories, where the core columns were more lightly loaded, the increase in utilization ratio is substantially larger and may have approached a value of 1.0. These conditions would have been made more severe by damage to one or more core columns.
184.108.40.206 Fire Development
Following the impact, fires spread throughout WTC 2, similar to the manner previously described for WTC 1. Extensive videotape of the fires' development through the building was recorded from various exterior vantage points. This videotape suggests that, in the minutes immediately preceding the collapse, the most intensive fires occurred along the north face of the building, near the 80th floor level. Just prior to the collapse, a stream of molten material--possibly aluminum from the airliner--was seen streaming out of a window opening at the northeast corner at approximately this level. This is of particular interest because, although the building collapse appears to have initiated at this floor level, the initiation seems to have occurred at the southeast rather than the northeast corner.
Although less time was available for evacuation of WTC 2 than for WTC 1, and the aircraft hit the building some 16 floors lower than in WTC 1, fewer casualties occurred within this building. The reduced number of casualties to building occupants in WTC 2 may be attributed to the movement of some of the building occupants immediately after the aircraft impact into WTC 1 and before the second aircraft struck WTC 2. Several survivors from WTC 2 stated that, following the impact of the aircraft into WTC 1, a message was broadcast over the loudspeaker system indicating that WTC 2 was secure and that occupants should return to their offices (Scripps 2001, BBC News 2001). Many of these survivors did not heed the announcement and continued to exit the building, using the elevators. Survivors also related reports of individuals who listened to the message, returned to their floors, and did not make it out after the second aircraft impacted WTC 2. Some survivors related that a small number of people traveled to the roof under the assumption that a helicopter rescue was possible (Cauchon 2001b).
220.127.116.11 Initiation of Collapse
The same types of structural behaviors and failure mechanisms previously discussed are equally likely to have occurred in WTC 2, resulting in the initiation of progressive collapse, approximately 56 minutes after the aircraft impact. Review of video footage of the WTC 2 collapse suggests that it probably initiated with a partial collapse of the floor in the southeast corner of the building at approximately the 80th level. This appears to have been followed rapidly by collapse of the entire floor level along the east side, as evidenced by a line of dust blowing out of the side of the building. As this floor collapse occurred, columns along the east face of the building appear to buckle in the region of the collapsed floor, beginning at the south side and progressing to the north, causing the top of the building to rotate toward the east and south and to begin to collapse downward (Figure 2-32). It should be noted that failure of core columns in the southeast corner of the building could have preceded and triggered these events.
18.104.22.168 Progression of Collapse
As in WTC 1, a very large quantity of potential energy was stored in the building, during its construction. Once collapse initiated, much of this energy was rapidly released and converted into kinetic energy, in the form of the rapidly accelerating mass of the structure above the aircraft impact zone. The impact of this rapidly moving mass on the lower structure caused a wide range of structural failures in the floors directly at and below the aircraft impact zone, in turn causing failure of these floors. As additional floor plates failed, the mass associated with each of these floors joined that of the tower above the impact area, increasing the destructive energy on the floors immediately below. This initiated a chain of progressive failures that resulted in the total collapse of the building.
A review of aerial photographs of the site, following the collapse, as well as identification of pieces of structural steel from WTC 2, strongly suggests that while the top portion of the tower fell to the south and east, striking Liberty Street and the Bankers Trust building, the lower portion of the tower fell to the north and west, striking the Marriott Hotel (WTC 3). Again, the debris pattern spread laterally as far as approximately 400-500 feet from the base of the structure.
As first WTC 2, then WTC 1 collapsed, nearly 600,000 tons of debris fell onto the Plaza level, punching large holes through the Plaza and the six levels of substructure below, and partially filling the substructure with debris. This damage severely compromised the ability of the slabs to provide lateral bracing of the substructure walls against the induced lateral earth pressures from the unexcavated side. This condition was most severe at the southern side of the substructure, adjacent to WTC 2 and WTC 3. In this region, debris from the collapsed WTC 2 punched through several levels of substructure slab, but did not completely fill the void left behind, leaving the south wall of the substructure in an unbraced condition over a portion of its length.
In early October, large cracks were observed along Liberty Street, indicating that the south wall had started to move into the failed area under the influence of the lateral earth pressures. Mueser-Rutledge Engineers were retained to review the situation and make suitable recommendations. As a temporary measure, sand fill was backfilled against the inside face of the south wall to counterbalance earth pressures on the unexcavated side. Following temporary stabilization of the wall, tiebacks were reinstalled through the wall in a manner similar to that used to stabilize the excavation during the original construction of the development. After these tiebacks were installed, it was possible to begin excavation of the temporary sand backfill and the accumulated debris. Tiebacks were similarly installed at the other exterior substructure walls to provide lateral support as the damaged slabs and debris were excavated and removed from the site.
2.3 Observations and Findings
The structural damage sustained by each of the two buildings as a result of the terrorist attacks was massive. The fact that the structures were able to sustain this level of damage and remain standing for an extended period of time is remarkable and is the reason that most building occupants were able to evacuate safely. Events of this type, resulting in such substantial damage, are generally not considered in building design, and the ability of these structures to successfully withstand such damage is noteworthy.
Preliminary analyses of the damaged structures, together with the fact the structures remained standing for an extended period of time, suggest that, absent other severe loading events such as a windstorm or earthquake, the buildings could have remained standing in their damaged states until subjected to some
Figure 2-32. The top portion of WTC 2 falls to the east, then south, as viewed from the northeast.
significant additional load. However, the structures were subjected to a second, simultaneous severe loading event in the form of the fires caused by the aircraft impacts.
The large quantity of jet fuel carried by each aircraft ignited upon impact into each building. A significant portion of this fuel was consumed immediately in the ensuing fireballs. The remaining fuel is believed either to have flowed down through the buildings or to have burned off within a few minutes of the aircraft impact. The heat produced by this burning jet fuel does not by itself appear to have been sufficient to initiate the structural collapses. However, as the burning jet fuel spread across several floors of the buildings, it ignited much of the buildings' contents, causing simultaneous fires across several floors of both buildings. The heat output from these fires is estimated to have been comparable to the power produced by a large commercial power generating station. Over a period of many minutes, this heat induced additional stresses into the damaged structural frames while simultaneously softening and weakening these frames. This additional loading and the resulting damage were sufficient to induce the collapse of both structures.
Because the aircraft impacts into the two buildings are not believed to have been sufficient to cause collapse without the ensuing fires, the obvious question is whether the fires alone, without the damage from the aircraft impact, would have been sufficient to cause such a collapse. The capabilities of the fire protection systems make it extremely unlikely that such fires would develop without some unusual triggering event like the aircraft impact. For all other cases, the fire protection for the tower buildings provided in-depth protection. The first line of defense was the automatic sprinkler protection. The sprinkler system was intended to respond quickly and automatically to extinguish or confine a fire. The second line of defense consisted of the manual (FDNY/Port Authority Fire Brigade) firefighting capabilities, which were supported by the building standpipe system, emergency fire department use elevators, smoke control system, and other features. Manual suppression by FDNY was the principal fire protection mechanism that controlled a large fire that occurred in the buildings in 1975. Finally, the last line of defense was the structural fire resistance. The fire resistance capabilities would not be called upon unless both the automatic and manual suppression systems just described failed. In the incident of September 11, not only did the aircraft impacts disable the first two lines of defense, they also are believed to have dislodged fireproofing and imposed major additional stresses on the structural system.
Had some other event disabled both the automatic and manual suppression capabilities and a fire of major proportions occurred while the structural framing system and its fireproofing remained intact, the third line of defense, structural fireproofing, would have become critical. The thickness and quality of the fireproofing materials would have been key factors in the rate and extent of temperature rise in the floor trusses and other structural members. In the preparation of this report, there has not been sufficient analysis to predict the temperature and resulting change in strength of the individual structural members in order to approximate the overall response of the structure. Given the redundancy in the framing system and the capability of that system to redistribute load from a weakened member to other parts of the structural system, it is impossible, without extensive modeling and other analysis, to make a credible prediction of how the buildings would have responded to an extremely severe fire in a situation where there was no prior structural damage. Such simulations were not performed within the scope of this study, but should be performed in the future.
Buildings are designed to withstand loading events that are deemed credible hazards and to protect the public safety in the event such credible hazards are experienced. Buildings are not designed to withstand any event that could ever conceivably occur, and any building can collapse if subjected to a sufficiently extreme loading event. Communities adopt building codes to help building designers and regulators determine those loading events that should be considered as credible hazards in the design process. These building codes are developed by the design and regulatory communities themselves, through a voluntary committee consensus process. Prior to September 11, 2001, it was the consensus of these communities that aircraft impact was not a sufficiently credible hazard to warrant routine consideration in the design of buildings and, therefore, the building codes did not require that such events be considered in building design. Nevertheless, the design of WTC 1 and WTC 2 did include at least some consideration of the probable response of the buildings to an aircraft impact, albeit a somewhat smaller and slower moving aircraft than those actually involved in the September 11 events. Building codes do consider fire as a credible hazard and include extensive requirements to control the spread of fire throughout buildings, to delay the onset of fire-induced structural collapse, and to facilitate the safe egress of building occupants in a fire event. For fire-protected steel-frame buildings, like WTC 1 and WTC 2, these code requirements had been deemed effective and, in fact, prior to September 11, there was no record of the fire-induced-collapse of such structures, despite some very large uncontrolled fires.
The ability of the two towers to withstand aircraft impacts without immediate collapse was a direct function of their design and construction characteristics, as was the vulnerability of the two towers to collapse a result of the combined effects of the impacts and ensuing fires. Many buildings with other design and construction characteristics would have been more vulnerable to collapse in these events than the two towers, and few may have been less vulnerable. It was not the purpose of this study to assess the code conformance of the building design and construction, or to judge the adequacy of these features. However, during the course of this study, the structural and fire protection features of the buildings were examined. The study did not reveal any specific structural features that would be regarded as substandard, and, in fact, many structural and fire protection features of the design and construction were found to be superior to the minimum code requirements.
Several building design features have been identified as key to the buildings' ability to remain standing as long as they did and to allow the evacuation of most building occupants. These included the following:
1). robustness and redundancy of the steel framing system
2). adequate egress stairways that were well marked and lighted
3). conscientious implementation of emergency exiting training programs for building tenants Similarly, several design features have been identified that may have played a role in allowing the buildings to collapse in the manner that they did and in the inability of victims at and above the impact floors to safely exit. These features should not be regarded either as design deficiencies or as features that should be prohibited in future building codes. Rather, these are features that should be subjected to more detailed evaluation, in order to understand their contribution to the performance of these buildings and how they may perform in other buildings. These include the following:
4). the type of steel floor truss system present in these buildings and their structural robustness and redundancy when compared to other structural systems
5). use of impact-resistant enclosures around egress paths
6). resistance of passive fire protection to blasts and impacts in buildings designed to provide resistance to such hazards
7). grouping emergency egress stairways in the central building core, as opposed to dispersing them throughout the structure
During the course of this study, the question of whether building codes should be changed in some way to make future buildings more resistant to such attacks was frequently explored. Depending on the size of the aircraft, it may not be technically feasible to develop design provisions that would enable all structures to be designed and constructed to resist the effects of impacts by rapidly moving aircraft, and the ensuing fires, without collapse. In addition, the cost of constructing such structures might be so large as to make this type of design intent practically infeasible.
Although the attacks on the World Trade Center are a reason to question design philosophies, the BPS Team believes there are insufficient data to determine whether there is a reasonable threat of attacks on specific buildings to recommend inclusion of such requirements in building codes. Some believe the likelihood of such attacks on any specific building is deemed sufficiently low to not be considered at all. However, individual building developers may wish to consider design provisions for improving redundancy and robustness for such unforeseen events, particularly for structures that, by nature of their design or occupancy, may be especially susceptible to such incidents. Although some conceptual changes to the building codes that could make buildings more resistant to fire or impact damage or more conducive to occupant egress were identified in the course of this study, the BPS Team felt that extensive technical, policy, and economic study of these concepts should be performed before any specific code change recommendations are developed. This report specifically recommends such additional studies. Future building code revisions may be considered after the technical details of the collapses and other building responses to damage are better understood.
The scope of this study was not intended to include in-depth analysis of many issues that should be explored before final conclusions are reached. Additional studies of the performance of WTC 1 and WTC 2 during the events of September 11, 2001, and of related building performance issues should be conducted. These include the following:
1). During the course of this study, it was not possible to determine the condition of the interior structure of the two towers, after aircraft impact and before collapse. Detailed modeling of the aircraft impacts into the buildings should be conducted in order to provide understanding of the probable damage state immediately following the impacts.
2). Preliminary studies of the growth and heat flux produced by the fires were conducted. Although these studies provided useful insight into the buildings' behavior, they were not of sufficient detail to permit an understanding of the probable distribution of temperatures in the buildings at various stages of the event and the resulting stress state of the structures as the fires progressed. Detailed modeling of the fires should be conducted and combined with structural modeling to develop specific failure modes likely to have occurred.
3). The floor framing system for the two towers was complex and substantially more redundant than typical bar joist floor systems. Detailed modeling of these floor systems and their connections should be conducted to understand the effects of localized overloads and failures to determine ultimate failure modes. Other types of common building framing should also be examined for these effects.
4). The fire-performance of steel trusses with spray-applied fire protection, and with end restraint conditions similar to those present in the two towers, is not well understood, but is likely critical to the building collapse. Studies of the fire-performance of this structural system should be conducted.
5). Observation of the debris generated by the collapse of the towers and of damaged adjacent structures suggests that spray-applied fire proofing may be vulnerable to mechanical damage from blasts and impacts. This vulnerability is not well understood. Tests of these materials should be conducted to understand how well they withstand such mechanical damage and to determine whether it is appropriate and feasible to improve their resistance to such damage.
6). In the past, tall buildings have occasionally been damaged, typically by earthquakes, and experienced collapse within the damaged zones. Those structures were able to arrest collapse before they progressed to a state of total collapse. The two WTC towers were able to arrest collapse from the impact damage, but not from the resulting fires when combined with the impact effects of the aircraft attacks. Studies should be conducted to determine, given the great size and weight of the two towers, whether there are feasible design and construction features that would permit such buildings to arrest or limit a collapse, once it began.
BBC News. 2001. "We Ran for Our Lives." Account of Mike Shillaker. September 13.
Cauchon, D. 2001a. "For Many on Sept. 11, Survival Was No Accident," USA Today.com. December 19. Cauchon, D. 2001b. "Four Survived by Ignoring Words of Advice," USA Today.com. December 19. Computers and Structures, Inc. (CSI). 2000. SAP-2000. Berkeley, CA. Dateline NBC. 2001. "The Miracle of Ladder Company 6." September 28. Hearst, D. 2001. "Attack on America: Survivors: Suddenly they started to yell out, `get out now': Bravery and fear mingled with disbelief," Guardian Home Pages, page 15. Account of Simon Oliver. September 13.
Labriola, J. 2001. Personal account. Channel 4 News, "Inside the World Trade Center," broadcast. September 13.
Masetti, A. 2001. Personal account received by email. December 21. Mayblum, A. 2001. Personal account. www.worldtradecenternews.org/survivorstory.html, World Trade Center Miracles section. September 18.
New York Board of Fire Underwriters. 1975. One World Trade Center Fire, February 13, 1975. Nicholson, W. J.; Rohl, A. N.; Wesiman, I.; and Seltkoff, I.J. 1980. Environmental Asbestos Concentrations in the United States, page 823. Environmental Sciences Laboratory, Mount Zion Hospital, New York, NY.
Scripps, H. 2001. "I walked out ... I made it out alive," Boston Herald.com. Account of John Walsh. September 14.
Shark, G., and McIntyre, S. December 5, 2001. ABS. Personal account. Smith, D. 2002. Report from Ground Zero. Viking Penguin, New York. p. 29. Zalosh, R. G. 1995. "Explosion Protection," SFPE Handbook of Fire Protection Engineering, 2nd edition. Quincy, MA.
Isn't it comforting that a supposedly scientific article about the collapse of the WTC (apart from articles on the 1975 WTC fire) only quotes survival stories from media sources?
2.1 Building Descriptions2-1
2.1.2 Structural Description2-1
2.1.3 Fire Protection2-11
22.214.171.124 Passive Protection2-12
126.96.36.199 Smoke Management2-13
188.8.131.52 Fire Department Features2-13
2.1.4 Emergency Egress2-13
2.1.5 Emergency Power2-14
2.1.6 Management Procedures2-14
2.2 Building Response2-15
2.2.1 WTC 12-15
184.108.40.206 Initial Damage From Aircraft Impact2-15
220.127.116.11 Fire Development2-21
18.104.22.168 Structural Response to Fire Loading2-24
22.214.171.124 Progression of Collapse2-27
2.2.2 WTC 22-27
126.96.36.199 Initial Damage From Aircraft Impact2-27
188.8.131.52 Preliminary Structural Analysis2-32
184.108.40.206 Fire Development2-34
220.127.116.11 Initiation of Collapse2-35
18.104.22.168 Progression of Collapse2-35
2.3 Observations and Findings2-36
Figure 2-1 Representative floor plan (based on 94th and 95th floors of WTC 1).2-2
Figure 2-2 Representative structural framing plan, upper floors.2-4
Figure 2-3 Partial elevation of exterior bearing-wall frame.2-6
Figure 2-4 Base of exterior wall frame.2-7
Figure 2-5 Structural tube frame behavior.2-7
Figure 2-6 Floor truss member with detail of end connection.2-8
Figure 2-7 Erection of exterior wall and floor deck components.2-9
Figure 2-8 Erection of floor framing during original construction.2-9
Edited 1 times. Last edit at 06/23/04 09:30PM by TheTruth.