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Collapse of the World Trade Center Towers.

G Charles Clifton, HERA Structural Engineer


17th September 2001, revised 19th September,
minor revision on impact force made 8th October,
minor revisions made 11 December (see elaboration (below))

Introduction

Construction of the World Trade Center Towers began on August 5 1966 and they were officially opened on April 4 1973. Fig 1. shows the two towers prior to the attack. As will have been forever seared on the memory of all readers, they were destroyed in a terrorist attack on 11 September 2001. The method of destruction was simple and devastating, namely suicide attack by aircraft. The resulting images of the towers burning and collapsing were ones no-one ever expected to see.

The first airplane hit the North Tower at 8.46 am local time and that tower collapsed at 10.28 am or 1¾ hours after the impact.

The second tower was hit at 9.03 am but collapsed more quickly, at 9.59 am. One curious aspect of the South Tower collapse was that almost all the staff and 940 registered guests of the 22 floor Marriott World Trade Center Hotel (WTC3) apparently anticipated the collapse and evacuated, even though no one had as yet predicted the collapse of the South Tower. Two staff and one guest died in the collapse. The hotel was totally obliterated when debris from the South Tower rained down upon it.




Fig 1 World Trade Center Towers Before Attack.

My background has given me some insight into what may have happened to these towers under the much more severe event of a direct hit from a near fully loaded large modern airplane. It is important to note that the explanation given is only my considered opinion, based on the information available six to eight days after the event.

Subsequent material received from then until 11th December has led to minor changes in terms of Before presenting those details, some details of the building are given, followed by brief details of the impact. The effect of the impacts can only be assessed in light of these details, in particular the devastatingly high local impact force on the buildings from the planes. This is followed by my assessments of the effects of this impact on each of the two towers, which showed some significant differences.

There has already been considerable speculation on the severity of the fire and its role in the collapses. On the basis of what I have seen and heard reported to date, it is my opinion that the effect of the fire was of much less importance than the effect of the initial impact, especially on the first tower to be hit (the North Tower). The reasons behind this opinion follow details of the effects of the impacts on each tower and the article ends with a personal footnote on the tragedy and a reference.

Details of the Buildings

Fig 1 shows the two towers in service. These towers were the principal buildings in a complex of city development. Fig. 2 shows the location of these towers on the World Trade Center site (along with the direction of impacts (the direction of impacts is missing from the current site-plan)).




Fig. 2: Site Plan of World Trade Center Development Showing Location of the Two Towers.

1. WTC 1 - North Tower - 110 floors
2. WTC 2 - South Tower - 110 floors
3. WTC 3 - Hotel - 22 floors
4. WTC 4 - South Plaza Building - 9 floors
5. WTC 5 - North Plaza Building - 9 floors
6. WTC 6 - US Custom Building - 8 floors
7. WTC 7 - World Trade Center Seven - 47 floors

Each tower was 411 meters high, 63.5 x 63.5 meters square on plan, core 24 x 42 m. There were 110 stories in each tower. The towers were one of the best examples of "tube tower construction", a structural form ideally suited to providing the strength and stiffness required for very tall buildings. On each facade a rigid moment-resisting frame was formed comprising 59 box-section columns, spaced at 1.02 meter centers, connected by deep spandrel beams. The frames did not run into the corners, however, there a shear connection between the two adjacent frames was provided so that the frames, together with the floors, formed a torsionally rigid framed tube fixed to the foundations. This framed tube carried all wind loads. The floors spanned without intermediate columns to the core, which was supported on 44 (other reports mention 47 core columns) box-section columns designed and detailed to carry vertical loading only. Fig 3 shows this concept in an isometric view, while one of the 450 x 450 mm exterior frame box columns is shown in Fig. 4.




Fig. 3: Isometric View of Building from [1]. Framed tube construction principle - load-bearing external walls stiffened by the floors to form a torsionally rigid tube

13 Load-bearing external wall - Perimeter frame
17 Core box column 450 mm square
20 Floor slab




Fig. 4: Cross Section Through Exterior Box Column with window frame connection, from [1].

36 External steel column
38 Fire resistant vermiculite plaster
39 Special fire-resistant plaster
40 Aluminium facade
41 Guide rail for window-cleaning equipment
42 Special tinted window glass
43 Aluminium window frame

The access and services were carried through the central core, as shown in Fig 5.



Fig. 5: Plan of typical floor, from [1].

10 Open-plan office
11 Express lifts
12 Local lifts

The floor system comprised 900 mm (35.5 inches) deep bar joists (the FEMA report claims the trusses where 737mm (29 inches) deep) spaced at 2.04 m centers and braced by secondary joists. These secondary joists then supported a profiled deck on which was poured a 100 mm thick light-weight concrete slab. The top of the bar joists stood above the soffit of the decking and was cast into the concrete slab to make the bar joists composite in a similar manner to the Speedfloor system.

The bar joists spanned between the perimeter frames and the core, as shown in Fig 6. Fig 7 shows an isometric of part of the floor and exterior wall , illustrating some of the details described above.

The gravity and lateral load-resisting systems were designed to deliver the strength and stiffness required from a 110 storey building with minimum dead load. This was achieved very well, with a steelwork weight of only 44.5 kg/m2 floor area. Clifton's figure is incorrect. The correct figure is 244.5 kg per square meter (of effective floor area) or 50 pounds per square foot. The very light and open structure, superbly engineered to meet the design serviceability and ultimate limit state conditions on a building of this height and size, probably made the buildings more vulnerable to collapse from the aircraft impact than would have been the case for a more inefficient and heavier structural system. The World Trade Center towers are not particularly light when compared to similarly built buildings. Clifton's false conclusion is based on the incorrect figure he quoted.

Passive fire protection was provided to the columns by vermiculite plaster and to the underside of the floor systems by a fire rated suspended ceiling.

Each tower had an effective floor area of 319,000 m2 and used 87,000 tonnes of steelwork. The reported quantity of structural steel used in the construction of the towers, varies from 175,000 to 220,000 US short tons, with the most commonly quoted figure being 192,000 tons, that is, 96,000 tons (87,091 tonnes) per tower.




Fig. 6: Structural system for typical floor, from [1].

13 Load-bearing external wall
14 Bar joist 900 m deep
15 Secondary joist
16 Horizontal floor bracing
17 Core box column




Fig. 7: Isometric View of Floor and Exterior Wall System.

The Impacts

Each tower was hit by a Boeing 767. The impact on the North tower was near the center of the North face at around the 95 th storey, with the plane hitting the side of the building square on and flying level. The impact on the South tower was some 15 stories lower, with the plane hitting the South face near the South East corner and impacting at an angle to the face of the tower. Fig 2 shows the impact directions and Fig 8 shows the plane about to hit the South Tower.




Fig. 8: Plane About to Hit South Tower

The potential force of the impact from each plane can be approximately calculated and the figures are very large. The weight of each plane would have been approximately 150 tonnes, according to the media reports and Boeing data on this type of plane. The plane would have been traveling at around 800 kms/hour at impact. This gives a momentum of

Momentum = 150 x 800/3.6 = 33,333 tonnes.m/sec.

If the plane was arrested by the building in effectively 0.6 seconds, which is a reasonable estimate based on a linear deceleration over the 63.5 m width of the building, then the force exerted on the building is the momentum/effective time to arrest, i.e.,

Force = 33,333/0.6 = 55,555 kN.

To put that in perspective, the ultimate limit state design wind pressure over the entire height of the building is 220 kg/m2 (45 lb/ft2). This gives a ULS wind force on one face of the building of 58,400 kN.

Wind Force = 220 x 63.5 x 411 = 5,741,670 kg-force = 5,741,670 x 9.8 N = 56,268,400 N = 56,268 kN

Thus the potential force of impact from the plane is 95% of the design ultimate limit state wind load on the building!

This is smoke and mirrors stuff from Clifton (and he knows it). The truth is that the uniform wind pressure over the face of the building cannot be compared to the impact of a high speed projectile. It is like concluding that because a slow moving truck smashed the glass completely out of a large window (by say backing into it), a bullet with the same momentum will also smash the glass completely out of the window. But this is not so. As we all know, the bullet will make a neat little bullet hole in the glass and the remainder of the window will remain unaffected. Because of this, Clifton's conclusion is erroneous.

The kinetic energy of the plane was only partly converted into the bending of the towers. Most of the energy went into the (localized) destruction or degradation of perimeter columns, concrete floor slabs and core columns. To fulfill Clifton's agenda, he completely ignores the proportion of the planes kinetic energy that was utilized to pulverize/degrade parts of the structure and implies that all the energy went into bending the towers. He then states that, if all the energy went into bending the towers, it would be almost (but not quite) beyond the limits the engineers had designed for. He then implies that this would be a dangerous situation, but this is unlikely given the "over-engineering" common to all such tall buildings.

Note that the speed (and shape) of a projectile determines whether the impact damage is localized or spread across a large area. The faster the projectile, the more localized the damage. Common examples illustrating this effect are, the driving of a nail through a piece of wood, and the firing a bullet through a fencepost. Both are done at speed and thus do only local damage. In both of these examples, the wood just a centimeter or two from the impact point, is essentially undamaged. Similarly, the aircraft impacts were at great speed and the damage localized.

Especially in the case of the North Tower, not much of the plane was ejected from the building, so it is reasonable to assume that most of that potential force was absorbed by the building. Also the above calculation also does not take into account any additional force generated inside the building from blast loading due to, for example, exploding jet fuel.

Unlike explosives, jet fuel does not have a source of oxygen built into it. Consequently, it has to mix with oxygen (before it can even burn). This mixing process takes far too much time for jet fuel to be an explosive (if you can keep the jet fuel away from any sources of ignition until it is throughly mixed with air, then it will explode, but this is impossible in a crash situation). Consider this statement from the FEMA report on the WTC collapses:

"Although dramatic, these fireballs did not explode or generate a shock wave. If an explosion or detonation had occurred, the expansion of the burning gasses would have taken place in microseconds, not the 2 seconds observed. Therefore, although there were some overpressures, it is unlikely that the fireballs, being external to the buildings, would have resulted in significant structural damage."

Having done this calculation it is more easy to understand what our eyes showed us - namely the planes slicing through the perimeter frames "like a knife through butter" as one reporter has stated.

Clifton's calculation does not make it easier to understand why the planes sliced through the perimeter frames "like a knife through butter". This is indeed easy to understand, but Clifton's calculation does little or nothing to help one's understanding of it.

I contend that, having penetrated the perimeter frames the planes would have done much more that just stripping the fire protection off the columns as has been surmised by some commentators. The effect would have been to completely shatter and eliminate large areas of floor slab and many of the internal supporting columns, thereby immediately destroying much of the vertical load carrying system and leaving the rest vulnerable to any subsequent fire attack.

This is simply wrong. It is clearly wrong in the case of the south tower where the plane only clipped the corner of the central core, but it is also wrong in the case of the north tower. The planes would not have "eliminated" many of the internal supporting columns, in fact, computer simulations of a Boeing 747 (much bigger and heavier than a Boeing 767 (maximum takeoff weight 875,000 lb, unloaded weight 670,200 lb, fuel capacity 57,285 gallons)) impacting a steel framed building, like the WTC core, show that very few of the 47 core supports would have been eliminated. Consider the following pictures from such a study.



Figure 5 (from the study). Results of simulation analysis of impact of a 747 jetliner crashing into a steel structure. Notice fracture of the steel column and breaking of the plane due to dynamic stresses (Graphics and analysis by MSC Software Corporation).

More on this simulation can be found by clicking here. Note that this particular simulation models the core of the WTC fairly well, however, it should be pointed out that the WTC core consisted of a grid, eight columns wide and six columns deep, whereas the model is six columns wide and three columns deep.

As a side note: Remember, that we were told that the "hijackers" wanted to cause maximum death and destruction. Then why didn't they hijack Boeing 747s? Boeing 747s weigh more than twice as much, they can carry more than twice the fuel and travel faster than the Boeing 767. Consequently, Boeing 747s would have caused much more death and destruction than the 767s.




Below floor 85, the World Trade Center central core columns were box columns fabricated from steel that was from 1 to 5-inches thick. At floor 85 they transitioned to H-columns fabricated from 3/4-inch thick steel. Above is a picture of an 85th floor box column. Although the H-column snapped off during the collapse its imprint is still clearly visible.

Another CAD (Computer Aided Design) simulation was carried out by Tony Fitzpatrick of Arup America. He assumed a 4.2-ton jet engine struck a single 16x14-inch (41x36-cm) steel H-column. Unfortunately, the steel thickness was not reported, but he did determine that it took a direct hit by the engine's shaft at 200 mph to punch through one of these columns.

The north tower impact floors were 94-98. In these floors the core columns were H-columns. These H-columns were most probably stronger than the H-columns in Fitzpatric's study (but if they were hit, they were probably hit at a higher speed). It is quite unlikely that a column received a direct impact by the engine's shaft (which is only a few inches wide (it is the green object in the graphic)) and even if it did, it is not clear that the impact would take out the column. However, to be conservative, we will suppose that two core columns were taken out by the engines.




The south tower impact floors were 78-84. In these floors the core columns were box columns. These box columns were considerably stronger than the H-columns in Fitzpatric's study. Since the box columns were wider, it is slightly more probable that a column received a direct impact from the engine's shaft (but it is still unlikely) and even if it did, it is extremely unlikely it took out the column. We know that one of the engines went right through the building and ended up at the corner of Murray and Church Street, so, in the case of the south tower, it is probable that no core columns were displaced by the engines, but to be conservative, lets say that one was.

In the case of the south tower the trajectory of the plane and design of the floor makes it clear that at most one core column was taken out by the fuselage.

In the case of the north tower, the trajectory of the plane took it right through the core, so between zero and four H-columns may have been taken out by the fuselage. Of course, the fuselage would have been considerably slowed by its impact with the perimeter columns. The reason that a maximum of four core columns may have been taken out is that the fuselage was 16 feet 5 inches wide and the core columns were spaced roughly 20 feet apart, and since the aircraft hit square on, at most one row (four columns deep) might have been impacted.

So, in the case of the south tower, at most 2 core columns were taken out in total. In the case of the north tower, at most 6 core columns were taken out in total. In either case, the remaining 40 or so core columns would have easily carried the extra load due to the missing columns.

Similarly, the remaining 213 or so perimeter columns would have easily carried the extra load due to the missing 23 perimeter columns in the south tower and the remaining 203 or so perimeter columns would have easily carried the extra load due to the missing 33 perimeter columns in the north tower.

This impact damage - not the severity of the fire - I contend is the principal cause of the ultimate collapse. However the nature and position of impact was different in each case and this led to different effects on each tower, with different collapse mechanisms. These effects are now discussed in as much detail as is possible based on the known information.

Clifton's contention is based on his guess that much damage must have been done to the core, and on his erroneous interpretation of the calculation he presented above. He does not produce a single piece of evidence to back up the claim that the impact damage was the principal cause of the collapse and he does not refer to (or run his own) CAD simulations, but simply expects people to believe his counter-intuitive conclusions.

Considered Effect of Impact on North Tower

The North Tower was hit first, with the plane slicing into the North face of the building, as shown in Fig.2. The video footage that has been shown of that impact shows the plane disappearing into the building followed by a fireball erupting from the West and East sides of the building and back out of the entry hole on the North side. Subsequent footage and photos of the South side - the side opposite to the impact - show a large amount of facade destruction and smoke being discharged. This shows that a considerable amount of material exited the building on the opposite side to the impact.




Fig. 9: North Tower After Impact Viewed From the Impact Side

Fig 9 shows the view looking into the impact hole. Given that the floor slabs are at 3.66 m centers and the facade column centers are at 1.03 meter centers with facade window widths of only 0.48 m, the number of facade columns and perimeter beams severed by the airplane in its passage though the perimeter frame is considerable (31 to 36 columns). From the major damage to the side opposite the impact it is also apparent that much of the airplane would have passed through the core.




It is likely that the impact destroyed most of the floors (partial collapse of floors in the impact zone appears to have occurred over a horizontal length of wall of approximately 65 feet) at least on the impact side, and the core on at least three levels, removing many of the core supporting columns, at least on the North side of the core, and leaving the remainder buckled and stripped of their passive fire protection.

The wings of the aircraft would have torn off and been chopped into small widths by the impact with the perimeter wall and floor slabs. The large initial mass of the wings (which enabled them to slice through the perimeter frames "like a knife through butter") was due to the jet fuel in the wing tanks. The impact with the perimeter wall and floor slabs quickly shredded the wings and turned the jet fuel into a fine mist which subsequently ignited. The expanding gases emptied the building of almost the entire jet fuel load, which then "exploded" outside the building, exactly as seen in the videos of the impacts. Consequently, the wings would have caused no damage to the core columns. The two engines and the fuselage had considerable mass, but were considerably slowed by their impact with the perimeter wall and floor slabs and were not very wide. It would have been possible (but unlikely) for them to pass completely through the building without hitting a core column. In any case, enough of the central core columns and perimeter wall columns survived the impact so that the building remained standing and showed no signs that it might collapse.

This would have caused the floors above the impact level to sag downwards in the center, with the gravity columns which had been severed or severed by the impact now acting as tension ties between each of the floors above the impact region, through to the top floor.

There is no evidence of such a sag. In fact, the TV tower on top of the North Tower showed no sign of movement until the collapse. So, there is documented evidence to the contrary. The TV tower sat directly above the central core. If the central core had sagged in relation to the perimeter, then this would have been clearly visible.

Core columns that were undamaged or with only minor damage would have now had to carry an increased compression load. Also, each upper floor would now have had to be at least partially supported off the perimeter frame to a greater extent than was allowed for in the design of the floor to perimeter frame connections.

If any core columns were taken out by the impact, then clearly, the undamaged core columns would have to carry an increased compression load. That this implies that the perimeter frame to floor connections must be put under severe stress, is false. The remaining core columns had ample reserve strength to handle the increased load (by themselves). The loading on these columns could be increased more than 400% before failure occurred (one would need to be remove 37 of the 47 core columns to increase the load on the remaining columns by 400%). The perimeter columns had even more reserve strength. Here is a quote from Engineering News Record, April 2, 1964.

A design procedure that will be used for structural framing of the 1,350-ft high twin towers of the World Trade Center in New York City gives the exterior columns (perimeter columns) tremendous reserve strength. Live loads on these columns can be increased more than 2,000% before failure occurs.

HERA has developed a method (based on UK fire research) allowing the design of unprotected secondary beams in composite floor systems by using the dependable inelastic reserve of strength from a region of floor slab supported around its perimeter. I have applied this in a very approximate manner to a typical WTC floor slab supported off only the perimeter frame and the result shows that this is just about possible with regard to the membrane capacity of the floor system, but not possible in terms of the vertical load carrying capacity of the connection between the floors and the perimeter frame. As the vertical load carrying capacity of the core diminished, requiring more load to be transferred from the floors to the perimeter frames, the mode of failure would have most likely been failure of this connection, leading to floor collapse.

I contend that, immediately following the impact, the core region of all the floors above the impact region would have sagged downwards due to the partial loss of vertical support in the core region. This sag would have progressively grown as the fire and ongoing yielding of the remaining damaged core columns reduced the core vertical load carrying capacity.

The jet fuel fire was 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. Indeed, the jet fuel fire was so brief that the concrete and steel simply could not absorb the heat fast enough and most of the heat was lost to the atmosphere through the smoke plume.

Since the central core area contained only lift shafts and stairwells, it contained very little flammable material. This meant that the core columns could have only been heated by the office fire burning in the adjacent region. The interior core columns would have been many feet from any possible fire and thus had no immediate source of heat. The exterior core columns would have been heated on one side only, with the corner columns being heated on two sides. This, and their large size meant that they would have heated quite slowly. As well as losing heat to the cooler core area, the columns would have lost heat by conduction to floors above and below the fire. Also, the fires of September 11, seemed to be patchy, so even the outer core columns may not have received much heating. For these reasons, the core columns would not have got remotely hot enough to fail. We know that the undamaged core columns would not fail since they did not fail in the 1975 World Trade Center north tower office fire.

This would have placed severe overstress on the connection between floor and perimeter frame around each floor at every level, with the greatest effect at the top floor, due to the core columns interconnecting each floor above the impact region now acting as tension ties. This would also have put extra vertical load on the perimeter frames, however these are sized to resist the lateral loading and would have had more than sufficient capacity to resist this extra load, especially as it would have distributed itself symmetrically around the perimeter frames.




This analysis of Clifton's is clearly incorrect. If it was correct, then the perimeter wall would initially be left intact as the core together with the floor slabs tore away from it (and collapsed within it). However, video of the North Tower collapse clearly shows that the central core collapsed simultaneously with the perimeter wall.

Remember, Clifton's contention is that the central core (including the floor slabs) broke away from the perimeter wall and collapsed internally. This implies that the (upper section of the) perimeter wall would be left standing for a few seconds as the TV tower sunk down behind it. This clearly did not happen.

The sagging of the core region on the upper floors could have been the reason for a phone call from the upper levels shortly after the impact saying that the building was breaking up. The sagging around the core and the impact damage would also have made the stairs impassible through the impact region, cutting off escape from the upper floors.

The strength, stiffness and redundancy of the perimeter frames would also have been more than adequate to redistribute vertical load around the severed members on the impact side, thus preserving the integrity of these frames above the impact region.

The likely influence of the fire in the time from impact to collapse would have been to progressively weaken the residual vertical load carrying capacity from the remaining core columns, increasing the need for slab panel action from the floor slabs above the impact region back to the perimeter frame. This would have been transmitted up through the floors above the impact region through the tension tie effect from the core columns, increasing the severity of shear action between the top floor or floors and the perimeter frame.

Finally, it is likely that the interconnection between one or more floors and the perimeter frame failed at or near the top of the building. This would have resulted in the immediate collapse of these floors (but not the collapse of the perimeter frame). From the video footage this collapse appeared to occur uniformly around the building and spread very rapidly down to the floor above the impact region. That region then pancaked causing a brief gout of flame to be expelled most noticeably from around the South and East sides as the areas within the impact region still on fire collapsed.

Clifton, falsely states that his analysis implies the uniform collapse of both the core and perimeter wall, which, of course, it does not. It is also of interest that Clifton can talk about the connection between floor and perimeter frame, but never describe it. It is just another unimportant detail I guess. All you need to know about this connection, is that Clifton considers it weak enough to suit his theory.




Fig. 10: Sequence of Collapse of North Tower

The collapse then continued down the building, with the floors pancaking leaving the perimeter frames briefly standing unsupported until they too collapsed. The effect of the floors pancaking nearly straight down inside the perimeter frames lead to the North tower effectively imploding, with some sections of the perimeter frame remaining standing unsupported for a few seconds before collapsing. This is seen from a number of video footages and pictures, including the collapse sequence shown in Fig 10.

Considered Effect of Impact on South Tower

At 9.03 am the plane impacted the South side of the south tower towards the South East corner. It struck the building at an angle across that corner and on a slight downwards heading, as shown in Fig.2. The plane passed into the building and then exploded out of the adjacent East side, causing a large fireball to erupt from both the entry and exit sides adjacent to the South East corner. Fig. 8 shows the plane immediately before impact and Fig 11 shows the expanding fireball erupting from both sides of the building.




Fig.11: Fireball From Impact on South Tower

First it should be emphasized that even if Clifton's explanation for the collapse of the north tower were correct, it would NOT explain the collapse of the south tower.

In contrast to the North Tower impact, in the case of the South Tower only one corner of the core would have been directly in front of the plane's path through the building, along with the floor slabs over several levels in the South East corner. It is likely that the initial impact destroyed all the floor slabs in that corner over at least four levels and maybe over as many as six. It would have also severely damaged the South East corner core, removing an unknown number of columns there, buckling many more and destroying most of the core walls (which are drywall construction). We do know that at least one stairwell in the core remained intact after the impact, as there have been five survivors from the floors above the impact floor who must have had access to an intact stairwell to escape.

Immediately after the impact the perimeter frame in the South East corner would have been severely weakened, being reduced to an unknown number of intact box columns in towards that corner on each of the two sides. However, these columns would have lost the lateral support from the floor slabs over many levels and would have had to function as isolated columns spanning multiple storey heights. They would likely have suffered blast damage and loss of alignment, however immediately following the impact they still retained sufficient compression capacity to resist their share of the loads from the 30 or so floors above the impact region.

The fires started by the impact would have then progressively weakened the vertical load carrying capacity of the remaining core, causing progressively more load to have to be carried by the perimeter frame system.

This is a stretch. In the South Tower impact, most of the fuel burnt off in a huge fireball outside of the building, so the central core would not have been heated very much by the jet fuel fire. After the small amount of jet fuel that entered the core region had burnt off any further heat would have had to have been radiated from the office fires in the adjacent region, or carried there by the cool breeze that was blowing on that day. Also, most the steel core columns fire protection would have still been in place. This means that the remaining central core columns were never in danger of failing.

In my opinion, based on the footage taken of the building over that time, the fire would have had little impact on the strength and stiffness of the perimeter frames, even in the damaged corner. The stiffness of this system above the impact region would have distributed this load approximately uniformly around the perimeter frames, increasing the loading on these frames through the impact region, including on the residual columns in the damaged corner.

Finally the combination of increasing compression load on these damaged columns, with second order effects from this load acting on the buckled shape of these columns over their unsupported length, would have caused their collapse.

Read this techno-babble as an admission of Clifton's inability to give a plausible explanation.

This collapse would have initiated in the damaged corner and spread rapidly over the impact region, causing the tower above to fail by toppling sideways with the floors above the impact region momentarily in an intact condition. This stage of the collapse is shown in Fig. 12.

If the collapse initiated in the damaged corner, then the building would have tipped in the direction of that corner. However, it actually tipped evenly along one of the faces (because explosives had been detonated along that face (as is clearly visible in various videos)). I like Clifton's statement "with the floors above the impact region momentarily in an intact condition". Here Clifton is trying to spin the disintegration of the top section of the upper portion of the tower as natural, even though it clearly is not.




Fig. 12: Top of South Tower Collapsing

The reason that the above photo, and others from the same video, are favorites with those defending the official line, is that the Millennium hotel hides the rows of explosions (which initiate the collapse) from view.

However, even with the top floors toppling sideways, sufficient material would have impacted straight down on the floors below the impact region to have caused these to start to pancake downwards, leading to the tower below the impact region collapsing in much the same manner as the North tower.

With both towers, the forces created by the falling floors above on the floors below would have been orders of magnitude greater than the resistance of these floors, leading to the complete collapses then observed.

How Severe Were the Effects of the Fires?

In my opinion the fires had a less important role to play in the collapse of both towers than the damage from the initial impact. It took both to cause the collapse, however the fire was in no way severe enough to have caused the collapse on its own. The reasons for this opinion are as follows:




Photo: Region glowing red hot. From the large compartment test at Cardington, towards the end of the fire (fuel 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)).
  1. If the temperatures inside large regions of the building were in the order of 700+ deg C, then these regions would have been glowing red hot and there would have been visible signs of this from the outside. Also there would have been visible signs of flames. If one looks at the photos of the Cardington fire tests, the flames and glowing of the steelwork is clearly visible even in the large enclosure test where the maximum fire temperature was only 700 Deg C. In contrast, the pictures of the towers after the impacts and prior to the collapses show signs of severe burning over only relatively small regions of the tops of the towers, even pictures taken from the air looking horizontally into the impact region (e.g. Fig.9).

    Photos of the First Interstate Bank fire in Los Angeles in the early 1990s? (May 4-5, 1988) show what appears to be greater heating effects and over larger regions than were apparent in either tower. This does not mean that there were no regions subjected to severe heating. It is likely that temperatures in some parts of the impact region would have exceeded 700 deg C for some or all of the time between impact and collapse, especially on the South side of the North tower. However, the extent of impact damage would have been such as to leave the residual vertical load carrying system within the core regions of both buildings vulnerable to further weakening at temperatures lower than 700 deg C.

    In contrast, had the columns in the core and the perimeter frames remained intact and protected (an impossible scenario given the magnitude of the impact) then it is expected that the building would have remained standing, with significant floor damage, even when subjected to fire temperatures of 1000 deg C and having suffered the loss of the fire rated suspended ceiling to the floor slabs.

  2. When fully developed fire conditions (temperatures of over 700 deg C) are reached within a region of a building, this results in the breaking of glass in any external windows within that region. This continuous breakage of glass as the fully developed fire spread through the floor of the First Interstate Bank, for example, was the most hazardous feature of the fire to those at ground level around the building.

    In contrast, once the blast and fireball effects of the impacts had subsided, there appeared to be little ongoing window breakage from either tower, either as evidenced from pictures/video footage or as reported from the ground. Significant areas of window even remained intact within the impact region (see e.g. Fig.9). This is further evidence that fully developed fire conditions did not spread much through and beyond the initial devastated region, following the impacts.

  3. If there had been severe fires burning in the core regions of the building due to the fire load from the plane combining with the fire load from the buildings, this would have adversely impacted on the conditions in the stairwells below the impact region. This would have especially been the case for the North Tower, where the core was destroyed by the impact, leaving the regions within the core below fully exposed to fire conditions within the impact region, such as the ingress of burning fuel and other combustibles. 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, as reported by survivors from the building. In the South Tower, at least one stairwell remained operable past the impact region after the impact. Given the damage that must have been done by the impact to the walls surrounding this stairwell, the resulting fire is unlikely to have been "incredibly severe", otherwise the few survivors from the South Tower above the impact region would not have been able to escape.

  4. When the North Tower finally collapsed, the collapse started from the top down onto the impact region. If the fire in this region had been very severe at the moment of collapse, then I would have expected to see a significant burst of fire and burning debris expelled from all around the perimeter of the impact region as it was compressed by the collapse. In reality, the footage of the collapse does not show much flame issuing from the impact region as it is compressed by the collapse.

  5. It is reasonable to assume that the force of the impact and subsequent fireball would have stripped the passive fire protection from most if not all of the steel members that remained in place within the impact region. If this is the case and the fire had been as severe as some have stated, the buildings would not have remained standing for as long as they did. Left unprotected, elements of any steel members exposed to severe fire conditions would have quickly reached temperatures close to the fire temperatures. We know this from the large-scale real fire tests conducted in recent years, in which the bottom flange and webs of unprotected beams and columns exposed to the fires reach 90% or more of the fire temperature and closely follow the fire time-temperature curve. The floor beams and exterior columns of these towers were relatively light members and would have heated up rapidly. Unprotected core columns would have heated up more slowly, but even in that case would not have survived 1 ¾ hours of severe fire exposure prior to collapse, especially if they had also been distorted by the impact. This is because the columns would have tried to expand with the heating and, being unable to do so by the surrounding cold building, would have instead buckled sideways leading to further loss of load carrying capacity. That was probably the mode of failure of any damaged core columns following the initial impact, however the fact that the buildings survived as long as they did after the impact indicates to me that most of the members that survived the initial impact were not then heated to very high temperatures.
The effect of the initial impact was so severe that the fire did not have to do much additional damage to collapse the buildings. It is likely that the fire was a more significant contributing factor for the South Tower, however for the North Tower this is less obvious and it was probably progressive shear failure of the system connecting the floors to the perimeter frames that initiated the final collapse.

In fact it is likely we will never be able to establish with absolute certainty the exact sequence between impact and collapse in either case. The above are simply my best guesses based on the information available and my experience of steel building behavior in extreme events.

A Personal Footnote

The two towers were exquisitely well engineered and, as we now know, very well built. It is a tribute to those involved in their design and construction that they absorbed the massive impact from the planes and remained standing long enough to allow many occupants to escape. It is however sobering to reflect on the 5000+ that have been killed in the attack and destruction of these magnificent buildings and the suffering that this has caused and will continue to cause for a long time to come.

Although I did not know anyone directly affected by the attacks, they have left a deep impact on me. As an engineer, part of my response to this is to try and provide my hopefully educated best guesses as to what may have happened to the two towers from the time of impact to collapse. The details given above come from my background of 17 years experience and practice in the research and education into steel building behavior and design, especially research into their behavior under severe earthquake and fire attack.

I hope that as much as possible can be learned from this tragedy so that whatever practical steps can be taken to lessen the vulnerability of this type of building to similar attacks (or accidents) in the future can be implemented.

Reference

1. Godfrey, GB (Editor); Multi-Storey Buildings in Steel, Second Edition; Collins, London, England,1985, ISBN 0 00 383031 4

Elaboration on Aspects of The Postulated Collapse Of the World Trade Center Twin Towers

G Charles Clifton, HERA Structural Engineer, December 13, 2001

Scope of Paper

This paper provides elaboration on aspects of my paper [1] entitled Collapse of the World Trade Centre Towers, written 17 September 2001 and revised three times since then, with Revision 3 dated 11 December 2001.

Since that paper was written, there have been several minor errors noted in it. Of more importance, two important aspects of it have been queried; these being:

The postulated structural load distribution in the North Tower (the first tower to be struck) following the impact and leading to the collapse ; and The intensity of the fire being underestimated because of distance and scale effects.

These points of concern require a response. The principal purpose of this paper is to provide that response.

It also briefly details some of the known errors in revision 2 of [1], dated 8th of October, and which have been either corrected or noted in Revision 3 to [1]. These details are given below.

This is followed by a brief coverage of the structural load-carrying system before impact, listing the assumptions made which impact on the postulated collapse mechanism for the North Tower.

Elaboration on the material presented in [1] relating to this collapse mechanism is then given.

This is followed by further material that has come to hand, in the two months since the attack, relating to the likely structural fire severity of the fires in each tower before the collapses occurred.

The paper ends with brief conclusions and references.

As with the original paper [1], the details herein are of a general nature. They do not present calculations or detailed metallurgical or structural observations. In the former case, this is because there are too many unknown to make a "correct" set of calculations possible. In the latter case, I don't have access to material / data from the wreckage of these buildings so I am not in a position to make detailed observations.

What I do have is a good general overview of the structural system and method of construction used in the Twin Towers, plus my knowledge from 17 years of research into and development of design guidance for the response of steel buildings to the extreme events of severe earthquake and severe fire.

The details presented in [1] and this paper are consistent with that knowledge and background, plus the material I have available on the attack and destruction of these two magnificent buildings.

Also, as with the original paper [1], the details presented herein are my opinions. It is for this reason that the paper and [1] have been written in the first person.

Inaccuracies Noted in The Original Paper.

These are as follows:

1) The times given for impact and collapse of each tower in the original version of [1] were based on published details as of mid-September. Some variation in these details was noted at that time. The generally agreed times are now given in Revision 3 of [1]. These are based on seismological recordings of the impacts caused by, first the plane hitting the building and, secondly, the building collapsing.

2) The directions shown on the site plan (Fig 2 of [1] ) are approximately 40° off the true directions. If one rotates the site plan 40° clockwise, thus making the direction shown as North read N40E, then the map is correctly orientated.

3) Some minor typographical errors have been corrected.

Structural Load Distribution Before Impact.

Overview

The section of [1] entitles "Details of the buildings " and presented on pages 2-5 therein provide an overview of the structural system used.

In summary, this comprised: Assumptions made about the structural system design

In postulating the collapse mechanisms for both towers (but especially relevant for the North Tower), I have made some assumptions about the structural system used.

These as follows:
  1. The bar joist/floor system is connected to the perimeter frame/gravity core using a top flange mounted seat system as roughly indicated in Fig.7 of [1]. This system is designed to support the applied dead load from half the floor joist clear span. The actual connection between each joist and the supporting girders at perimeter frame and core is a typical bar joist connection detail, designed to carry principally its tributary design vertical load and with some nominal lateral load transfer capacity.

  2. There is a shear stud or some form of physical connection of the slab into the perimeter frame girders, through eg. shear studs from girder into slab and slab reinforcement passing beyond the line of connectors or alternatively some type of starter bar arrangement from the perimeter frame into the slab. Some form of this is evidenced from photos of the still intact perimeter frame remnants at the site. This would presumably have been sized on the greater of diaphragm transfer forces from floor into perimeter frame at each level or code minimum requirements for the interconnection, as applying at the time of design

  3. The visco-elastic dampers added from the bottom chord of the bar joists back to the perimeter frame did not noticeably increase the shear or tension capacity between floor slab and perimeter frame.

  4. There was a network of beams interconnecting the 44 core columns, such that the core formed an effective moderately rigid box. Some details of these are given in [2].

  5. The perimeter frame assemblages were spliced by mid-span bolted web connections (this known) and columns spliced with bearing splices having some moment/tension capacity (this assumed).

  6. The gravity columns were spliced by slot and tab splices, with the ends prepared for compression bearing. That much is known; I have assumed that these column splices also had an integral connection capacity for shear (possibly 15% of design shear capacity, as would be stipulated by NZS 3404 [3]). If so, this could have been formed by blind bolting of the tabs of the supporting column through the walls of the supported column, or more likely, by incomplete penetration site butt welds between the abutting surfaces, with these welds of small size and formed using a ductile weld metal.
Prior to the impact, the applied vertical loads on the core would have been carried to ground through the core columns. The applied loads on the perimeter frame would have been carried to ground through the perimeter frame. The vertical loading on the floor system spanning between core and perimeter frame, including its self-weight, would have been carried effectively equally by the supports at core and perimeter frame.

Postulated Structural Load Distribution in the North Tower Following Impact

The plane hit the North-East face of the North Tower, flying near level, at around the 94th and 95th floors. It cut through the perimeter frame leaving a hole in this frame some 2-4 floors deep and impacted into the core, causing an unknown amount of damage there.

In my original paper [1], I stated that the impact on the core would have "removed many of the core supporting columns, at least on the North side of the core, and leaving the remainder buckled and stripped of their passive fire protection". At that time (mid-September) I did not want to speculate in the paper on the number of core columns destroyed in the impact region. Since then I have been advised of estimates up to 40%.

This loss of core columns would have had the following immediate effects:
  1. The load from the still intact upper floors would have had to be carried by alternative load paths to the still intact core columns. This redistribution would have required vertical sagging of the core region to become effective, with this sagging occurring immediately on impact and to a very noticeable extent on the floors above, especially above the impact side of the core. Only a massive redistribution of load would have lead to vertical sagging. There is no reason to believe this happened. No evidence of vertical sagging has ever been presented. The loading on these columns could be increased more than 400% before any noticeable sagging occurred.

    I believe that this immediate sagging offers a possible explanation as to why many people jumped from the North Tower upper floors before the effects of fire on those floors became very apparent. If this sagging hypothesis is correct, then it would have cut access to the stairs by jamming doors, etc. It would also have given a clear message of impending collapse, leaving the people trapped on these upper floors with a terrible choice; jump and die or stay in the building and die in the collapse. Lame. Real lame. So people choose to commit suicide in preference to waiting and seeing how things turned out (and this because the building might collapse like South Tower (but then it might not)). Yeah, sure, sure.

  2. The compression load on the remaining core columns would have significantly increased. Not true. Since the core was well braced, any additional load would have been distributed among all the remaining core columns. Hence, the extra load carried by each would be small.

  3. The vertical load being transmitted through the floor system into the perimeter frame would have increased, as the core region sagged immediately following the impact and in a progressive manner from then on to the final collapse. Only true if the core sagged and the fires then progressively weakened the core columns. However, as mentioned above, the core contained very little flammable material, so there is little reason to believe that the fires significantly heated the core columns, let alone caused them to fail.

  4. The gravity columns which had been severed by the impact would now act as tension ties between each of the floors above the impact region, through to the top floor. The amount of tension so transmitted upwards would have been limited by the strength of the column splices to transmit tension, and possibly by the amount of redistribution occurring within the core region.

  5. At the top floor, these "tension" columns would have exerted as additional unbalanced downward load on this floor, adding to the overload stress on the floor to perimeter frame connection.
Following the impact were the fires. Fires adjacent to the damaged core region would have impacted on core columns already suffering various extents of local and member buckling from the plane's impact and loss of insulation material. As these columns heated up, they would have been subject to additional compression forces from restrained thermal expansion. In the case of columns with member buckling, this would have increased their lateral deflections between points of effective lateral restraint, leading to increased P-delta (P-δ) actions and reduced compression capacity. The temperatures required for this effect to become significant, on already damaged columns, is not high - I would estimate no more than 400-500°C would have been needed to cause significantly reduced compression capacity in the residual core columns.

In my opinion, based on the available evidence, there appears no indication that the fires were as severe as a fully developed multi-storey fire in an initially undamaged building would typically be. (More on this below). However, the observations show that fire temperatures of over 500°C would have been probable over enough of the core to cause an ongoing loss of compression load carrying capacity in the remaining core columns.

This would have increased the pull-down action of the floors on the perimeter frame, with this effect greatest on the top floor. I believe that final failure would have been through detachment of the top floor from the perimeter frame, starting at one point and rapidly spreading around the top floor. This would have been followed by near instant tearing away of all floors above the impact region from the perimeter frame, with the pancaking effect then proceeding to the ground.

As detailed in [1], the above relates to the North Tower. The explanation of collapse for the South Tower is quite different, as given in that paper.

How Severe Were the Fires: Revisited

Having carefully studied all available material available to me and collected since the original version of [1] was written, there is nothing in points 1-5 of [1] under the section "How severe were the effects of the fires?" that I would amend on the basis of this new material.

In fact, the new material provides further support for the fires not being particularly severe. Additional points to 1-5 of [1] in this regard are:
  1. Almost every building occupant below the impact floors (including up to the 91st floor on the North Tower) survived. Given that the fire separating walls around the stairs & lifts over the impact region were destroyed, then there must have been negligible spread of burning fuel/debris down into the breached stairwells to allow them to remain tenable.

  2. There is a survivor account of a group trapped in a lift, which had the cables severed by the impact. They were able to stop the lift on the 53rd floor, open the doors/escapes hatch in the top & cut their way out through the drywall fire separation. Their account says little about the effects of any fire above; nor could these effects have been significant or they would not have survived.

  3. Close scrutiny of close-up views of the burning buildings that I have received since mid-September (i.e much more detailed pictures than Fig 9 from [1]) show little evidence of temperatures above 600°C (i.e. fully developed fire conditions) within the impact region. These same pictures also show that, on some of the upper floors where fully developed fire conditions are observed, the fire-rated suspended ceilings appear largely in place and so the likely effect of these fires on the structure is minor.

  4. A fire engineer (Martin Feeney from Holmes Fire and Safety, Auckland NZ) has advised that theoretical consideration of the fire severity based on the quantities of combustibles in the planes and impact region of the buildings give answers that are not supported by observation. The observed fire behavior points to temperatures in the building not being particularly severe - say no more than about 600 to 700°C. Possible reasons for this may involve the coating of combustible material in dust from pulverised concrete and wall linings and the volatility of the aviation fuel leading to large amounts of fuel being pyrolised but not burnt in the interior of the building.
Conclusions:

This paper, which should be read in conjunction with Revision 3 of [1], provides elaboration on aspects of the postulated collapse of the World Trade Centre Twin Towers that are given in [1] and which have been queried by readers.

The details presented in [1] & herein are my opinions and are put forward to stimulate discussion and consideration of all aspects of this tragedy, in order that we can learn as much as possible to make buildings safer & more resistant to deliberate or natural acts of an extreme nature.

References:

[1] Clifton, GC; Collapse of the World Trade Centre Towers; HERA, Manukau City, New Zealand; written 17 September 2001; revised 19 September, 8 October and 11 December 2001.
[2] Godfrey, GB (Editor); Multi-Storey Building in Steel, Second Edition; Collins, London, England, 1985, ISBN 0 00 38 3031 4.
[3] NZS 3404:1997, Incorporating Amendment No.1: 2001, Steel Structures Standard; Standards New Zealand, Wellington, New Zealand.