14 November 2001: Add comments and links to New York Times and The New Yorker reports on the collapse.
30 October 2001: Add additional comments on the collapse and link to call for independent investigation of the design failure:
7 October 2001. Add comments on alternative.
6 October 2001: Add WTC collapse alternative.
13 September 2001: Add comment on the Port Authority.
12 September 2001. See related comments on hazards of the WTC collapse.
Comments on the WTC Collapse
John Young practices archtitecture in New York City.
Professional structural engineers are pre-eminent on building structural matters. Architects usually are not, except on small projects, though architects are co-equally responsible for the overall safety and health of buildings they design, assisted by a variety of special consultants, principally structural and building systems engineers. On a large project like WTC, there are dozens of special consultants involved. For construction and renovation of buildings in New York City, design by state-licensed architects and/or engineers is mandatory who are required to file construction documents with municipal agencies for governmental approval. Licensed design professionals are governed by the same law that governs building construction codes. New York State Education Department oversees licensing; the agency publishes on the Web a list of its currently-licensed professionals from all fields.
Comments welcome. Send to: firstname.lastname@example.org
To: John Young <email@example.com>
I've seen two messages [below] you sent, forwarded by Declan, but there's nothing on your web site that I can find (including the architectural sections). I think it'd be useful if your messages appeared there, in some easy to find place, and started a further discussion and structural analysis.
Do you think it possible (or likely) that these buildings were deliberately targeted BECAUSE it was more likely they would fall down if pierced? What other buildings have similar construction and thus might also be targets?
Young's response: It is possible the buildings were targeted because of their distinctive construction. The buildings were very well known in architectural and engineering circles and were heavily publicized during design and construction. Construction drawings are readily availble to the public.
If you wanted to destroy a building and had access to an airliner, would you do it the same way, or differently?
An airliner with explosives would be more effective. Crashing into the base of the building might cause it to collapse immediately if there were sufficient explosives to destroy the base structure which is strongest due to maximum loading.
Did the second tower fall because of its penetration, or because the first tower's fall (and/or intense burning at ground level) weakened the second one?
Would the towers have stayed up if internal fire suppression had overwhelmed the initial fires resulting from impact? I.e. was it the heat of the fires that started the chain resulting in total collapse, or was it stresses created by the penetration (and subsequent motion of the building, in reaction to the impact, the wind, as people fled from it, etc)?
Yes, if the fires were suppressed it likely the structure could have survived the impact except for localized damage. However, conventional architectural fire-suppression systems are not designed to suppress the types of fires caused by plane crashes and cascading fuel.
Declan's second forwarded message from you was unclear in spots:
Young's prior messages: I modify my first evaluation to speculate that the interior supports appear to have given way before the exterior lattice (whose girdle of closely-space columns and thin vertical windows between gave the buildings a unique look compared to use of large panes of glass elswhere) The lattice amazingly contained the interior collapse and the whole mess dropped vertically, almost, as newscasters report, as if executed by a demo expert.
The first what? The collapse of the first tower?
Yes, the first tower.
... above the crash fell. Then the smoke cleared momentarily to show the totality. Then the second tower, collapsing in a near-perfect copy of the first. The sudden dropping of the floors above the crash, that impacting load overpowering the remaining system, and the straight drop collapse, neither tower falling much to the side, indicated what had happened.
To you, perhaps, but not to us. Please explain. Do you mean that seeing this is what provoked you to modify your first evaluation (as explained one paragraph up)? Or are there further things it tells you?
Studying the replays of the collapse caused me to see that the interior gave way before the exterior. The exterior structure appeared to resist collapse; some multi-story sections remained standing at the base for a brief period.
Some eyewitness accounts said that they saw or heard explosions on the upper stories just before the whole thing came down. If you knew more about these explosions (e.g. were they in the exterior wall or in the interior of the building?) would this tell you more about the failure mode?
I don't recall seeing explosions, but when the upper floors above the crashes suddenly dropped, that forced balls of flame from the buildings. While it is possible there were pre-positioned explosives in the building, I do not think that was the case due to it being very difficult to assure that a plane would hit the correct location in the building.
From reading Why Buildings Fall Down a few years ago, I got a strong impression that this sort of initial-failure-overstresses-the-next-part-which-then-fails chain reaction failure is a very common design defect -- so common that you'd think a highly experimental $700 million building would have been analyzed six ways from Sunday to make sure it wouldn't happen. If the interior contents of ten upper stories all suddenly fell on the floor of the next story down, was the building engineered to take that?
When the WTC towers were built there was extensive controversy over their safety in emergencies. The NYC Fire Department protested, as did a host of other agencies and professional associations. The buildings were constructed in bulk and height far in excess of what municipal constuction and zoning codes allowed. However, the Port Authority, a quasi-governmental agency with exceptional powers inherited from the regime of Robert Moses, was specifically exempt from compliance with municipal codes. The real estate, construction and finance industries were powerful supporters of the project.
Suppose not only the interior contents of the ten upper stories fell, but also the upper ten stories of exterior structure? Would the loss of those top ten stories reduce or improve the strength of the exterior load-bearing structure? (Was the building specially tied together at the top, for example, or was every story tied together in the same way, perhaps even stronger at lower levels than at upper levels?)
If the upper floors fell without damaging the structures along the way or at the base, that would not have destablized the buildings. That is what I expected to happen, and thought had happened when the first tower collapsed.
Close-ups of the exterior show the latticework bridging the crash penetrations,
Not sure when you mean here. After the airliners penetrated, but before the buildings fell? Or can you see things in the rubble after the fall that tell you things about how it failed?
After the crashes knocked out portions of the exterior structure lattice, the remaining exterior structure bridged over the crash openings, though not necessarily designed to do that.
Assuming you mean before the fall, what does that tell you? Wouldn't you expect the latticework that wasn't impacted to survive? What do you mean specifically by "bridging"?
I expected the undamaged latticework below the crashes to survive, as well as parts above. Bridging means that the remaining structure spanned the crash openings like a bridge spans a river.
Date: 11 September, 11:14 AM
E-mail in Upper West Side of Manhattan is okay, local and national TV too. Radio is out due to loss of transmitters on the collapsed WTC towers. Our wire and wireless phones are on and off. Subways are partly out. We cannot see the WTC area from the roof of our building, so TV and Internet are all we got. I was about to leave for a meeting in the WTC area when the news broke.
Trying to locate a daughter who has a dot.com job near the WTC. Many friends nearby too.
A word on the structure of the WTC towers:
The WTC towers had a distinctive structural system which utilized the exterior wall framing for lateral bracing -- a so-called lattice framework. This allowed minimization of internal lateral bracing and opened up the floor plans. You can see the effect of that when the buildings collapsed, with the lattice framework crumbling and the interior imploding. The lattice works so long as it remains intact as a system: if a part of it goes, then the whole system goes.
The planes punched holes in the lattice, one tower punched on two sides, maybe the other too. Portions of the lattice of the second tower briefly remained standing after the collapse, then fell.
The system was considered daring at the time of construction, for it distributed loads more efficiently than legacy column-and-beam-supported systems. Probably the legacy systems would not have totally collapsed due to damage at upper floors, although floors above the damage would have come down if columns were weakened.
Date: 11 September, 1:25 PM
The '45 Empire State Building plane crash is oft-studied in architectural and structural engineering to learn why the building withstood the hit. The plane was a B-26, I believe, but in any case a much smaller craft than the ones which hit the WTC and the Pentagon. The '45 plane's engines did penetrate the building, shooting out the far side and falling to the ground and killing passersby, but most of the plane remained inside the structure for it, like those which hit WTC, was made of far more fragile materials than a building. A relative small amount of damage was done to the structure of the building though fire was devastating, especially from flaming gasoline cascading inside.
The fireball that shot from the second of the WTC towers hit -- opposite where the jetliner penetrated -- blew out windows and part of the latticework exterior structure. Flaming fuel probably cascaded down the shafts for elevators, ductwork and stairwells whose fire-protective enclosures would have been destroyed by the explosive crash and ballistic heavy plane parts. These fuel flames, and fires started from them, would have weakened interior structural support beyond protection provided by code-required fireproofing. Once the interior structural supports were weakened, and the exterior lattice framework lost its integrity, collapse was inevitable.
I modify my first evaluation to speculate that the interior supports appear to have given way before the exterior lattice (whose girdle of closely-space columns and thin vertical windows between gave the buildings a unique look compared to use of large panes of glass elsewhere) The lattice amazingly contained the interior collapse and the whole mess dropped vertically, almost, as newscasters report, as if executed by a demo expert.
I did not expect the Twin Towers to collapse. To suffer terrible fires and localized interior damage but not total collapse. The first collapse was unbelievable, and as I said, I thought only that portion above the crash collapsed. Then the smoke cleared momentarily to show the total disapperance. Then the second tower, collapsing in a near-perfect copy of the first. The sudden dropping of the floors above the crash, their impacting load overpowering the remaining system, and then the straight-drop collapse, neither tower falling much to the side, indicated what had happened.
Close-ups of the exterior show the latticework bridging the crash penetrations, reminding of sales pitches from the 19th Century when cast-iron manufacturers promoted their architecture with structural compoments missing with no apparent destabilization -- the load automatically shifting to remaining components. Their prognostications failed at the first intense fire which overheated and cracked the cast iron, sometimes collapsing more quickly thanpredecessor masonry bearing wall and wood floor system composites.
Humbling news: My daughter is safe and sound. She heard the first crash and saw the tower blazing on the way to work and thought it was merely (!) a fire. Her office remained at work unaware of what had happened, and was happening, without TV or radio, until telephoned from overseas headquarters which ordered everyone home. At first the office dismissed the alarm, saying, hey, this is New York, no problema, we have work to do, our customers come first, sure that would impress the venal bastards. Then someone was sent outside to check reality.
Date: 11 September, 8:28 PM
Architectural critic Michael Sorkin's studio is located blocks away from the WTC on Hudson Street. Looking down Hudson the North Tower filled its terminus. Michael said a studio colleague and his daughter were in a bookstore in one of the towers at the time of the first crash but escaped with most others. Michael was at home in the West Village at the time, about a half-mile distant. He went to his studio to gather e-mail before being evacuated with most of the neighborhood. The vacate view down Hudson was incomprehensible.
Michael Kaplan is right that the towers were vilified during construction in 66-68, that was the time I first came to New York, and the controversy was intense. The buildings were terribly out of scale to anything and were considered to be a power symbol of the Port Authority -- the inheritor of the arrogance of Robert Moses which proclaimed without shame that the people respected the strong. Yamasaki suffered terrible attacks in the architectural and poplular press, and according to co-workers was immensely affected by it, and indeed may have died of heartbreak.
Prior to WTC, Yamasaki was a stellar US architect and presumably was picked for the job because of his outstanding reputation and to window-dress the power display. He was also vain and susceptible to flattery by similarly self-important titans. During design of the project and the criticism, he repeatedly defended the towers on aesthetic grounds, and also invoked his personal courage to take on a project no other reputable architect would dare.
Yamasaki was the design architect, with the bulk of the work being done by one of NY's giant commercial hacks well versed in real estate ideology and its sordid practices to maximize land use. The NYC Zoning Resolution would have prevented the towers but the Port Authority did not have to comply with local laws, a legal precedent orchestrated by Moses. The NYC Fire Department protested that it would be impossible to fight fires in the building and attempted to prevent its construction, to no avail.
No doubt, there will be calls for restoring them, perhaps even bigger, in the name of history, as if what they were was a great American dream.
Date: 13 September 2001
The Port Authority of New York and New Jersey (its official name) has a large, highly qualified archtectural and engineering department, which overseas design, construction and operation, including security, of the two states' shared airports and seaports.
The PA is not obliged to submit its projects to state and municipal authorities for approval but most often coordinates with these authorities and generally meets -- or exceeds -- construction requirements of conventional governments. This is true of most of these distincitive authorities as far as I know.
In New York City it is the Department of Buildings which oversees construction, along with Departments of Fire, Health, Landmarks Preservation and Highways. The DoB promulgates and enforces the Building Code, one of the strictest in the world.
The Planning Commission oversees land and building use, and building bulk and height regulations, as promulgated by the Zoning Resolution.
Designs for construction must comply with the Building Code and Zoning Resolution, but as elsewhere, there is intense negotiation over compliance. New York City has an entire industry, separate from design professionals, which negotiates compliance due to large costs involved and what are believed to be arcane, even corrupt provisions in regulations. This leads to much daring innovation, to put it politely, at every step of review of construction documents and during construction.
As a young architect I was told and believed that construction in the city was irreversibly corrupt. No longer, 30 years on. What I know now is that a small number of people at all times continue to work to improve the system for public safety and security, not just in NYC but in all human settlements. I am humbled to know and work with and learn from these few. I was never told about them in professional training, and you seldom read about them in the media. They work in government and private industry, in universities and hospitals, yes, and in what appear to be 100% venal professional firms. Many labor for much less than they could earn in the private sector, some pro bono, and some are those are in the Port Authority and similar public authorities for which I am most grateful knowing that the private and commercial real estate industry falls well below their standards of safety and security.
What is odd about the WTC debris is how few larger fragments of concrete it contains. Reports on the structure say reinforced concrete was used for floor slabs and customarily in demolition such material breaks apart in relatively large chunks, the broken concrete remaining attached by steel reinforcing bars or wire mesh.
However, there is a structural design where plain concrete is placed on metal floor decking with little or no reinforcing. Also, lightweight concrete, whose course aggregate is cinder rather than stone, is often used to limit the weight of the structure, especially in high-rise buildings. This lightweight material does not usually have the strength of regular concrete and strength is provided by metal decking below it. The large amount of dust produced by the towers' collapse and the relative lack of larger concrete shards raise the possibility that the building disintegrated at least in part due to lesser strength of the type of concrete used in the floors. That remains to be examined.
Observation of the towers' remains show that while main steel vertical structural members of the exterior wall and central core withstood the collapse, the horizontal floor-supporting trusses broke away from these vertical supports at the points of attachment (probably made by welding). One view of the North Tower (1 WTC) shows the exterior and core remnants and the ruptured points of floor structure attachment.
What this suggests is the possibility that the structure of each floor collapsed due to load of collapsing floors above them, and that only afterwards did the vertical supports at the exterior and core collapse. Review of video of the collapse appears to confirm this sequence as well as the consequent supposition that the floor structure was the weakest part of the buildings -- which would not be uncommon for floor structure supports only a single floor while the vertical members support all floors above them. However, a slow motion examination of visual recording would be needed to confirm exactly what collapsed first.
There has been speculation about the initial step in the collapse of the buildings, most commonly attributed to the intense heat of burning jet fuel softening structural steel, usually the steel of the core. However, it is possible that collapse of the core steel was not the initial phase, but instead it was the floor structure breaking away from vertical supports. The collapse in this scenario would be that of floors dropping one after the other onto floors below, the load of the upper floors overwhelming the relatively weak attachments of floors to vertical supports -- the attachments customarily being designed to support only a single floor load.
For example, a single floor dropping onto the one below could have ruptured the next lower attachment, thus setting off a disastrous sequence. This could have occurred without fire initially weakening the vertical steel structure as has been speculated. The impact of the crash, and/or subsequent swaying of the buildings, could have ruptured floor structure attachments, and only one floor breaking away would have been enough to precipitate the collapse.
Alternatively, the fuel fire, and flaming building contents, could have weakened floor structure and/or its attachment to vertical supports, in particular if the crash destroyed fire-protection materials of the floor structure. Thus, with heat weakening floor structure along with the attachments being ruptured by the crash, the collapse sequence commenced.
One significance of these speculations is that weakening of the core steel by intense heat may not have been the initial cause of collapse. A New York Times report of October 6 describes an investigative engineer "finding what appears to be a few pieces of the south tower that were directly hit by the Boeing 767 jetliner, and the discovery poses a few new puzzles. While the impact sliced through half the column, the column did not buckle; each column is designed to support the weight even if half is missing. The column also exhbits no outward signs of smoke or heat damage."
Another signficance is that more steel reinforcing in the concrete could have increased the strength of the floor structure and better withstood the initial step in the floor-by-floor collapse sequence. And, the immediate and long-term adverse affects of the huge dust clouds of the collapse might have been lessened. It is likely that some victims were suffocated by these clouds.
-- John Young, Cryptome
Comments welcome: firstname.lastname@example.org
> You are saying that 4 martyrs in strategically selected corner
> Come to think of it, no martyrs needed, just fedex packages addressed
> It's a fragile world.
It would not take a lot of explosives to breakaway an entire floor constructed with steel joists like WTC; it is often done in planned demolitions with a small charge placed at each joist support -- customarily a shelf angle either continuous or segmented.
What would do it though without explosives is the twisting torque and vertical flexing in the tower tubes caused by an aircraft hitting off center of the tower's axis at 300 mph.
Survivors tell of the tower swaying several feet, but they are not likely to have perceived the torque as such. The hit on the South Tower (the second hit) was more off center than that on the North Tower.
Tall buildings are designed with some torque resistance against turbulent weather and earthquake but probably not for torque coupled with high impact.
The WTC towers were made of two tubes, the exterior wall system (the more flexible) and the interior core (the more rigid), the two tubes connected by light-weight floor structures and at about 1/3 intervals, heavy-weight transfer and mechanical equipment floors.
From examining the wreckage the steel floor joists rested on shelf angles welded to vertical supports. These angles are designed to support vertical loads not much twisting.
The joists could have lost their support by tower twisting of no more than a few inches, the shelf angle welding beads popping, or the joists slipping off the angles that did not break loose (as photos show many did not).
If the floor slab concrete was not reinforced it would have crumbled easily under torque. Again, most building structure is designed to resist vertical loading not substantial twisting. (Structural connections are designed to resist relatively minor twisting caused by static and dynamic loading.)
There is a valid question of why the buildings did not collapse immediately if floor structures were damaged and serial pancaking set in motion. The raging fires surely contributed to further weakening of the building structure, but probably due to adverse effect on floor structures and in particular on the light-weight shelf-angle welded connections between the floor system and the heavy-weight vertical supports.
It is at these connections that demolition designers place light-weight charges, as well as heavy-weight cutting charges on vertical supports.
For comparison, a WTC shelf angle may be 1/2 to 3/4 inches thick welded with a bead about the same while a vertical columnon WTC could average 2-3 inches thick, increasing in thickness from top to bottom.
The WTC designers claim the buildings were capable of withstanding a 707 hit, asserted at original construction and after the 1993 bombing. But the design criteria for that protection have not been published so it cannot be determined what crash scenario was used for design.
-- John Young
A reader of my site suggested the following: In order for the two towers to have pancaked straight down in demolition fashion, the damage from the fuel burn needed to be even. In other words, if the burn had managed to dislodge or destroy enough support structure on one side of a tower first, the floors above would have collapsed with a "lean," and the resulting total collapse of the tower would have occurred at an angle, not straight down. In fact, at some point part of the tower might have broken loose from the overall building and launched itself out, not down. If this rough analysis is true, then we are looking at a rather unlikely (to say the least) event: the fuel burn managed to destabilize each tower in an exceedingly overall, coherent, "side-to-side and back-to-front" fashion. I would welcome any comments.
Your reader's analysis is correct as far as it goes.
I've been looking at photos of the collapse to try to determine the exact collapse sequence for each tower.
The South Tower's floors above the crash did indeed collapse toward the east side of the tower -- that is the side where the plane hit. A photo caught this fall and it is confirmed by eye-witnesses as well as site surveyors that much debris of the top of the South Tower fell easterly rather than straight down.
Then the collapse gets complicated, for as the upper portion fell easterly its off-center impact kicked the floors below toward the west and a slew of floors fell westerly toward the World Financial Center where such debris has been identified.
Similarly, study of the North Tower shows that its upper floors collapsed northly rather than straight down due to the plane hitting close to the center of the north side of the building (there is a heartbreaking photo of the huge hole made with people inside waving for help at the photographer).
I assume that as the upper floors of the North Tower slammed off-center into the floors below, those floors were kicked southerly, although I haven't seen accounts of that.
No matter that these off-center events were occurring in both towers, the great mass of them probably dropped more or less straight down due the huge overload of falling debris and contained by the strong girdle of the exterior facade's tube-like structure, much of which had not been punctured or torn by the crash impact or by the fall of upper stories. These lower floors plunging down the tube exploded it more or less in all directions, though you can see remnants of ground level exterior and core elements still standing at both towers (or were a few days ago).
I am sure will be a computer image made of the two collapses based on photos, eye witness, structural analysis and forensic evidence. I expect it to be fairly complex sequence due to the multiple forces involved.
Early suppositions, mine among them, are probably too simple, though not necessarily wrong, but instead are merely aspects of the full picture. I continue to envision more complex sequences as I study photos, read accounts and ponder what little the forsensic engineers are disclosing.
It will probably be a while before a full picture is made public due to restrictions of official investigations and contending claims of responsibility and negligence. In the meantime all we've got is our collective genius, and apprehension that we better learn to protect ourselves from experts before and after disasters.
-- John Young
Cryptome, November 14, 2001:
The NY Times had a lengthy report on the WTC collapse on Monday, with swell diagrams, and the current New Yorker has a long report featuring Leslie Robertson, the WTC structural designer, which also describes the likely cause of collapse.
There are still missing parts to the collapse scenarios I've read, though it may be quite a while before a well-rounded, wholly credible explanation emerges.
Books of photos and special issue mags showing the collapse are now starting to appear and I got a couple today. One photo* shows an unusual event occuring several floors below the collapsing floors of the South Tower (first to fall). It shows the exterior columns of the east face of a single floor buckling outward, with columns for many floors above and below the buckle remaining intact. East is the direction the upper portion of the tower fell. But it is strange that the load concentrated at that particular floor to buckle the columns. This appears to be a mechanical equipment floor with an atypical structure or it has some other unique characteristic.
Before studying photos of the North Tower collapse, I had thought its upper floors had fallen northward (on the side of the crash) but apparently not. It appears to have collapsed more or less straight down. That the collapse did not tilt toward the side of the crash, as did the South Tower, is quite amazing for that appears to indicate the huge load of the collapsing floors was contained equally on all sides by the square-tubular design of the structure. Had the building been rectangular or irregular in plan that equilateral distribution of the load might not have happened.
I read that the towers were designed to take the hit of a plane like a bullet hitting a pipe: penetration but not rupture or collapse. That pipe-like, square-planned, equal-sided tube appears to have had other virtues in containing the collapse so the interior fell down a giant demolition chute, smartly pulling the chute down as the final step in a brilliantly designed demolition.
I'm now finding hard to believe that the collapses were unexpected by the attackers, or the masterminds behind the attack. And perhaps not unexpected by the designers, despite the many learned assurances that the collapses could not have been predicted.
I do know from my own experience that a common way to cover-up culpability of architectural failure is to claim the failure could not have been predicted under standards of the profession. That "standards of the profession" is the clue to seeing how blame is diffused so widely that no individual(s) is culpable. There is a goodly amount of that diffusion taking place among a host of New York interest groups, not only by engineers and architects, but cheerled by them to continue designing unsafe structures as though no wrongdoing has been demonstrated by WTC.
What appears to be proven by the collapse of the structures is that they were designed to collapse just the way they did. How widely that prospect was known is not clear, but the price to be paid for that homicidal design remains to assessed against all the parties who exploited the knowledge. -- John Young
* Day of Terror, September 11, 2001, American Products Publishing, Beaverton, OR. ISBN 1-58583-1-1-8. p. 15