WTC core – steel, concrete or both?

Two things bug about the “WTC had a concrete core” idea:

(1) If it did not have a concrete core, why do some experts believe it did?
For example, John Knapton says, “The core comprises steel beams and columns with reinforced concrete infill panels.” http://www.john-knapton.com/wtc.htm
Knapton is a consulting civil, structural and highway engineer. He does not believe the towers were destroyed by explosives. Where did he get his concrete core idea from?

(2) Why use so much explosives? I think the buildings exploded, rather than imploded in the manner of a usual controlled demolition; they certainly didn’t collapse neatly into their own footprint. If explosives were used, then there were far more than were needed to cut enough steel supports to bring the building down. However, if the core were concrete, not steel, then the amount of explosives used would make more sense. Alternatively, if the bombers believed the building had a concrete core, they would use lots of explosives, even if this belief was mistaken. But why would the bombers think the building had a concrete core, if it didn’t?

I’ve been studying the design of the WTC at the University of Google and I came across this:
Vincent Dunn is a retired deputy firefighter chief. His piece here http://vincentdunn.com/wtc.html discusses how the planes knocked the WTC over. When explaining the background of the building code according to which the WTC was allegedly built he says, “Then the masonry smoke proof tower was eliminated in the 1968 building code. It contained too much concrete weight and took up valuable floor space. Then the solid steel beam was replaced by the steel truss. And finally the concrete and brick encasement of steel columns, girders and floor supports was eliminated.”

And he goes on to say of the 1968 code, “This performance code signaled the end to concrete encasement fire protection and allowed a spray on fire protection for steel and plasterboard enclosed stairs and elevator shafts. Builders hailed the New York City building code of 1968 as a good performance code.”

So when did they start work on the WTC then?
“Groundbreaking for construction was on August 5th, 1966. Steel construction began in August 1968. First tenant occupancy of One WTC was December, 1970, and occupancy of Two WTC began in January 1972. Ribbon cutting was on April 4, 1973.”
http://www.greatbuildings.com/buildings/World_Trade_Center.html

And which building code applied to the towers?
“As part of the Investigation, it has been determined that WTC 1 and WTC 2 were constructed in accordance with the Building Code of the City of New York (BCNYC) that was enacted by Local Law No. 76 for the year 1968, effective December 6, 1968.”
http://wtc.nist.gov/pubs/NCSTAR1-1F.pdf

“The current Building Code was enacted by the City Council on October 22, 1968, and approved by the Mayor on November 6, 1968. It became effective on December 6, 1968.”
http://www.nyc.gov/html/dob/html/reference/code_internet.shtml

Let’s get this straight, they started work on the towers in August 1966 and started on the steel in August 1968, but the building code according to which the towers were built came into effect over two years after work started?

I’m not familiar with planning procedures in New York, but surely an application for a construction permit must have been filed at some time in the mid-1960s under the old building code. If this is so, then the plans should have included the concrete core demanded by the then-current building code.

When were the WTC plans unveiled?
“The breakthrough, however, came from Albany in January, 1964, when Rockefeller announced a consolidation of state offices in New York City into 1.9 million square feet of space in the Trade Center.

The Tallest

Five days after securing the state's commitment, the Port Authority revealed its plans for a $525 million, 10 million square foot project consisting of two 110-story towers that would be the tallest buildings on earth.”
http://www.greatbuildings.com/buildings/World_Trade_Center_History.html

I guess the architects would have had to know what they were going to make the core out of by then. This was nearly five years before a steel-only core became lawful, so they must have had a concrete core in the original design.

After the new code came into effect during construction, perhaps the builders requested a new construction permit (or an amendment to the old one) in accordance with the new code, to make the WTC cheaper to build (because steel would have been cheaper than concrete). However, by this point several floors would have already been constructed and they should have a concrete core. After having read umpteen articles on the WTC’s construction, my impression is that they changed lots of things during the 6 years of construction work and that this was sometimes the result of regulatory amendments (for example regarding asbestos fireproofing). If the core were changed from concrete to steel part of the way up, this would only be one of a long list of such changes. In addition, some buildings which have a concrete core do not have it from top to bottom (like the CN Tower, which is 553m high, but whose concrete core is only 335m high), so it may just have been a question of moving the concrete/steel transition down a few dozen floors, rather than redesigning the towers from scratch while they were under construction.

Therefore, my answer to the question, “Did the towers have a concrete core or not?” is that some of the core was concrete, but some wasn’t. If none of the core were steel, why would there be so many people who think it was?

Judging by the ratio of experts who believe in a steel-only core and those who believe in a core with vertical concrete elements, it would seem that most of the core (i.e. more than 110 floors in the two towers) was steel-only (except the flooring, obviously). My hip shot guess for the number of concrete floors is around 40.

This idea can explain why:
(1) There was “too much explosives” in the towers. The bombers were confused by the conflicting reports and figured they’d use lots of explosives, because they couldn’t work out which floors were concrete and which weren’t.
(2) Why some of the evacuees thought the stairways were concrete – they were in some places, but stairways higher up weren’t.
(3) Why the 1990 documentary contained clips of concrete being poured – it was poured into the core in some places, just not everywhere.
(4) Some experts believe the towers had a concrete core.


By the way, I saw (but can’t now find) a reference to the number of box columns on the outside of the core being doubled at one point from 24, presumably to 48 (47?). This would explain a lot.

By the way II, photos of concrete cores are not common. Just try looking for a photo of the concrete cores of the CN Tower or the Petronas Towers (or other building with a concrete core). Most pictures of skyscrapers are of them majestically silhouetted against the setting sun – it seems that punters are not interested in looking at photos of concrete.

That's not a concrete core in the way Christophera means it. The concrete core theory proposes just that: steel-reinforced concrete, where the concrete is the main load bearing component.

What you describe in your first point is a steel core with concrete elements that are not load bearing. Officially the WTC towers had such a core, and it seems most here believe that.

"That's not a concrete core in the way Christophera means it."
No, it isn't.

"The concrete core theory proposes just that: steel-reinforced concrete, where the concrete is the main load bearing component."
There are various ideas about concrete in the core circulating. This one, for example, is closer to Christopera's:
“Each of the towers, in other words, was held up by its reinforced concrete core and the world's strongest curtain walls.”
http://salwen.com/wtc/
This site is run by the Society for New York History and does not believe the towers were destroyed by explosives. Its info seems to come from the Essential New York by John Tauranac.

"What you describe in your first point is a steel core with concrete elements that are not load bearing. Officially the WTC towers had such a core"
Do you have a link to an official site saying there were vertical concrete elements in the core (which were not load-bearing)?
NIST describes the core on page 62/8 and the only material it mentions is steel. Not a word about concrete.
http://wtc.nist.gov/pubs/NISTNCSTAR1Draft.pdf

No one pays much attention to the construction methods in the basement and lower part of the WTC's because the plane hit high on the structure.

We need to remember that there were seven basement levels, and that the core columns were connected to bedrock. There is a very good chance that part of these were either reinforced concrete construction or concrete covered structural steel columns. No matter what contraction method was used it is a sure bet the steel columns were connected to reinforced concrete piers. At what level those piers ended is a question, but it is the only way I know of to connect the steel to the foundation.

Covering steel with poured concrete is commonly used for fireproofing that is structurally critical, and has a chance of being exposed to fire. In the basement level with loads of machinery it is a good chance that some or most of the steel was fireproofed in this way. The construction method lowered "grade" to the basement levels in pictures I have seen. It is possible those people that speak about concrete cores saw the early stages of construction.

As for the upper levels it is without question steel construction with a spray on fireproofing applied after it was erected.

Where would the transition from concreted core not non-concreted core be? On the ground floor? the first basement? or the mechanical floors (AFAIK 42 + 43 and 75 + 76)? If there was a reduction in the concreting of the core during construction, then the "logical" move would be from the upper two mechanical floors to the lower two mechanical floors.

Even if the concreting ended at the upper mechanical floors, there wasn't any present in the impact zone. I read somewhere that the mechanical floors were supposed to have more concrete than normal floors (as mechanical floors do in other buildings), would this just mean thicker floors, or does it mean vertical concrete elements in the core area?

The extra mass is needed to help dampen the vibrations from all the rotating equipment (fans, pumps, turbines, etc). Usually the equipment is mounted on spring isolators (Mason Industries is one manufacturer) and the extra mass affects the period and amplitude of the resulting vibration. I don't remember forcing functions from DifEq that well but it isn't complicated to figure out. I'll look it up this week if I have time.

It's been mentioned that the Port Authority was not obliged to follow NYC fire and building codes because it was Congressionally chartered (or some damn thing). Locking the roof exits, for example, in violation of every fire code in the country, was technicaly not illlegal. So it's likely that the city building code was modified in 1968 to meet the WTC design, not the other way around.

I don't think they'd mix concrete and steel in the cores because it would be a nightmare to engineer, at least above ground. A 47-column core (an 8x6 grid minus one to accomodate elevator lobbies) seems more likely. Concrete would have been heavier than steel and Robertson and company seem to have been aiming for as lightweight a design as possible. In any case weren't the lateral loads resisted by the perimeter columns, which would make a concrete core unnecessary?

That's a good point. Transitioning from a concrete tube to an array of
steel columns would be quite involved--unless there were a couple of
utility floors and you could devote the non-elevator core space to
bridges that support the inner core columns.

I've always been extremely skeptical of Christophera's photo, but I
can't seem to kill it off logically.

there. Christophera's picture of what looks like a concrete core standing for 14 seconds might fit the part concrete and part steel scenario. Kevin's idea seems very plausible to me.

But note that WTC7 was "rebuilt" very rapidly with a concrete core very much like Christophera's concrete core.

Are people suggesting that WTC7 probably had Christophera's concrete core, but WTC1 and WTC2 had either a combination or mostly steel?

The fact that firemen survived in the lower floors of WTC1 and WTC2 seems to fit concrete core on lower floors.

that if the only concrete in the buildings was in the floors, that virually all of the floor concrete would have been pulverized by pancaking and expeled explosively and very rapidly for many blocks?

and likewise some bigger stuff expeled explosively for several blocks.

Has anyone heard of anyone buiding a model and seeing what happens, even if a large mass falls on a reinforced concrete floor from a distance? What direction would the pulverized concrete be expeled at, and how would this occur?

If one floor was mostly pulverized, what pulverized the next floor?

It doesn't make sense to me and doesn't seem to match what I've seen in the videos.

I'm not talking about some of the quick reference models suggested here (cinder blocks falling from ten feet, etc) but a fairly well-detailed model. I'm talking about a digital model, with correct dimensions (or at least close) and material properties. I think it would have to be created and tested using some version of a freeware tool, because the commercial versions of these tools run in the thousands of dollars (if not tens of thousands) and - more importantly - if the tool is freeware, others in this forum could acquire the model from the originator and verify it.

Unfortunately this is neither an easy nor a quick task.

http://covertoperations.blogspot.com/2005/09/great-must-read-but-loooooong-and-very.html

Basically, the guy calculates that pulverizing the concrete takes WAYYYY more energy than can be accounted for by a gravity-driven collapse model. It requires explosives.

Also, you might want to check out this post:
http://covertoperations.blogspot.com/2005/10/can-hydrocarbon-fires-weaken-steel.html
Here I show that hydrocarbon fires cannot weaken even a flimsy steel structure.

I have done a follow up expt with kerosene and a different steel structure and obtained the same result: hydrocarbon fires cannot significantly weaken steel structures, even when they are damaged.

with the video evidence and the available analyses such as this, I'm convinced at this time that the "collapse" was due to explosions

"It's been mentioned that the Port Authority was not obliged to follow NYC fire and building codes because it was Congressionally chartered."
Yes, that's right. However, NIST says it was built in accordance with the 1968 code, which applied throughout most of the construction period and I'm sure I read somewhere that the Port Authority decided the building had to comply with the code (but I can't find this excerpt now). Perhaps they were ignoring the old code but decided to go along with the new one, but this seems a strange way of doing business to me.

"I don't think they'd mix concrete and steel in the cores because it would be a nightmare to engineer, at least above ground."
Why would it be a nightmare to engineer aboveground? If the basement core was steel, then there had to be transition somewhere.

"In any case weren't the lateral loads resisted by the perimeter columns, which would make a concrete core unnecessary?"
Sorry, I don't understand this point. Why does placing the bit that deals with the wind loading outside the core mean that the core doesn't have to be concrete?

Those are great questions and I hope this does justice to them:

On the codes: it looks like the PA had carte blanche to build whatever they wanted however they pleased, but evidently somebody decided it would look better if the buildings were in compliance with the city building code (maybe out of market considerations), so they amended the code.

Engineering nightmare: what I was thinking is that they had to consider gravity AND wind loads above ground, which would make the calculations unbelievably complicated if they also had to figure in the various separate behaviors of reinforced concrete, steel, and some kind of transitional structure between them.

(Come to think of it, I'm pretty certain that the core columns were steel all the way to the bedrock. I remember seeing a drawing of a steel-to-bedrock column connection somewhere, maybe in one of the FEMA/NIST reports, not that that means anything, but I also remember seeing steel column stubs sticking out of the bathtub floor after the cleanup. I'll try to find a pic.)

Wind loads: my understanding of the concrete core theory is that the concrete would resist lateral loads, but I think it's widely acknowledged that the purpose of all those view-obstructing perimeter columns was to take the wind loads so the cores wouldn't have to, i.e., to make a concrete and/or masonry core unnecessary.


So I agree with your original objection, but not with your conclusion.

I was thinking of something I noticed when I was down there once, after they'd finished cleaning up but before they'd rebuilt the PATH station. You could just barely see the column stubs, which had been sawn off flush with the slab.

Regarding the code, my understanding is that it was going to be amended anyway - many codes in other cities were amended in the same way - from a specifics code (fireproofing must be concrete/asbestos/ whatever) to a performance code (fireproofing must last for 1/2/3 hours). I guess the PA could have started following the new code before it actually came into force (but obviously not before it was drafted). However, if it was PA practice to voluntarily follow the relevant building code, then the original design from the mid-60s should have a concrete core (or should assume one in some way if all the details weren't worked out then). What I'm getting at here is some people (not just Christophera) think there's a concrete core and, if the original plans assumed a concrete core, then this might be a reason why. If there wasn't any concrete in the core, then how come all these people think there is?

Engineering nightmare: AFAIK there are three concrete core beliefs in circulation: Christophera's all-concrete core, concrete poured around the columns (say as fireproofing) and concrete infill panels. Surely, the engineering problems would only apply to the first one? Also, neither concrete infill panels nor a concrete coating around the panels prevent the steel being fixed to the bedrock (which some articles claim).

Like the concrete core, there are differing opinions about the function of the perimeter wall and some articles do say it was to take the wind loads. However, I believe it's more accurate to say that there were sort of rubber squeezy bits in there (the floors) somewhere which kind of contracted and expanded when the wind got up and dampened the swaying of the towers in the wind. The perimeter walls seem to have had some sort of role in this process, but as far as I can tell they had other functions including holding themselves up and ensuring the Yamasaki didn't get vertigo (that's why the windows were so small) when he went up his building. Some articles even claim they were load-bearing! I don't think the wind-damping function that was undoubtedly somewhere in the towers' design precludes the use of vertical concrete elements in the core (or makes it unnecessary given the quality of concrete available).

My two question here are:
(1) If there was no (vertical) concrete in the core, why do so many people think there was?
(2) Why use so much explosives? My previous explanation was that the bombers wanted to take the towers down quickly in the event they became unstable following a terrorist attack at the base of the tower, but perhaps the bombers thought there was a concrete core and so used more explosives than necessary.

and maybe they used some wood in the roof. I'm not saying it's impossible but I can't think of a reason why they'd want to.

I don't think the explosions are determinative because whoever set the charges probably had access to the blueprints, and if they didn't, all they had to do was look around with a flashlight.

I take it you think all the concrete core people are mistaken.

"I don't think the explosions are determinative because whoever set the charges probably had access to the blueprints, and if they didn't, all they had to do was look around with a flashlight."
What do you mean "determinative"? WTC 1 and 2 certainly didn't fall like a normal controlled demolition (like WTC 7). More explosives than necessary seem to have been used. How come? There was absolutely no attempt to hide the squibs.

Blueprints
What are these blueprints? Are they the architects' drawings or the as-built documents or what?

But I haven't seen any evidence of a concrete core, and I've seen plenty of evidence of a steel-column core.

Why do people think there was a concrete core? I'm guessing that (1) right after the attack, when people starting wondering about the WTC construction, there wasn't as much information out there as there is now, so they made a lot of guesses; and (2) I think one of the TV shows like Nova tried to put over the idea that the cores were finished with gypsum board because the builders were too cheap to spring for concrete, or in other words, that the lack of concrete was a sign of shoddy construction, which the core theory disputes.

My guess is that cost was not a consideration, but weight was, since the whole philosophy behind the design was to eliminate as much weight as possible. I also think the steel core was a lot stronger than reinforced concrete would have been anyway.


The explosions: I think the reason they used so many explosives was just to make sure they got the job done, partly for dramatic purposes, and partly to cover their tracks.

"The explosions: I think the reason they used so many explosives was just to make sure they got the job done, partly for dramatic purposes, and partly to cover their tracks."
Getting the job done: If the bombers had access to the plans and were half-way competent, then they should be able to do it with the usual amount of explosives.
Dramatic purposes: The highest buildings in New York fell down in 15 seconds, how much more dramatic could you get?
Covering their tracks: Surely, the large amount of explosives (and the related squibs) don't cover the bombers' tracks, but expose the collapse as being caused by explosives. So what's the point?

Christophera's explanation (that the towers had a concrete core) explains the amount of explosives used, which is why I'm pursuing it. My original explanation for the large amount was that the explosives were there to take the towers down in the event of another truck bomb and may have to have destroyed the towers after they had already been damaged. Christophera's explanation is neater, but is it true?

when you said, "Why use so much explosives? I think the buildings exploded, rather than imploded in the manner of a usual controlled demolition." That's my feeling too: it WASN'T a usual demolition, where the foundations are kicked out and the building falls into the basement (like WTC 7), probably because there was just too much structure and too much likelihood of a tipover. So each floor or group of floors was simply blown apart.

I hadn't heard the "too many explosives" idea before but it seems like they would have every reason to err on the side of too many rather than too few. If the buildings hadn't been completely demolished it would be difficult to cover up all that dynamite in the core.

Frankly I don't think there was anything accidental or unsuccessful about the demolitions themselves. Whether WTC 7 was supposed to get its own plane crash is another question but I personally doubt it, considering it's tricky location.

The method of demolition of the twin towers is a problem for those (myself included) think the three buildings were demolished with explosives. 7 looks like a standard controlled demolition, but the first two don't, so we should advance some sort of explanation for this.

Why put too many explosives in 1 and 2, but not in 7? 7 was the one with the weird structure built over a subway station. 1 and 2 were just tube-in-tube steel (allegedly) towers, albeit big ones. It almost looks as if 1, 2 and 7 were demolished by different teams with different methods.

If the bombers have the plans (and what are they doing bombing the building if they haven't got the plans?) why use more than necessary? All that C4 (or whatever) gives the game away. Why not have the damn things tip over? How can anybody commit the crime of century, but then risk exposure by being too squeamish to do the job properly (by tipping them over sideways, more slowly) and make it look like a "natural" fall?

The best way to do it would be to have one of the towers not collapse completely, then blow it up the next day as a safety hazard, thereby explaining the presence of signs of explosives in the wreckage. Hell, if there were only partial collapses of the twin towers from the impact zones, nobody would have doubted it was genuine for a second.

WTC 1 and 2 were a quarter mile high. That's 110 stories of steel to grind into haulable chunks in just a few seconds. Gravity couldn't do that, and a sideways tip might have destroyed some very profitable real estate, like the World Financial Center, which is privately owned, unlike the WTC and Pentagon, which are gov't property.


WFC 3, 4, 2, and 1

"That's 110 stories of steel to grind into haulable chunks in just a few seconds. Gravity couldn't do that,"
It seems to me that there were only explosives every 10 or so floors, so it wasn't the explosives that snapped the steel into haulable chunks - the columns just snapped "naturally" on the way down (except for a few that were broken by explosives). The steel obviously came to the WTC in haulable chunks and it went out in haulable chunks, I really don't see anything odd about this: the columns should snap on the way down and they should snap at the weakest part, which is the join. Also, to snap steel into chunks you don't need a few big explosions, but lots of little ones (every three floors or so).

"a sideways tip might have destroyed some very profitable real estate"
Very good point and I'm sure this is important. However, the large amount of explosives resulted in debris being thrown out of the building's footprint and damage to adjacent buildings. A more conventional controlled demolition with less explosives would have been better from this viewpoint.

Being a newbie, I couldn't start a new thread, so I had to pick which of the several concrete core threads I wanted to respond to. I put together a rebuttal to the concrete core theory in reply #12 to this thread:
http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=125x56852

I think there's a lot of supporting information in that reply that shows that the WTC 1 and 2 towers were built entirely of structural steel and did not use concrete as a structural element.

A really interesting and thorough analysis. Thanks for clearing that up!

that is a nice, thorough and well written argument, and I appreciate that you don't at any point drag the debate down on a personal level.

I have a few questions,

1. Is it normal that the drawings are kept in the buildings? Seems like a practice that would prohibit research in building performance in extreme situations.

2. What is typical strengths of the type of weld you use in core columns. I mean I realise they are not one piece of steel but I was under the impression that the welds would not be that much weaker than the steel. Most of the columns would likely not be heated significantly even though heat was transported around.

3. Just outta curiosity, Do you believe the towers were brought down on purpose? Feel free to ignore this question if you don't feel like broadening the debate beyond the concrete core question at this time.

Thanks. And a few answers.

1. I don't know if it's "normal" to keep architectural drawings in the building. But, I wouldn't go so far as to claim it's unusual either. These were complex buildings with many unique elements to the design and construction. As such, I think it would be expected to have drawings available for review. Face it - no one individual is going to know earthing about the electrical, plumbing, HVAC, steam, fire protection, data communications and other subsystems. Both towers totaled about 200 acres of rentable space that were customized by the various tenants, and by a large number of tenants over the years. They only way to know what went where is through drawings and other paperwork.

A similar question could be placed to anyone who was a sailor on a submarine or aircraft carrier and worked in the engineering departments. Do you think they had drawings of the equipment and how everything worked? I would presume that they do.

2. Not a clue about welding and strengths.

3. Personally, I do not believe that the towers were destroyed on purpose. I do think that the collapses of WTC 1 & 2 were a "luck of the draw" events that were unforeseen by whomever hijacked the planes. And yes, I do believe that four commercial jets were hijacked and flown into the buildings, or were intended to be flown into a building.

I base these statements based on my personal background. At one time, I had a pilots license. Just your basic Single Engine Land, but I am a plane junkie. Something flies overhead and I look for it. If there's an aviation museum where I'm traveling, I'll go visit. My father had planes when I was growing up, so flying was fun. Dad also sold hydraulic equipment to the Lockheed GA plant where the C-130, C-141, C-5 were built. He would have drawings on the kitchen table for these aircraft. A neighbor five houses away was a primary test pilot on the C-5 program. I've flown into Oshkosh a couple of times. I've seen plane crashes and the explosions when the gas caught on fire. I live underneath one of the four major approach paths into the Atlanta airport and easily identify aircraft type and airline as they're crossing overhead. I read NTSB accident investigation reports for fun.

All that is to say that from what I've seen, the two planes that hit WTC appeared to be real. WTC 1 had the worse quality video, but it looked like a heavy commercial jet. It sounded like one too. That doesn't preclude that it was a purloined military version of the same type, but I have a real hard time believing those theories. Who knows, I may be wrong.

WTC 2 had a lot better video. There's one video sequence where the jet comes out of the horizon towards the camera. What most people don't notice is the amount of maneuvering that is done in those final seconds. The plane descends, it rolls, all with realistic motions. It looks to me that the pilot came real close to missing the building entirely, and only lined things up at the last second. I've studied the photos that argue that there was a "pod" attached to that plane, and come to the conclusion that people are seeing what they want to see. The underside of that plane had an unusual paint job, in particular a stripe down the centerline of the belly of the aircraft. Good pictures showing the underbelly stripe are at:
http://www.airliners.net/open.file/780842/M/ and
http://www.airliners.net/open.file/699766/L/

I really think that the pod and pipes that some people claim to see are optical illusions due to the paint job, speed of the airplane, and poor resolution of video. I also think some of the analysis of that airplane are wrong because they don't take into consideration the bending of the wings as the final roll is completed. You're welcome to disagree.

For larger facilities it is important to keep a set of building plans up-to-date (known as the "as-built" set) and on-site for use as a reference both to facilities management, consultants and contractors. Otherwise it would require re-visiting the area for a proposed change and documenting everything (a momentous task). I have run into problems before where the building systems didn't match up to the "as-built" set and I had to do an extensive examination in order to determine what would be affected by the proposed changes (and the intermediate construction).

I guess I meant is it normal that the building plans are lost in natural disasters?

I realise there would likely be drawings present in the building, but it seems very weird that no copy can be produced for the public to see.

In most (at least western) countries, this would be public information.

The plans aren't public information at all - they are owned by the person who stamped them. There is usually more than one set, but the ones kept at the site are the most up-to-date. The government can't choose to release the information to the public because the government doesn't have that right.

(1) One of the things I am trying to do is to understand why these various people think there was a concrete core. One of my suggestions is that the original intention (say in 1964) was to build using a concrete core and a set of plans with a concrete core was prepared, which would account for some of the confusion. Whilst the building code revision would be known months (possibly even a year or two in advance), you're stretching it to suggest it would be known almost 5 years beforehand. Obviously, the PANYNJ could start following the new code before it came into force, as it wasn't binding on them anyway.

(2) "You have to coordinate the disparate material properties in such a way that costs shoot up tremendously. It would be much cheaper to use steel in both the exterior and interior, using one major set of trades for the construction work."
That's true, but there were hundreds of tons of concrete in the building anyway (in the floors), so the argument isn't particularly convincing.

(3) As for the drawings, they and your comments on them are interesting, but, if there were a concrete core, they could be for floors above where the concrete stopped.

(4) "The new code required that the steel should stand fire temperatures for certain periods of time. Thus, spray on fireproofing came into use."
Actually, I think they first used asbestos, but that went out during construction and the south tower was finished with spray on. AFAIK the non-asbestos fireproofing in the north tower dates from a refit.

(5) Many/all the reports of concrete stairwells come from lower floors, whereas all the reports of breaking through the walls come from the impact zones - above the second pair of technical floors. Just because one floor has a concrete wall, doesn't mean that another does and vice versa. Concrete and plasterboard walls can look the same (provided they've been plastered, for example), but you can tell the difference by knocking. Why would people specify the walls were concrete, if they weren't? Do we know how the escape routes were surfaced finished? Painted? Plastered? Millions were spent on the emergency escape routes after the 1993 bombing.

Postscript:
Most articles, including those by Robertson and the architectural team, talk about the WTC as having a steel core and I find it hard to believe they're shitting us. Maybe sometimes building designers and contractors make up stuff about what they build (i.e. building not built to standard to save on costs), but I doubt this in the case of the WTC (at least I have a hard time beleiving they invented a ficticious core). However, if there was some concrete in the core (less than 50%), they would have a reason to ignore this, as the majority steel part of the core would be the most important bit.

It looks pretty certain that the core columns were steel from bedrock to roof, but as you say, that doesn't rule out the possibility of some additional structural concrete on the lower floors.

On the upper levels, though, it seems unlikely, especially since this plan appears to show some of the core being used as rentable floor space:



Too bad the photo cuts off the floor number. Still, it's hard to see how the whole question amounts to much.

Here are partial drawings for two of the basement level floors:




They are from this document: http://wtc.nist.gov/pubs/NISTNCSTAR1-1C.pdf   (pdf pages 153 and 154)

Now compare the core column layout and numbering to this floor diagram from the same pdf file (pdf page 95):



The layout and numbering look the same to me. (The resolution is slightly better in the pdf file. Also, it is easier to read the column numbers on the drawing found on pdf page 90. Additionally, throughout the document there are many text references to the column numbering.)
____________________

And I'd also like to add this picture:

-Make7

in the above-ground floors than the NIST narrative mentions, and a heck of a lot more than the TV shows mention.

Funny how that plan also cuts off the floor number(s).

The two basement pictures only show the southeast corner of the WTC 1 basement (presumably they were done or annotated after the 1993 bombing), but provide a very good match with the upper floor. I think this is evidence of the same columns being used all the way up, as is often stated, from the bedrock to the top. However, there's nothing there that specifically says that the columns weren't encased in concrete, at least to some point.

It strikes me that it would be unusual to have plasterboard walls in a parking garage. Most garages I've been in have concrete walls, to discourage drivers from crashing into them. Maybe this is where the "infill panels" idea comes from.

I don't recall any reference to the vertical core columns being encased in concrete (although they very well might have been), but there is this reference to horizontal beams in the core using concrete for fireproofing:


And there are references to concrete walls in the section about the damage from the 1993 bombing:


I think it is safe to assume the walls in the basement were concrete. And since this early phase of the construction would be more visible to passersby, I agree that this may be where some of the "concrete core" and "concrete infill panels" information got started after being repeated enough times to cause misinterpretation. Maybe not.

-Make7

I'm no engineer, but does the construction of the core REALLY prove anything one way or another? Why is this an issue?

Personally I think a concrete core would be more likely to produce a "progressive collapse" than steel, but it seems virtually impossible in either case, at least as a result of the 9/11 plane crashes.

We've invested well over 1000 posts on this and I really don't know what difference it makes.

things:

1. There's been a major coverup involving, as Christophera alleges,
a scrub of construction photos and presumably silencing people
including Leslie Robertson, who should know how it was built.

2. IMHO the photo, if genuine, proves explosives, because a concrete
core like that if still standing after all the steel was stripped off
it would not then simply collapse for no reason. And even if it did,
it would have sprinkled the top of the pile with its shattered pieces
like jimmies on ice cream. If it had a 55-story concrete core, then
even if the steel skeleton collapsed, the core should have remained.

It is interesting, though, with all the bytes we put into the WTC
demolition theories, how little energy we've put into Mohammed Atta and
his playmates in Venice FL, the owners of that flight school where Atta
and Marwan al-Shehhi trained, etc.

Generally, the people who think the WTC was destroyed with explosives think that far too many explosives were used. If the core were concrete, however, the amount of explosives would be about right. So what I'm looking for here is primarily a reason for the amount of explosives - the core was concrete or the bombers (may have) mistakenly believed that the core was concrete.