The 9/11 Forum

Aside from the columns, core and facade... and those trusses and the hat truss, what do we know about the other steel... mostly in the core?

What sort of lateral bracing was there?

Was there any diagonal bracing aside from the lower sections of the core?

What were to splices between columns, especially when they changed from one section to another?

Were the core columns aligned on center in each axis? If not why not and what is the implication?

Were the facade corners acting as columns(supporting floor loads) or gussets between adjacent facades? Why were they only 24' tall? Why did only alternate floors have a center vertical bar?

How important are the above details in understanding the failure of the structure?

That is a major sore point in this entire issue.

Here is Lon Waters' site:
http://wtcmodel.wikidot.com/start

Search it and see what you can find about horizontal beams connecting core columns.

Very curious considering how much info he has about the columns.

psik

Thanks, I am familiar with that site and have found that there is a paucity of information about anything except the trusses, the perimeter columns and the core columns.

We can't do any serious forensic analysis of the structure with ONLY the columns and the floor trusses.

SanderO wrote:We can't do any serious forensic analysis of the structure with ONLY the columns and the floor trusses.

NO SHIT! And almost NINE YEARS after the event!!!

psik

SanderO wrote:Thanks, I am familiar with that site and have found that there is a paucity of information about anything except the trusses, the perimeter columns and the core columns.

We can't do any serious forensic analysis of the structure with ONLY the columns and the floor trusses.

If the information was available Lon Waters would have it posted on his site. However, the
NIST report is quite silent about the horizontal beam sizes and their connections to the columns in the core and there has been no separate release of this information, although it has most certainly been asked for via FOIA.

I have been in back and forths on other sites about diagonal bracing in the core which I believe was only in the first 10 stories and others claim the core was full of it. I can't see it in any photos or on the plans it would show and it doesn't. We do have hat truss information.

Why do you support they are withholding the structural plans?

Is it possible that there were inherent flaws in the design concept which allowed for the progressive collapse once the floors got overloaded? What was the load at which a single floor would collapse locally onto the one below?

SanderO wrote:I have been in back and forths on other sites about diagonal bracing in the core which I believe was only in the first 10 stories and others claim the core was full of it. I can't see it in any photos or on the plans it would show and it doesn't. We do have hat truss information.

Why do you support they are withholding the structural plans?

Is it possible that there were inherent flaws in the design concept which allowed for the progressive collapse once the floors got overloaded? What was the load at which a single floor would collapse locally onto the one below?

I don't see diagonal bracing above the first few floors in any photos either and I don't believe it was used above there.

I think structural details are being withheld concerning the connections of the lateral bracing in the core as they were probably very robust moment resisting connections, which would not easily fail. The core was actually a self supporting grid structure.

I think the floors outside of the core would fail once overloaded. However, these floor assemblies were significantly more robust than many realize. The NIST report shows the factor of safety of the floor to column connections for the outside of the core office areas to be about 12 to 1 and the double main trusses could handle about twelve times the load they had on them, plus there were transverse trusses. I think it would take at least about six or seven floors from above impacting a lower floor to cause it to sever and fall. That is not going to happen early in the collapse and the notion by some so-called debunkers that the entire upper section got misaligned and landed on the floors is impossible with the inertia of the upper section.

Tony,

We looked into floor connection failure in terms of energy to shear the connections a while back, and the outcome was around the 70MJ mark.

Do you agree with that figure ?

If not, what value would you place upon it ?

Tony,

Are you saying the weakest link in the floor system had a FOS of 12?

The core may have been self supporting up to a point. Without diagonals it would not resist wind loads. It obviously was quite stiff... especially in the long axis which is probably why the box columns had very long flanges oriented in the short direction to provide stiffness in that axis.

The longest surviving lateral supports in the core were the ones in the short axis (87') between rows 500 and 600. There were no floor loads on those that survived in the "spire" from my observations.

SanderO wrote:I have been in back and forths on other sites about diagonal bracing in the core which I believe was only in the first 10 stories and others claim the core was full of it. I can't see it in any photos or on the plans it would show and it doesn't. We do have hat truss information.

Why do you support they are withholding the structural plans?

I noticed that the core columns immediately above ground level were thicker and therefore heavier than the core columns in the basement. At first I thought it had to be some kind of mistake. But the columns in the basement only had to deal with gravity load. The columns immediately above ground must deal with the sway of the building which can go on for hours or days and they would have to do it for as long as the buildings were supposed to last. The Empire State Building will be 80 years old next year and it will be TEN YEARS since the destruction of the WTC.

I am of the opinion that it is IMPOSSIBLE for any skyscraper that could hold itself up for ten years and withstand the wind to have collapsed like that. The structural diagrams don't really matter because the endurance would have proved that the design was adequate. People are trying to rationalize the impossible after the fact but accurate information would just show how IMPOSSIBLE IT WAS. So the facts must remain obfuscated.

This is now an emotional propaganda issue not a Newtonian physics issue. so how do our schools do a competent job of teaching Newtonian physics for the next 100, 200, 1000 years.

Consider how funny it is that the Empire State Building was designed without computers but we haven't resolved this 40 years after the Moon landing with all of the computing power we have today. Grade school kids all over the world should be laughing at the United States.

psik

The Empire State building was a completely different design concept. It's base was much broader and it had steps/ set backs for one and it had columns every 25' or so in a grid. it was also clad in masonry. The ESB design could not have a runaway floor collapse because of the "dense" framing". And it was probably over designed to boot.

The twins were a clever design tube inside of a tube with the inner tube a 3 D grid of steel. The outer tube did the work of wind shear with some help from the core damping the motion through the floor system. But the twin's design was vulnerable to a progressive collapse of the floors. The fast and cheap assembly using pre fab components of the same design for every floor method was ultimately the undoing. Even with the large FOS Tony mentions for the floors, if that was exceeded on any floor confined to even one area, this would cause a local collapse of that area right to the ground. There was little to arrest it and it might even propagate around the open floors. No one expected those sorts of loads on these floors outside the core.

It collapsed "like that" not because the columns could not support the loads, but because the floors could not support the excess loads... loads they were not designed for.

The facade columns fell (away) not because they were crushed... though some were.. but because they could not stand without the floors to brace them laterally. And the core below the destruction... the cold undamaged standing part was subjected to a similar assault of loads landing on beams which were not capable of supporting those loads. These were the lateral supports for the core columns and so the core columns were stripped of lateral support as well and what remained was too tall and thin (very high aspect ratio) to be stable.

You don't think if you set out to build even the strongest column line - 501, for example it could stand alone 110 stories or even 80 stories without any lateral bracing? That's why radio towers have guys and stays to hold them "in column".

The towers would stand for 80 years if their floors up top were not subjected to 10 or 20 times their design loads without consideration of the kinetic energy of those loads dropping on them.

Open long span floors supported as those in the twins were is a vulnerable design.... much more so that the traditional floor framing system used in the ESB.

SanderO wrote:The facade columns fell (away) not because they were crushed... though some were.. but because they could not stand without the floors to brace them laterally. And the core below the destruction... the cold undamaged standing part was subjected to a similar assault of loads landing on beams which were not capable of supporting those loads. These were the lateral supports for the core columns and so the core columns were stripped of lateral support as well and what remained was too tall and thin (very high aspect ratio) to be stable.

You don't think if you set out to build even the strongest column line - 501, for example it could stand alone 110 stories or even 80 stories without any lateral bracing? That's why radio towers have guys and stays to hold them "in column".

The core did not need the lateral support from the floors outside of it. Self-support is a function of being able to take both the compressive loads and buckling loads. The important features of self supporting structures are the area, large moment of inertia, and low slenderness ratio of the combined columns and their interconnection, to withstand their own load. The compressive stress on the central core columns was about 11,000 psi and they were about 60% 36 ksi yield strength steel and 40% 42 ksi yield strength steel columns, which means they had a factor of safety against compressive rupture of at least 3 to 1. As a unit the core of the twin towers had a fairly large moment of inertia and quite low slenderness ratio allowing it to take much more than 3 times its load before buckling would occur. Theoretically if one were to increase the load on the central core it would have failed due to compressive rupture long before buckling.

Guyed radio towers are designed to take the compressive stress but do not have a large moment of inertia or low slenderness ratio and are not capable of resisting buckling at their height and thus need to be guyed.

I agree that what remained of the core after the collapse front had passed did not have a high enough moment of inertia for its height and had a high slenderness ratio, which would not allow it to support its own weight and thus it was unstable and buckled. However, it can be shown that the intact core had hundreds of times the moment of inertia of the remnant and a vastly lower slenderness ratio.

I think the core alone was certainly self supporting but might have required diagonals to deal with the wind as it would bend / flex in high winds and that could stress the connections and cause failures perhaps.

My reference to stays is that high aspect and very tall thin structures need to deal with the wind loads which can be quite substantial. The windward guys are in tension to resist the wind loads. N'est pas?

I believe the core was intended to provide additional stiffness and resistance to sway in the twins which would have made them uncomfortable without it and the damping of the vaso elastic compensators / connectors on the bottom chord of the floor trusses.

I thought I read that tower 1 swayed something like 12 feet at the top in response to the plane strike. Is this true and how was this determined?

SanderO wrote:The twins were a clever design tube inside of a tube with the inner tube a 3 D grid of steel. The outer tube did the work of wind shear with some help from the core damping the motion through the floor system. But the twin's design was vulnerable to a progressive collapse of the floors. The fast and cheap assembly using pre fab components of the same design for every floor method was ultimately the undoing. Even with the large FOS Tony mentions for the floors, if that was exceeded on any floor confined to even one area, this would cause a local collapse of that area right to the ground. There was little to arrest it and it might even propagate around the open floors. No one expected those sorts of loads on these floors outside the core.

I am fully aware that the the WTC was a tube-in-tube design and that the ESB was not.

I also cosider that attempt to blame the collapse on that design difference to be a bunch of rubbish. It is used to avoid the the that it is obvious from watching the videos that other energy sources beside gravity had to be involved in that DESTRUCTION it was not really a collapse. A collapse could not have hurled so much material from the north tower to the Winter Garden.

But the ESB had to hold itself up just like the WTC and they did not have electronic computers in 1931. I have not found a level by level specification of the TONS of STEEL and TONS of CONCRETE on any skyscraper in the world and yet every one has to cope with the same gravity. We are being given the silent treatment by all of the people in the skyscraper business. I got a LAME excuse from Richard Gage when I asked him about it.

I have never even seen anyone compute how much the south tower had to deflect at the 81st floor due to the impact. Since the NIST says it moved a 12 +/- 1 inches at the 70th floor that extrapolates to 15 inches at the 81st. So why doesn't experts want to know the distribution of mass to compute how much of the planes kinetic energy that took. It doesn't seem like too complicated an idea to me. How could they make a reasonable guess bout core damage without that information?

Since my computer Python program gives about a 12 second collapse with no structural resistance the core alone should have increased collapse time to considerably more than that.

psik

The debris outside the towers - the facade and the aluminum cladding largely fell over and was not expelled explosively. Some columns perhaps sprung off at the beginning of the collapse, but the max speed I calculated for the WFC debris was 34.4 mph.

The floors collapse did not involve the core which stood a bit longer and finally fell. The floors collapse was about 14 seconds.

When half the tower had collapsed over 200,000 tons of debris were what was collapsing through the floors. something like 10,000 pounds per foot.