Floor connection question

[leftysergeant]

I have been trying to model the manner in which the floor slabs in the towers failed and in what attitude they descened to the next slab. It seems obvious to me that they would NOT fall flat, because one end or the other would become detached first, thus allowing that end to strike the next floor first.

I have seen photographs of the seats in the perimeter columns to which the floor trusses were secured, and can draw some comclusions from the shape thay were left in, but I have not seen what the connections to the core columns looked like, post-collapse.

Has anybody looked at pics of those connections and reached any conclusions from them?

 

[MG1962]

I think the first couple of floors acted as you mentioned. Once things got rolling though, the speed energy of the fall pretty much caused each floor to fail across its area vitually simultaniously.

 

[sheeplesnshills]

I have been trying to model the manner in which the floor slabs in the towers failed and in what attitude they descened to the next slab. It seems obvious to me that they would NOT fall flat, because one end or the other would become detached first, thus allowing that end to strike the next floor first.

I have seen photographs of the seats in the perimeter columns to which the floor trusses were secured, and can draw some comclusions from the shape thay were left in, but I have not seen what the connections to the core columns looked like, post-collapse.

Has anybody looked at pics of those connections and reached any conclusions from them?

Given the impact it is likely large section of at least two floors would have been heavily damaged and collapsed on to the floor below dangerously overloading it. It is possible that fire on the floor below would have softened the floor trusses enough in one point that the floor failed at that one point and the failure progressed sideways from that point zippering around the perimeter and that entire floor would have fallen on the one below triggering the same failure. The exterior columns would then have failed in bucking or through failure of the joints (design of these was poor in hindsight) and the top started falling.
Even if the initial floor collapse had arrested, the weight of the floors above would have restarted the process. The static mass of each floor would quickly have exceeded the design load and given that the loading would have been dynamic even one floor collapsing would likely trigger a progressive failure.

 

[paloalto]

re: the failure of the floor slabs

These 6,000 ton floors slabs were held up by floor trusses connected to the outside columns by truss keys, welded to 4 ½ foot high spandrels that were then welded to the outside columns. Each floor truss was 3 to 3 1/2 feet tall made by welding wire rope, 1 1/8th inch steel bars, between plates at the top and bottom of this plate or web truss. The floor trusses were bolted onto the truss keys. These trusses had the vermiculite fire proofing knocked off by the force of the aircraft collisions.

These trusses depended of the wire rope or the 1 1/8th inch steel bars staying intact and not becoming deformed. But these bars were deformed by the floor fires from the flammable material on each floor, which then caused the web trusses to sag, and this sag pulled the outside columns into the buildings or completely break away the floor trusses from the outside columns. Once the floors started to sag and pulled the outside columns into the build the compression force on the outside columns exacerbated this sag and the ultimate collapse of the outside columns.

Once a floor broke away from its truss keys, it would fall down onto the next floor. The floor trusses for any entire floor were designed to hold between 18,000 and 30,000, tons. Since each floor weighed 6,000 tons, and was carried down by gravity as the floor trusses collapsed, when it hit the next floor below, its force was much greater than 6,000 tons. With the fact that the fall of the floors was in no way symmetric, one or two falling floors would bring the whole building down. The truss keys broke away like unzipping a zipper, because of the asymmetric nature of the collapse of the floors. The vertical integrity of these buildings and outside columns depended on the horizontal integrity of the very long floor trusses, and the floor trusses depended on the vertical integrity of the wire rope.

These builds were death traps from the very time they were first built and were just waiting for a tragedy such as the hijacked aircraft to bring them down.

Why the New York City officials ever allowed these buildings to be build in the first place is the real question.

 

[leftysergeant]

What I was getting at is that I thought one end might have failed first, thus putting the debris falling from above on a ramp to collide with the perimeter columns (if the outer end failed first,) or with the cores (if the inner end failed first.)

If the outer end failed first, this would also subject the cores to a horrendous pounding as debris slammed floor pans flat against them, thus further stressing and breaking cross-members in the cores.

That so few objects outside the footprint appear to be floor pans, I would think that the outer ends failed first, otherwise they would have been pulled ouward as perimeter columns fell away or blown out by the expanding mass of debris inside the perimeters.

 

[Sam.I.Am]

Why the New York City officials ever allowed these buildings to be build in the first place is the real question.

1. The NY/NJPA was allowed to bypass NYC building codes. They could've built it out of Legos and there wasn't a thing that the city could do about it.

2. Hindsight is 20/20.

All that aside I don't think that the vulnerabilities were glaringly obvious, otherwise I don't think that the people doing it would've done it. This line of thought also might tie into why NIST has not released some of its data. They might have discovered some previously unidentified weaknesses that are present in other structures and don't want to promulgate them so that some other group of terrorists can use them against potential targets in the future. These weaknesses might not be vulnerable to what I'd loosely call "Normal" conditions (office fires, earthquakes etc.) but might be exploited if specifically targeted.

Everything has its Achilles heel(s) and the more complex the "Thing" is the more likely it is to not be able to protect it (or them) and prevent intentional damage. There are things that we build that are well protected against just about every conceivable scenario but they are inherently expensive to build and maintain and so are not the norm.

 

[leftysergeant]

I am still kind of puzzled as to why they need that much uncluttered open space in a room full of cubicles. Seems to me that a few extra load-bearing columns would have made a bit more sense.

 

[DGM]

I am still kind of puzzled as to why they need that much uncluttered open space in a room full of cubicles. Seems to me that a few extra load-bearing columns would have made a bit more sense.

Simple answer = $

 

[Justin39640]

I am still kind of puzzled as to why they need that much uncluttered open space in a room full of cubicles. Seems to me that a few extra load-bearing columns would have made a bit more sense.

Simple answer = $

That is the answer. They were designed to have the maximum rentable office space per floor. It also made it very convenient to tenants that they would have a lot of flexibility in how they could design their office space.

 

[Sam.I.Am]

I am still kind of puzzled as to why they need that much uncluttered open space in a room full of cubicles. Seems to me that a few extra load-bearing columns would have made a bit more sense.

Not all floors were 100% cubicle farms. Some were divided into offices and conference rooms or whatever with partition walls. Some were a mixture of offices and cubicles. Some even had high end offices (material-wise) for executives.

The only constant on all of the floors were the outer diameters and their window sizes and locations, the flooring system design (on the office floors anyway) and the location of services in the core. The layout after that was pretty much at the discretion of the tenants (within reason or course).

Less vertical columns in the way of those tenants choices meant more flexibility and more money. After all who's going to rent a building with a column right in the middle of the conference table? Someone will, but not for as much money.

 

[Reactor drone]

I have been trying to model the manner in which the floor slabs in the towers failed and in what attitude they descened to the next slab. It seems obvious to me that they would NOT fall flat, because one end or the other would become detached first, thus allowing that end to strike the next floor first.

I have seen photographs of the seats in the perimeter columns to which the floor trusses were secured, and can draw some comclusions from the shape thay were left in, but I have not seen what the connections to the core columns looked like, post-collapse.

Has anybody looked at pics of those connections and reached any conclusions from them?

NCSTAR 1-3B fig A-11 shows sample C-129 (page 81, near the end) that is a piece of channel that the connected the trusses to the core columns. I'd guess the failure of the interior connections would have the same variety as the exterior with the added possibility of the connecting channel being smashed off the columns.

 

[njslim]

Fewer columns = more rentable floor space

Also allows unobstructed access and permits tenants to subdivide space as want

 

[Reactor drone]

NCSTAR 1-3C section 4.2 from page 199-209 gives more examples and a summary of the damage types on the recovered core channels.

Table 4-2 gives the summary.