OOS Collapse Propagation Model

[JSanderO]

Live Load Reduction is a very well understood statistics based analysis and is allowed by both the modern code and the code the WTC was designed to. They don't need scare quotes around them.

I am no code expert and I don't know what the climate was about this when the towers were designed back in the late 60s. The code I believe has been revised since then. Although the anticipated live loads may have been less than the NYC code required... the net result of this was floor slabs which were essentially weaker and this may have been one of the reasons that that they shattered and collapsed so easily as they did.. not to mention how they were *anchored* to the steel structure.

Obviously even a 100# floor will not fare well with several floors and contents dropping on them. And isn't it true that there were several floor re-inforcements done because of service loads exceeding the reduced / approved load design?

 

[Major_Tom]

How was that a response to my post, Major_Tom?

There are two distinct ways to view the physics of a ROOSD-type process, as a progressor or as an initiator.




Obviously, this structure is composed of 3 main components:

The perimeter
The core
Flooring


The initiation of collapse, in general, can be classified as

1) Perimeter failure initiated
2) Core failure initiated
3) Floor slab failure initiated



One would think that the NIST would carefully try to find out which of the three cases actually occurred. The ROOSD process is vital to understanding the collapse progressions of WTC1 and 2, but it is also vital to understanding the initiation sequence.

Progressive floor collapse initiated by failed floor slabs has always been one of the 3 basic logical possibilities for the initial failures of WTC1 and 2.

 

[JSanderO]

The initiation of collapse, in general, can be classified as

1) Perimeter failure initiated
2) Core failure initiated
3) Floor slab failure initiated

One would think that the NIST would carefully try to find out which of the three cases actually occurred. The ROOSD process is vital to understanding the collapse progressions of WTC1 and 2, but it is also vital to understanding the initiation sequence.

Progressive floor collapse initiated by failed floor slabs has always been one of the 3 basic logical possibilities for the initial failures of WTC1 and 2.

The initiation process was of different durations in each tower and they had different mech damage from the planes. It possible that there were MULTIPLE initiators working synergystically but the release appears to include:

miss alignment of multiple columns lines
loss of load paths
warping of frame
collapse of floor and contents onto intact floors

 

[Newtons Bit]

I am no code expert and I don't know what the climate was about this when the towers were designed back in the late 60s. The code I believe has been revised since then. Although the anticipated live loads may have been less than the NYC code required... the net result of this was floor slabs which were essentially weaker and this may have been one of the reasons that that they shattered and collapsed so easily as they did.. not to mention how they were *anchored* to the steel structure.

The modern design live load for an office is 50psf. The designers used a higher number on the WTC to allow tenants to have different configurations for their spaces.

Obviously even a 100# floor will not fare well with several floors and contents dropping on them. And isn't it true that there were several floor re-inforcements done because of service loads exceeding the reduced / approved load design?

Very few floors will fare well with several floors falling on them. Most buildings will experience a catastrophic collapse in this situation (see Ronan Point or L'Ambiance Plaza for examples).

Some of the floors on the WTC were reinforced to accommodate tenant requirements that exceeded the design. Things like high-density file storage system are heavy. Again, I'd recommend actually reading the NIST report instead of just making up like you always do.

 

[Newtons Bit]

snipped

Please try to focus on the points expressed in this post (and the quoted portion): http://www.internationalskeptics.com/forums/showpost.php?p=10222634&postcount=2476

No one is interested in the constant spam you revert to when frightened off a particular point.

 

[1stClassAlan]

I work at a TV station and its a very old building so there's not much opportunity to run cables in walls(plaster & lathe). We then run them above the drop ceiling tiles but that requires entry/exit holes in the tiles. Last time we renovated some offices had cables removed but the ceiling tiles remained as is. Fire Dept came and did an inspection (voluntary on our part) and recommended replacing all tiles with holes in them.

Oh I'm glad you posted that! I'm sure your codes are roughly the same as over here - and any fire rated suspended ceiling has to be complete - i.e. no holes; however; as you mentioned that to get cables around the place you ran them above the ceiling - I bet that there's nothing holding the tiles down!
Your average Armstrong type lay-in grid acoustic system can give FR30 (30mins Fire Resistance) but only if there are hold down clips fitted around each tile on all four sides - this inhibits fire draft or draw from lifting them. Ceilings without clips are merely cosmetic.

Over here there are also specifications for the total area above the ceiling at FR30 without further fire barrier work - many fire officers will insist on structure to structure compartmentation for enclosures (rooms) containing sources of heat, electrical equipment etc., etc. and I'm very glad they do as providing this sort of thing to businesses helped me put my daughters through private schooling!

Frankly would't make a big difference to fire spread IMHO. Its a 3 storey masonry exterior and timber interior built in 1919. If there were a fire here , the masonry exterior walls would very quickly become one of the biggest chimneys in the world. Our fire drills have everyone out in 90 seconds including fire wardens. IMHO that gives us a buffer of about 8:30 before the building would become a funeral pyre for anyone still inside.

You are so right! I've taken part in fire tests at both building research centres here - ironically, to show the benefits of plasterboard partition systems! Old timber partitioned offices burn like tinder - a blaze in a waste basket becomes an out of control raging hurricane in less than 90secs.

 

[1stClassAlan]

The towers did have an extensive grid system that redistributed the loads very well. An open grid needs to redistribute the loads through not so optimal paths. I think the ESB would have failed immediately on impact.

I firmly with you on this one now! From the very first viewings of the collapses, I and my discussion group here pointed the wagging finger of doubt at the floor to column connectors ( as we all agreed that the walls parted company with the trusses at the outer end first) there was also some derogatory remarks about the buildings in general - BUT the more I looked into the way they were designed and constructed - the more impressed I became. I still think that they were thinned down beyond the bounds of reason to build so tall and the fires spread upwards far, far to quickly - but nevertheless - they both stood being hit by heavy aeroplanes deliberately flown into them.

Despite what I've said about the ESB's better fire resistance - I too think that it would not have survived the initial impact as its birdcage steekwork does not have the load redistribution capacity the Towers did.

 

[DGM]

Despite what I've said about the ESB's better fire resistance - I too think that it would not have survived the initial impact as its birdcage steekwork does not have the load redistribution capacity the Towers did.

I think more of the ESB would have remained standing but, the point would be moot. It would have been a hollowed out shell (no floors remaining)

 

[jaydeehess]

Oh I'm glad you posted that! I'm sure your codes are roughly the same as over here - and any fire rated suspended ceiling has to be complete - i.e. no holes; however; as you mentioned that to get cables around the place you ran them above the ceiling - I bet that there's nothing holding the tiles down!
Your average Armstrong type lay-in grid acoustic system can give FR30 (30mins Fire Resistance) but only if there are hold down clips fitted around each tile on all four sides - this inhibits fire draft or draw from lifting them. Ceilings without clips are merely cosmetic.

Over here there are also specifications for the total area above the ceiling at FR30 without further fire barrier work - many fire officers will insist on structure to structure compartmentation for enclosures (rooms) containing sources of heat, electrical equipment etc., etc. and I'm very glad they do as providing this sort of thing to businesses helped me put my daughters through private schooling!



You are so right! I've taken part in fire tests at both building research centres here - ironically, to show the benefits of plasterboard partition systems! Old timber partitioned offices burn like tinder - a blaze in a waste basket becomes an out of control raging hurricane in less than 90secs.

Both rooms with equipment racks are in corner rooms, same location on first and second floor, making at least two walls the outside double layer brick. It also reduces the possibility of trapping people from egress points.

Last summer I was aghast to learn that some techs and office staff did not know the proper usage of fire extinguishers. Most thought you tried to fight the flames from the top down. We arranged for the FD to come in and demonstrate proper usage and stress that you only even try it on a small fire, like a waste can. An email blast to all employees was sent stressing that order number one was "get out".
Thankfully we had banned smoking in the lunch room ten years ago.

But this is all a derail of course. 10000 gallons of acellerant, even a modern office tower, make fire extinguishers a moot point.

 

[jaydeehess]

I think more of the ESB would have remained standing but, the point would be moot. It would have been a hollowed out shell (no floors remaining)

Interesting and macabre thought experiment. How wide was the ESB at the 78th floor? What if a 500 mph 767 could cut through far enough and fully across one face and collapse began as the building was reacting to the impact. Might the sway impart enough lateral momentum to cause the upper portion to fall away from the lower section such as to avoid progressive floor failure as in the WTC towers?

 

[JSanderO]

Interesting and macabre thought experiment. How wide was the ESB at the 78th floor? What if a 500 mph 767 could cut through far enough and fully across one face and collapse began as the building was reacting to the impact. Might the sway impart enough lateral momentum to cause the upper portion to fall away from the lower section such as to avoid progressive floor failure as in the WTC towers?

HAHAHA I worked on the 74th floor of the ESB... I didn't measure the plan!

 

[DGM]

Interesting and macabre thought experiment. How wide was the ESB at the 78th floor? What if a 500 mph 767 could cut through far enough and fully across one face and collapse began as the building was reacting to the impact. Might the sway impart enough lateral momentum to cause the upper portion to fall away from the lower section such as to avoid progressive floor failure as in the WTC towers?

I would tend to doubt it (more of a guess than fact). I think the design would be too rigid to allow that much movement before collapse.

BTW: If memory serves, the wingspan of a 767 is still about the same size as the face at that height.

 

[jaydeehess]

In trying to find the varying widths of the ESB I found many sites that give the width of the base but one was quite incredible. It stated the building was 6000 feet wide! Dang close to a nautical mile.
ETA :

 

[jaydeehess]

I would tend to doubt it (more of a guess than fact). I think the design would be too rigid to allow that much movement before collapse.

BTW: If memory serves, the wingspan of a 767 is still about the same size as the face at that height.

]

Right, its not designed with the same flex as the towers.
On the other point it did look like it was significantly less wide at the 78th floor than the towers were. So a significantly larger percentage of perimeter columns would be cut on impact. In addition interior columns being more regularly spaced than in the towers, it may well lose more of them , as a percentage of the total, than the towers did. Its a good chance that this building, being hit by a 500 mph 767 would be immediately compromised across the impact level.
My thought then, though now seemingly moot, was that the building would be almost cut through, and in reacting to the transfer of momentum, sway in the direction of the aircraft track then the lower part would rebound back but as its loosely connected to the upper section, that upper part would not come back to vertical with the lower section.

No falling mass impacting lower floors means no runaway floor collapse progression.

 

[Richard the G]

Check the pictures more closely, parts of the structure is bar joists. There are bar joists shown very clearly in this picture:

http://urbantoronto.ca/sites/default...0195-36437.jpg


There are assemblies other than spray on FRM. And in some types of construction, the joists and deck are pre-assembled before lifted into place, these assemblies could already have the protection applied with easier application methods.

Its not clear. They may have trusses at some level but they are not bar joists. When I researched all I could find was pics of beams.

And you probably know about the Broadgate fires where steel truss floors stood a fire for many hours without any fireprotection or sprinklers.

The point I am trying to make is that bar joists are very difficult to fire protect and particularly vulnerable to fire compared to beams.

 

[Newtons Bit]

Its not clear. They may have trusses at some level but they are not bar joists. When I researched all I could find was pics of beams.

And you probably know about the Broadgate fires where steel truss floors stood a fire for many hours without any fireprotection or sprinklers.

The point I am trying to make is that bar joists are very difficult to fire protect and particularly vulnerable to fire compared to beams.

I think your google foo is weak. There are clearly joists in that building. Your confusion may be based on what you think the word "bar" means, however. Bar does not mean "rod" or "cylindrical".

 

[Richard the G]

I think your google foo is weak. There are clearly joists in that building. Your confusion may be based on what you think the word "bar" means, however. Bar does not mean "rod" or "cylindrical".

Hmm yes looks like a terrible design

 

[Newtons Bit]

Hmm yes looks like a terrible design

 

[Major_Tom]

What is the minimum threshold to set off a ROOSD progression in the most vulnerable OOS floor spaces in the buildings?


One would think the NIST would have looked into this question. A review of available literature on this subject brings up this paper, published in 2007:

"Progressive Collapse of Multi-story Buildings Due to Failed Floor Impact"

by A.G. Vlassis, B.A. Izzuddin, A.Y. Elghazouli and D.A. Nethercot, available at this link.



ABSTRACT

This paper proposes a new design-oriented methodology for progressive collapse assessment of floor systems within multi- storey buildings subject to impact from an above failed floor. The conceptual basis of the proposed framework is that the ability of the lower floor for arresting the falling floor depends on the amount of kinetic energy transmitted from the up per floor during impact. Three principal independent stages are employed in the proposed framework, including : (a) determination of the nonlinear static response of the impacted floor system, (b) dynamic assessment using a simplified energy balance approach, and (c) ductility assessment at the maximum level of dynamic deformation attained upon impact. In order to calibrate the proposed method, the part of the kinetic energy of the impacting floor that is transferred to the impacted floor is first theoretically determined for the two extreme impact possibilities, namely fully rigid and fully plastic impact. Moreover, a series of numerical studies is carried out to further refine the accuracy of this new approach with respect to different impact scenarios, whilst the effects of detailed joint modelling and redundancy are also investigated. The application of the proposed methodology is demonstrated by means of a case study, which considers the impact response of a floor plate with in a typical multi-storey steel-framed composite building. Several possibilities regarding the location of the impacted floor plate, the nature of the impact event and the intensity of the gravity loads carried by the falling floor are examined. The application study illustrates the extremely onerous conditions imposed on the impacted floor resulting in an increased vulnerability to progressive collapse for structures of this type. Importantly, the likelihood of shear failure modes in addition to inadequate ductility supply under combined bending/axial actions is identified, thus establishing mthe need for further research work on the dynamic shear capacity of various connection types subject to extreme events.





The best mathematical approach to the study of ROOSD as initiator I have found within all available literature is contained within this paper.

 

[Newtons Bit]

How can that be, Major_Tom? That paper was written in 2007, but you hadn't yet invented the concept!