Structural Safety Of Freedom Tower

[LightbulbSun]

I haven't heard much about this in detail, but does anyone know what changes they made in the structural design of the Freedom Tower to prevent the collapse of the tower in the case of it being attacked via jetliners? I know it can't be 100% certain, but I would think they would have done something to the design to increase the likelihood of the structure standing in case of another attack.

[Mech_Engineer]

Details are somewhat fuzzy from what I've found. Nontheless, searching Google will turn up lots of results.

Other new safety features will include 3-foot (90 cm) thick walls for all stairwells, elevator shafts, risers, and sprinkler systems; extremely wide "emergency stairs"; a dedicated set of stairwells exclusively for the use of firefighters; and biological and chemical filters throughout its ventilation system.
...
The robust, redundant steel moment frame, consisting of beams and columns connected by a combination of welding and bolting, resists lateral loads through bending of the frame elements. Paired with a concrete-core shear wall, the moment frame lends substantial rigidity and redundancy to the overall building structure while providing column-free interior spans for maximum flexibility
http://en.wikipedia.org/wiki/Freedom_Tower

[LightbulbSun]

Details are somewhat fuzzy from what I've found. Nontheless, searching Google will turn up lots of results.


http://en.wikipedia.org/wiki/Freedom_Tower

So if I'm reading that second paragraph correctly, if one side of the structure gets overloaded in the case of an attack, this structure will have a more robust design to handle that than the old WTC towers?

[faux]

So if I'm reading that second paragraph correctly, if one side of the structure gets overloaded in the case of an attack, this structure will have a more robust design to handle that than the old WTC towers?

yes. Post 9/11 structures are now designed to consider robustness. The scenarios being undertaken could either be one or few columns removed in the perimeter frames (either by bomb attack or any other terrorist action) and check the structure for a range of combinations of removed columns if the structure satisfies & meets the performance objective set by the building owner, either life-safety or collapse prevention.

[mgb_phys]

It's difficult to find among all the conspiracy rubbish - but my understanding was that the original world trade centre design was a bit marginal even by the building standards of the 70s.

There is also a shift to using elevators to evacuate in a fire, with modern buildings the elavator shaft is actually one of the strongest and most fire proof parts - it's just that people have been conditioned for so many years not to use the elavator.

[LightbulbSun]

yes. Post 9/11 structures are now designed to consider robustness. The scenarios being undertaken could either be one or few columns removed in the perimeter frames (either by bomb attack or any other terrorist action) and check the structure for a range of combinations of removed columns if the structure satisfies & meets the performance objective set by the building owner, either life-safety or collapse prevention.

So if in the case of 9/11 repeating (North Tower like hit), would the Freedom Tower just burn, but never collapse?

[LightbulbSun]

http://www.civil.usyd.edu.au/latest/aibs_2002_wtc.pdf

On page 2 it said the original towers core carried vertical loading only. Is it possible for newer structures to have both vertical and horizontal loading?

[LightbulbSun]

It's difficult to find among all the conspiracy rubbish - but my understanding was that the original world trade centre design was a bit marginal even by the building standards of the 70s.

There is also a shift to using elevators to evacuate in a fire, with modern buildings the elavator shaft is actually one of the strongest and most fire proof parts - it's just that people have been conditioned for so many years not to use the elavator.

Are you thinking of the asbestos that was replaced in the mid-1990s?

[mgb_phys]

I think it was one of the first very large core+curtain wall / cantilever floor buildings and there was a concern that if one floor failed it would take out the floor below in a domino effect.

A similair accident happened in a residential tower block in the UK soon after, a corner floor section failed and took out every corner beneath it, fortunately they all snapped neatly and didn't destroy the main floor slabs.

[faux]

So if in the case of 9/11 repeating (North Tower like hit), would the Freedom Tower just burn, but never collapse?

supposedly, yes as this is the case for any structure that has been designed on performance-based philosophy which is gaining acceptance on the design of tall buildings.

[faux]

I think it was one of the first very large core+curtain wall / cantilever floor buildings and there was a concern that if one floor failed it would take out the floor below in a domino effect..

That's why post 9/11, a lot of research/studies has been undertaken to mitigate the effects of progressive collapse (i.e. domino effect) on structures there have been a lot of ammendments in the structural and building design codes to address this issue.

A similair accident happened in a residential tower block in the UK soon after, a corner floor section failed and took out every corner beneath it, fortunately they all snapped neatly and didn't destroy the main floor slabs.

I'm not sure of any recent event. Perhaps you were talking about the not-so recent event such as the Ronan Point incident?

http://en.wikipedia.org/wiki/Ronan_Point

[LightbulbSun]

supposedly, yes as this is the case for any structure that has been designed on performance-based philosophy which is gaining acceptance on the design of tall buildings.

I'm really curious about this subject matter so pardon all of the inquiry, but how will the loads be more robust in the newer skyscrapers to reduce the likelihood of a collapse?

[mgb_phys]

I'm not sure of any recent event. Perhaps you were talking about the not-so recent event such as the Ronan Point incident?
Thats the one, I meant soon after the WTC was built, I remembered it as rowan point so I couldn't find any detals.

[LightbulbSun]

http://www.nypost.com/seven/06302008/news/regionalnews/report__wtc_faces_up_to_3_year_delay_117912.htm

It appears there will be a three year delay to the opening of the Freedom Tower due to "over budget and unrealistic estimates."

[faux]

I'm really curious about this subject matter so pardon all of the inquiry, but how will the loads be more robust in the newer skyscrapers to reduce the likelihood of a collapse?

the thing is, it's not the load that is robust but the structural elements and structural system become more robust to mitigate progressive collapse. The structural elements become more robust by achieving a ductile detailing and careful attention of the load path when critical elements (such as columns) are taken out of the system (for any reason including terror attacks and fire).