CTBUH Comments on Nist Recommendations for Performance Based Design of Tall Buildings

[ctbuh]

This thread is managed by the CTBUH Fire & Safety Working Group Co-Chairs: Simon Lay and Daniel O'Connor

[ctbuh]

Last week the National Institute of Science and Technology (NIST) issued its draft report on the fire and collapse of World Trade Center 7. This document is very interesting reading for all tall building designers and it can be found at www.nist.gov

Tower 7 collapsed as a result of the fire that was ignited during the 9/11 terrorist attack. The report concludes that the collapse was solely a result of the fires that started on ten levels following the initial attack. The failure occurred approximately eight and half hours after the first attack. The collapse of the WTC1 and 2 severed the water mains and the firefighting effort was abandoned after the collapses of the twin towers.

NIST notes that the failure was caused by the effects of thermal expansion. While fire engineers are well aware that the effects of thermal expansion and thermal contraction (during the cooling phase) are often substantially more significant than the effects of heat reducing the strength of materials. This understanding needs to extend to architects and engineers that are involved in the high rise industry and an introduction to the subject is described in the paper David Scott presented at the NIST Chicago workshop. The paper “Fire Induced Progressive Collapse “ by Scott, Lane and Gibbons can be found online and on the CTBUH website.

[ctbuh]

The NIST report recommends that in future buildings should be designed so that they do not collapse, even in an extreme fire, even if the sprinkler system fails or is overwhelmed by the fire. The approach recommended by NIST is essentially a performance based approach which explicitly checks the performance of structure in fire. Although this approach has already been used on many tall buildings it is much less common on normal towers, and it marks a substantial change of design philosophy, from the prescriptive code method of applying a thickness of fire protection to all elements.

[DScott1]

The Council is collecting comments and ovservations from the NIST report with a view to presenting the opinion of our members to NIST.

The report is very interesting but it does not describe the detail failure mechanism nor does it explain to designers what were the main charecteristics of the building that led to the collapse. What the industry needs is to learn what types of details and configurations create poor performance, and why.

The fire induced failure of WTC 5/6 show that short slotted holes in primary beams do not work. What did the failure of WTC 7 show ? and what can be done to make a similar building perform better.?
1. if the primary beam had shear studs would it have failed..
2. If the girders had fin plates or end plates would the building have survived
3. Did the floors fail on the heating or cooling cycle… and theoretically what was worse?
4. How effective was the slab to tie the floors over the column… what were the catenary forces and how effective was the reinforcement
5. Normal fire codes assumes a fire only occurs on one floor and much of the fire protection design is to stop flame spread between floors. WTC 7 started with fires on 10 floors and the report is vague if this had an impact on the failure.
6. Would the tower have failed if the fire was only at one level. The report is not clear on this issue

We need to understand what happened at WTC 7 and how the collapse could be prevented. In general the industry needs to share much more information on performance and how we achieve it and where things do not work. This is where the WTC report is lacking and needs to be expanded.

D Scott

[HenryM]

I cant understand why people say that Tower 7 performed well just because it lasted 8 hours without sprinklers and fire fighting. If you look at the report you can see the following

Chapt 9-9
If you look at the fire spread diagrams you can see that the fire was on level 7 for only an hour. I thought towers were supposed to last at least 2 hours before there was any failure.

You can also see that their was a full flashover fire at level 12 that lasted until it burnt out. So why did level 7 fail and not level 12?

Chapt 10
If you look at both simulations A and B, the floors are subject to critical heat for less than an hour.

So does this imply that the tower would have fallen down under a normal fire if the sprinklers didn't work?

[HenryM]

So NIST want us to design all our building by modeling fire performance. The implication is that this is not difficult and can be part of the normal process.

So why dont NIST take the WTC 7 floor plan, model it in fire and change the design to make it work in a fire, and show the public what it takes and how easy it would be.

That way designers can see the type of changes that would need to be incorporated in the design

[HenryM]

How would NIST estimate the max design fire for a building Sure it is a function of the contents, but the Nist report says the WTC 7 fire started on 10 floors simultaneously. What are Nist recommending for fire modelling

Normal codes assume that there is a fire on only one floor at a time. Are Nist recommending that all floors are on fire. Based on the WTC 7 fire I would assume a maximum of 2 floors, but some other fires had many floors on fire. What guidance would Nist give.

If we keep on adding up extreme approaches we could get some extreme buildings
- assume sprinklers do not work
- assume fire fighting does not occur
- assume 2, 5, 10 levels on fire simultaeneously

[HenryM]

Can someone tell me how much it would cost to do the type of analysis that Nist are suggesting... on say a 40 story 800,000 sq ft building.

Is there enough people who can do this type of analysis and who are they.?

[live2c]

My take on the Nist report was that they are saying that the building fell down when Col 79 collapsed. And Col 79 collapsed when the floors failed... So the real cause was the floor failure... any ideas how many floors failed?

The Nist report says that the floor failed because of the joint detailing... but also after shear stud failure.. and it doesnt really say what happened.

If the floor fails in a ductile way, surely the catenary forces the floor puts on the column are massive. And its that pulling that causes the collapse.

Why doesn't the Nist report document the key events that started the initial failure. Sounds like the building fell down because of a shear connection failure in a primary beam.. or was it because it was a few shear studs too few... not good

[live2c]

Originally Posted by HenryM

How would NIST estimate the max design fire for a building Sure it is a function of the contents, but the Nist report says the WTC 7 fire started on 10 floors simultaneously. What are Nist recommending for fire modelling

Normal codes assume that there is a fire on only one floor at a time. Are Nist recommending that all floors are on fire. Based on the WTC 7 fire I would assume a maximum of 2 floors, but some other fires had many floors on fire. What guidance would Nist give.

If we keep on adding up extreme approaches we could get some extreme buildings
- assume sprinklers do not work
- assume fire fighting does not occur
- assume 2, 5, 10 levels on fire simultaeneously

...........

If you look at the report the fire took several hours to work its way around a floor and it looks to me as if only two floors immediately adjacent to each other were on fire at any one time. If you based your design on WTC 7 fire then it would be reasonable to only design for two adjacent floors on fire at any one time.
What do the fire experts say and what do NIST say? ...when they design skyscrapers for fires, do they consider more than one floor on fire? How many? I remember seeing pictures of a tower on fire and it looked like all the floors were on fire at the same time.

[HenryM]

On page 330 NIST say that "the challenge was to determine if a fire-induced floor system failure could occur in WTC7 under an ordinary building contents fire" But I can not find where they address this challenge in their report.

If NIST are advocating that engineers analyze building performance as part of the normal design process, then surely they can answer the challenge they set themselves. Did their analysis show that the building would fail under a normal contents fire?

[HenryM]

The NIST analysis pg 353, shows that shear studs and the bolts holding the primary to Col 79 failed before the temperature of the steel reached 200degC.

These temperatures are very low compared to a fire protection test that assumes that steel loses strength at 550degC.

-------------------------------------------------------------------

It looks like the finite analysis model they did was very small Fig 8-22, and it seems to me that this is too small to pick up all the local effects around col 79.

-------------------------------------------------------------------

I am surprised to see failure of shear studs, since I would expect the beam to simply bend out the way and for the end connections to buckle like they did at Cardington.

-------------------------------------------------------------------

I cannot see why the top bolts of the girder would fail at connection to Col 79, because if it did then that would mean the slab had moved relative to col 79.

------------------------------------------------------------------
Did NIST look at any cooling cycle effects? Because if cooling had started after the bolts connecting to Col 79 had failed would the connection be stable?

-------------------------------------------------------------------

But I am most surprised about the failure initiation event which NIST seem to say was the local buckling of a secondary beam away from the floor slab that led to a global buckling of the floor.

Nist also do not describe how the local floor failure led to the global failure.?

----------------------------------------------------------------

I would interested in any opinions on these issues from a fire expert?

[ctbuh]

Hello. Am I to understand that you find the NIST report believable, and that fire can be seen as a real cause for the symmetrical collapse of a steel framed skyskraper? Thank you. Joseph Ciolino NYC Joseph Ciolino

[DScott1]

I am not sure I understand your question. I believe that the NIST report is a responsible attempt to find the cause of the failure, however there are many questions that are not answered in any detail and several of these questions are already on the discussion forum. I think that with a responsible dialog and debate that the NIST report can be much better and clearer than it is in the current form.

However, that being said, I would like to be clear that I see no credibilty whatsoever in the 911 truth movement and I believe, like the vast majority of tall building professionals, that all the failures at the WTC ( WTC 1, 2, 5, 6 and 7) were a direct or indirect result of the planes that were flown into the two towers. I have carefully looked at the evidence that the 911 truth movement presents and I cannot see any evidence of a controlled demolition. Unfortunately the 911truth movement web site does not allow any opinions contrary to their own, or I would have presented my views.

David Scott - CTBUH Chairman

[Profjoe223]

David,
Thanks for the reply. Really, what I find has not been explained to even my own understanding are the following:

) How do we account for the evenness, totality, and swiftness of all thee collapses, in 2 entirely different types of structures?

2) How do we account for the pools of molten metal at all 3 sites of destruction, when office fires can't possibly get hot enough to cause that?

If anything, the NIST cartoon depicted (if it would have allowed itself to play out to the collapse itself) looked like a very asymmetrical collapse. This is very disturbing along with Dr. Sunder's statement that "Buildings were not constructed to withstand airline impacts." (sic!)

Very disturbing and usettling.

Thanks.

[DScott1]

Originally Posted by Profjoe223

David,
Thanks for the reply. Really, what I find has not been explained to even my own understanding are the following:

) How do we account for the evenness, totality, and swiftness of all thee collapses, in 2 entirely different types of structures?

2) How do we account for the pools of molten metal at all 3 sites of destruction, when office fires can't possibly get hot enough to cause that?

If anything, the NIST cartoon depicted (if it would have allowed itself to play out to the collapse itself) looked like a very asymmetrical collapse. This is very disturbing along with Dr. Sunder's statement that "Buildings were not constructed to withstand airline impacts." (sic!)

Very disturbing and usettling.

Thanks.

This forum is for a technical debate about the report and its recommendations. We do not want to talk about general opinions unless they are detailed and focused. These comments are too general and not technical and they do not help the debate. Future comments like these will be removed. Also the forum is also about WTC 7 only.

"1. How do we account for the evenness, totality, and swiftness of all thee collapses, in entirely different types of structures?"
The inside of WTC 7 collapsed first, followed by a buckling failure of the perimeter in the lower 15 floors. A non-linear collapse analysis would verify the speed of failure. From my perspective the building behaves as expected. Once the building collapse had started and momentum gained there is nothing of substance to arrest it.

"2) How do we account for the pools of molten metal at all 3 sites of destruction, when office fires can't possibly get hot enough to cause that?"
I dont know, but what hypothesis is being proposed?

The forum does not wish to engage in a debate about conspiracy theories and the 911truth movement. However if there is enough interest we will start a separate thread. Please do not post any views on 911truth in this forum, but write separately to info@ctbuh.org

David Scott CTBUH Chairman

[Profjoe223]

David,

I apologize if I have violated the rules of this forum. It was your invitation to post my question so I thought... well, it doesn't matter what I think.

However, I am not part of any movement, nor have I espoused any "conspiracy theories" either my own or of any "movement." If you read my post carefully there are none present.

I believed that I was questioning the report juxtoposed to particular and specific verifiable facts.

I had hoped that this, in and of itself, does not violate the spirit of this forum.

[simonflay]

I think there was up to 10 floors on fire at any one time, that may well have been critical to the overall collapse. After all, that's a major deviation from the standard fire tests.

[simonflay]

The shear stud failure does seem a bit odd. I tihnk that the temperature regime of the slabs may have impacted on the stud failure prediction. The top of the slabs were I think modelled as being hotter than the underside of the slabs, that might cause an upward bow, against the typical downward deflection of the steel deck, adding stress to the shear studs?

[simonflay]

I think we need to be careful when we talk about 'the studs failed' or 'the beam collapsed'. Remember, the modelling that NIST undertook was extremely complex and yes, produced results that appear to mimic to a good approximation the overall effects. However, we must remember that a model is a model and where partial modelling results are fed back into the model (shear stud failure being such an example), caution needs to be taken.

It is a brave effort by NIST to try to model 'reality' so closely, but I am sure that like any modelling exercise, it is ultimately a model, not the real thing?