Another paper from the folks at Arup/Edinburgh

[cmcaulif]

Not sure if this paper has made the rounds yet since it was published in 06, but it did not come up on a search of the forum:

G. Flint, A.S. Usmani, S. Lamont, J. Torero and B. Lane, Effect of fire on composite long span truss floor systems, Journal of Constructional Steel Research 62 (4) (2006), pp. 303–315.

The study creates FE models of the floor systems, one model is of a single floor, and one is of several floors, and for different fire temperatures, 500C and 800C. In both cases the trusses are considered unfireproofed and the supporting columns have intact fireproofing.

The most interesting result is the comparison of the performance of the single truss model vs the multi floor model with equal thermal loading. The authors plot the midspan deflection vs time of the single truss and the eight floor of their multi floor model. As they note, the deflection rates are about the same for both models until about 240 seconds into the simulation, when the multi story truss has an increase in its rate of deflection, and continues to diverge(though the divergence slows at 400s) until collapse occurs.

While this is clearly not a fair representation of the towers, IMO it shows that behavior of a subassembly alone does not quite capture the behavior of the entire structure. The authors of this study were certainly justified in their previous criticisms of the NIST when they said it was a disadvantage to have the the floors modeled as plates in the NISTs FE simulations.

[Gravy]

Very interesting.

[R.Mackey]

I've seen references to those calculations, but hadn't dug up the article itself. Thanks for finding that.

Some other reader of my whitepaper asked me (through e-mail, and through an intermediary; what brave souls these Truth Movement folks are) whether I thought the Towers were vulnerable to ordinary fire, e.g. arson. My answer -- which is speculative, since not all studies agree on this -- is that fire alone probably could have felled the Towers. NIST says impact damage to the structure was required, but on the basis of work such as that you've cited, I believe a multifloor, well-ventilated fire would be capable of causing a collapse.

Of course, your garden variety accidental fire won't be multi-floor, and you won't get hundreds of windows smashed out like we had in the plane crashes, so the likelihood of this would be rather slim. Still, the search goes on...

[cmcaulif]

Very interesting.

Its quite a good study, and makes me wonder if the NIST had modeled the towers differently, the base cases would cause collapse, or the base cases would lead to global instability in burnout. Hopefully Arup/Edinburgh will continue their research.

[cmcaulif]

Of course, your garden variety accidental fire won't be multi-floor, and you won't get hundreds of windows smashed out like we had in the plane crashes, so the likelihood of this would be rather slim. Still, the search goes on...

yes I think the aircraft simply breaking windows and providing ventilation is quite often overlooked as one of the major factors that doomed the towers. Surely this was a large factor for building 7 as well.

[R.Mackey]

Its quite a good study, and makes me wonder if the NIST had modeled the towers differently, the base cases would cause collapse, or the base cases would lead to global instability in burnout. Hopefully Arup/Edinburgh will continue their research.

My understanding of the NIST results is that the base cases would lead to collapse, just not in the correct length of time. Only the "less severe" impact cases are claimed to not lead to a collapse, if I read it correctly.

NIST of course has its own critique of the early Arup results, among them noting that you can't just set all the floors to 800oC at once, but nonetheless acknowledges its own shortcomings regarding thermal expansion, creep, and more detailed modeling of the trusses.

Reality is somewhere in the middle. I think it's closer to Arup myself, but it'll take more work.

[cmcaulif]

My understanding of the NIST results is that the base cases would lead to collapse, just not in the correct length of time. Only the "less severe" impact cases are claimed to not lead to a collapse, if I read it correctly.

I see, I think I mistook NIST's meaning then, I'll have to check it out some more.

NIST of course has its own critique of the early Arup results, among them noting that you can't just set all the floors to 800oC at once, but nonetheless acknowledges its own shortcomings regarding thermal expansion, creep, and more detailed modeling of the trusses.

True, the uniform heating is a pretty big drawback for the Arup models, but in this case, there is no fireproofing on the truss, actually achieving a steel temp of 800C is certainly reasonable, and the temperature is governed by the function:

$$ T(t) = T0 + (Tmax - T0)(1 - {e^{(-at)}}) $$

which predicts truss temps of 600C at around 250 seconds, with heating rate tapering significantly after that, so the time to 800C is not as rapid, or near instantaneous as it would seem from what Dr. Lane mentions in her presentation.

[BenBurch]

Firemen also distrust that sort of truss system.

[LashL]

<snip>Reality is somewhere in the middle. I think it's closer to Arup myself, but it'll take more work.


Reality, in this case, delivered never-before seen parameters, which led directly to catastrophic results. :(

[cmcaulif]

Firemen also distrust that sort of truss system.

I have heard that as well. The University of Sheffield (http://fire-research.group.shef.ac.uk/publications_frameset.html) in the UK has done a ridiculous amount of research on fire performance of structures, and one recent paper on this type of truss system, though not on 9/11 per se, notes that the trusses are uncommon in Europe, partially due to difficulties in fire resistance design.

[Architect]

I have heard that as well. The University of Sheffield (http://fire-research.group.shef.ac.uk/publications_frameset.html) in the UK has done a ridiculous amount of research on fire performance of structures, and one recent paper on this type of truss system, though not on 9/11 per se, notes that the trusses are uncommon in Europe, partially due to difficulties in fire resistance design.

If you keep quoting Sheffied, Arup, and Edinburgh then people are going to think I've gone and got myself a sock puppet.....

:p

[jaydeehess]

True, the uniform heating is a pretty big drawback for the Arup models,

slight derail,
NIST, in the WTC 7 prelim report , touchs on non-uniform heating of columns under load, and the bending effect that it would have. This would be different on columns of differing geometry and construction as well as the type of non-uniform heating experienced.

Has anyone seen a paper on tests of columns under load experiencing non-uniform heating?

A senario would be that a column is exposed to fire on one side with a fire block wall running between the columns keeping the room on the other side of the columns relatively cooler.

I could see this being very much the case where there is a specific fire danger in one room that carries heavy equipment such as a generator room.

The room could easily have a row of columns within one wall. If the wall is designed with extra protection against transfering heat and sound from the generator room to the columns or the next room, then if a fire occurs on the other side rather than in the generator room, one face of the column will be exposed more to heat if the sides of the column perpendicular to the wall are behind extra fire protection.
For instance;

(fire on this side)
_________________ office space wall
#####[]#######insulation with embedded column
-------------------generator room wall

as opposed to;


(fire on this side)
_________________ office space wall
. [] column
-------------------office space wall


a welded box column [] would also be more prone to non-uniform heating defomation than an "I" shaped column since it would be less able to transfer heat to the far side and a plate weld on the side facing the fire would then be subject to bending stress towards the heat while losing its strength. If that plate weld fails anywher along the height of the column, the box column is now greatly compromised.

[cmcaulif]

Not sure if it is quite what you are looking for but this power point (http://www.mace.manchester.ac.uk/project/research/structures/strucfire/DataBase/TestData/BRE/Nov11_2003/Arsonist%20to%20Fire%20Engineer.pps) from the Cardington experiments is of a test cell that had the peak temperature of the front of the compartment about 300o greater than the back. The beams are unfireproofed though.

It looks like the of web/flange buckling and other local failure modes are comparable for both ends, but the amount of deflection in the floor system definitely favors the side with greater heat, there is also more cracking/spalling on this side. I suppose this makes sense since the local failure modes will occur as soon as the steel reaches a certain threshold, while the deflection is largely dependent on the peak temp and the rate of heating.

You could try to dig through the Sheffield publications and find something further as well.

(Since I have also mentioned Cardington, I will probably have to dispel any suspicion that I am a sock of Architect by mentioning how throughly pissed I was that England lost the RWC):D

[Architect]

(Since I have also mentioned Cardington, I will probably have to dispel any suspicion that I am a sock of Architect by mentioning how throughly pissed I was that England lost the RWC):D

You'll want to mention the Building Regulations and the Corus fire performance data in a bit, too. And no-one will fall for that "England lost" line, it's too obvious..... ;)

[jaydeehess]

Did I miss it or was that pps mostly concerned with slab and beams. Its the effect on vertical , load bearing columns that I was concerned with. I can find some papers on the effect on non-load bearing metal stud walls (they bend and buckle towards the heat source).

That does suggest that a similar effect would be seen in load bearing columns. Although such a column would take longer to heat than a metal stud it would also be in compression due to the axial load.

[cmcaulif]

Did I miss it or was that pps mostly concerned with slab and beams. Its the effect on vertical , load bearing columns that I was concerned with. I can find some papers on the effect on non-load bearing metal stud walls (they bend and buckle towards the heat source).

That does suggest that a similar effect would be seen in load bearing columns. Although such a column would take longer to heat than a metal stud it would also be in compression due to the axial load.

It was mostly concerned with the slab and the beams, although the relation between the performance of the slab and beams and the performance of the columns are connected since the sagging action of the floor systems pulls on columns. When you increase the eccentricity of an axial load, the critical stress drops. Compressive strength is also a function of the modulus of elasticity and the yield stress(for inelastic buckling), both of which are reduced with heat, and given the columns will be pulled by the floor systems as well it sounds likely that buckling would also occur towards the heat source. I believe the uneven thermal load and subsequent effects also changes the way the frame redistributes loads, I'm not fully sure on that though.

[njslim]

Originally Posted by R.Mackey
Of course, your garden variety accidental fire won't be multi-floor, and you won't get hundreds of windows smashed out like we had in the plane crashes, so the likelihood of this would be rather slim. Still, the search goes on...


One of the frightening things revealed at the Meridan Plaza fire in 1990 was the degree
autoexposure played in spreading the fire. In firespeak an exposure is flammable material
threatened by fire ("exposed") - fires venting out the building broke the windows on
floor above allowing fire to enter and ignite contents, Also found that heat would cause
the beams supporting the floors to sag and warp buckling the lightweight concrete
floor slabs. Fire would then heat the carpeting on the floor above to ignition point
spreading the fires.

Considering light truss beams supporting floors fire could easily spread through this
method .