[Moderated]Another engineer criticizes NIST & FEMA

[Hokulele]

In NCSTAR1-5D, NIST suggests that 66% of the airplanes KE was transferred into the building's KE (movement of the building). This would mean that only 34% of the KE is left to actually damage the building and SFRM, unless of course the movement of the building caused damage.

I find nowhere else in NIST, including the 1-2 Airplane Impact Analysis series, that this is taken into account. In fact, they compare the NIST results to Wierzbicki's study which applied the entire energy to destroying the airplane and damaging the building and had fairly similar results.

I think that one thing you may be missing here, is that this calculation was used to determine the acceleration to be used on the shaking tables that were used to simulate what happened to ceiling tiles on floors above and below the impact area. (The formal title of NIST NCSTAR 1-5D is "Reaction of Ceiling Tile Systems to Shocks".) The report clearly states that this was not meant to be a comprehensive analysis, but an approximation to be used for a specific reason (determing the wave-form and other parameters for the shaking table).

Why are you trying to apply this set of calculations to the SFRM simulations?

 

[GregoryUrich]

Thank you fot the quote.
So, Gregory Urich... How does one get from "the kinetic energy transferred from the airplane to the tower. .... It was assumed that two-thirds of the energy was transferred from the airplane to the structure" to "NIST suggests that 66% of the airplanes KE was transferred into the building's KE (movement of the building). "
The energy ransfer was energy transfer. It takes a different comprehension to read "..energy was transferred from the airplane to the structure" as "KE energy was transferred from the airplane to KE of the structure"

You may, in fact, find yourself with an entirely different comprehension if you actually read p. 43-45 of NCSTAR1-5D.

 

[Apollo20]

The important thing to note in NIST's treatment of the aircraft impacts, as described in NCSTAR 1-5D, is the notion of a "participating mass".

Thus NIST state that about 0.6 seconds into the impact "the tower and the airplane debris were moving together at the same velocity ... 42.3 ft/s (12.9 m/s)."

(This statement turns out to be quite misleading as we shall see.)

On page 44 of NCSTAR 1-5D NIST state that the "participating mass" was 31 x 10^6 kg or only about 10 % of the mass of a tower!

Now let's check these numbers remembering that the mass of the aircraft was 124,000 kg and its velocity was 250 m/s:

K.E. after impact =1/2 Mv^2 = 1/2 x 31 x 10^6 x (12.9)^2 = 2.6 x 10^9 J

K.E. before impact = 1/2 x 124 x 10^3 x (250)^2 = 3.9 x 10^9 J

2.6/3.9 = 2/3

Q.E.D.

 

[PhantomWolf]

Um....., GregoryUrich how do you think that energy was transfered to the building?

 

[Furcifer]

Or NIST is saying 1/3 of the KE exited the building as flying debris. The rest went into destroying the building. As far as the visco-elastic dampers go, I don't know of any that are designed to operate at such high KE's over such short impulse durations. Anyone know?

 

[GregoryUrich]

This is not quite accurate, the building will not be displaced 13 ft, nor anywhere near it. In this study:

Structural Responses of World Trade Center under Aircraft Attacks. Omika, Yukihiro.; Fukuzawa, Eiji.; Koshika, Norihide. Journal of Structural Engineering v. 131 no1 (January 2005) p. 6-15

The accelerations and displacements are calculated for the building, and they find that WTC 1 and 2 experienced maximum accelerations of 2g and 3g respectively, which corresponded to displacements of approx 30cm(.98ft) on the impact floor and 50cm(1.64ft) at the top floor.

If the building was accellerated to 42 ft/s after 0.63 seconds as NIST describes, with constant accelleration the displacement will be around 13 ft. Actually, taking into account that the accelleration is highest earlier in the impact the displacement would be even greater. You can do the math if you don't believe me. Keep in mind this is only true in NIST's world.

I agree that, in reality, the building was not displaced 13 ft. This is just one example of the NIST investigation not conforming to reality.

 

[GregoryUrich]

Or NIST is saying 1/3 of the KE exited the building as flying debris. The rest went into destroying the building. As far as the visco-elastic dampers go, I don't know of any that are designed to operate at such high KE's over such short impulse durations. Anyone know?

Interpretation of NIST and contribution to the discussion is often enhanced by reading the source being discussed.

 

[GregoryUrich]

I think that one thing you may be missing here, is that this calculation was used to determine the acceleration to be used on the shaking tables that were used to simulate what happened to ceiling tiles on floors above and below the impact area. (The formal title of NIST NCSTAR 1-5D is "Reaction of Ceiling Tile Systems to Shocks".) The report clearly states that this was not meant to be a comprehensive analysis, but an approximation to be used for a specific reason (determing the wave-form and other parameters for the shaking table).

Why are you trying to apply this set of calculations to the SFRM simulations?

Have I said anything about SFRM in this discussion?

 

[GregoryUrich]

Um....., GregoryUrich how do you think that energy was transfered to the building?

In general, energy was transferred to the building in the manner described by NIST. It is the amount of energy (66% of the planes KE) that went into the KE of the "participating mass" of the building that is in error. The "participating mass" pointed out by Apollo may also be incorrect.

 

[Furcifer]

Oh so it's my p-poor interpretation. I get it.

 

[GregoryUrich]

Oh so it's my p-poor interpretation. I get it.

Have you read p. 43-45 of NCSTAR1-5D?

 

[Spins]

The important thing to note in NIST's treatment of the aircraft impacts, as described in NCSTAR 1-5D, is the notion of a "participating mass".

Thus NIST state that about 0.6 seconds into the impact "the tower and the airplane debris were moving together at the same velocity ... 42.3 ft/s (12.9 m/s)."

(This statement turns out to be quite misleading as we shall see.)

On page 44 of NCSTAR 1-5D NIST state that the "participating mass" was 31 x 10^6 kg or only about 10 % of the mass of a tower!

Now let's check these numbers remembering that the mass of the aircraft was 124,000 kg and its velocity was 250 m/s:

K.E. after impact =1/2 Mv^2 = 1/2 x 31 x 10^6 x (12.9)^2 = 2.6 x 10^9 J

K.E. before impact = 1/2 x 124 x 10^3 x (250)^2 = 3.9 x 10^9 J

2.6/3.9 = 2/3

Q.E.D.

I agree, also you are airing on the side of caution and using an impact velocity right at the top of the estimated range. The impact velocity is estimated at 545mph +/- 18mph, so the impact velocity could have easily been only 236 m/s:

K.E. before impact = 1/2 x 124 x 10^3 x (236)^2 = 3.45 x 10^9 J

2.6/3.45 = 3/4

I've no doubt a considerable amount of the impact energy was transferred into the structure as movement and absorbed, for example Stanley Praimnath mentioned the building swaying back and forth several times, but that value of 12.9 m/s (based on Greenings calculations) is just wrong, it has to be or there would be virtually no energy left to cause the damage we all witnessed.

Doesn't mean 9/11 was an inside job, just means that values they used to calculate the amount of ceiling tile loss for floors not directly damaged by the airplane from building motion alone are possibly incorrect.

The ceiling tiles at and around the impact zone would have been obliterated by the plane impact.

 

[R.Mackey]

In general, energy was transferred to the building in the manner described by NIST. It is the amount of energy (66% of the planes KE) that went into the KE of the "participating mass" of the building that is in error. The "participating mass" pointed out by Apollo may also be incorrect.

It seems to me that one reasonable way to estimate the energy transfer would be to consider the momentum -- if we model the aircraft impact as fully inelastic, we should be able to work out the peak velocity seen after the impact, and then backsolve against their simple estimated acceleration curve to estimate the peak acceleration. This in turn could give us a more defensible energy estimate. It will be hampered by pass-through of debris and absorption of momentum by the lower structure, but it should be good to +/- 25% or so.

I understood the extremely rough 66% estimate in NCSTAR1-5D as being a worst-case estimate limited to the ceiling tile dislodgement problem, not relevant to the Tower evolution as a whole. After all, 1-5D essentially concludes that tile dislodgement was expected to be minor, so it makes sense to run an excessively aggressive bounding case.

As such, I agree this is a bit sloppy, but unlikely to have any effect on the overall conclusions of the Report. I still consider the Report as a whole a B+ effort.

 

[cmcaulif]

If the building was accellerated to 42 ft/s after 0.63 seconds as NIST describes, with constant accelleration the displacement will be around 13 ft. Actually, taking into account that the accelleration is highest earlier in the impact the displacement would be even greater. You can do the math if you don't believe me. Keep in mind this is only true in NIST's world.

I agree that, in reality, the building was not displaced 13 ft. This is just one example of the NIST investigation not conforming to reality.

The entire building will not experience this maximum acceleration, but rather only the impact floors will experience it, since it is a concentrated impulse load on one portion of the structure. The farther away you get from the impact, the acceleration will drop quite substantially. Also, the floors that do experience high accelerations will have their displacements minimized, since the horizontal movement will be resisted by the action of the framed tube perimeter. This is illustrated quite well in Omika et all(who also take into account 3% damping) where it was shown that displacements will occur all the way down to the bottom of the building due to the impact.

 

[rwguinn]

The entire building will not experience this maximum acceleration, but rather only the impact floors will experience it, since it is a concentrated impulse load on one portion of the structure. The farther away you get from the impact, the acceleration will drop quite substantially. Also, the floors that do experience high accelerations will have their displacements minimized, since the horizontal movement will be resisted by the action of the framed tube perimeter. This is illustrated quite well in Omika et all(who also take into account 3% damping) where it was shown that displacements will occur all the way down to the bottom of the building due to the impact.

OHHHH-Kay.
I am wondering just where this "2.5%" and 3%" damping is coming from?
% damping is expressed as the fraction 0f critical damping: z/zcrit. This value is usually less than 1.0 which is 100 %. Values greater than 1 (100%) means the system is over-damped and it will not complete a full oscillation cycle, instead, deflection will decay from maximum back to zero. 100% (1.0) means the system goes through 1 oscillation from excitation to rest, with amplitude decaying all through the cycle.
0% damping means that the system oscillates forever, at the same amplitude.
I cannot see the designers allowing the towers to oscillate for a very long time 5-10 minutes) at 1/11 Hz, with the subsequent high deflection and slow decay.
I will believe that a "Q" of 3,or a "Q" of 2.5 is real. I will not believe a "Zeta" of .02-.03 until someone provides me with a link showing how the value is derived.

Damping is a function of friction and velocity. The higher the velocity, the higher the damping value. at the single chunk of homogenous material level, this is a function of molecular friction.
Steel, for most shapes and fabrications, 2% of critical can be safely assumed. USAF and NASA allow up to 5% on most aluminum/titanium structures, without having to justify it.
Rubber, depending on composition, can provide 20->100% damping, depending on utilization. The tires on your car are about 15-20% damping, not considering the shock absorbers, which can push the overall suspension system up to around 30-50%

 

[Hokulele]

Have I said anything about SFRM in this discussion?

This is the post/response I was commenting on.

Yes, Kevin Ryan has been telling this lie for some time. I cover it on Page 19 of my whitepaper.

The NIST Report says that the aggregate KE of impacting fragments need to be roughly 1 MJ (actually they say 0.1 to 1 MJ) to shake loose a square meter of SFRM. But this does not mean that the SFRM absorbs all of this energy. The vast majority of energy remains to damage the structure underneath or ricochet the impacting fragments into other SFRM somewhere else.

Kevin Ryan not only uses 1 MJ / m², disingenuously using the top of the range, but also assumes all of that energy is absorbed. If a three-quarter inch layer of SFRM could do that, we should use it as armor on main battle tanks.

However, since this is "Bash NIST Day," I will add that I don't understand why they used energy and not momentum in the above expression. I believe that momentum is actually the correct quantity, and their use of KE leads to further confusion. Probably has no impact at all on their overall conclusions, though.

In NCSTAR1-5D, NIST suggests that 66% of the airplanes KE was transferred into the building's KE (movement of the building). This would mean that only 34% of the KE is left to actually damage the building and SFRM, unless of course the movement of the building caused damage.

I find nowhere else in NIST, including the 1-2 Airplane Impact Analysis series, that this is taken into account. In fact, they compare the NIST results to Wierzbicki's study which applied the entire energy to destroying the airplane and damaging the building and had fairly similar results.

To me, it looks like you are discussing the dislodging of the SFRM rather than the ceiling tiles, but applying the ceiling tile calculation. If I misread, I apologize.

 

[Apollo20]

If you consider an impacted tower to be a vertical cantilever undergoing simple harmonic oscillations, the velocity at the mid point is 2pi y / T, where y is the amplitude and T is the vibrational period.

For the Twin Towers T was about 11.5 seconds, and for the aircraft impact on WTC 2, y was 0.76 meters.

It follows that the maximum velocity of a tower after impact was only about 0.4 m/s.

 

[Dave Rogers]

If you consider an impacted tower to be a vertical cantilever undergoing simple harmonic oscillations, the velocity at the mid point is 2pi y / T, where y is the amplitude and T is the vibrational period.

For the Twin Towers T was about 11.5 seconds, and for the aircraft impact on WTC 2, y was 0.76 meters.

It follows that the maximum velocity of a tower after impact was only about 0.4 m/s.

Doesn't that assume that only the fundamental vibrational mode is excited? I'd have thought there would be many modes available, and since the two planes impacted at different heights the vibrational spectrum excited by each would be very different. How much of the energy of the impact would have gone into higher modes - does anyone have a feel for this?

Dave

 

[rwguinn]

Doesn't that assume that only the fundamental vibrational mode is excited? I'd have thought there would be many modes available, and since the two planes impacted at different heights the vibrational spectrum excited by each would be very different. How much of the energy of the impact would have gone into higher modes - does anyone have a feel for this?

Dave

I have not run a modal analysis of the towers, not having a model.
However, for similar Types of structure, i.e., slender columnar structures, over 90% of the kinetic energy is in the fundamental mode.
It is pretty difficult to get any energy into excitation of secondary modes.
In the case of the towers, a SWAG on my part would put 85-90% of the energy into exciting te fundamental, and maybe 5% into the torsional mode. But I would have to have enough data to model the thing to be sure.

 

[cmcaulif]

OHHHH-Kay.
I am wondering just where this "2.5%" and 3%" damping is coming from?
% damping is expressed as the fraction 0f critical damping: z/zcrit. This value is usually less than 1.0 which is 100 %. Values greater than 1 (100%) means the system is over-damped and it will not complete a full oscillation cycle, instead, deflection will decay from maximum back to zero. 100% (1.0) means the system goes through 1 oscillation from excitation to rest, with amplitude decaying all through the cycle.
0% damping means that the system oscillates forever, at the same amplitude.
I cannot see the designers allowing the towers to oscillate for a very long time 5-10 minutes) at 1/11 Hz, with the subsequent high deflection and slow decay.
I will believe that a "Q" of 3,or a "Q" of 2.5 is real. I will not believe a "Zeta" of .02-.03 until someone provides me with a link showing how the value is derived.

Damping is a function of friction and velocity. The higher the velocity, the higher the damping value. at the single chunk of homogenous material level, this is a function of molecular friction.
Steel, for most shapes and fabrications, 2% of critical can be safely assumed. USAF and NASA allow up to 5% on most aluminum/titanium structures, without having to justify it.
Rubber, depending on composition, can provide 20->100% damping, depending on utilization. The tires on your car are about 15-20% damping, not considering the shock absorbers, which can push the overall suspension system up to around 30-50%

The authors cite the same paper by Mahmoodi et all that Apollo made note of a few posts back:

A Rayleigh damping of 3% was assumed for the
fundamental and secondary vibration modes, based on measurements
taken at high wind speeds (Mahmoodi et al. 1987).