[Moderated]Another engineer criticizes NIST & FEMA

Critical Damping and Vibrational Modes of WTC 2

I am sorry I do not have a link, but I do have TWO references, (in addition to the already quoted Mahmoodi paper), that discuss the effectiveness of the viscoelastic dampers in the Twin Towers:

1. B. Samali et al. "Use of Viscoelastic Dampers in Reducing Wind and Earthquake Induced Motion of Building Structures" Engineering Structures 17(9), 639, (1995).

On page 650 we read: "The performance of the (WTC) dampers was tested during Hurricane Gloria ... for which the total damping of the building was calculated and found to be in the range 2.5 - 3.0 % of critical damping."

2. P. R. Montpellier. "The Maximum Likelihood Method of Estimating Dynamic Properties of Structures" Masters Thesis at the University of Western Ontario (1996). Table 6.2 on page 103 provides data on measurements of wind conditions, vibrational frequencies and damping (as a % of critical) based on measurements made at the WTC between 1980 and 1993. The % damping data are in the range 1.5 % - 3.4 % with an average value ~ 2.5 %.

As for the modes of vibration of the Twin Towers, while the first translational mode of WTC 2 was excited by the aircraft impact, the second translational and the torsional modes were also significantly excited. The 1st translational mode had a vibrational period of ~ 11.5 seconds; the 2nd translational mode's period was ~ 4 seconds and the first torsional mode was ~ 5 seconds. The 2nd translational mode of vibration was excited because the aircraft hit WTC 2 well below the top of the tower. As for the torsional mode, this was excited because the aircraft hit WTC 2 off center.
 
Perhaps it is just me, but surely any energy that went into moving the towers did so AFTER the energy form the impact damaging the towers was expended. The initial overload of energy would do the damage, then once the wreakage had lost enough energy to actually damage the structure the impacts would transfer the kenitic enery from one to the other, rather than it breaking the structure. That would mean that the structure took the full impact energy load of the plane, then after the ~33% was removed by things breaking, the rest was transfered into the building as motion.
 
Greg: Sorry, a lame attempt at being cryptic and sarcastic at the same time. There seems to be an issue with interpreting "absorbed" and "transferred" as well as what was used as a working number as opposed to a calculation.
 
PhantomWolf said:
Perhaps it is just me, but surely any energy that went into moving the towers did so AFTER the energy form the impact damaging the towers was expended. The initial overload of energy would do the damage, then once the wreakage had lost enough energy to actually damage the structure the impacts would transfer the kenitic enery from one to the other, rather than it breaking the structure. That would mean that the structure took the full impact energy load of the plane, then after the ~33% was removed by things breaking, the rest was transfered into the building as motion.
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Actually, that's not a bad layman's attempt at an axplanation.
What happens is that the structure, through its frequency response characteristics, can only absorb energy at a certain rate.
If the exciting impulse occurs at a faster rate than the structure can absorb through deflection, the excess goes into breaking and bending stuff, or back into the excitation source.
You an easily demonstrate this by finding yourself a wooden Yardstick (or, find a Meter stick if you want to be that way). Holding it by one end and shaking it, you can excite the first mode at a particular shaking frequency. Shake it faster, and the end will actually quit moving as much, and your effort will seem greater, and you will get tired quicker.
Basically, when the aircraft hit the towers, the towers couldn't get out of the way" quickly enough. The excess energy ended up shredding the aircraft and the local area of the towers where the impact took place
 
RWGUINN:

But strong vibrations of WTC 2 were indeed excited by the aircraft impact!

In NCSTAR 1-5D NIST use a questionable theoretical approach to show that the peak acceleration of the building was as high as 0.25 g, but remarkably, NIST do not allow for this energy sink in it's structural damage model as described in NCSTAR 1-2. Given the time and money NIST was given to investigate the collapse of the twin towers I don't know why it did not perform a proper vibrational frequency analysis of the towers - even a simplified calculation such as has been published by K. Zalka would have been useful, (See Engineering Structures 23, 1544, 2001). This would at least have provided an effective flexural rigidity (EI) for the towers from which the fundamental vibrational frequency could have been determined.

I would say that the observed deflections of WTC 2 are not consistent with the aircraft impact KE. This is made worse by the fact that NIST, (and Wierzbicki!), assume that ALL the impact KE was used for plastic deformation and fracture of the aircraft and building structures in the impact zone, leaving nothing to excite elastic vibrations of the entire building. Could the fuel-air explosion induced by the aircraft impact on WTC 2 have given the building an extra kick?
 
Apollo20 said:
RWGUINN:

But strong vibrations of WTC 2 were indeed excited by the aircraft impact!

In NCSTAR 1-5D NIST use a questionable theoretical approach to show that the peak acceleration of the building was as high as 0.25 g, but remarkably, NIST do not allow for this energy sink in it's structural damage model as described in NCSTAR 1-2. Given the time and money NIST was given to investigate the collapse of the twin towers I don't know why it did not perform a proper vibrational frequency analysis of the towers - even a simplified calculation such as has been published by K. Zalka would have been useful, (See Engineering Structures 23, 1544, 2001). This would at least have provided an effective flexural rigidity (EI) for the towers from which the fundamental vibrational frequency could have been determined.

I would say that the observed deflections of WTC 2 are not consistent with the aircraft impact KE. This is made worse by the fact that NIST, (and Wierzbicki!), assume that ALL the impact KE was used for plastic deformation and fracture of the aircraft and building structures in the impact zone, leaving nothing to excite elastic vibrations of the entire building. Could the fuel-air explosion induced by the aircraft impact on WTC 2 have given the building an extra kick?
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Again, getting high-velocity into a dynamic load case is not something that happens.
If the energies input are below those required to actually cause mechanical failure (material yield) then you would be correct. A suspended yard (or meter) stich will go though all kinds of dances and jigs if you support the ends and drop, say, a rubber eraser on it.
Suspend it by the ends and drop a brick on it, and it breaks. It never had time to respond.
By a similar phenomenon, a 1/2 inch lead ball, hitting a 2 foot square steel plate (that is simply stood upright on one edge) at 800 feet per second will knock it over. At 3000 feet per second, a lead ball 1/3 inch in diameter will go right through the same steel plate, leaving it upright

It is somewhat akin to pouring 5 gallons of liquid from a wide-mouth bottle into a 3 gallon narrow-mouth bottle. The likelihood of actually getting even 3 gallons into the narrow-mouth bottle are slim unless you do it very slowly. If you simply invert the big one over the little one, you pretty much end up with 2 empties.
Mechanical energy is the same way.

This is not totally complete, but I have not the time to go into response to a step function.
Shock and Vibration Handbook, Section 23 is pretty good--An example is Figure 23-15, equation 23.31
 
Or...this one time I had to take a strut assembly off my car. I got the spring off, but the shock absorber itself wouldn't come off. Not having oxy, I took a hammer to it. Despite being a "shock absorber" it was mangled by the time I broke it away and got the nut off the top. I guess it only operated under the conditions it was designed for, and not as I had abused it. It operated at a certain angle to dampen certain oscillations. Out of it's range, it was useless. After a hard blow from my hammer it could only absorb the transient effects of the wave induced..
 
Hokulele said:
This is the post/response I was commenting on.

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.
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My bad. Sometimes I forget what I have written a few days ago. I didn't intend to apply the assumptions from NCSTAR1-5D to the impact analysis (SFRM), but rather indicate that the assumptions seemed purposterous.
 
Apollo20 said:
RWGUINN:

But strong vibrations of WTC 2 were indeed excited by the aircraft impact!

In NCSTAR 1-5D NIST use a questionable theoretical approach to show that the peak acceleration of the building was as high as 0.25 g, but remarkably, NIST do not allow for this energy sink in it's structural damage model as described in NCSTAR 1-2. Given the time and money NIST was given to investigate the collapse of the twin towers I don't know why it did not perform a proper vibrational frequency analysis of the towers - even a simplified calculation such as has been published by K. Zalka would have been useful, (See Engineering Structures 23, 1544, 2001). This would at least have provided an effective flexural rigidity (EI) for the towers from which the fundamental vibrational frequency could have been determined.

I would say that the observed deflections of WTC 2 are not consistent with the aircraft impact KE. This is made worse by the fact that NIST, (and Wierzbicki!), assume that ALL the impact KE was used for plastic deformation and fracture of the aircraft and building structures in the impact zone, leaving nothing to excite elastic vibrations of the entire building. Could the fuel-air explosion induced by the aircraft impact on WTC 2 have given the building an extra kick?
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I think they even got the 0.25g value wrong. In think it should be 2.5g.
 
GregoryUrich:

Well you have a point.

In the paper by Y. Omika et al. in the January 2005 issue of the ASCE Journal of Structural Engineering there is a graph showing a calculated acceleration of ~ 3g and a displacement of 30 cm for the 80th floor of WTC 2.

NIST has some 'splaining to do!
 
an order of magnitude is exactly what it sounds like. giving it up is very hardcore.
 
metamars said:
Perhaps you could submit your concerns to the Union of Concerned Scientists, or, better yet, to an analogous organization of engineers? I kind of doubt that there is one, though. OTOH, UCS must have engineers, as they are concerned about the safety of nuclear reactors.

Considering the recent bridge collapse in Minneapolis, it looks like we need a Union of Concerned Engineers. Governments at various levels are not doing their jobs ito maintaining infrastructure, as became common knowledge after the Minneapolis catastrophe.

While I certainly don't agree with many things you have said re 911 collapses, I believe that you're sincere. Is your sincerity likely to be honored at JREF? Indeed, are you honoring your own life, when there are probably many scientists and engineers who, like yourself, are more interested in using their talents for public service? (As opposed to nit-picking and superficial 'apologetics', e.g.)

Perhaps you should start an NGO which focuses on engineering safety issues. Then you could get $$ from foundations, which you are going to need to do many of the kinds of work you can't do, otherwise.

I wonder where UCS gets funds from. I really don't know much about them - mostly I know of them from interviews of their members on Michio Kaku's 'Explorations in Understanding'.
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I got a PM from a poster regarding American Society of Safety Engineers (ASSE) as a "starting place".

Also, the USC has a pie chart of their funding sources in their annual report.

http://www.ucsusa.org/assets/documents/ucs/annual_report_06.pdf

Membership & Contribution 56%
Foundations 28%
Planned Gifts 10%
Operations & Net Investments 7%
 
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