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My contentions with the NIST report due not lie within the framework of LIHOP or MIHOP. In fact, my contentions don't attach to any conspiracy theory at all. Where I take great umbrage with the NIST final report is in the fact that the conclusions drawn within the report do not align with, and at times soundly contradict, the data presented in the report. Since this investigation, analysis and resultant report were paid for by the American people, which include me, I take issue with being handed a report that reflects insufficient data collection (insufficent collection of failed members and testing on same), scientifically unsound statements (creep played a significant role in the failure of the supporting elements), and a failure model that contradicts the data they present themselves on what little testing they did perform.
What makes the incongruities of the NIST report even more exasperating is the fact NIST refuses to share their model and database with the U.S. citizens who wish to vet the process used. I noticed you state in your paper that reviews Dr. Griffin's work (a work I must admit I know little to nothing about), that while Dr. Griffin does not provide calculations to support certain accusations (calculations I fully agree are needed), you state "the NIST Report  is readily available, and while the report may be criticized, errors that Dr. Griffin makes regarding its contents may be factually verified with no uncertainty." While this statement is true as refers to the REPORT, it is absolutely untrue as regards the actual important information (i.e. the model, the empirical data, the assumptions, etc.) Yes, we can verify the CONTENT of the NIST report, but that content is made up of only sparse references to actual data, and by and large is INTERPRETATION of that undisclosed data.
Stepping off my soap box I get to the matter hand. One particularly ornerous incongruity is the fact that the final failure mode on the macro level presented by NIST is in unacceptable contradiction (as far as this learned reader can tell) to the failure mode of certain components analyzed. In particular, the floor truss connectors at the perimeter walls and at the central core columns of WTC 1/2. The NIST report states that the floor truss connectors at the perimeter walls were sheared in a downward motion while none of the floor truss connectors on the central columns exhibited this failure mode. This would require that the floors fall in a downward motion at the perimeter wall connection while staying attached to the core columns. Since the perimeter walls couldn't possibly have "jumped up" to cause a downward shearing motion at the connectors, and since the connectors on the core columns did not shear in a downward motion, this would require that the floor trusses and core columns move in concert - which would be in a downward motion relative to the perimeter walls. The failure mode that has been presented by NIST does not account for this situation.
Originally posted by CaptainObvious
If the complaint is about the failure modes observed during collapse, it is quite possible -- the perimeter columns essentially buckled inwards, whereas the core, being the heaviest part of the structure and the most compromised in the moments leading up to failure, would have been leading the collapse.
The work of Bazant and Zhou proved this long before the NIST investigation began.
And this is from "Griff" who claims you are wrong for stating that "Wind load is not considered as a live load."
Source: "Structural Analysis", Alexander Chajes, 1990, Prentice Hall
My structural analysis book from college.
"It is similar but not identical. In particular, the design rules allow "live load reduction" with height, which was a shorthand approach to avoid double-counting safety margin in tall structures. Wind load is not eligible for live load reduction, and therefore must be bookkept separately. Wind load is not a dead load or a superimposed load, so it is perhaps most similar to a live load, but for tall structures it is simply false to state that it is live load. "
Then why does my structures book say that wind load is a live load?
Because, for whatever reason, your textbook saw fit to group all loads into either "live" or "dead." I certainly won't argue that, if given only these two choices, a wind load is more like a "live" load than a "dead" one, but this grouping is insufficiently precise.
The actual definitions -- the requirements for the actual WTC design and construction -- are not responsive to this textbook. Instead, they follow building codes, which clearly provide different definitions regarding wind load.
For example, ASCE 7-02 states the following: "Live loads are those loads produced by the use and occupancy of the building or other structure and do not include construction or enviromental loads such as wind load, snow load, rain load, earthquake load, flood load, or dead load. Live loads on a roof are those produced (1) during maintenance by workers, equipment, and materials, and (2) during the life of the structure by movable objects such as planters and by people."
So, as you can see, wind load is NOT generally considered part of the live load. Perhaps there is some building code somewhere that uses such a definition, but it would be exceptional. There is no reason to assume this is true for the WTC construction, and no reason to assume it applies to Skilling's offhand comments from 1964.
Ryan's quote:"In modern design practice this is irrelevant as we no longer use load factors and ultimate strength design."
Also wrong. In modern times we use LRFD (Load Resistance Factor Design). Notice the load and factor in there? Also, modern times don't govern what was designed for back in the 60's and 70's. We used ASD (Allowable Strength Design) back then. Don't know where he gets Ultimate Strength Design from.
I'll concede this point, since I was imprecise. The ASD practice from the 60's and 70's (and still in use in some places) is concerned with the maximum elastic capacity of a structure. LRFD (also known as "Ultimate Stress Design," which is how my professors referred to it) can plausibly go beyond this limit, but uses multiple formulas to calculate likely worst cases rather than a single monolithic worst case, as in ASD.
The point I failed to clearly articulate is that, under ASD, since there is only one formula in use -- that of the maximum elastic stress -- there is a well defined maximum load as well, and thus we can easily express the total margin as a function of the total service load, the dead load only, or (as was done in Skilling's quote) a function of service live load only. In LRFD, the limit is not as well defined due to the competing limit cases, and it would be harder to make a "2000% margin" quote that had any meaning. We would also have to specify a scenario, i.e. minimal wind load, maximum earthquake load, etc. Under ASD there are no different scenarios.
In any case, I see no error with the respondent's corrections in this regard, so I'll leave it at that. I should have been more careful in my first reply.
What is relevant is that the "2000%" claim cannot refer to both service live loads and wind loads. If it did, that means the perimeter columns were overdesigned by more than a factor of 10. This is simply not true, as found by NIST in its baseline models, and can be verified by hand from the column cross-sections and composition provided in NCSTAR1-1. We therefore understand that the "2000%" could only have referred to the service live loads, in which case it isn't at all inconsistent with our expectations.
On the relatively windless day of September 11th, the perimeter columns were about six times stronger than the total load at the time, and that is consistent with our interpretation that the "2000% of live load" remark only refers to service live loads, not wind loads. This margin is why the Towers stood as long as they did, but it was not enough margin to be invincible, contrary to what Dr. Griffin claims.
Originally posted by CaptainObvious
This is awfully long-winded and short on actual questions. There are no "sound contradictions" between the conclusions and the data
There is nothing unscientific about creep playing a role in failure, and this is an expected result given the anticipated temperatures and loading of the structural elements.
Steel beams in standard fire tests reach a state of deflections and runaway well below temperatures achieved in real fires. In a composite steel frame structure these beams are designed to support the composite deck slab. It is therefore quite understandable that they are fire protected to avoid runaway failures. The fire at Broadgate showed that this didn't actually happen in a real structure. Subsequently, six full-scale fire tests on a real composite frame structure at Cardington showed that despite large deflections of structural members affected by fire, runaway type failures did not occur in real frame structures when subjected to realistic fires in a variety of compartments.
Originally posted by bsbray11
I would love to see a free body diagram on one of those, too. Apparently you even get a dynamic load from heating a truss, motion being almost completely horizontal and enough to yank out a column at both ends at once.
Finding 13: Impact of an empty wing segment from approximately mid-span of the wing normal to the exterior wall produced significant damage to the exterior columns but not complete failure. Impact of the same wing section, but filled with fuel, resulted in extensive damage to the external panels of the tower, including complete failure of the exterior columns.
I want to know where the meat is.