An Independent Analysis Of NISTs Scientific Methods And Assumptions
Summary of Key points
NISTs official report says global collapse is inevitable following the establishment of the conditions for collapse. This is dependent on a 2002 paper proposing a mechanism for progressive collapse of a steel structured building. However the 2002 paper also clearly states that it is based on a hypothesis that the upper section is rigid at the instant of impact with the lower section. NISTs report fails to show that the upper section is rigid at the instant of impact and there is circumstantial evidence that the hypothesis is not true in the case of the twin towers, hence NISTs report is in error when it claims global collapse is inevitable following the establishment of the conditions for the initiation of collapse.
NIST’s “Final Report of the National Construction Safety Team on the Collapses of the World Trade Center Tower” (hereafter referred to as “The Final Report”) states its first objective was to “Determine why and how WTC 1 and WTC 2 collapsed following the initial impacts of the aircraft”1 (emphasis mine). To determine why and how the buildings collapsed The Final Report needs to explain the events following the initial impact until the collapse of the buildings is complete. NIST state that their approach was to simulate the behaviour of the tower using “four steps:
1. The aircraft impact into the tower, the resulting distribution of aviation fuel, and the damage to the structure, partitions, thermal insulation materials, and building contents.
2. The evolution of multi-floor fires.
3. The heating and consequent weakening of the structural elements by the fires.
4. The response of the damaged and heated building structure, and the progression of the structural component failures leading to the initiation of the collapse of the towers.”2 (emphasis mine)
Their approach appears to fall short of that which would be required to determine why and how the initial impacts resulted in the collapse of the towers as their stated approach stops at the initiation of collapse, not at the completion of it. There is no explicit explanation why they took this approach in The Final Report or in any of the supporting documents, however their footnotes state that they conducted “little analysis of the structural-behaviour of the tower after the conditions for collapse were reached and collapse became inevitable.”3 (emphasis mine – NOTE: in another footnote (footnote 13, page 82), “conditions for collapse” is replaced with “conditions for collapse initiation”, these are assumed to be one and the same). Although I can find no analysis of the structural behaviour of the tower after “the conditions for collapse” were met in The Final Report or supporting documents, this sentence implies they have made an assumption, or have unstated proof, that once the conditions for collapse are reached, the collapse that occurs must, inevitably, be the collapse that was observed. There is no justification offered for why this assumption was made and no evidence or sources given to support its validity.
However, elsewhere in The Final Report they state that their recommendation for further research into the “Prevention of progressive collapse” is “related” to 9/114 (emphasis mine). The Final Report also states that “Progressive collapse (or disproportional collapse) occurs when an initial local failure spreads from structural element to structural element resulting in the collapse of an entire structure or a disproportionately large part of it.”5 From this statement, and given the lack of explicit explanation, it can be inferred that “the conditions for collapse” were defined as the initial conditions for progressive collapse, which The Final Report states to be “an initial local failure [that] spreads from structural element to structural element”. To check the validity of this definition it is necessary to examine the peer reviewed scientific papers modelling progressive collapse to see whether “an initial local failure [that] spreads from structural element to structural element” does inevitably result in progressive collapse of the entire structure.
Reading the peer reviewed work on the subject of progressive collapse reveals that there has in fact only ever been one paper modelling the mechanics of the collapse of the twin towers (or the global collapse of any steel framed structure) after initiation of collapse, a paper called “Why Did the World Trade Center Collapse? – Simple Analysis” by Zdenek P. Bazant, Fellow ASCE, and Yong Zhou (hereafter referred to as “The Paper”). All other papers on the subject reference this paper directly (including Bazant authored papers on the subject discussing questions others have asked) and use it as a proven basis for further analysis or they state (paraphrased)“it has already been well established that at the moment of impact the steel in the lower section is overcome by an order of magnitude” i.e. The Paper is correct in all circumstances. Therefore if The Paper conclusively supports NISTs definition of “the conditions of collapse” then the definition can be assumed to be valid and hence NISTs conclusions are valid.
Using a simplified model the paper presents an analysis of the overall collapse of the twin towers and purports to show that the towers were doomed “if prolonged heating caused the majority of columns of a single floor to lose their load carrying capacity”6. In examining the paper I found that its internal logic was consistent and it seemed to support NISTs inferred definition. However, as stated in The Paper “an important hypothesis implied in this analysis is that the impacting upper part, many floors in height, is so stiff that it does not bend nor shear on vertical planes, and that the distribution of column displacements across the tower is almost linear, like for a rigid body. If, however, the upper part spanned only a few floors (say, 3 to 6), then it could be so flexible that different column groups of the upper part could move down separately at different times, producing a series of small impacts that would not be fatal (in theory, if people could have escaped from the upper part of the tower, the bottom part of the tower could have been saved if the upper part were bombed, exploded or weakened by some ‘smart’ structure system to collapse onto the lower part gradually as a mass of rubble, instead of impacting it instantly as an almost rigid body).”7 The paragraph is quoted in full as it has two fundamental implications directly linked to NISTs definition of “the conditions of collapse”.
The first implication is that, despite admitting that the flexibility of the upper section is dependent on the number of floors, and that it admits the upper section “could be so flexible that different column groups of the upper part could move down separately at different times, producing a series of small impacts that would not be fatal” no justification is offered as to where the threshold of flexibility is. Hence the assumption that 15 floors “is so stiff that it does not bend or shear on vertical planes, and that the distribution of column displacements across the tower is almost linear, like for a rigid body” must be proven to be true for the world trade centre towers before this paper can be proved to explain the towers complete collapse after initiation of collapse.
The second implication is that, as “the bottom part of the tower could have been saved if the upper part were bombed, exploded or weakened by some ‘smart’ structure system to collapse onto to the lower part gradually as a mass of rubble, instead of impacting it instantly as an almost rigid body”, then it must be shown conclusively that the ‘system’ suggested (but not referred to in these terms) by both NIST and The Paper, namely rising heat from the fires started by the jet fuel from the planes, did not weaken the upper section sufficiently to “collapse onto the lower part gradually as a mass of rubble” in the specific case of the world trade centre before The Paper can be proved to explain the towers collapse.
Studying the calculations used to determine the overload ratio’s shows that it is dependent on the mass at the instant of impact (assumed to be approximately 58000 tonnes), if the upper section was broken into, say, 3 identical pieces then the overload ratio would have to be calculated for an impact of 19333 tonnes, then another, separate 19333 tonnes, and then again.
The importance of whether the upper section is rigid or not at the point of impact can be easily demonstrated by visualising a simple model. Consider a solid tower with a density equal to that of sand. If we drop a solid object with a mass of, say, 1 Kg, and the density of sand on top of that tower then it is easy to see how the mechanism proposed in The Paper applies (shown in Fig. 1 in The Paper8), however if we allow 1 Kg of sand to fall on top of the same tower then it is clear that mechanism proposed in the paper does not apply.
The importance of whether the upper section of the tower was rigid at the instant of impact can also be seen when comparing the diagrams in Fig. 1.3, 1.4 and 1.5 from The Paper8 showing the stages of ‘progressive collapse’ with the visual record:
Clarification or important points
In their supporting documents9 NIST do discuss other collapse hypotheses, The Paper already mentioned, Weidlinger Associates 2002 study which was to show that the two collapses were independent of each other and deals only with the collapse initiation, Maryland which they state they disagree with and two students from Edinburgh and Arup, both of which deal with cross-sectional areas and so don’t impact the implied hypothesis in The Paper. Hence my assertion that the only justification for their collapse initiation conditions inevitably leading to global collapse is Bazants 2002 paper.
The Final Reports flow chart10 also shows that only three of their studies fed into the global collapse scenario, NCSTAR 1-2 which studies specifically what the impact damage was and hence where the load above impact was redistributed to, NCSTAR 1-3 which studies the structural steel response of the additional heat and additional load caused by the redistribution to determine the point of failure of the supporting steel, again used to determine if initiation of collapse was due to impact and fire only, and NCSTAR 1-6 which determines the structural fire response to show where the fires were located around the impact site to find where the collapse was initiated. None of this research is used to determine whether the fires in the upper section were enough to weaken it to the point where the initiation of gravitational collapse was enough to break it apart before it impacted the lower section.
Further evidence of their assumption is found in NISTs FAQ released in 200710, which states:
“1. Was there enough gravitational energy present in the World Trade Center Towers to cause the collapse of the intact floors below the impact floors? Why was the collapse of WTC 1 and 2 not arrested by the intact structure below the floors where columns first began to buckle?
Yes, there was more than enough gravitational load to cause the collapse of the floors below the level of
collapse initiation in both WTC Towers. The vertical capacity of the connections supporting an intact floor below the level of collapse was adequate to carry the load of 11 additional floors if the load was applied gradually and 6 additional floors if the load was applied suddenly (as was the case). Since the number of floors above the approximate floor of collapse initiation exceeded six in each WTC Tower (12 and 29 floors, respectively), the floors below the level of collapse initiation were unable to resist the suddenly applied gravitational load from the upper floors of the buildings.” (emphasis mine)
So the assumption, once again, is that the upper section remained intact, the only analysis performed is the difference between gradual application of the load and sudden application. This does not test the implied hypothesis in The Paper, it is merely the difference between the steel support slowly giving way or giving way quickly. In fact this statement suggests that (assuming 15 floors above the impacts site) if indeed the upper section did break up under the force of gravity before impacting the lower section, and each individual section to impact the lower section was less than 6 complete floors in mass, and more than 4 floors worth of material was ejected and didn’t impact the lower section, then collapse due to gravity only wasn’t possible. Given the visual evidence of both the extent of the fires above the impact site and the collapse itself I would suggest that this scenario is possible.
“10. Why didn’t NIST fully model the collapse initiation and propagation of WTC Towers?
The first objective of the NIST Investigation included determining why and how WTC 1 and WTC 2 collapsed following the initial impacts of the aircraft (NIST NCSTAR 1). Determining the sequence of events leading up to collapse initiation was critical to fulfilling this objective. Once the collapse had begun, the propagation of the collapse was readily explained without the same complexity of modeling, as shown in the response to question #1 above.” (emphasis mine)
So once again their argument is circular, their response to question one is dependent on the assumption that the upper section had not been weakened enough by the fires to suffer break up under gravity alone before it impacts the lower section, a scenario that hasn’t been shown to be the case.
The crucial implied hypothesis stated in The Paper means the inferred definition of “the conditions for collapse” in The Final Report is incomplete and should have been “an initial local failure [that] spreads from structural element to structural element, and proof that the upper section of the tower was a rigid body at the instant of impact with the lower section”.
Furthermore, if indeed there are no other demonstrable mechanisms for explaining the total collapse of the lower section of the towers then the inferred definition of “the conditions for collapse” is irrelevant. NISTs model must show that at the instant of impact with the lower section the upper section was still a “rigid body”. Anything less than this means that the mechanism for progressive collapse demonstrated in The Paper has not been shown to inevitably follow on from the model in The Final Report and the model in The Final Report would therefore be a scenario based on an unproven and, in the specific case of the twin towers, suspect hypothesis.
As The Final Report and supporting documents (including the WTC 1 collapse sequence as described in the supporting documents11 and 12) do not show that the upper section must have still been a “rigid body” at the instant of impact with the lower section, The Final Report does not, as it claims, explain how the twin towers collapsed from impact and fire damage alone, it is only a possible sequence of events based on an unproven and suspect hypothesis and it will remain so until they rectify the fatal flaw in the formulation of their approach.
1 Section E.1, page xxxv of The Final Report
2 Section E.2, pages xxxvi and xxxvii of The Final Report
3 Footnote 2, page xxxvii of The Final Report
4 Table E-1, page xliv of The Final Report
5 Footnote 19, page 206 of The Final Report
6 page1 of The Paper (page is labelled XVII in linked document)
7 pages 2 and 3 of The Paper (pages are labelled XIX and XX in linked document)
8 page 8 of The Paper (page is labelled XXV in linked document)
9 Section 9.4.4 beginning on page 322 of NIST NCSTAR 1-6D
10 page xxxi of the Final Report
11 Section E.3.1 beginning on page lii of NIST NCSTAR 1-6D
12 section 9.4.5 beginning on page 326 of NIST NCSTAR 1-6D
The Final Report: http://www.nist.gov/customcf/get_pdf.cfm?pub_id=909017
Supporting document NIST NCSTAR 1-6D: http://www.nist.gov/customcf/get_pdf.cfm?pub_id=101366
Other supporting documents to The Final Report: http://www.nist.gov/el/disasterstudies/wtc...inalreports.cfm
NIST information on progressive structural collapse: http://www.nist.gov/el/topic_collapse.cfm
The Paper: http://www.mae.ncsu.edu/eischen/courses/ma...s/BazantWTC.pdf
Selected list of Scientific papers on the progressive collapse of the twin towers: http://sites.google.com/site/wtc7lies/peer...hewtcimpacts,fi
Weidlinger Assocites Study: http://www.wai.com/articles_pdf/webAS_abbo...rensic_2003.pdf