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Explosive demolition?  A response to Bazant and Zhou.

By Jerry Russell  

 

An article "Why Did the World Trade Center Collapse?—Simple Analysis" by Zdenek P. Bazant and Yong Zhou appeared in the on-line version of Journal of Engineering Mechanics ASCE, with the first version dated 9/13/01 and revisions dated 9/22/01 and 9/28/01.  With such rapid publication after 9/11, it is evident that the paper must have been rushed, with no time for full peer review.  Let's take a critical look at this paper.  The basic theory of Bazant & Zhou is explained as follows, and sketched in their Fig. 1:

 

In stage 1 (Fig. 1), the conflagration caused by the aircraft fuel spilled into the structure causes the steel of the columns to be exposed to sustained temperatures apparently exceeding 800°C. The heating is probably accelerated by a loss of the protective thermal insulation of steel during the initial blast. At such temperatures, structural steel suffers a decrease of yield strength and exhibits significant viscoplastic deformation (i.e., creep—an increase of deformation under sustained load). This leads to creep buckling of columns (e.g., Bazant and Cedolin 1991, Sec. 9), which consequently lose their load carrying capacity (stage 2). Once more than about a half of the columns in the critical floor that is heated most suffer buckling (stage 3), the weight of the upper part of the structure above this floor can no longer be supported, and so the upper part starts falling down onto the lower part below the critical floor, gathering speed until it impacts the lower part. At that moment, the upper part has acquired an enormous kinetic energy and a significant downward velocity. The vertical impact of the mass of the upper part onto the lower part (stage 4) applies enormous vertical dynamic load on the underlying structure, far exceeding its load capacity, even if it is not heated. This causes failure of an underlying multi-floor segment of the tower (stage 4), in which the failure of the connections of the floor-carrying trusses to the columns is either accompanied or quickly followed by buckling of the core columns and overall buckling of the framed tube, with the buckles probably spanning the height of many floors (stage 5, at right), and the upper part possibly getting wedged inside an emptied lower part of the framed tube (stage 5, at left). The buckling is initially plastic but quickly leads to fracture in the plastic hinges. The part of building lying beneath is then impacted again by an even larger mass falling with a greater velocity, and the series of impacts and failures then proceeds all the way down (stage 5).

 

The first problem with the model proposed by Bazant and Zhou is that there is no evidence that the columns were ever heated uniformly to anywhere near this temperature.  An article "Collapse of the World Trade Centre Towers" by engineer G Charles Clifton of New Zealand argues that temperatures must have been less than 700 Deg. C, well less than the 800 degree threshold noted by Bazant and Zhou. 

 

Clifton states:

 

In my opinion the fires had a less important role to play in the collapse of both towers than the damage from the initial impact. It took both to cause the collapse, however the fire was in no way severe enough to have caused the collapse on its own. The reasons for this opinion are as follows:

 

1. If the temperatures inside large regions of the building were in the order of 700+ deg C, then these regions would have been glowing red hot and there would have been visible signs of this from the outside. Also there would have been visible signs of flames. If one looks at the photos of the Cardington fire tests, the flames and glowing of the steelwork is clearly visible even in the large enclosure test where the maximum fire temperature was only 700 Deg C. In contrast, the pictures of the towers after the impacts and prior to the collapses show sign of severe burning over only relatively small regions of the tops of the towers, even pictures taken from the air looking horizontally into the impact region ....

 

Photos of the First Interstate Bank fire in Los Angeles in the early 1990s show what appears to be greater heating effects and over larger regions than were apparent in either tower.

 

[....]

 

2. When fully developed fire conditions ( temperatures of over 700 deg C) are reached within a region of a building, this results in the breaking of glass in any external windows within that region. This continuous breakage of glass as the fully developed fire spread through the floor of the First Interstate Bank, for example, was the most hazardous feature of the fire to those at ground level around the building.

 

In contrast, once the blast and fireball effects of the impacts had subsided, there appeared to be little ongoing window breakage from either tower, either as evidenced from pictures/video footage or as reported from the ground. Significant areas of window even remained intact within the impact region .... This is further evidence that fully developed fire conditions did not spread much through and beyond the initial devastated region, following the impacts.

So the basic premise of Bazant and Zhou is seriously flawed.  But let's move on to their next analysis:

For our purpose, we may assume that all the impact forces go into the columns and are distributed among them equally. Unlikely though such a distribution may be, it is nevertheless the most optimistic hypothesis to make because the resistance of the building to the impact is, for such a distribution, the highest. If the building is found to fail under a uniform distribution of the impact forces, it would fail under any other distribution. According to this hypothesis, one may estimate that C 71 GN/m (due to unavailability of precise data, an approximate design of column cross sections had to be carried out for this purpose).

Why would all the impact forces go into the columns as compression?  In a tube structure, the columns would bend outwards, placing stress on the perimeter frame and the floor trusses.  The entire tube and core system is a complex box which is much stronger than the columns alone.  Therefore, the number 71 GN/m is probably a very serious underestimate of the strength of the underlying building.  

 

As if to recognize the problems with Bazant and Zhou, Clifton proposes a buckling theory for the South Tower which is similar, but is based on locally focal structural defects rather than any uniform distribution of stresses, and which does not require uniformly high temperatures.

 

Immediately after the impact the perimeter frame in the South East corner would have been severely weakened, being reduced to an unknown number of intact box columns in towards that corner on each of the two sides.

 

However, these columns would have lost the lateral support from the floor slabs over many levels and would have had to function as isolated columns spanning multiple storey heights. They would likely have suffered blast damage and loss of alignment, however immediately following the impact they still retained sufficient compression capacity to resist their share of the loads from the 30 or so floors above the impact region.

 

The fires started by the impact would have then progressively weakened the vertical load carrying capacity of the remaining core, causing progressively more load to have to be carried by the perimeter frame system. In my opinion, based on the footage taken of the building over that time, the fire would have had little impact on the strength and stiffness of the perimeter frames, even in the damaged corner. The stiffness of this system above the impact region would have distributed this load approximately uniformly around the perimeter frames, increasing the loading on these frames through the impact region, including on the residual columns in the damaged corner.

 

Finally the combination of increasing compression load on these damaged columns, with second order effects from this load acting on the buckled shape of these columns over their unsupported length, would have caused their

collapse. This collapse would have initiated in the damaged corner and spread rapidly over the impact region, causing the tower above to fail by toppling sideways with the floors above the impact region momentarily in an intact condition. 

Clifton's argument seems almost reasonable, from a structural engineering point of view.  But is it correct?

 

Videos of the South Tower collapse show the upper portion of the building appearing to slide downwards, possibly disappearing behind the standing lower portions of the building for a few moments as explosions emerge from the windows.   I have spoken to an eyewitness of the collapse, and his statement was that shards of glass flew out from the sides of the building, and then the face of the building appeared to bow out for a moment either before or during the  collapse of the upper portion. 

 

A recent Nova television program ("Why The Towers Fell") suggests that the core of the South Tower may have remained intact momentarily as the tube wall collapsed.

 

If the initial phase of the collapse involved primarily sliding and shearing effects, it would account for the low friction and high speed of this phase.

How it might have been done.

The World Trade Center was leased by  Westfield America and Larry Silverstein, on  April 26th, 2001.  Zim Israeli Shipping moved out of the buildings around that time.  With a certain amount of shuffling of tenants from floor to floor, it should have been possible for the owners to gain access to various parts of the building.  Critics of the demolition theory have often remarked on the difficulty and expense of explosive demolition, requiring tens of thousands of pounds of explosives, drilling into structural members, and months of time.  However, a particularly diabolical structural engineer with a clear understanding of the unique flaws of the WTC architecture, might have hatched on the plan of exploding the trusses supporting a single floor.  This would have been sufficient to bring about the sequence of events which destroyed the towers, with the added benefit that if an airplane strike had occurred, the upper stories would appear to fail at the location of the strike.

Have we been lied to?

At the very least, there has been plenty of confusion and controversy.  The first question was that the role of the load of fuel from the aircraft.  Early reports were that the hot fire was responsible for the collapse, but other observers pointed out that  no kerosene fire can burn hot enough to melt steel.  In point of fact, most of the fuel in the jets was contained in their wing tanks.  The thin aluminum of the tanks was pierced or stripped as the airplanes penetrated the walls of the towers, and the result was the huge fireball which was seen on national TV, where most of this fuel was burned.

A hot, vigorous fire would have blown out many windows in the building and would have burned a red or white color.  This was not what happened.  The fire in the World Trade Center was an ordinary smoldering office fire.

But let's suppose that the fire was hot enough to melt steel.  What would have happened in that case?  Before it breaks, hot steel begins to bend.  This redistributes the forces in the structure and puts elastic stress on those parts that are still cool.  The process is asymmetric, so that the structure should visibly bend before breaking.  

Let's suppose the structure were sufficiently weakened that it did fail catastrophically near the point of the airplane strike.  In this case, the intact structure below would exert an upward force on the base of the upper story portion of the building (the part that has been broken loose), while any asymmetry would allow the force of gravity to work uninhibited on the tip of the skyscraper.  Thus, the top section of the skyscraper would tip and fall sideways.  This seems like common sense, and the analysis of Bazant & Zhou may not be sufficient to disprove it.

A thorough forensic examination of the wreckage, might have provided a confident answer to the questions of why the WTC towers collapsed.  Have we had such an investigation?  Bill Manning in Fire Engineering wrote: 

Fire Engineering has good reason to believe that the "official investigation" blessed by FEMA and run by the American Society of Civil Engineers is a half-baked farce that may already have been commandeered by political forces whose primary interests, to put it mildly, lie far afield of full disclosure. Except for the marginal benefit obtained from a three-day, visual walk-through of evidence sites conducted by ASCE investigation committee members- described by one close source as a "tourist trip"-no one's checking the evidence for anything.

 

Here are links and supporting documentation:

Bazant & Zhou

G. Charles Clifton part I

G. Charles Clifton part 2

PsyOps articles -- now back on the Web.

Article on wind sway stabilization

North Tower collapse video

New York Times article 1

New York Times article 2

Justin Raimondo http://www.antiwar.com

Michael Rivero http://www.whatreallyhappened.com

Eric Hufschmid http://geocities.com/erichufschmid/index.html

Jared Israel http://emperors-clothes.com/news/albu.htm

Bill Manning   Fire Engineering

Jim King on WTC collapse

Powder analysis

Posted 3/28/2002 by Jerry Russell; Updated 3/31/2002 and 4/4/2002

Updated 4/6/2002: reference to WWII airplane strike to Empire State Building deleted, documentation added.

Updated 5/1/2002: Video labelled as "South Tower Collapse" was actually a video of the North Tower!!  This error led to the mistaken argument that the South Tower floors must have pancaked before the tube walls collapsed.   

Updated 2/14/2003: removed redundant arguments to sharpen focus of article.

The author has a master's degree in Electrical Engineering from Stanford University, and a Ph.D. in Psychology from the University of Oregon.