Tower construction

The key to understanding the collapse of the Towers is in their design, most notably how the elements carried the load and interacted with each other.  They were generally simple in their construction, in essence consisting of a core, a perimeter wall, a hat truss [the top], and a number of floors. 

The rectangular core consisted of 47 vertical columns arranged with horizontal and diagonal cross bracing.  Its job was to carry the weight of itself, the enclosed elevators and core floors, as well as a proportion of the hat truss, and about half of the main floor load. 

The perimeter columns formed a square with horizontal bracing and chamfered corners, and carried themselves and about half the main floor load.  The perimeter columns had a design safety factor such that they would have been capable of carrying the entire load themselves without the help of the core, although this would never be the case under normal circumstances. 

The floors carried their own weight and that placed upon them.  They transferred this load to the perimeter structure, and the core structure to act vertically, although there would also be a small horizontal load under normal circumstances of loading.

There are some important details to know in order to understand the collapse.

1/  The perimeter structure could carry all of the vertical mass above it at any point with ability to spare.  So immediate collapse would not occur even in the event of a failure of all of the load carrying ability of the core.

2/  Similarly the corners alone of the perimeter walls could theoretically support the total vertical mass above them.  The corners can be regarded as the most rigid and strongest part of a structure.  Note how in demolished or fallen down buildings, a corner is often the last part standing.

3/  The design of the towers was such that the floor trusses connected outwards from the faces of the rectangular core structure to connect only with the mid wall perimeter columns. The floor trusses carrying the floor sections in each corner were not connected directly from the core to the perimeter, but rather from the perimeter to a transfer truss and then to the core. The relevance of this is that if the core pulled downwards it would act effectively only through the floors to a proportion of the perimeter columns - those in the middle of each wall inside the lines of the core. The perimeter columns at and about the corners, those outside the line of the core, would not be connected via the floors directly to the core, but via the floors via the transfer truss to the core, and would therefore be less affected by a force caused by downward movement of the core structure.

Phase 1

The object of Phase 1 was to weaken the Tower by attacking the central core structure, disassociating the core structure  columns at a level below that of the impact [Plane 1]

Effects of of phase one

  The result of a failure of the core such as this prior to collapse initiation would be to allow the mass of the core and floors above the failure to act downwards on the hat truss and the load would then be transferred to act vertically on the perimeter columns.

The downward movement of the upper disassociated core structure would also allow a horizontal force to act on the mid wall perimeter columns through the floors to cause inward bowing of the mid wall perimeter columns.

Evidence of phase one

Upper core failure, lower core survival

     The still photographs taken by Aman Zafar during the collapse show that the core structure suffered a total and complete failure resulting in a complete disassociation of the core structure through a horizontal plane.  [plane1]    The photograph shows that part of the core remained upright as the collapse front progressed to ground level.

These two screen captures also show that the lower setion of the core structure was upright after the collapse front had progressed.   One shows the core structure, and the second shows the same thing highlighted with added graphics.  They are taken from,

http://video.google.com/videoplay?docid=-1615521411849861778

This is more revealing than the still photograph in that we can see no downward movement of the remaining section.  Collapse of this section was initiated at a later point in time.

Perimeter column bowing

Note that the bowing identified by Nist was only on one side of each tower. It was not generalised across all of the tower.
For the example of WTC1, Nist report bowing only on the South face, storeys 94 - 100. For WTC2 bowing only on the East face, storeys 77 - 83.
Note that for both Towers only the MID-WALL perimeter columns were bowed.The corners were not visibly bowed.

If the bowing was being caused by a pure vertical movement of the upper core structure, that is with no tilt, the bowing would be present on all four sides.

Angle cut columns

Note the important point mentioned above that the design of the towers was such that the floor trusses connected outwards to connect directly only with the mid wall perimeter columns.   The relevance of this is that the core pulling downwards would act effectively only through the floors to a proportion of the perimeter columns - those in the middle of each wall inside the lines of the core. The corners which were the strongest section of the perimeter structure were not placed directly under this load.

Sagging floors

Further doubt is thrown on the Nist theory that the bowing was caused by sagging floors by examination of the photographs contained in their own report on WTC2, which shows floors sagging in a direction perpendicular to that which would be required to cause the bowing. 

http://wtc.nist.gov/WTC_Conf_Sep13-15/session6/6McAllister.pdf

If the bowing was caused as Nist describe, then that would require that the floors are still connected to the East perimeter wall columns in order to pull them in, but the report shows disassociation of many of these connections.  Those floor areas could not both be disconnected and contribute to a pulling action.  This reduction in the number of floor to column connections would severely limit the total load which could be applied

Note also that while there is evidence in their report to show sagging did take place perpendicular to the direction required, there is no evidence presented that any sagging took place actually in the direction required.

The animation shown to the left demonstrates graphically that all of the core structure and perimeter structure must bend to accomodate the tilt.  Sagging floors cannot be regarded as being responsible for this level of pre collapse damage.

Early movement of antennae

Examination of the video evidence shows that the antennae began to move downwards a fraction of a second before the roof line.  This can only be achieved through a downward movement of the core to which it was directly connected. 

The early movement of the antennae in WTC1 when regarded in relation to the roof line shows that it was the movement of the upper core structure acting on the perimeter columns through the floors, which was driving the collapse in the early stages, and not vice versa.

Phase 2

The object of Phase two was to achieve collapse initiation by an attack on eight initial points.   These were located at each of the four corners of the perimeter structure on two storeys and caused crushing of the volume between.  An ancillary attack was made to disassociate the hat truss.

Effects of Phase two

The result of these attacks was a radical reduction in the corners ability to carry load and this resulted in failures at the points of attack, the corners of the perimeter structure.  The full weight of the tower above these attacks was then transferred to the midwall perimeter columns.  These were already suffering under both vertical and horizontal forces and immediately suffered failure between the points of attack.  The hat truss was disassociated allowing the upper core structure to drop downwards and thus pull inwards on the perimeter structure.

Evidence of Phase Two

Changes to colour and characteristics of smoke emissions 

Immediately prior to collapse initiation there is a distinct and obvious colour change to an area as highlighted in this photograph.  There are several of these to be seen on other videos and photographs.  These emanate from the same general position relative to the Tower corners and it is in these areas that the attacks occur.

Three of these areas are very obvious in the next photograph.  Two are evidenced by the short but intense flashes of light and the other is in the smoke emission highlighted.  

The corner column areas were by their nature one of the most rigid section of the towers and were to a large extent exempt from the loading applied by the core through the floors.

Removing the ability of these columns leaves only the mid wall sections to carry all of the load above Plane 2 and all of the core load above Plane 1. The load would act both downwards and inwards through the floors on these remaining columns, and at that point, collapse of the area between these planes began. This was the disintegration of the upper section that commentators have noted. In actuality a partial disintegration.

The planes were located as follows; Plane 3 at or about the storey where the bend in the upper section occurred and Plane 2 at or about the storey from where the molten metal was seen to be expelled. These represent the upper and lower bounds of the volume which was first crushed.

The following graphics obtained from the excellent terrorize.dk site show this very clearly.  These photographs in and of themselves destroy the arguments of Dr. Bazant and his fellow advocates of the "crush-down crush-up" theory of collapse.  They demonstrate that the first storeys to undergo crushing were those between the two planes indicated.  The lower collapse front did not accelerate at the same rate as the roof line.

It is exactly at these planes that the first obvious failures are seen to occur. 

The corner of  WTC2 is seen to bend at the point where the smoke emanates in the photograph above and at the lower plane indicated by the second photograph.

Examination of video evidence shows that the following phenomenon coexist in time and space.

1/ distinct colour changes in smoke

2/ behavioural changes in the smoke emissions

3/ flashes of light

4/ local and definable damage.

Rotation of upper section.

 This can be envisaged by imagining a Christmas tree being pulled downwards through a packaging funnel.  The force only has to be generally downwards to have the trunk passing through the centre and it will tend to straighten or arrest any tilt of the tree.

Note that the collapse front, plane 2 does not begin to move downwards immediately, there is a distinct time difference between the first movement of the upper section and the first movement of the collapse front. 

The collapse front also exhibits strange behaviour which has not been addressed by the official inquiries.

Phase 3

Behaviour of ejections

Almost immediately after collapse initiation there followed a rapid series of expulsions of dust and debris issuing simultaneously from horizontal lines across all walls of the tower simultaneously.  These progressed rapidly downwards and in some instances their progression was faster than that of objects falling outside the tower.  

The photograph above shows the first early stages of these expulsions on the same level as the highlighted flash.

It has been argued that the expulsions in the early stages of collapse were caused by air pressure between pancaking floors but the tilting of the upper section would not allow the symmetry in the collision between floors required for the symmetry of the expulsions. 

   Simultaneous emissions of trapped air across whole floors could only be caused by straight down collapse of one floor onto another and thus the tilting of the upper section rules this out as a cause for the behaviour of the expulsions.  In other words the tilting rules out pancacking floors as an explanation for the ejections of dust and debris.

     Further refutation of theories which explain these expulsions as being part of a gravity only collapse is found by examination and identification of the storeys from which these expulsions are emitted.  The following analysis shows that the expulsions do not occur on every storey but rather from storeys about four or five apart with no such comparable expulsions from storeys between.

Note also that the corner columns below plane 2, unlike the mid wall columns, do not collapse with the collapse front, but remain standing for a period of time, before then falling inwards.

These corners do fail at a slightly later stage and in at least some cases fall inwards after collapse has progressed below their level again showing the spacing and pattern of the planes under attack.  It would not be necessary to continue these attacks all the way to ground level.

The survival of these corner sections of the perimeter structure, show the order of collapse.  To allow this two vertical lines of spandrel plates - the perimeter horizontal bracing - have to be disassociated at each corner.  The mid wall columns disassociated from the floors, can fall outwards, while the corners remain upright.

The later failure of these corner sections, and particularly the arcs which they describe as the fall, showing that further failures took place above ground level, are evidence of further attacks on the corners as in Phase two, at other planes below the impact levels.  This is again reinforced by flashes of light and smoke emissions.

These two photographs, again from terrorize.dk, graphically show the eventual failure of one such corner section.  The failure is again seated at the point in time and place of the flash of light. 

Expulsions from lower floors

As mentioned previously, in the area of the maintenance floors there was a more robust structure to satisfy local load considerations.  These required special attention from the demolition.  The following photographs showing large expulsions from those areas before the collapse front approaches their position shows the independence of these effects from the collapse front.

In short the progress of the collapse below the aircraft impact level was facilitated by detaching the floor to column connections below this level, and by attacking the spandrel plates connecting the corner perimeter columns to the mid wall perimeter columns. This is accompanied both in time and place by the expulsions of dust/debris. The mid wall perimeter columns below the aircraft impact, fall outwards as the upper section is pulled down inside them by the falling core. 

Phase 4

     This second photograph by Aman Zafar shows the collapse of the remaining section of the core.  Taken only a few seconds later the top of the core structure is only just visible.

    Most notable and revealing aspect of the survival of the core until this late stage of the collapse lies in how they behaved when they did fall.  The columns fell independently of each other showing that all of the horizontal bracing which previously connected them had been severed. 

No gravity only collapse could cause such precise disassociation of the core structure into constituent pieces, nor could it be responsible for the smoke and flashes of light.

Conclusion

This analysis shows the order of the collapse from initiation to completion of the collapse.  It has identified the outline procedure which was used to cause the towers to fall.  A consciencious investigation would have revealed all of these pieces of evidence and explained what each piece meant in terms of the initiation and progression of the collapse.

The 911 Commission, FEMA, and NIST examined the collapse and were charged with explaining the mechanisms of the collapse.  Not only did they fail to adequately do so, they failed even to identify and incorporate the various pieces of evidence detailed in this analysis.

This page is under construction and further details will be added as work on the final version progresses.  These details will include the evidence upon which my deductions have been made.