WTC Collapse Alternative

[John Young]

What is odd about the WTC debris is how few larger fragments 
of concrete it contains. Reports on the structure say reinforced 
concrete was used for floor slabs and customarily in demolition 
such material breaks apart in relatively large chunks, the broken 
concrete remaining attached by steel reinforcing bars or wire mesh. 

However, there is a structural design where plain concrete is 
placed on metal floor decking with little or no reinforcing. Also, 
lightweight concrete, whose course aggregate is cinder rather than 
stone, is often used to limit the weight of the structure, especially
in high-rise building. This lightweight material does not usually 
have the strength of regular concrete and strength is provided by 
metal decking below it. The large amount of dust produced by the 
towers' collapse and the relative lack of larger concrete shards 
raise the possibility that the building disintegrated at least in 
part due to lesser strength of the type of concrete used in the 
floors. That remains to be examined.

Observation of the towers' remains show that while main steel 
vertical structural members of the exterior wall and central core 
withstood the collapse, the horizontal floor-supporting trusses 
broke away from these vertical supports at the points of attachment 
(probably made by welding). One view of the North Tower (1 WTC) 
shows the exterior and core remnants and the ruptured points
of floor structure attachment. 

Photos: http://cryptome.org/wtc/wtc035.jpg
        http://cryptome.org/wtc/wtc047.jpg

What this suggests is the possibility that the structure of each 
floor collapsed due to load of collapsing floors above them, and 
that only afterwards did the vertical supports at the exterior 
and core collapse. Review of video of the collapse appears to 
confirm this sequence as well as the consequent supposition that 
the floor structure was the weakest part of the buildings
-- which would not be uncommon for floor structure supports only 
a single floor while the vertical members support all floors above 
them. However, a slow motion examination of visual recording would 
be needed to confirm exactly what collapsed first.

There has been speculation about the initial step in the collapse 
of the buildings, most commonly attributed to the intense heat of 
burning jet fuel softening structural steel, usually the steel of 
the core. However, it is possible that collapse of the core steel 
was not the initial phase, but instead it was the floor structure 
breaking away from vertical supports. The collapse in this scenario 
would be that of floors dropping one after the other onto floors 
below, the load of the upper floors overwhelming the relatively 
weak attachments of floors to vertical supports -- the attachments
customarily being designed to support only a single floor load. 

For example, a single floor dropping onto the one below could have 
ruptured the next lower attachment, thus setting off a disastrous 
sequence. This could have occurred without fire initially weakening
the vertical steel structure as has been speculated. The impact of 
the crash, and/or subsequent swaying of the buildings, could have 
ruptured floor structure attachments, and only one floor breaking 
away would have been enough to precipitate the collapse.

Alternatively, the fuel fire, and flaming building contents, could 
have weakened floor structure and/or its attachment to vertical 
supports, in particular if the crash destroyed fire-protection 
materials of the floor structure. Thus, with heat weakening floor 
structure along with the attachments being ruptured by the crash, 
the collapse sequence commenced.

One significance of these speculations is that weakening of the 
core steel by intense heat may not have been the initial cause of 
collapse. A New York Times report of October 6 describes an 
investigative engineer "finding what appears to be a few pieces 
of the south tower that were directly hit by the Boeing 767 
jetliner, and the discovery poses a few new puzzles. While the

impact sliced through half the column, the column did not buckle; 
each column is designed to support the weight even if half is 
missing. The column also exhbits no outward signs of smoke or 
heat damage."

Another signficance is that more steel reinforcing in the concrete 
could have increased the strength of the floor structure and 
better withstood the initial step in the floor-by-floor collapse 
sequence. And, the immediate and long-term adverse affects of the 
huge dust clouds of the collapse might have been lessened. It is 
likely that some victims were suffocated by these clouds.