WTC Collapse Alternative

[John Young]

It would not take a lot of explosives to breakaway an entire floor
constructed with steel joists like WTC; it is often done in planned 
demolitions with a small charge placed at each joist support --
customarily a shelf angle either continuous or segmented.

What would do it though without explosives is the twisting torque 
and vertical flexing in the tower tubes caused by an aircraft hitting 
off center of the tower's axis at 300 mph. 

Survivors tell of the tower swaying several feet, but they are not 
likely to have perceived the torque as such. The hit on the South 
Tower (the second hit) was more off center than that on the 
North Tower.

Tall buildings are designed with some torque resistance against
turbulent weather and earthquake but probably not for torque 
coupled with high impact. 

The WTC towers were made of two tubes, the exterior wall system 
(the more flexible) and the interior core (the more rigid), the two 
tubes connected by light-weight floor structures and at about 
1/3 intervals, heavy-weight transfer and mechanical equipment 
floors.

>From examining the wreckage the steel floor joists rested on shelf
angles welded to vertical supports. These angles are designed to
support vertical loads not much twisting.

The joists could have lost their support by tower twisting of no more 
than a few inches, the shelf angle welding beads popping, or the 
joists slipping off the angles that did not break loose (as photos
show many did not).

If the floor slab concrete was not reinforced it would have crumbled
easily under torque. Again, most building structure is designed to
resist vertical loading not substantial twisting. (Structural connections 
are designed to resist relatively minor twisting caused by static
and dynamic loading.)

There is a valid question of why the buildings did not collapse immediately 
if floor structures were damaged and serial pancaking set in motion.
The raging fires surely contributed to further weakening of the
building structure, but probably due to adverse effect on floor structures
and in particular on the light-weight shelf-angle welded connections 
between the floor system and the heavy-weight vertical supports.

It is at these connections that demolition designers place light-weight
charges, as well as heavy-weight cutting charges on vertical supports.

For comparison, a WTC shelf angle may be 1/2 to 3/4 inches thick 
welded with a bead about the same while a vertical columnon WTC 
could average 2-3 inches thick, increasing in thickness from top to
bottom.

The WTC designers claim the buildings were capable of withstanding
a 727 hit, asserted at original building and after the 1993 bombing.
But the design criteria for that protection have not been published
so it cannot be determined what crash scenario was used for design.