CarolC,

The cores of the WTC towers were not the principal load-bearing members - that was done by the external steel columns. The cores provided some static (gravity) load bearing, but were mainly a central structure for tying the external columns (by way of the floor trusses and slabs) to keep them from splaying in or out.

The planes sliced each sliced a large proportion of the external columns and, for what it's worth, sliced the cores. That is why people could not get up and down past the crash-sites.

With the load bearing connections from the upper floors to ground cut at the crash sites, and the raging fires, it was only a matter of time before the partly or wholly unsupported upper floors collapsed.

As the floors are tied together, the collapsing floors immediately above the crash sites, dragged down the higher floors and that dyamic load caused the cascade failure of everything below it.

That is what you see when the top parts of the towers keel over slightly before dropping like pancakes and tearing down the rest of the buildings.

It's what you see as the floors continue to fall, tearing away from the cores, like a banana being peeled. The disintegration of the cores can be seen proceeding at a pace just behind the falling away of the floors. The cores are what guided the collapsing floors and external columns down in a straight line.

I'm sorry, CarolC, it just isn't at all complicated.

Now if people are going to make an extraordinary claim that this was a controlled demolition - for that is an extraordinary claim - then this is what must be done to support it.

* Find a way of rigging the buildings with explosives at a particular floor without anyone noticing.

* Find a way of protecting the explosives from being involved in a high energy impact, jet-fuel explosion, and then general fire that burns for hours in an unpredictable and uncontrolled way.

* Get two planes to crash into exactly the place that you planted your explosives, without interfering with the explosives.

Sorry CarolC - I am not some sort of leading expert, but I have a degree of practical experience and knowledge of these issues. What I saw and what I have read makes perfect sense. The controlled demolition theory does not. The cores are not the big deal, people are wrong to think that they provide a lot of support for the gravity load of a super-tall building. They do not. The people who propose controlled demolition as a theory, need to explain how the explosives were installed, installed in just the right place for the subsequent plane-attacks and how they were protected from early detonation.

WTC 7 was another structure supported largely by its steelwork. It was on fire from the early morning until it collapsed later that day. It was hit by the collapsing North Tower. It had a deficient sprinkler system. Firefighters were in the building throughout the day and reported the spreading fire, lack of water pressure and structural damage from falling debris. at 14:00, fireighters noticed the SW corner of the building was bowing outwards and they evacuated it at 15:30. It collapsed at about 17:20.

It just isn't complicated.