Below is a very clear and crisp you tube clip of WTC7 collapsing.
- Look closely at the roofline as it implodes inwards and all the windows popping out.
- Looks like the inner core columns are removed first then the outer columns the way the roof peels inwards first.
Looking a the floor plan of WTC 7 (Tower 7) bellow you can imagine what is going on; columns 58..81 are being blown apart, causing the hollowing out of WTC 7.
The exoskeleton of 60 columns stay intact until they too are demolished a few seconds