Contrary to what they assumed, there were signs of bowing and sagging in the towers well before the collapse of the towers. The very thing that Griffin points to as a feature of a fire caused collapse we can see in photos of the World Trade Center. Griffin writes, "in fire-induced collapses---if we had any examples of such---the onset would be gradual. Horizontal beams and trusses would begin to sag; vertical columns, if subjected to strong forces, would begin to bend. But as videos of the towers show, there were no signs of bending or sagging, even on the floors just above the damage caused by the impact of the planes." But contrary to what Griffin claims, there were indeed signs of bending or sagging. Witnesses reported it and photos document it. Griffin is simply wrong.
The very first video I uploaded to YouTube which should have put an end to David Ray Griffin's ignorant claims several years ago: https://www.youtube.com/watch?v=sGAoRrBoPRM&list=PLfrlsC1yJ2dSIYYEW5LRZx-m25GW9xAps
The very same area that we can see in a photo here: https://sites.google.com/site/representativepress/east-face-of-wtc2-9-21-am
then you can see that the bowing has progressed in a photo here: https://sites.google.com/site/representativepress/east-face-of-wtc2-9-53-am
Then you can see that very same bowed in area where it reaches the point of total failure in this video here: https://www.youtube.com/watch?v=BQmaGCxHxj8 We can see the early stages of the bowing in the two pictures linked above and then we can see how that bowed in area reaches the point of total failure in the video linked above.