@Olivier5,
I've already told you that I don't know what the physical causes were. I only know what the observable effect was. This thread concerns the physics of what happened on 9/11. You are implying that if I don't know who did it, how they did it, and why they did it, then the physical anomalies mean nothing and need not be addressed. That's a ridiculous idea. Now let's get back on topic and pick up where we left off.
You've declared that the explosion--which can be heard at the
10:55 mark on the video below--was the sound of the Tower cracking open. However, you've never ever heard the sound of a building cracking open. You see your problem there?
https://www.youtube.com/watch?v=VJMPN66FHwY&t=1s
10:55 mark.
Next, the law of conservation of energy dictates that the upper block of the North Tower would have exhibited
at least a visible jolt as it descended, but video of the collapse shows no jolt and no slowing down whatsoever as it met with the lower intact core structure. When a falling structure hits a stationary and intact structure, two things happen. The lower structure will resist, and the upper structure will be slowed or arrested. The video below clearly shows that the upper structure was not slowed down in the least. Watch the antenna at the 30 second mark. This means that either the lower structure was neutralized before the upper structure descended, or the laws of physics were violated. Which do you suspect is the more likely explanation? Your "The core beams had been heated enough to weaken their strength significantly" is certainly not an explanation for what was observed.
https://www.youtube.com/watch?v=xGAofwkAOlo
30 second mark.
The South Tower, whose upper block fell over the edge, leaves you with nothing to cause its collapse, unless you want to hold on to your idea that the floors dragged down the core structure and perimeter columns even though those structures were designed to support the floors, and not the other way around.