The towers each survived the impact. The structural steel is covered with fireproofing rated at 2-3 hours. The steel is also rated at twice the load it is designed to carry: i.e. the weight of everything above any point in the building. The towers were also designed as stiff tubes due to the tremendous wind load at the higher elevations.
The south tower was hit closer to the middle. I expected that tower to fall from the damaged floor up, and it should have fallen at whatever corner or side the fireproofing burned through first, thus buckling the steel and causing a collapse. That's not what I saw on TV. I saw a straight down collapse, similar to an implosion caused by demolition charges. In reality, the top of the tube should have fallen to one side.
If we say that the steel at the bottom gave way, which is possible due to the first four floors being engulfed in flames, thus the fireproofing could have been burned through and the steel weakened, I would have expected the building above to fall in a semi-intact state.But it seemed that the building disintegrated downward, as it fell. That doesn't add up. The steel in the other parts of the building had not been engulfed in flames, thus the "tube" in some parts should have been intact.
Then the north tower fell within minutes of the south tower. That tower had much less damage, and the damage was much higher up than the south tower. Yet it fell in the same pancake fashion, straight down. One tower collapsed right down to ground level, one down to twenty floors. Controlled demo charges seemed to be at work here.
My take is that the buildings were to be hit by the planes-plan A- and then plan B would be to demo the towers an hour later. Plan B would happen whether plan A succeeded or not, as plan A had more visual impact to the world then a straight demo job.
It's truly shocking that the structure failed in such a major way, but I'm not sure it was secondary explosions, as I saw or heard no evidence of that on the videotapes.