I'm pretty sure that it is delicate work. And a bit dangerous. It takes a lot of expertise to know which columns can be (safely) weakened (by how much) and which ones must be left for the final charges.
The building that comes down is much weaker than the one that was originally built.
Do you have a source for that?
Ideally one that gives us some quantities (such as "30% of columns severed" or "all columns reduced by 50%", something to that effect)
In the meantime, I want to remind you that Danny Jowenko, in the 2006 interview so often quote-mined by Truthers, also expressed zero surprise that the towers would collapse completely once the top gets going, for that is what gravity does also in controlled demolitions. No mention is made of a requirement to pre-weaken the structure below.
I certainly don't see how it's relevant here. If you're up to it, you can enlighten me.
Yikes.
Seriously?
A gravitational load applied dynamically exerts a force AT LEAST TWICE as large as the same mass does statically.
The factor 2 only applies if the mass is applied coming from a vertical distance (or, equivalently, an initial velocity) of ZERO.
Once the mass already is moving down as it loads a bit of structure, the forces multiply quickly, and easily far beyond static capacity.
Let me elaborate the concept of dynamic loading from a height/velocity of zero:
Imagine WTC1 being built up to the 95th floor, and the top 15 floors being assembled separately, then lifted by a giant crane to be placed onto the lower 95 floors.
Case 1: You carefully make the column ends touch, with just a wee bit of force experienced by the lower structure. Then slooooowly lower the crane, such that the load is applied (increased) linearly, until the elastic starin of the lower 95 floors are equal to the weight of the 15 upper floors, at which point the crane no longer bears any weight: The 95 floors now "feel" the static weight, that is mass times g (gravitational acceleration) of the top.
Case 2: Again, you gently lower the crane until the top column ends juuust touch the bottom column ends, but almost no force between them, the crane still carries the weight of the top, it's just hanging at a height of zero above the target. Now cut the cables between crane and top in one instance: The weight of the top will start to bear on the bottom, which therefore starts to "feel" the weight; in reaction to this, both structures get compressed elastically - strain energy builds up, columns shorten, the top moves down and accelerates - until the recoil force is as large as the simple weight of the top - this is exactly the point where the static whole tower is in equilibrium and not moving - but our case 2 dynamically applied top is at this point still moving downward, at its maximum velocity - from this point on, as the bottom gets compressed even more and recoil force increases beyond the weight of the top, the top now starts to get decelerated, until it comes to a stop. At that point, elastic compression (strain) is at its maximum - and the force experienced by the columns at the intersection between top and bottom happens to be TWICE the weight of the top, hence TWICE the force of a statically applied top
Case 3: The top is lowered by the crane, with columns perfectly vertically aligned, but not all the way: This time, we cut the cables simultaneously an inch or a foot or twelve feet before column ends meet. This way, the top already has some velocity before the structures get loaded and elastically compressed, the columns end up getting loaded MORE THAN TWICE when they reach the point of maximum compression - and there really is no limit to the factor by which load is multiplied through dynamic loading. Bazant&Zhou's result in 2001 was that 12 feet would very definitely result in an overload / buckling.
Case 4: Like case 3, except that columns no longer align vertically, instead the top is leaning out of plumb, and column ends either meet imperfectly or miss each other entirely, such that load bearing capacity is nowhere near where it was before. In this case, the multiple load of Case 3 is compounded by a fraction of capacity.
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I hope you now will never again pretend that it hasn't been explained to you why the fact that the towers were able to stand as built (static loading) in no way predicts "no total collapse" once a 15-floor top part has begun to fall (dynamic loading, compounded by deteriorated alignment/loss of capacity).