Does anyone have a more precise analysis of exactly when Building 7 began to tilt noticeably southward? My best guess was two seconds of straight-down, and based on eyeballing the windows lined up against the building silhouetted in front of it, the tilt began in the third second.
I really don't believe that this matters in the slightest. This does become "trying to calculate where all the matches land when the match box hits the floor" level of irrelavent detail.
The important question is "what knocked the match box off the table?"
The only significant inference from its tilt to the south (and possibly south-west) is that the external shell (not the whole building) failed first in the southern-most part of the west wall.
Also, does anyone know how much strength is lost when a column buckles? I'm guessing a very high percentage, like 98% or so.
Sure.
From Bazant & Verdue, Mechanics of Progressive Collapse.
Fig 3.
The "Crushing Force" (i.e., height of the curve, also called "Crushing Resistance F(u)) is equivalent to the force that the column can generate.
Buckling begins
immediately after the curve has reached its peak, and the slope of the curve BEGINS to go negative. Essentially one iota to the right of the peak value F
0.
You can see that a fully formed, 3 hinge buckled column will support somewhere <5% of its straight load carrying capacity. Your 98% strength loss is a good estimate.
Note well that "buckling" is not the same as "breaking". A column (even a wooden stick) can buckle, but not break. There is a famous shot (in one of the BBC videos on 9/11, IIRC) that shows an MIT professor demonstrating buckling using 4 wooden sticks, a couple of spacers & iron weights. He shows that the sticks (columns) can support a certain weight as long as they are laterally supported 3 times up the side (representing the floors). But when he removes the lateral supports, the sticks cannot support the same weight, they buckle & drop the weight.
The slope of the force vs deflection curve went negative. That defines "buckling". The sticks did NOT break.
But be careful how you think about it.
Buckling is crucial to collapse initiation.
Buckling is relatively irrelevant to collapse progression, since the columns did not buckle. The connections fractured. Fracturing connectors takes a MASSIVELY reduced amount of energy compared to column buckling.
That's why the idea that other forces like torquing and leveraging can bring the speed of collapse of one small part of the building to freefall acceleration.
It's simple: Free bodies (those that have no forces other than gravity acting on them) will fall at a constant acceleration of g (decreased by air friction). See the red line in the graph below.
The external wall were NOT free bodies. They were attached at many points to the internal, already fallen core, by beams & girders.
There is NO law of physics, or Newton, that says that they cannot fall at accelerations less than g, equal to g or greater than g.
People like Gage, ergo & chris7 simply don't know what they are talking about.
Chandler knows what he's talking about. He simply oversimplifies the situation to the point that he is simply, completely wrong.
Also Tom, like you I have asserted in my video 18 that the lines connecting the data points show slightly faster-than-freefall for a total of around one second. Chris7 says that is within the margin of error of the measurements, which seems possible to me. That's why I'm always careful to say "possibly faster than freefall, but within the margin of error."
I do not believe that the "faster than g" segments are the result of data errors.
The use of 7.5x as many data points shows clear trends in the data.
Once again, the green line is the actual instantaneous acceleration, and the red line is something that is really falling at FFA.
Do those two lines look anything alike to you, Chris?
They don't to me.
Amazing how the correction of my minor mistake takes me even further from chris7's assertion, isn't it?
Not amazing in the slightest.
The more you understand, the further from Chris7's flawed opinions you'll be.