Originally posted by ANOK
reply to post by IrishWristwatch
But buildings are designed to hold themselves up many times over. FoS!
FoS applies to as-built geometry. Right? What happens when that geometry is lost? I asked above, do you think the components of the towers could be
put together
any which way and still stand? Let alone resist momentum? Surely you can see that a column end on a floor slab does not offer
the FoS of columns end-to-end. Did you know capacity of even the column ends is proportional to the contact surface area? That if the column ends of
the towers were displaced horizontally only a few inches they would only have a small fraction of design capacity with correct alignment?
They are never built so a slight over loading would cause failure.
Obviously true. But we're not talking about a slight overloading once the upper section is motion, it's a gross overloading even for the nominal load
paths. Take away the nominal load paths and there isn't even the capacity to support it statically. And that's ostensibly how the whole thing thing
initiates. Quasi-static loss of capacity and load redistribution until global failure conditions emerge. Once capacity drops below load, the load
begins moving. Once motion occurs, misalignment increases and effective capacity is reduced because it is largely bypassed. Not totally, but does
that matter? No.
You can talk about what ifs all day long.
What "what ifs"? I'm relating to you facts concerning physics and engineering mechanics, as opposed to just making **** up and calling it physics.
The only speculation I've indulged in is expressing my opinion that core failure precipitated collapse, and I stated it as such.
Facts speak for themselves and when you put all the facts together in context with steel framed buildings the OS fails.
There are times when I have to part company with the OS. Quite a few instances, And, yes, I'm on my own in those circumstances, all I can do is
state my opinion and why.
Not sure what you're on about with columns hitting columns on their ends?
You need to be or you'll never even come close to understanding what I say. Does this make it clearer for you?
Total cross-sectional area versus load-bearing cross section. You know why it's called 'load-bearing'?
How would columns hit columns that way?
Almost none would! That's the point! What on earth do you think the nominal load paths are? The paths that provide the FoS you talk about? That
FoS comes
exclusively from column end bearing on column end, in perfect alignment and perfect plumb. Offset the upper block a few inches and
that capacity is nil by comparison!
Why is this so difficult to understand?
How would a gravity fed collapse cause columns to break, in order for them to hit end to end?
They don't, as I keep saying. So, apparently you don't think they hit in perfect axial alignment, either. So tell me where the FoS you're going on
about comes from?
Steel would be resisted instantly if it hit another steel column that way.
This is completely made up pseudo science. Do I need to dredge up
thousands of YT videos showing steel objects in collision which do not
"resist instantly"? Can you think of a few everyday examples yourself?
But there is no reason the columns would break from their own weight. They had successfully held their own weight for years. What severed the
columns to begin with?
First of all, it is advantangeous to conceptually separate intiation from progression. There is a difference between the conditions which could lead
to the upper section moving due to insufficient capacity, and the conditions which could stop it once it attained a certain threshold velocity. To me
(and to anyone who's examined the mechanics in depth), this distinction is paramount. It is very easy to conclude that there is no viable load path
to arrest after a certain point, whereas the conditions leading to
initiation are subject to no end of speculation. That's another subject and
one I was not addressing in replying to you.
If the planes severed the columns why did NIST make up the sagging truss hypothesis?
Again, I'm not a spokeshole for NIST. I disagree with much of their work. I also happen to disagree with some of your claims, that's why I
posted.
edit on 29-1-2012 by IrishWristwatch because: (no reason given)
edit on 29-1-2012 by IrishWristwatch because: (no
reason given)
edit on 29-1-2012 by IrishWristwatch because: (no reason given)