DIYbanter - View Single Post - Reichstag Fi 102 minutes that changed America

The Natural Philosopher[_2_]

wrote:
On 9 Sep, 23:00, Theodore wrote:

I really don't see why people have difficulty with all this. It only
takes one floor to give way and the weight and force of the building
above collapsing onto the next floor causes the pancaking effect.

Yes, if the one floor goes straight down.
If one side of a floor collapses, the floor below will collapse at the
same side; the building falls sideways.

No it doesn't.

You simply dont understand what a building is, engineering wise,
especially a tall steel and concrete one.

For it to fall sideways implies it retains enough structural integrity
as it falls to transit lateral loads to upper storeys to accelerate them
sideways. It cant do that if its effectively been sliced in half in the
middle.

Think of a pile of really heavy concrete biscuits, separated by cocktail
sticks. balanced one on top of the other. Now at a given layer start
removing coccktail stiicks. You cant push the lot over, because there is
simply no strength in bending: remember the ONLY lateral loads it was
designed for were high winds. Even an airliner ramming into it didnt
knock it over, because its so HEAVY. nothing is going to make it fall
sideways.

The question remains the same. How would you get one floor to go
straight down to initiate a straight-down collapse of the building? Is
it likely that would happen?

Its *guaranteed* to happen. If you lose structural integrity due to fire
at one level, affecting a large fraction of the 'cocktail' sticks,
sooner or later one, two three go, the floor sags, and at some point the
rest go more or less simultaneously - as one goes there is overload on
the next..you take out a complete layer of cocktail sticks..

Without a redundant, and much more expensive structure, progressive
collapse is almost 'designed in' to any large structure.