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SL4V3M4YB3 -> RE: SENATE BURGLARY: CIA DOMESTIC BLACK-OP TEAM ARRESTED (2/10/2010 8:27:06 AM)
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ORIGINAL: Real0ne
joint rotation wont twist the column sideways.
Depends on the relative stiffness of the beam and column, along with what you mean by sideways.
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The only way you can get a global collapse of that system is if the slabs are prestressed and even then it would be a very specific failure mode with precise conditions.
little 4" conrete nonstressed slabs just break up and crack with any kind of significant flexure of its base.
You need to do some research on composite structures, the slab is acting as a compression flange so adds to the overall bending resistance of the beam. When that compression flange fails in crushing there is an immediate redistribution of that moment into the beam below. If the beam is incapable of taking that new moment then it will fail. The truss alone does not withstand the bending stress (steel is expensive and concrete is cheaper) this is the whole point of composite design; to reduce beam depth and amount of steel. So in your WTC example the steel is losing its bending strength and passing some of the bending stress into the slab above.
Think of the failure type along the lines of an R.C. beam. YOU NEVER OVER REINFORCE CONCRETE because you want the concrete to fail before the steel rebar within it does. If the steel rebar reaches yield point first then the collapse is sudden because the redistributed moment jumps directly into the concrete and it can’t take it. The moment has to go somewhere once the resistance of one component changes.
I don’t know how to explain this in more simple terms to a simpleton such as yourself. You aren’t using your imagination and you are still thinking of slab and truss acting as separate elements. The main problem I know of regarding pre-stressing is sometimes you can relieve the load from below causing it to snap upwards but this is a different issue altogether. Also in the modern sense this risk is now mitigated by ensuring better end fixity.
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secondly you would need enough fire to at a minimum envelop the whole floor, not just one or 2 or 10 and it would need ot be uniform and it sorta was in 1975 on the 11th floor and oops didnt come crashing down now did it.
The failure mechanism for the WTC is well documented. The short version is: the impact of the plane stripped off fire protection to columns and exposed it directly to fire. You will not find many steel buildings that remain standing for long if directly exposed to fire. The buildings themselves also didn't have much of a key element design philosophy to them either. Also as I pointed out previously; steel conducts heat quite well so only small portions of it need to be exposed directly for the heat to then be conducted throughout the frame locally. Also perhaps you have water from sprinkler systems spreading around a liquid fuel fire (no foam in such systems as they are not expected to deal with liquid fuel fires.)
Columns are quite critical, the example I showed had fire protection to the columns because they were only looking at composite beam deflection. Those test buildings may have collapsed further if there wasn’t any fire protection to the columns.
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Long story short I do not believe you know anything about structural anglneering.
Well I have difficulty believing you know how to wipe your own arse, so we are all entitled to our opinion of one another aren't we?
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aww what a bummer the gif of the sagger proving my point is to large for this lame board. Oh well enjoy this one instead:
I forget what your exact point was, I think it goes somewhere along the lines of 'Look everyone I'm a Prat.'
You've talked of many things. If you ultimately believe a plane crashing into a building then causing a fire throughout a whole floor or two can't release all the potential energy of the structure above and turn it into kinetic energy, then you is a bit mentally challenged. Your point as to which direction columns should or shouldn't buckle is a complete diversion. Like I said earlier columns need lateral support at each level to work efficiently. So even if (and it’s a big unrealistic if) floor deflection doesn't cause joint rotation then the column at least is going to lose its lateral support at that level meaning the effective length is too great to support the load above.
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