Challenge: Demonstrate Sagging floor Trusses Pulling in Perimeter Columns
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Newtons Bit
Penultimate Amazing
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Thank you.
Your input loads look correct, but I can't make heads or tails of the boundary conditions you have set: the axes aren't labeled.
Can you post the output principal stresses? I can garner a bit more information from that than I can from your previous von Mises stress output.
Newtons Bit
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I'd hope so! I've used RISA to design countless structures!
eta: if you want to turn off the AISC Appendix 7 stiffness modifications, you can go to Global->Codes and change the Adjust Stiffness ComboBox to "No".
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jaydeehess
Penultimate Amazing
Depends on what you would call buckled. If a column reachs sufficient plasticty that it has very little axial strength then yes it is failed and will deform relatively easily and if pushed off axis and downwards will bend over like a licorice stick.
However, up to that point it will still have strength to hold some load. It will deform only as much as the redistribution of loads causes the entire upper section to change orientation. IOW it cannot buckle, that is fold over, until the rest of the structure comes far enough down to allow that. If there is sufficient strength in the rest of the columns to support the upper section then a completely plastic column is being suspended from the hat truss.
Columns with full or near full strength will compress when loads get high enough, they will bulge equally on all sides if loads are axial similar to a wire under axial tension will thin as it stretches. If the load is nonaxial then it will bulge preferentially on the side under greater compressive force, with enough force and being unconstrained, it will then fold over.
Do not confuse effects on columns below the initiating event(s) with those involved in the initiating event.
Its unlikely also that all core columns, at the level of initiating events, were compromised equally or by the same mechanism. A few may well have been relatively unaffected by heat or impact damage and these would be as you say, last men standing at initiation of collapse and would fold like a cheap lawn chair when over the course of a few seconds, the bulk of the gravity load of the upper section was now on them. They would look different than the others which had suffered creep first.
Odd phrasing, 'dropping out'. Of course they were still there but incapable of carrying load which would have to be redistributed. That redistribution, as ozeco describes, will never be even. not even close. It will be transmitted preferentially to the nearest columns and preferentially to columns on the side towards which the upper section leans (because the center of gravity ahs shifted that direction).
The hat truss cannot evenly distribute gravity loads. What it does do is tie the perimeter column system with the core column system forcing them to move in unison under wind loading and probably combatting twist.
But you can actually test this out yourself in an experiment.
Building yourself an 8 foot tall tower out of 2X4s with a few perimeter and a few core 'columns'. Tie them together at the top with horizontal 2X4s and a3/4 inch plywood. That should make a good hat truss. Place a bathroom scale under each column. Now saw a gap in a few perimeter columns on one side and check on load redistribution. I highly doubt that you will see each scale move up by the same amount. I fully expect some will actually have less weight on them than before.
ozeco41
Philosopher
Credit pass.
Correct on "cannot evenly distribute gravity loads" - the same principles of load distribution apply whether under "designed for" conditions OR conditions prevailing with failures occurring several storeys lower in the tower.
Take care with the claimed "move in unison" when we are considering collapse initiation. For the same reasons I have cautioned JSanderO. The whole top block under failure is flexible so concepts of "rigidity" don't apply and "move in unison" has to be understood as "flexible unison" depending on all the structural elasticities/forces etc which apply under the conditions prevailing.
I backed off the statement about load redistribution....there was asymmetry for it to equal. How ever if you removed all the strength of the core 12 stories from the roof... the loads they supported... is then hung from the hat truss... the steel is in tension and the connections clearly are not designed for tension. But the loads of the 12 stories will be redistributed to the facade... before/as the connections fail and the core drops. Is like pulling at the hat truss from below with a chain... the chain breaks but before it does there is force applied to where its anchored and then to where the HT is supported.
This happens very rapidly and then/as the buckling of the facade occurs. The core col which lost strength likely caused the floors connected to the to drop and this provided the inward force... to the buckling expressing it as in inward bow.
That is how I see it.
This happens very rapidly and then/as the buckling of the facade occurs. The core col which lost strength likely caused the floors connected to the to drop and this provided the inward force... to the buckling expressing it as in inward bow.
That is how I see it.
Newtons Bit
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None of the architects I've ever worked with used the word "facade" in associated with steel columns...
It was a structural facade as opposed to a curtain wall... I know precisely what it was... and it the assemblies acted like vierendeel trusses... knitted together into what was a diaphragm.
The design was completely novel.
What the point here?
a rose by any other name has just as sharp a prick
jaydeehess
Penultimate Amazing
my bad wording, I was referring to the pristime building and its reaction to wind loads. A ridgid hat truss would work against leading or lagging between perimeters and core column systems, during wind sway periodic motion by keeping opposing top level perimeters parallel.
ozeco41
Philosopher
Understood.
In the current context of helping JSanderO work through some of the issues I'm being careful that none of us overstate the role of the hat truss under collapse initiation conditions.
Plus I also get nervous about "rigid" esp when flexibility is a key issue in the load redistributions associated with a cascade failure.
Recall that, in the cartoon model I posted for Sander, I made a clear distinction between "rigid" and "flexible" THEN showed how significant the distinction was in that model. It is probably more significant in the real situation of multiple interacting mechanisms.
jaydeehess
Penultimate Amazing
I understand you to mean the core column sections above this core failure, and the connections you refer to as the core to floor truss connections. The intercore beams would basically not be affected if all core columns suffered such a failure.
Yes, in a situation such as this the entire 12 storey load of the portion above the plastic failure would now have to be borne by the perimeter.(whats left of it after iimpacts had removed some). However, there is that lean, which is causing CoG to move towards the lean and thus more of that redistributed load will be borne by perimeter columns on that side and the perpendicular perimeter between centerline and the corner in the direction of the lean, not the side opposite the leaning.
Thus one would expect sudden perimeter failure to initiate in these regions
Such a large core drop would put the same drop on every floor and on all sides, between the core weak spot and the rooftop. It would also follow the trig that was posted earlier which had a 20+ foot core drop equal to the task of that inward bowing, which was not seen in the videos.
The way I see it;
heat causes floor deformation in the areas most affected by heat output and fire durations(the area opposite the impacts where rubble would be more dense)
Light weight trusses are more susceptible to heat than are the heavy core columns because their ratio of surface area to volume is greater. Or the concrete floor pans expanded ala Usmani if you prefer. Either way the perimeter columns were bowed inwardly.
Some core columns reached temperatures at which they were susceptible to creep plastic deformation and slowly deformed causing a small amount of core shortening as they did.At collapse initiation the inward bowing reached a critical tipping point at which the non-axial loading caused the perimeter columns that were bowing to be unable to support further loading and they failed. This meant redistribution of the hanging load (from hat truss) to pass a point at which the remaining columns of both perimeter and core were unable to sustain the total load and they failed . Transmission time for this would be dependant only on the inertia of the moving mass, slowed only by the spring constant in steel columns.(or whatever engineers call it, my background is in physics, I only hung around with the engineers)
All columns have failed by buckling or sudden fracture at or near impact levels.
The part in italics would tend to contribute to inward bowing and if the timing was right could have been the tipping point for perimeter column failure. If so then its a matter of POV as to which caused collapse initiation.
We could have the same arguement of a glass half full/empty
What I cannot envision is that core creep could be large enough to in itself cause all perimeter columns to fail close to simultaneously. I see them as deforming slower and less than, the perimeter
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jaydeehess
Penultimate Amazing
wrt to the hat truss it is much more rigid than the horizontal connections from perimeter, through core, through to the opposite or perpendicular perimeters.
(oops, have I been spelling 'rigid' wrong?
)ozeco41
Philosopher
Understood. I have made no assumptions (so far) about the degree of flexibility (or lack of rigidity). Only that it is flexible, not rigid, and that load distribution depends on the flexibility. If we get to "How much load goes where?" we will need to assess the effects of flexibility. And at that point if not sooner "Rigid" would lead us in totally wrong directions>>>wrong conclusions.
...(oops, have I been spelling 'rigid' wrong?
- I've had my "spelling pedantry switch" in the "Off" position. Hard to do for my generation.jaydeehess
Penultimate Amazing
My generation is not much younger than yours. Spelling and punctuation are important to me though I take no prizes for it certainly
ozeco41
Philosopher
In the setting of a cascade failure, à la WTC1 and WTC2 on 9/11, I think the concept of one element "causing" others is highly suspect. We should look very hard at the meaning we assign to "cause" OR better still IMO not use it at all.
Sequence yes but preceding does not equal causation.
ozeco41
Philosopher
I rely on the spellchecker to a large extent. And "their" v "there" confusion is one of the most embarrassing errors for old farts like me. I make that one often at the draft stage - when my mind is a couple of sentences ahead of my typing fingers.
Then "complement" v "compliment" would be more embarrassing but I rarely make that error.
Naturally as a Brit born Aussie I take secret delight in avoiding most American spelling. I use English AU dictionaries when available, English UK as second choice. I don't use Mr Gates' US centric software as my first preference so I don't have to fight the inbuilt US language bias. Including the "s" (ess) versus "z" (zed) stuff.
)In that context ridgid is a mere misdemeanour - the others are felonies.
And I decided to tolerate "9/11" and similar back to front date formatting. A pragmatic choice. I wouldn't get much support referring to "11/9" Took me a lot longer to understand what it meant than it did for me to conform. I still have to force myself to remember that it means September not November.
I suppose that is why the Brits decided to have their little event on 7/7
- or was it 7/7?
Then my biggest irritation is the constructions in the form "I would of thought that...." and similar. I had the idea that it was a flaw of current Aussie teenagers...but I have been disillusioned. It's international and persons of apparent adult age seem to use it.
Be cautioned that anyone who types "would of ..." or "could of..." in a post visible to me is automatically downgraded two full levels in my estimation...and his/her comments treated at that lower level...
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Newtons Bit
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No, the facade was aluminum. The structure was not exposed.
Idiotic comment. You can get 2 gold stars on your forehead for knowing that the towers were clad in aluminum.
Splitting hairs is your game... have a ball...
From wiki:
"A facade or façade (pron.: /fəˈsɑːd/) is generally one exterior side of a building, usually, but not always, the front. The word comes from the French language, literally meaning "frontage" or "face".
In architecture, the façade of a building is often the most important from a design standpoint, as it sets the tone for the rest of the building. Many façades are historic, and local zoning regulations or other laws greatly restrict or even forbid their alteration....
Highrise façades
In modern highrise buildings, the exterior walls are often suspended from the concrete floor slabs. Examples include curtain walls and precast concrete walls. ....
In general, the façade systems that are suspended or attached to the precast concrete slabs will be made from aluminum (powder coated or anodized) or stainless steel. In recent years more lavish materials such as titanium have sometimes been used, but due to their cost and susceptibility to panel edge staining these have not been popular."
But we know it was not a curtain wall:
https://en.wikipedia.org/wiki/Curtain_wall
Newtons Bit
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There's more ways to skin a building than just curtain wall. You might want to keep reading up on jargon...
Thanks Bit... I am well aware of the technology used in the WTC buildings.
Archi(tect) Bunker