OOS Collapse Propagation Model

[ozeco41]

Sorry Ozeco41, your wrong and that is shown in the seismic data, the towers were highly
Efficient at absorbing and redirecting the impacts energy.

Wow.

Try reading what I said CC

Then tell me which bits of my post that you quoted are wrong. Here I'll parse it for you:

Please take care - all of you - that you don't build false implications out of partial truths.

Is it wise to not build false implications out of partial truths?

@DGM "If the design was not able to flex and respond to the impact it could have collapsed immediately." flexure is a reality in any structure. Your conditional could makes the first assertion a truism.

Does the conditional "could" make the statement a truism?

But the causal relationship "flexure" >> "not collapse" is not necessarily true. Possibly isn't true but is definitely unproven at this stage.

Is the causal relationship unproven if proof has not been given? Excuse the circularity but there was no proof in the argument as presented. Which was what I claimed. NOR reference to external or previous proof.

AND
"...the WTC design flexed" and "did not collapse right away" is the true sequence

I'll insist that bit is true.

but the two issues are not directly causally related.

If you disagree with that bit you need to address the factor I identified i.e. whether or not extra WTC flexibility ALONE determined the delay before collapse.

We cannot say "the Twins did not collapse immediately because they were flexible".

Well we could say it but my point is that it is not proven by the statements made in the argument up to that stage.

You may not be intending that implication but is is there - so Sander has queried the implication and it looks like Bravin has accepted it. I may be wrong on that point.

Which of those three assertions and one conditional concession do you disagree with?

It may be that WTC design had more flexibility which helped it more than other designs but the causality is not proven at this stage.

You assert that I am wrong but you do so by trying to provide the proof of causality which I said was missing? Why are you providing it if it wasn't missing?

Begging the question about whether some proof or even "the" proof lies in the seismic data - the fact that YOU see the need to add some proof actually proves my point. The proof wasn't stated. It is needed to ensure the validity of the reasoning and to avoid unsupported inferences.

AND I note that you don't query the last point I made for Bravin. There is still no proof referred to in the series of posts that collapse would have immediately followed impact if the building had been less flexible. How much less flexible would be needed to guarantee collapse immediately on impact?

As I said for Sander "So I am cautioning - disregard the implication of proven causality. The question is much more subtle."

Don't miss the subtlety of "the question" or my comments.

 

[JSanderO]

You would have induced weld damage in core columns to an unknown existing, near the impact zone that could have been fatal to the design.

This is a unsupported and if I must say "crazy" statement. So you think all the welds of the column to column splices would fail? What would happen? These connections were unrestrained and saw no lateral forces.

Are you referring the the welds of the bracing to the columns? The bracing would fall off?

How many steel frames are welded together????

"Column splice joints usually occur at approximately mid-floor level permitting ease of access for connecting the pieces and avoiding clashing with the connections of beams at floor level. Column lengths are usually made as long as practicable to minimise the number of splices and hence reduce costs.

The typical inner column of a multi-storey, multi-bay frame will have four beams connected to it at each floor level: usually two main beams and two tie beams. It would further complicate matters if there were also a joint in the column at the same point. It is usual, therefore, to:

Make the column in one piece for the height of the building, where the height does not exceed about three storeys.
Introduce column splices for taller columns, putting the joint not at mid-height of a storey - the theoretically optimum position, but at least 500-1000mm above the floor beams to afford easy access for the erectors. (The joint should be easily accessible to the erectors standing on the floor beam).

The lowest section of such a column normally carries the most load, and it needs to be stronger than higher sections. To provide for this, different sections with the same serial size are sometimes used, with the largest at the bottom. If this is done, it must be remembered that the serial size is only nominal: the overall dimensions in a serial size group may vary considerably, and shims will be needed to pack out at a splice joint. Countersunk bolts will help to reduce the bump effect of the splice at this point, but they are expensive.

Beam-column joints are usually considered as pinned.

If the lateral stability to a frame is provided by vertical bracing, the joints can be regarded as ‘pinned’, and effected using simple details. The site connection will generally be made with ordinary 'black' bolts. Such a joint uses the bolts mainly in shear: this action is not dependent on the tightness of the nuts or the order or tightening."

and then theres's this:

Published in
Structural Engineer
magazine – February 2009
Steel Moment Frames – History and Evolution
By Paul McEntee, S.E., R & D Engineer for Simpson Strong-Tie

Steel-frame structures were developed during the 19th century in response to the limitations of masonry-bearing wall structures, which were the common method of commercial construction at the time. These bearing wall structures were limited to about 10 stories high and allowed for only small openings because of the strength of the masonry materials. Since property owners back then wanted the same things that developers and architects want today—the maximum amount of rentable space on their land and plenty of natural light—new solutions were explored. Steel-frame building construction using rigid frames, or moment frames, was the answer to these
demands......

A Brief History

The earliest steel moment frame buildings used riveted connections
with angles or T-sections connecting the beam flanges to the
columns to create moment connections. The development of new
welding technologies throughout the 20th Century and the introduction of high-strength bolts in the 1950s ended the use of riveting in building and bridge construction in the United States. The Welded Flange connection became one of the most common moment frame joints, using either a bolted or welded shear tab for vertical loads, and complete joint penetration welds for the beam flange-to-column flange moment connection. (See Figure 1) Steel moment frames are expected to achieve ductility through yielding beams or columns, and the connections must be capable of remaining intact through several cycles of inelastic rotation due to seismic loading. The 1994 Northridge Earthquake demonstrated that the standard connection (shown in Figure 1) did not perform as
expected, in many cases fracturing at low levels of plastic deformation. Failures in the moment frame connections were attributed to multiple causes and are well documented, but are beyond the scope of this article to discuss.

In mid-1994, the SAC Joint V enture was established, bringing together SEAOC, ATC and CUREE. Funded by FEMA, SAC was tasked with developing new design recommendations for welded steel moment frames. Several documents were published to address the design of moment frames in new structures, repair and evaluation of existing structures, and quality control guidelines

 

[Crazy Chainsaw]

It's a matter of record that he did no such thing, as I understand it. After the design was more or less complete, a political campaign to block construction of the towers raised the question of an airplane strike, and Robertson responded by calculating the response of the structure to an impact of the magnitude expected from a 707 travelling at slightly higher than stalling speed (simulating a plane lost in fog on a landing approach, because otherwise it wouldn't be that low). No specific account was taken in the design stage of such an impact.

Dave

True but Robertson did state that the flexibility inherent in the design would allow for the
Buildings survival, the perimeter columns taking most of the damage, and he was right.
He was right about them, only it was assumed the plane would be low on fuel, not a
Terrorist attack act of war, on a nuclear power.
It scares me when I remember the people saying just nuke them, right after 9/11/2001.
Bin laden had to be one of the most stupid people on the entire planet, great he is gone.

 

[JSanderO]

It's a matter of record that he did no such thing, as I understand it. After the design was more or less complete, a political campaign to block construction of the towers raised the question of an airplane strike, and Robertson responded by calculating the response of the structure to an impact of the magnitude expected from a 707 travelling at slightly higher than stalling speed (simulating a plane lost in fog on a landing approach, because otherwise it wouldn't be that low). No specific account was taken in the design stage of such an impact.

Dave

Well there ya go.

I would like to see what those calculations were. And how the tower was treated... Wind shear calcs assume a uniform wind load on the face of the building which is treated as a vertical cantilever I assume. A plane impact would be a concentrated load (more or less). My hunch is the calc revealed how many columns would be severed and how much capacity remained.

My guesses are often incorrect.

You can see that a 100mph window would exert 55 psf on the facade. That is almost the same as the "design live load" of the floors

 

[JSanderO]

True but Robertson did state that the flexibility inherent in the design would allow for the
Buildings survival, the perimeter columns taking most of the damage, and he was right.
He was right about them, only it was assumed the plane would be low on fuel, not a
Terrorist attack act of war, on a nuclear power.
It scares me when I remember the people saying just nuke them, right after 9/11/2001.
Bin laden had to be one of the most stupid people on the entire planet, great he is gone.

FLEXIBILITY IN ALL HIGH RISE DESIGNS WOULD ALLOW THEM TO TAKE THE FORCE.... otherwise they would fall down in hurricanes.

 

[Crazy Chainsaw]

This is a unsupported and if I must say "crazy" statement. So you think all the welds of the column to column splices would fail? What would happen? These connections were unrestrained and saw no lateral forces.

Are you referring the the welds of the bracing to the columns? The bracing would fall off?

How many steel frames are welded together????

"Column splice joints usually occur at approximately mid-floor level permitting ease of access for connecting the pieces and avoiding clashing with the connections of beams at floor level. Column lengths are usually made as long as practicable to minimise the number of splices and hence reduce costs.

The typical inner column of a multi-storey, multi-bay frame will have four beams connected to it at each floor level: usually two main beams and two tie beams. It would further complicate matters if there were also a joint in the column at the same point. It is usual, therefore, to:

Make the column in one piece for the height of the building, where the height does not exceed about three storeys.
Introduce column splices for taller columns, putting the joint not at mid-height of a storey - the theoretically optimum position, but at least 500-1000mm above the floor beams to afford easy access for the erectors. (The joint should be easily accessible to the erectors standing on the floor beam).

The lowest section of such a column normally carries the most load, and it needs to be stronger than higher sections. To provide for this, different sections with the same serial size are sometimes used, with the largest at the bottom. If this is done, it must be remembered that the serial size is only nominal: the overall dimensions in a serial size group may vary considerably, and shims will be needed to pack out at a splice joint. Countersunk bolts will help to reduce the bump effect of the splice at this point, but they are expensive.

Beam-column joints are usually considered as pinned.

If the lateral stability to a frame is provided by vertical bracing, the joints can be regarded as ‘pinned’, and effected using simple details. The site connection will generally be made with ordinary 'black' bolts. Such a joint uses the bolts mainly in shear: this action is not dependent on the tightness of the nuts or the order or tightening."

and then theres's this:

Published in
Structural Engineer
magazine – February 2009
Steel Moment Frames – History and Evolution
By Paul McEntee, S.E., R & D Engineer for Simpson Strong-Tie

Steel-frame structures were developed during the 19th century in response to the limitations of masonry-bearing wall structures, which were the common method of commercial construction at the time. These bearing wall structures were limited to about 10 stories high and allowed for only small openings because of the strength of the masonry materials. Since property owners back then wanted the same things that developers and architects want today—the maximum amount of rentable space on their land and plenty of natural light—new solutions were explored. Steel-frame building construction using rigid frames, or moment frames, was the answer to these
demands......

A Brief History

The earliest steel moment frame buildings used riveted connections
with angles or T-sections connecting the beam flanges to the
columns to create moment connections. The development of new
welding technologies throughout the 20th Century and the introduction of high-strength bolts in the 1950s ended the use of riveting in building and bridge construction in the United States. The Welded Flange connection became one of the most common moment frame joints, using either a bolted or welded shear tab for vertical loads, and complete joint penetration welds for the beam flange-to-column flange moment connection. (See Figure 1) Steel moment frames are expected to achieve ductility through yielding beams or columns, and the connections must be capable of remaining intact through several cycles of inelastic rotation due to seismic loading. The 1994 Northridge Earthquake demonstrated that the standard connection (shown in Figure 1) did not perform as
expected, in many cases fracturing at low levels of plastic deformation. Failures in the moment frame connections were attributed to multiple causes and are well documented, but are beyond the scope of this article to discuss.

In mid-1994, the SAC Joint V enture was established, bringing together SEAOC, ATC and CUREE. Funded by FEMA, SAC was tasked with developing new design recommendations for welded steel moment frames. Several documents were published to address the design of moment frames in new structures, repair and evaluation of existing structures, and quality control guidelines

Not all the welds, JSO, but enough to cause significant damage to cause the building to
Transfer more load failing critical floor sections, causing major floor collapses.
If the floors collapse they could pancake inside the perimeter columns.

 

[JSanderO]

Not all the welds, JSO, but enough to cause significant damage to cause the building to
Transfer more load failing critical floor sections, causing major floor collapses.
If the floors collapse they could pancake inside the perimeter columns.

Welding is expensive and not practical...and may not have the full strength required for a connection... Maybe?

read about weld calculations...

http://www.gowelding.com/calcs/calc.htm

 

[Crazy Chainsaw]

Welding is expensive and not practical...and may not have the full strength required for a connection... Maybe?

read about weld calculations...

http://www.gowelding.com/calcs/calc.htm

Welds are subject to sheer lag, breakage of the welds in off center strikes, as I remember
from the movie "Building the world trade center marvels of modern enginering", the oridginal achetectual drawings, called for a concrete core, and traditional floor joists not trusses,
With bolted and welded perimeter columns, this was shown to be a problem because constant wind loading and sway would fracture welds.
Both in the perimeter and in the core, so the design was switched to trusses, and bolted
Perimeter columns.
Trusses and bolted perimeter columns, allowed more freedom of movement to dissipate
Wind energy,
However that lead to the conspiracy theory that the buildings without the more ridid structure would not survive a plane strike, Robertson's calculations showed the reverse to be true, by not transmitting the full shock to the core and localizing the damage the structure,
Was more prone to survive an impact than a traditional structure, with minimal damage.

 

[JSanderO]

Welds are subject to sheer lag, breakage of the welds in off center strikes, as I remember
from the movie "Building the world trade center marvels of modern enginering", the oridginal achetectual drawings, called for a concrete core, and traditional floor joists not trusses,
With bolted and welded perimeter columns, this was shown to be a problem because constant wind loading and sway would fracture welds.
Both in the perimeter and in the core, so the design was switched to trusses, and bolted
Perimeter columns.
Trusses and bolted perimeter columns, allowed more freedom of movement to dissipate
Wind energy,
However that lead to the conspiracy theory that the buildings without the more ridid structure would not survive a plane strike, Robertson's calculations showed the reverse to be true, by not transmitting the full shock to the core and localizing the damage the structure,
Was more prone to survive an impact than a traditional structure, with minimal damage.

Steel frames are not welded together. They include SOME welding but never ONLY welding. One of the main reason is erection efficiency.

They were not bolted to dissipate energy.

 

[Dave Rogers]

I would like to see what those calculations were.

So would a few other people, if only out of simple curiosity. IIRC the calculations, according to Robertson, were written out over three pages, but no record of them survives.

Dave

 

[Crazy Chainsaw]

Steel frames are not welded together. They include SOME welding but never ONLY welding. One of the main reason is erection efficiency.

They were not bolted to dissipate energy.

I didn't say only welded I said bolted and welded perimeter columns with conventional floor joists instead of trusses.
A much stiffer construction, for the first tube in tube frame sky scraper.

 

[WilliamSeger]

It's a matter of record that he did no such thing, as I understand it. After the design was more or less complete, a political campaign to block construction of the towers raised the question of an airplane strike, and Robertson responded by calculating the response of the structure to an impact of the magnitude expected from a 707 travelling at slightly higher than stalling speed (simulating a plane lost in fog on a landing approach, because otherwise it wouldn't be that low). No specific account was taken in the design stage of such an impact.

Dave

After looking into this some more, it appears that in '93 Skilling talked about an analysis that may have been done in early '64 during the design phase, and then Robertson did a separate analysis in late '64 to verify the design. It doesn't really matter though: Robertson's conclusion that the structure would survive the initial impact was proved correct, twice.

 

[Newtons Bit]

Thank you for that brilliant answer!

Says the guy who asked a question that was directly answered in the post he was replying to. FFS JSanderO, it was just two sentences. Try reading instead of shooting your mouth off.

 

[tsig]

Forget MT's agenda... which you are projecting.

If you watch the PR about the "innovative engineering" design and the several comments about a structure being able to "absorb" the impact of a big jet... the "pitch" was these were extremely strong structures... even over built. That may or may not have been the case... but that WAS the pitch AND the design has this super strong skin which could "stand on it's own" with the help of the floor system brace it to the self supporting and super strong core. How "strong" could the design have been if it collapsed to the ground in 10 seconds? (rhetorical)

There was no discussion in the PR about how robust and strong the floor system was. The PR was about the clever off site manufacture and easy erector set light weight construction... right? (rhetorical)

Do you think a plane strike to the top of a typical high rise would "knock it over" or cause it's instant collapse? If so why? (not rhetotical)

So what?

 

[Newtons Bit]

There is more than shaking because ground can heave and create moments and and cause toppling. They say:

Low Height to Base Ratios
Equal Floor Heights
Symmetrical Plans
Uniform Sections and Elevations
Maximum Torsional Resistance
Short Spans and Redundancy
Direct Load Paths

For everyone elses information, every single one of these items is wrong except the last one. Some, like the equal floor heights, are just misunderstanding the concepts. In seismic design, one wants the stiffness of each floor to be similar or stronger than the floors above. Floor-to-floor heights are one component of stiffness. There are no ill-effects from an increase in floor-to-floor heights so long as the lateral stiffness is appropriately tweaked.

"Maximum Torsional Resistance" actually makes me laugh. There's no such thing. Either a structure has a torsion in its first two nodes or it doesn't. After the Northridge Earthquake, the very simple UBC earthquake analysis methodology was discovered to be inadequate for structures that behaved torsionally and that was changed in future codes.

JSanderO: once again proving he has no freaking clue what he's talking about.

 

[tsig]

I am barely interested in this discussion at this point... it's like beating a dead horse.

Thank you very much!

Probably isn't much fun when one is the dead horse.

 

[tsig]

well.... if the was some manner of arrest or not the entire oos floor went....and not total collapse in 10 seconds... maybe more would have survived.

I agree completely, if things had been different they would be different.

Utterly profound. Next I will propose that water is wet and up is up.

 

[tsig]

Disclaimer... I am not going to argue a paper I haven't read and probably can't understand if I did.

HOWEVER.. did Mr B assume all the mass from the top section would "drop" or crush or whatever and this would destroy the tower?

Why assume all the mass from above the impact is in play? Why not assume 1/3 of it was "in play?

It's not an assumption, it's a fact unless you know some way to counteract gravity on 2/3 of the mass above.

 

[tsig]

Obviously, the only correct design is to position all the columns away from the airplane's path. Any other arrangement is vulnerable!

We coat them with no-plane paint and they bend out of the way when they see a plane coming, just like in the Bugs Bunny documentaries.

 

[tsig]

So how did Robertson design the building to take a 707 strike at x miles per hr? Does anyone here know what he did to make it "strong enough" to not????? fall down immediately on impact?

What would the difference be had he simply ignored a plane strike? Was this the same sort of design for wind shear?

Does flex mean that the joints have some "give" like an expansion joint in a bridge span?

We have heard many times that the towers were remarkably strong and were designed to take a plane strike... multiple ones one person stated and remain standing.

My sense is that the facade was like a huge membrane... that WAS a design innovation. It was a large plate / bearing wall because the staggering of the column joints and the very deep spandrel panels/beams which were extremely effective in redirecting vertical loads around missing/damaged facade columns. It was dense not open and at the time some believed this also made the high floors feel more secure with smaller windows.

OK experts explain how one adds the ability to sustain a plane strike other than beefing up the amount of steel?

He didn't. The fact that you still use this bit of misinformation says a lot about your position.