SF Tower settlement Part II 18

[1503-44]

"Appreciation has dropped to 2%"
Well that's less than inflation, but more than interest rates.

Although as I said, probably nobody bought in for either of those reasons.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[geochase]

So, I watched the entire Board of Supervisors Committee meeting online yesterday out of morbid curiosity and interest...and, I was quite surprised by some of the very honest statements that Hamburger made, including alleging the original geotech made a mistake in their calculations when originally estimating the potential consolidation settlement of the Old Bay Clay (which anyone who has any geotechnical knowledge already well knew, but that was never admitted publicly...). He also clearly stated that dewatering of "adjacent sites" (plural) contributed to increased settlement as result of increasing effective stress with loss of buoyancy. Sites, not just the one prominent and publicly-funded project site that the Millenium HOA has pointed their fingers at all along...I have always wondered about the effects of dewatering for the buildings to the north and west of Millenium that was ongoing during time of increased rate of settlement, yet that never seems to be discussed in any of the news stories or any of the documents I can find online.

 

[Craig Neth]

Grady Sullivan has done another one of his great summary videos on YouTube:

What Really Happened at the Millennium Tower?

 

[spsalso]

Towards the end of the video, there's a drawing showing how the current attempt to right the building is supposed to work (at around 9:30). It includes adding an additional piece of concrete under the sidewalk, and keying that piece to the existing foundation, and then lifting both with hydraulic jacks.

My problem is with the design of the key. The "upper lifting surface", because it is angled, would appear to cam the new additional concrete away from the foundation. This would increasingly lessen engagement, and lower the effectiveness of the concept.

Or so it appears to me.


spsalso

 

[Craig Neth]

I noticed that too, but I assume that Grady's illustration is simplified and not based on any drawings.

 

[spsalso]

"simplified"?


Not the word that comes to my mind.


spsalso

 

[dik]

The direction of the keyway is incorrect in the sketch.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Ingenuity]

Grady has it consistent with SG&H:

PRACTICAL ENGINEERING schematic:

SG&H detail:

 

[dik]

I have no idea of what they want the keyway for, but if they are uplifting, the keyway is in the wrong direction. Maybe part of the problem is that they don't know what they are doing.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[TugboatEng]

I think the upwards slope of the keyway allows for better filling without voids.

 

[dik]

With properly consolidated concrete this would not be an issue... even a slope of 5deg would suffice if they needed one. A slope of 45deg is way too much.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[hamjohn]

Drill/core on a sloped concrete surface will be difficult.

 

[TugboatEng]

Having the key cut at an angle like that increases the cross section area of the rod in the shear plane.

 

[hamjohn]

The link to Google Maps shows two new concrete curb walls above the sidewalk located at the southwest corner of the 301 Mission site. What functions do they serve?

https://www.google.com/maps/place/O...8ce9fde06c3b5!8m2!3d37.7898133!4d-122.3962033

 

[Ingenuity]

The "keyway" is there to also expose the original bottom rebar mat and mechanically couple to new added rebar.

Whilst the moment component of the uplift reaction (a horizontal force at the keyway location) tends to 'open' up the joint, the key does provide reaction for the vertical component via bearing.

What alternative direction/location would you make the key, dik?

 

[dauwerda]

I agree with Ingenuity, the keyway is not meant to transfer shear at all, it is there to allow them to tie into the existing reinforcement.

 

[dik]

You can still achieve that without the excessive slope without causing a significant 'wedging' action.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[spsalso]

As the slope of the keyway allows the new section of concrete to pull away horizontally, that will then tension the new horizontal rebar.

Seems maybe clever. Sure hope the concept has been thoroughly tested before this building--not really a good time for an experiment.



spsalso

 

[dik]

OK, I'm maybe corrected. I still don't think there's any reason to pretension the steel, and it just adds to the connecting force caused the the cantilever jacking.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[MechinNC]

Just a dumb question from a mere mechanical: Those perimeter piles are very long and relatively thin going through a deep layer of clay, which has "give" to it. The piles will be loaded heavily in compression to lift then support the corner of the building.

Is not buckling a concern? Does the lower layer of clay give enough lateral support to prevent this?

Seems the whole problem is the clay "oozing out" laterally from underneath a very heavy load. So the stuff does move. Worry that the pile once loaded will deflect laterally mid span.

 

[dauwerda]

Buckling of the new piles is definitely a design concern. According to the video posted above (here is the clip:

https://youtu.be/ph9O9yJoeZY?t=544),

the piles are actually being kept from contacting the clay at all, so I would think the design is assuming the full height of the pile is unbraced. That is, they are not counting on the old bay mud to brace the new piles.