Miami Beach, Champlain Towers South apartment building collapse, Part 12 60

[SFCharlie]

A continuation of our discussion of this failure. Best to read the other threads first to avoid rehashing things already discussed.
Miami Beach, Champlain Towers South apartment building collapse
Miami Beach, Champlain Towers South apartment building collapse, Part 02
Miami Beach, Champlain Towers South apartment building collapse, Part 03
Miami Beach, Champlain Towers South apartment building collapse, Part 04
Miami Beach, Champlain Towers South apartment building collapse, Part 05
Miami Beach, Champlain Towers South apartment building collapse, Part 06
Miami Beach, Champlain Towers South apartment building collapse, Part 07
Miami Beach, Champlain Towers South apartment building collapse, Part 08
Miami Beach, Champlain Towers South apartment building collapse, Part 09
Miami Beach, Champlain Towers South apartment building collapse, Part 10
Miami Beach, Champlain Towers South apartment building collapse, Part 11

SF Charlie
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[gonefishing2]

The core sample in this image shows disruption of rebar to concrete bond, but I just don't see the corrosion necessary, in this sample, to cause the expansion. Now with this small of core hole, it is possible drilling core broke it up, but it would seem proper concrete strength, mix and installation would have far better bond that what this image shows. It appears concrete itself is more suspect or perhaps how it was installed? If same crew as Demented's watery low aggregate looking mix, did this repair, then installation and materials seems more likely?

Edit: Ignore the marks and comments, as I just grabbed this from the lead in image to another long winded read and think out loud video for profit, which is just a bunch of hot air.......

 

[SFCharlie]

The core sample in this image shows disruption of rebar to concrete bond

Now who's resorting to power-pointy do-dads

I recommend The Importance of Punching Shear at the Surfside Collapse starting at 9:55

SF Charlie
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[TheGreenLama]

The core sample in this image shows disruption of rebar to concrete bond, but I just don't see the corrosion necessary, in this sample, to cause the expansion.

The concrete for that particular core sample does look particularly degraded (powdery). And at least from this view the steel looks relatively clean. It is still odd that the separation is occurring at the steel. Core samples don't normally break up. I'm not a materials expert, so I'll keep an open mind on causation until some actual testing is done.

Here is an image of that core from earlier in the thread Part 7, 18 Jul 21 1:10.
By the way, could anyone provide the link to the Morabito Phase IIA report where these come from. I can no longer find it. Is it within the Surfside document trove?

 

[Reverse_Bias]

Here's the corroded column base from the NIST vid. I'm guessing it's one of the deck cols. Before anyone says a car hit it, it could have been hit with a bulldozer a few times during debris removal.

 

[gonefishing2]

Now who's resorting to power-pointy do-dads

I don't have the literary skills of some of posters, so I had to join "Team" do-dads, to get my points across!

link to the Morabito Phase IIA report where these come from. I can no longer find it. Is it within the Surfside document trove?

I looked in Surfside document trove and could not find it either. I thought that is where it was originally found, as it was buried in the 80 page Association Meeting Minutes.

 

[Vance Wiley]

Too much water in the mix makes concrete weak and powdery. Is there yet available any test reports of the strength of cured cylinders cast from the concrete used at this site and cured in the lab? Any test results of cores taken in the last year? Pre or post collapse.
There have been comments on rainwater possibly causing weakening or dissolving rock strata below the building.
Now the question - about aggregates in Miami. Are the aggregates sensitive to degradation by salt water exposure? In the FIU photos of broken concrete almost all the aggregates I saw were split by the fracture. Good bond, not so good aggregates? Can we see aggregates in the crumbles here?
A comment on "congestive rebaritis" - I do not see anything out of the ordinary by viewing the pics of the columns - and if rebar is tight there is a max aggregate size to be used. At column bar splices, it is common practice to offset the bars from below to do a contact lap just above the floor slab pour , in this case. There is a detail of such a condition and it appears correct to me - bars offset at 1:6 slope and the offset should be offset toward the center of the column (I do not recall seeing the direction to orient the offset as a requirement on the drawings, but it is the practice). Reinforcing needs to be separated enough to allow good bond and load transfer between the concrete and reinforcing, and the codes address this. Mix properties as placed and proper consolidation of the concrete is required. Measurements will confirm compliance or non-compliance.
I am not sure the rebar spacing was a major contributor to the collapse. One column on line "L" is 24X12 and has 12 - #12 bars for about 5.3% steel, with the maximum allowed being 8%. Column on Line G , 14X18 and 10 - #11 is 6.07%, less than the 8% maximum.
Thanks,

 

[dik]

Unless things have changed... at the splice you can have 4% (main steel)... 4%+4% spiced bars = 8%.

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

-Dik

 

[TheGreenLama]

I do not see anything out of the ordinary by viewing the pics of the columns

That pic above that SFCharlie posted looks odd to me. In it there are 4 bars all in line and basically touching each other at the corner. Bundling of bars in EQ zones is done all the time. Usually the shape is triangular w/ 3 bars per group. Here, with them all in line, it seems like concrete placement could be a problem.

 

[Optical98]

All About Money

Link for - Phase IIA, Summary of Work Performed

https://s3.documentcloud.org/documents/20985354/20201014bodminutesapproved.pdf

 

[gonefishing2]

Thanks Optical98!

 

[Optical98]

Regarding columns with too little or too much rebar, here are examples of each.

Too much.

Too little

 

[Vance Wiley]

The amount of steel within the lap zone is not used to determine the capacity of the column.
The longitudinal reinforcing up the column is the determining factor and the focus of the 8% limitation.
If there are moments at the top of the slab/bottom of the column, the development length for any bar in tension should be checked.

BIGTIME EDIT - - I do not see the logic in this but is correct - the limit of 8% DOES apply to splice zones in the ACI318- 2014 Section 10, as follows (with comments). What this does today is require splice collars so all reinforcing stays in line and reduces the congestion if more than 4% is needed
From

http://aghababaie.usc.ac.ir/files/1506505203365.pdf

10.6.1.1 For nonprestressed columns and for prestressed
columns with average fpe < 225 psi, area of longitudinal
reinforcement shall be at least 0.01Ag but shall not exceed
0.08Ag.
Discussion/Comments
R10.6.1.1 Limits are provided for both the minimum and
maximum longitudinal reinforcement ratios.
Maximum reinforcement—The amount of longitudinal
reinforcement is limited to ensure that concrete can be
effectively consolidated around the bars and to ensure that
columns designed according to the Code are similar to the
test specimens by which the Code was calibrated. The 0.08
limit applies at all sections, including splice regions, and
can also be considered a practical maximum for longitudinal reinforcement in terms of economy and requirements
for placing. Longitudinal reinforcement in columns should
usually not exceed 4 percent if the column bars are required
to be lap spliced, as the lap splice zone will have twice as
much reinforcement if all lap splices occur at the same
location.

I learned something toeday. Thank you,

 

[SFCharlie]

Report: Evidence of extensive corrosion in collapsed condo

“The corrosion on the bottom of that column is astronomical,” Dawn Lehman, a professor of structural engineering at the University of Washington, told the Miami Herald. She said that amount of corrosion should have been obvious and documented as part of the 40-year inspection that was ongoing when the building in Surfside, Florida, collapsed June 24.

SF Charlie
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[Demented]

he concrete for that particular core sample does look particularly degraded (powdery). And at least from this view the steel looks relatively clean. It is still odd that the separation is occurring at the steel. Core samples don't normally break up. I'm not a materials expert, so I'll keep an open mind on causation until some actual testing is done.

Here is an image of that core from earlier in the thread Part 7, 18 Jul 21 1:10.
By the way, could anyone provide the link to the Morabito Phase IIA report where these come from. I can no longer find it. Is it within the Surfside document trove?

Concrete was removed at partial depth to where rebar could be inspected. Where no full depth repairs were required, steel was sand blasted and coated. A random combination of repair mortar, bagged concrete mixed on site, and pumped concrete with the slump of a snow man in Florida, was used to fill back the excavated areas. In some areas, new steel was just laid besides, on top, or below existing.

Partial depth repairs were done from above and below in the same locations, mostly around the planter locations. They went 3-6" down, and 3-6" up in their excavation to expose the rebar.

 

[gonefishing2]

The following is an excerpt from:

An ACI Standard
Building Code Requirements for Structural Concrete
(ACI 318M-14) and Commentary (ACI 318RM-14)
Reported by ACI Committee 318
Chapter 10 Columns, page 156

I don't know if this is latest or correct source, so I attached document to this post.

R10.6—Reinforcement limits
R10.6.1 Minimum and maximum longitudinal reinforcement
R10.6.1.1 Limits are provided for both the minimum and maximum longitudinal reinforcement ratios.

[highlight #FCE94F]Maximum reinforcement—The amount of longitudinal reinforcement is limited to ensure that concrete can be effectively consolidated around the bars and to ensure that columns designed according to the Code are similar to the test specimens by which the Code was calibrated. The 0.08 limit applies at all sections, including splice regions, and can also be considered a practical maximum for longitu- dinal reinforcement in terms of economy and requirements for placing. Longitudinal reinforcement in columns should usually not exceed 4 percent if the column bars are required to be lap spliced, as the lap splice zone will have twice as much reinforcement if all lap splices occur at the same location.[/highlight]

 

[Red Corona]

This 'too much rebar' thing. It's pretty conclusive that it's against code (thanks to people above who've looked up the rules). But those columns only had too much rebar in the splice region, right? Which was immediately above each floor. Is there any indication that a column initially failed in that area (immediately above the ground floor, presumably, as the load on the columns is highest at the bottom of the building)?

Does it apply at the base of the columns in the garage as well? I wouldn't have thought there would be any splicing there, as there isn't a column beneath that slab, and it didn't look like it in the pictures where they'd cleared the rubble but the columns were still present. And we know that, apart from maybe M12.1, all those columns were still standing at basement level.

 

[Vance Wiley]

I stand corrected. Thanks for correcting - I had just edited my original reply to @dik, with the comments pasted there.
The 4% limit was hiding in the back of my mind, and I was surprised when I saw the 8% - really! But there it was in bold - but without the comments.
The takeaway is a good thing - increase the size of the column if you need to exceed 4% reinforcing OR use splice couplers.
Now to find a copy of ACI318 that was in effect in 1980.

 

[dik]

Not a problem... one of the features of this website. I haven't done a real concrete tower for two decades, but at that time the general maximum practical reinforcing ratio was about 2.5% to 3% (and max reinf at splices was about 6%) with column sizes and concrete strength only changing as needed to accommodate greater loads.

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

-Dik

 

[Vance Wiley]

I have corrected my original post about % and added one just minutes ago.
The condition of lapped reinforcing generally applies to the top of foundations also because it is difficult to set and hold a large tall cage of reinforcing over a deep pile cap or footing and while placing concrete. The dowels can be tied to mat reinforcing and most of the time no bracing is required.. But there are a lot of times the dowel bars are not straight vertical and in the right place.

 

[raymiepo]

in other news:

Millennium Tower sank another inch this past month.
Underpinning to bedrock.
shoulda in the first place.