[conspire] What I learned about concrete from Champlain Towers South
Deirdre Saoirse Moen
deirdre at deirdre.net
Sat Jun 11 14:37:18 PDT 2022
We're coming up on the anniversary of Champlain Towers South aka CTS (Jun 24, 2021), the third deadliest structural failure in the US. 98 people died. Four were rescued from the rubble (but one later died from injuries), and 35 rescued from the surviving building.
When I worked in Ft. Lauderdale (and was frequently on ships), one of my bosses lived in Champlain Towers South, and so I've been there several times. He lived in one of the penthouses, and in the part that collapsed last.
I finally had some bandwidth to ask: *why* did it collapse, and then, after watching some of that I thought, hey, as a non-expert in concrete, how would I know that a building was in imminent danger?
For that last, here are some of the warning signs to prevent you or your loved ones being caught in a pancake stack of concrete, and then at the end I'll discuss some of the specifics that led to this particular collapse.
My primary sources are three YouTube channels:
1. Building Integrity, a YT channel of a working structural engineer in Florida, who's done a lot of analyses - he has great analysis, but he's very much a "we don't know enough to say *why* the building collapsed, but these are what concern me" type of person. You won't get conclusions from him.
2. Jeff Ostroff, who's a contractor. He's prone to leaping to conclusions when the reality is more complex, but he has gathered the best set of footage of what happened.
3. Additionally, while his presentation is dry (and three ~30 min episodes), Structural Stories talks about some of the specifics to look for in concrete failure.
With that in mind, here's what led up to collapse in general terms:
1. Concrete has great compressive strength, but very poor tensile strength. The Roman stuff that still stands was carefully engineered to be all under compression, so it stands *without reinforcement* to this day. Modern concrete, however, is used in mixed compression/tension situations.
2. When subjected to tensile stress in excess of its limit, it'll start to crack. In a slab, tensile cracks tend to be diagonal if they're between columns. https://www.civilax.com/concrete-tension-tensile-strength-concrete/
Catch is, it's fairly easy to see failures in a slab, but it's less easy to see failures of concrete in a column since it's mostly under compression.
3. Concrete is reinforced with rebar, but that rebar needs to be correctly anchored. It also needs to be tied (perpendicular to the rebar) to reduce the chance of failure in the case of excess load. It appears that in CTS this was not done. At all.
4. Concrete works partly due to a high pH (typically 12-13). It has to get down to a pH of about 10 before it starts to fail. Water is a pH of 7, but that alone is not enough to cause concrete to fail despite the fact that concrete is porous.
5. Water intrusion, however, can start to make rebar rust, and rust takes up ~6x the volume that the rebar takes before rusting. This pushes outward on the concrete causing spalling, where concrete is split off from the rebar or the rebar rust stains a crack in the concrete:
6. Despite concrete's incredible compressive strength, it does leave some visible signs of failure. An example would be CTS's remaining standing part (before it was demolished). You can see that there are cracks on the side wall, and those cracks correspond to where every floor joins the wall:
As he points out, you'll see these generally starting from bottom to top, and indicate the possibility of imminent collapse.
7. If concrete gets saturated and starts spalling, it *can* be repaired, and the usual way of repairing is to a) fix the cause of the water; and b) inject an epoxy mixture. That's usually done through concrete injection ports, which are sometimes (and in the case of CTS, they were) left in, leaving a visible sign of the repair: https://info.chemcosystems.com/blog/bid/165642/Injection-Ports-for-Concrete-Crack-Repair
8. Stalactites below a slab are an indication that calcium carbonate is leeching out of the concrete and is structurally failing. CTS had stalactites in the garage at least back to 2020.
9. Concrete failing is not silent. *The night before* the collapse, people heard the building moaning. So they had at least 24 hours warning. Here are examples of the moaning in this 13-second Ring video of the collapse. In the last few seconds, the room is actually collapsing (just not super obviously so) as the rain of particulate matter becomes diagonal.
In that video, you hear two low moans, and a snap. The snap may be rebar breaking (this was in the column of apartments over the column that collapsed first), and the moans are concrete failing.
10. Standing water bad.
11. Visible evidence of uneven settling and/or compression bad. Look at the still used as the key for this YT video:
That was captured from a video shot in 2020, about a year before the collapse.
12. Building failures are more likely to happen in the middle of the night due to expansion and contraction differences. The way a building reacts when cooling is more stressful.
Recap, in order of most to least peril.
1. Unexplained sharp or moaning sounds in the building. Run.
(If you do hear the building moaning, don't stand out on your balcony talking to your spouse on the phone.)
2. Horizontal cracks where the floors join the walls, especially on lower floors. Run.
3. Shifting of a wall where a significant crack suddenly opens up. This happened in CTS 16 days before. https://www.youtube.com/watch?v=M0UtmbRAL9M
4. Diagonal cracks between columns, especially on lower floors. This can be failure in months or years rather than immediate.
5. Stalactites, as that's further degradation than spalling.
6. Signs of rebar or staining from rebar. You should not see rebar in a concrete structure.
7. Signs of standing water not explainable by rain, etc.
Champlain Tower South's specific failure, earliest to latest:
1. They changed 24" columns to 16" (this is sometimes done to put in more parking spaces according to one video I watched) but did not recalculate the load. Further, when they did reduce the column size, *they didn't recalculate how much rebar was needed*. Reinforced concrete needs not only steel, but also a significant amount of concrete, and the ratio is super important.
2. The columns did not appear to have horizontal reinforcement bands that prevent the rebar from bulging under stress.
3. A beam was removed after the plans changed, which affected how the building's load would be borne, but the columns weren't recalculated to account for that.
4. During construction (so long after the concrete plans were finalized), they had the bright idea to add a penthouse level. More load. Building Integrity estimates it was at 100% dead load (meaning the building was already handling all the load it *could* handle) from the start, which is not a place you want to be.
5. In 2011, they got the bright idea to put in planter boxes with palm trees on the pool deck. Right over the most overloaded columns, naturally. With heavy concrete planter boxes: https://www.youtube.com/watch?v=28YAZstU8DI
7. By 2017, the palm tree's roots were punching through and they were removed around 2018.
8. Where one tree was punching through is believed to be the column that collapsed first.
9. Buildings have to be re-certified in Miami-Dade county after 40 years, and the prelim report was generated in 2018.
10. The HOA took the report and its estimate of $9 million and made a special assessment. Was it for fixing the engineering problems? Why no, it was for making the lobby spiffier.
11. Enraged, the residents and owners got together to replace the board, which they did. By that time, the estimate to repair had increased to $15m, and they were starting to address issues, but they ran out of time.
12. In 2020, video taken by a prospective condo buyer showed uneven settling. Look at the screencap that heads this video:
Also note that exactly halfway between the columns is where the stalactites start. The left half is under the building; the right half is under the pool deck (which had many photos of standing water over time). The right front column is the one that is believed to have collapsed first, and visible evidence of structural issues was there a year before.
There were numerous signs of impending collapse that even the non-technical should have been able to parse, but…only one woman and her kid did the right thing and got the hell out of there before the collapse.
Deirdre Saoirse Moen
deirdre at deirdre.net
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