The Importance of Punching Shear at the Surfside Collapse

In this video we discuss why most of the columns at the pool deck punched through the slab and why it didn't happen as expected and what that could mean.

[@BuildingIntegrity]

For the more technically advanced viewers. The contribution of the steel to punching shear in this building is not as significant as the concrete's contribution. Codes around the world limit most of our punching shear formulas to JUST the concrete. Shearhead reinforcement is only used if the math doesn't work for the concrete alone and this type of reinforcement must be specially designed. This is built into the codes more as a precaution than because of any idea that the steel does not contribute at all. The reason for this is because concrete failing in shear is sudden and catastrophic and should be used as the limiting factor. Therefore In this video, we look at losing the steel reinforcement, which may be argued as insignificant, but nonetheless relevant, AND the loss of the top layer of concrete due to delamination, which would reduce punch-through resistance by as much as 25%. For more information, I recommend reading the ACI commentary on 2-way slab shear from ACI's historical records. Thanks for watching!

[@ilkkak3065]

What If punch-truogh caused pool deck collapse? And there is clip that shows water flow from pipe to carage. Could 7 min water leak to damaged carage cause hole building collapse? Atleast if there was void space and cagare slab broke and cause suddenly huge flow of water to void space that could cause damage to columns stability.

[@yodaiam1000]

BTW, I liked the video but I just wanted to clarify some misconceptions about shear in concrete. With any significant bending (Mf>Mcr) and full delamination or corrosion you loss 99% of your punching shear capacity and not just 25%. The top bars are there for bending but they are absolutely critical to punching shear capacity. Without them, the shear capacity approaches zero. With a high bending force, even without delamination, any major corrosion in the top bars will essentially eliminate the punching shear capacity (Although with very low bending stresses, you will still have some punching shear capacity without top bars). It is a misconception to think that there is a steel (vertical steel) and concrete contribution. This idea came from older codes. The concrete portion was a fudge factor since the steel portion significantly under estimated the predicted shear capacity from a 45 degree strut and tie model. Newer codes (I am not sure about the US codes since I am not from the US) use a variable strut angle models that more closely models the actual behaviour. If you had pure bending without shear, the concrete crack in a slab or beam is vertical. With low bending but high shears, the crack angle might be 30 degrees from the horizontal. With varying bending and shear, the angle starts vertical out the edge of the failure cone (the hammer head) and then develops into a shallow angle where the column intersects the bottom of the slab. The compression strut is close to parallel to the cracks in the concrete (Mohr's circle: the highest compression is perpendicular to the highest tension). Vertical steel crosses through the cracks so you can develop a "truss" to increase the capacity. However, most countries don't allow for vertical shear steel in thinner slabs (it has no effective contribution since it generally can not develop over a short height). Understanding the relationship between the top steel and punching shear is critical to understanding the probable initial failure mechanism.

[@reallyme3573]

13:40 The sand+paver+ waterproofing + tile+mortar + topping slab together form a layer of 4 6/8 inch which is already pretty thick. I wonder if that's the delaminated layer that we see on the pictures pointing upwards, discussed later in this vid ( 22:40 ) --- What happens, if a 12 floor building collapses on top of a completely intact pool-deck. Would that lead to the collapse of the pool-deck ?

[@ilkkak3065]

@@reallyme3573 Second hand eyewithness has told that pool-deck collapsed first.

[@21anthem12]

I’m wondering if the builder even followed the planners rebar schedule.

[@BuddyTobyTV]

I took a full suite of undergraduate structural engineering coursework at a university highly ranked for their civil engineering program. Mr. Josh Porter outshines every professor and instructor I had, particularly in the ability to explain basic concepts of reinforced concrete design. He’s living up to the name of his channel. We need more like him.

[@user-me8hc3bs7i]

Engineering professors think way too highly of themselves. I’ve noticed that in my IE degree adventure I’m on is that they are so determined to show off what they know that they end up doing the worst job of explaining things.

[@peterpalmer7014]

It seems that you did not pay attention in class If you believe that top and bottom flexural slab reinforcement restrians two-way diagonal shear.

[@BuddyTobyTV]

@@peterpalmer7014 Where did I say that???? Did you watch the video? I don’t think Josh makes that assertion either. If you think there is an error, provide an explanation rather than a snarky comment.

[@jockoharpo2622]

BuddyTobyTV Did you use that knowledge to anaylize the Solomon Bros. Building that collapsed into it's own footprint that fateful day on sept 1, 2001?

[@zubuxstein]

he needs a better microphone... everything else ok.

[@susanajimenez9030]

If all of the teachers in the world had this personality nobody would quit school. I could see a lot of younger people wanting to become engineers / architects thanks to this channel.

[@ctr289]

You're talking nonsense. Teaching isn't just fun, it implies formulas, maths, a lot of work, problem solving etc. This is 10% learning, 90% fun, so of course it's compelling to watch. Engineers don't become what they are by watching YT clips. But this is the excuse of most lazi or low-iq people that blame school for their failings

[@johnhoward4977]

I'm a structural concrete repair and strengthening contractor, and I can't count how many structures I have submitted proposals on that have not been repaired. They are sitting there with corroding reinforcing steel. $100m building that does not have the budget for a $100k concrete repair scope that is exponentially growing as it sits unrepaired.

[@dinaboop]

Scary. It's called budgeting people! When I lived in CT, they would always try to raise taxes to pay for plowing every winter. Like, c'mon, you had to have known to factor that in since it happens every year? And here in OR, they knew there was lead in the pipes of the water fountains in schools. But they waited for the pot money to start flowing in, then blamed one guy for having known and nothing having been done about it until they had that extra 42 million.

[@elizabethowens8548]

People want to live high on the hog with a Wal-Mart budget

[@KristaMae]

@John Howard maybe publish letters to local news saying how building owners are asking for a Miami collapse of their own? If it's kept factual, our responsibility as engineers to human life should outweigh the owners' perceived shame.

[@philippa_burgess]

@@KristaMae Agree. I have a media and PR background and know little about engineering (until now - thank you Mr. Josh Porter) and have been thinking recently that engineers + media = greater knowledge, accountability and support for decision makers. I am studying urban planning in a Masters Program so definitely have a growing interest in cities, buildings, budgets, and safety and look forward to applying my previous skills to the field.

[@boringpolitician]

This has been my impression pretty early on as well. That this has all been done, in full knowledge, with the hope that, "meh, just put a bandaid on it, it'll hold, we don't want to spend any money on this after all". I can't read minds, so I don't know. It's just the impression I've got.

[@allosch9]

How the hell do you manage to make this topic so interesting to me?! Hahaha I'm a veterinarian, so this is completely out of anything I've ever learned and I feel like I actually understand what you're talking about and can follow it and find it fascinating. Thanks! My heart goes out to everyone affected by this condo collapse ❤

[@NickanM]

He is a brilliant lecturer! 😊👍

[@fnhc2023]

I agree 100%. Bizarre. I feel I missed my calling at 59. I love the investigation and the tiniest of detail.

[@Lauragreat913]

Accountant here. And totally drawn in. Video after video.

[@jinxigrl]

Anesthesiologist... Same

[@papawspistoloo6984]

Anyone else miss this guy after a few days?

[@Popcatobjecthingy2024]

I literally check several times a day for Josh’s new videos. Out of all youtubers discussing the surfside collapse, he provides the most comprehensive and extensively researched analysis, always looking at all aspects before drawing any conclusions.

[@shopiefraccount9782]

Oh my god - YES

[@papawspistoloo6984]

He managed to make an engineering nightmare interesting and comprehensible. Like NDT on astrophysics or JP on human behavior. Impressive

[@NickanM]

Yep. Brilliant lecturer.

[@susanajimenez9030]

Yes I need a new update every day lol. I watch them every night

[@williamboyle8918]

Did a shot for everytime you said rebar, now I'm Spalling over.

[@71degrees]

We drank for "shear" 😂🤢👍wasted

[@sierraskye913]

@@71degrees Are yall still alive? 😂

[@71degrees]

@@sierraskye913 halfway through the video right now 🤢😂😂😂

[@claudermiller]

😆

[@TrumpedUp888]

😆

[@jonahbrame7874]

The more I watch this series, the more I understand why you have been an expert witness in 30 cases. You excel at communicating technical information in a way that a non-technical audience can understand. Thank you so much for sharing your valuable time and energy and experience with the internet. I have learned so much watching these videos, and I am really looking forward to the next two videos that you mentioned at the end.

[@GeraldMMonroe]

Imagine would an explanation like this would play like in a courtroom. Not only does he explain things clearly, but once he develops a theory, he shows how all these little details in the pictures fit the theory. It would be devastating for the side against his explanation.

[@rogerwilco2]

Yes, the communication and explanation is excellent.

[@KristaMae]

@@GeraldMMonroe IMHO it is "simple engineering"... Try to best understand every way possible an idea can be wrong. It builds much stronger case FOR your idea. It is the "negative" side of theory and process which makes engineering shine. It's easy to prove a thing ... Often much harder to prove the insufficiency of a thing.

[@BronZeage]

My father was a professor of Civil Engineering, specializing in structures, so I am familiar with all of this. The first thing that struck me when I saw the photos of the pool deck was how clean the tops of the columns were. There's nothing of the slab on top. I think you've identified the root cause of the failure.

[@razeezar]

When I was in school at just nine years of age, for a class assignment we had to write a fictitional news report article based on a subject of our choice. Out of ALL the things I could have chosen (e.g. cat stuck up tree, bank robbery, storm / flooding etc.) I chose to write about a skyscraper in my home city dropping a chunk of concrete due to the rusting rebar. I have no idea where on Earth I got such inspiration from (I somehow learnt what spalling was without learning the word 'spalling'), but structural engineering has always been a casual interest. That and I was a weird kid... I never did get into construction work (let-alone civil engineering), but perhaps I should have! :-)

[@mrbluesky2050]

@T.J. Kong , perhaps one or more of those cube planters slid into one of the columns. . . two or three tonnes, sideways shear, onto already weaken column ? ( sorry, tonnes in U.K. speak )

[@trinitylane2202]

@T.J. Kong That makes sense because a man on a business trip received a phone call from his wife who was on her balcony and told him the pool deck had collapsed and the building was shaking. She screamed and then the line went dead. So that's a witness describing the pool deck collapsing before the building. And there were witnesses from a building across the street who heard a noise and could see into the garage that there was debris and a broken water pipe. So that area definitely is where the collapse started.

[@rolandemorgan5992]

This building was lucky to have lasted 10 years. Imagine what those palm trees did before removed.

[@rolandemorgan5992]

@@razeezar excellent story. It shows how learning various things can mold our thinking.

[@Ronnie86753]

Love this guy and his articulate information. Very professional

[@Mowgli15221]

I totally agree! I'm fascinated with the Champlain Building Collapse story. I've enjoyed these videos explaining what probably happened. Very informational to folks like me who are not engineers!!! I love this channel...

[@ED-es2qv]

Yep

[@vzburns1]

But is said in a way you don't have to be an engineer to understand. It's awesome and very interesting.

[@jameshanenburg9591]

@@vzburns1 YesI am very interested in it also. Very informative!

[@rollothecat2010]

He is an excellent speaker. He uses examples. He explains clearly the concepts he is covering. I am very impressed with Josh Porter's videos.

[@TheMelamia]

I've never had the slightest interest in structural engineering, but I am finding this stuff fascinating. Thanks!

[@jrosebud2021]

A former employee that worked at the towers when they were newly contracted said both buildings had leaking basement ceilings from the very beginning. Residents complaining about the damage to the paint on their cars from the calcium/minerals dripping onto them. So both North and South towers had water leakage from day one.

[@jrosebud2021]

Newly constructed…

[@absurdengineering]

It’s so stupid that they didn’t waterproof it correctly. It was so simple to do and would have spared so much waste and later also lives…

[@parallelsandtangents938]

Incredible presentation! Astounding research and analysis! Your study tops all other KZbin coverage and estimations...

[@KatGMaine]

How is it that I know nothing about architecture, but I watch Josh's videos with absolute absorption? The half hour goes by like it was 5 minutes each time. What a talent you have for teaching, every one of these videos have been fascinating. It's a horrible, tragic subject but it's been very helpful to see the reason why this happened. Thank you so much for doing these, Josh! You are an amazing speaker!

[@sarahhollister150]

Same here. I never thought I’d be so interested in engineering/architecture. It takes the right person I guess.

[@paolstuart-thomson2990]

Mr Josh you are a gifted teacher and we are lucky to have your insights.

[@SandrA-hr5zk]

I love that you're continuing this series. It feels like the public (especially outside of Surfside area) lose interest rather quickly, or maybe just the news loses interest rather fast. I love all these concepts as building blocks to fully see why the structure failed as it did.

[@kanamisprs4330]

My first job back in the mid 1980s was in civil engineering construction. It was there I learned that concrete is not waterproof. One of the many tasks I had was to paint the inner face of a concrete retaining wall with bitumen. No one explained why, though I was aware of steel reinforcing rusting and causing the concrete to burst. Now I know just how important it is. If only we had videos like this back then.

[@midcenturymodern9330]

This is one of a handful of channels where I can give a video thumbs up after just two seconds, because I know it will be good! I am an electrical engineer, but you got me interested in structural engineering. Thank you! Never stop learning, folks.

[@BuildingIntegrity]

Thank you, thank you!

[@arthurcoleman6908]

This was a very clear presentation. In fact, it is the best explanation I've heard so far of potential failure elements!!

[@kmagnussen1052]

The elephant in the room is were the slabs above the adjacent parking garage also de-laminating? This would be in addition to failing at the base due to water, rust, and spalling. The fix in high salt (corrosive environments) would be to use stainless steal at the ground and below ground levels. It seams to me that a more substantial stirrup is required at the bottom of these lower level columns as well. Comments?

[@davidhenningson4782]

@@kmagnussen1052 I bet the delamination ran right through the top sections of the parking garage columns. Spalling around the rebar there and "any" lateral movement (such as the collapse of the slab under the planter boxes... creating an imbalance of forces, from a horizontal vector, acting on the weakened columns)... would have started the domino style collapse. The affected columns would have slid past from the top of the delaminated garage ceiling (1st floor) and off the garage level columns. So 1st floor (ground floor) buckling and point of failure.

[@michaelcorvin4330]

Josh, thanks for keeping these vids free of conjecture, drama and creepy background music. It's refreshing to have an actual engineer explain structural engineering to the lay person in a simpler fashion. I wish more videos followed your formula. Thanks for all that you've provided.

[@testeksystems1483]

Josh Your method of teaching far surpasses all my university professors Please please please Consider becoming a professor The world needs engineers like you High praises Randy

[@testeksystems1483]

Josh Would it make a difference if a larger beam is used from pile cap to first floor and round instead of square And does it make sense to eliminate basements since the water table is so high in Florida

[@vipahman]

My heart skipped a beat when you explained the cracks in those core samples. I thought they cracked while drilling the sample, but closer inspection does say otherwise. It's a chilling realization.

[@deedewald1707]

I agree and core samples performed in 2018 ! Most of this structural damage was hiding !

[@kiwidiesel]

Core samples should never look as shattered as the ones in the first pics. If you have good concrete and good integrity there should be zero fractures in those samples.

[@johnbroadwell2603]

I was a rebar inspector among other duties, I did inspections for commercial, bridge and nuclear construction. My attention is to focus on the base mat, thinking of what I have seen and read about the history of the bottom slab being wet, especially it being salt water.

[@TheBandit7613]

As a heavy equipment operator in Las Vegas, I've done plenty of demolition cleanup. The rebar was a twisted mess and actually kind of a pain to work around. The first thing I noticed when this building collapsed is not as much rebar as I'm used to seeing in a rubble pile. I wonder if they skimped on the rebar.

[@christinefolgmann3772]

Best KZbin coverage of this issue, by far. Thanks!

[@christophercripps7639]

Very good analysis. The garage was screaming for years time is running out. The question almost seems to be not why did it collapse in 2021 but how did it not fail sooner? Or perhaps what precipitated the failure of the first critical element?

[@lynnmaupin-simpson1215]

I saw 2 news programs via You Tube. One mentioned the change in building codes post hurricane Andrew and the other mentioned the level of corruption that was going on when these condos were built. Then you add in climate change and you have building failures. It was eluded to that the building codes changed during the construction of these buildings. Also, the building managers quit due to arguments among the tenants. The repair bill was $9 million. The tenants thought it was too high.

[@colincampbell767]

@@lynnmaupin-simpson1215 The HOA had already secured a $15 million line of credit to pay for the work. At the time of the collapse they were waiting for the contractors to submit their bids. What I do find interesting is that a couple of months prior the prospective bidders had gone over the entire building. Apparently none of them noticed anything that caused them to sound the alarm.

[@chemech]

​@@colincampbell767 In a previous video, Josh discussed the potential problems with the soil and water conditions below the slab and piling caps underneath the underground garage. It is extremely likely that the soil was washing out and "working" below the slab, causing cracking, which in turn allowed saltwater intrusion into the bases of the building columns, leading to rust-jacking. None of that would be visible from above until the situation was beyond critical.

[@colincampbell767]

@@chemech If the problem was at the base of the columns - why are they still standing?

[@tubester4567]

@@chemech There were some cowboy contractors in Florida in the 1980's. A 12 storey building collapsed in 1981 while under construction in Florida, killing several workers. The cause looks clear to me. Poor quality work during construction, lack of rebar, corner cutting, and probably poor quality concrete too. One of the investigators who drilled the core samples said the concrete was very soft and easy to drill. The salt water and air around the building exacerbated the existing problems.

[@happyhome41]

This is GREAT -- maybe I had to see the others to get this one -- this one was clear as a bell.

[@peternicholsonu6090]

I’m not an engineer but father brother were and it’s in my bones. Having been involved in multi-storey construction in Australia I became accustomed to the methods you described. Also used to hang suspended ceilings from under slabs. About 10-12 years ago I visited an older high rise for remedial floor topping to drains. I parked in the underground car park and naturally looked up!! Absolute horror at radiating cracking from all column tops.....multiple cracking like never seen before. This is a Casino on Queensland Gold Coast. Have not been back, now retired anyway but figure a large number of qualified men have seen what I saw over THE YEARS. Sometimes I figure I’ll hear of it being closed for demolition or worse collapse but no, life goes on. Love your analysis so clear and understandable....thankyou

[@JustMeUpNorth]

I know nothing about buildings or architecture (I'm an English teacher), but after watching many videos on this channel I'm absolutely fascinated. Thank you so much for the excellent explanations and clear information, put across so well!

[@Raptorman0909]

Josh, I've been following your analysis for some time and I have to say this video is the clincher for me. Clearly there was punch through and the delamination of the top of the structural slab around the top mat would weaken the punch through resistance. The poor or non-existent water proofing explains the delamination from corrosion, spalling etc do to water as well as weakening the concrete itself doomed the slab to fall eventually. And, once the slab begins to fall it can pull some of the building columns towards the falling slab and once those columns are rendered useless the building comes down. The fact that some of that water may well have been salt water and the fact that rising and lowering water tables of saline water ... it's just a nightmare. I think Florida and other coastal cities around the world need to learn from this tragedy and revise codes and do potentially many other things to account for the fact that sea levels are rising at about 3mm per year. Over the 40 years this building was standing sea levels will have risen about 80mm or over 3 inches with the pace of the rise increasing.

[@uploadJ]

re: "the fact that sea levels are rising at about 3mm per year" Land subsidence; The longer-existing sea level 'markers' around the world don't show this ...

[@shAnn0n1]

The way they explain everything is great. It's great to learn something new when a tragedy like this happens. We'll be waiting with bated breath for the next several videos.

[@Varangian_af_Scaniae]

"The way he explain everything is great" You know the gender of the man in the video, stop unperson people.

[@shAnn0n1]

@@Varangian_af_Scaniae I say they because he has a partner with him at BUILDING INTEGRITY. They both do videos for KZbin. I'm sorry if I offended you.

[@toomanymarys7355]

I think you're the first person on KZbin comments to ever spell "bated breath" right. ....

[@shAnn0n1]

Thank you for the comment. It's funny because the spell check gave me a hard time when I was writing it.

[@sisboombah9595]

I have absolutely no background in engineering, but I found myself captivated by this video. Amazingly good job laying everything out for the layperson.

[@scottbc31h22]

Take a look at column 72 (@4:36). Note that there is two long (4' to 5') pieces of rebar sticking out of the column in each direction. These rebar appear to be part of the top layer of rebar, of the structural slab. Note that there is only two pieces in each direction instead of four. On the right side of the column, marked 73, it appears to show a short piece of rebar (1' or so) that is part of the lower reinforcement. If the structural slab had de-laminated, as described, these bars would have pulled out of the slab like a loose thread from a piece of cloth; when the structural slab fell out from underneath. The weight of the overburden layer (as well as the inch or two of the de-laminated concrete would be enough weight to bend these rebar to the shape shown. This further supports the theory in this video.

[@stevemiller995]

Plus, those sections of rebar (protruding from pillar #72 (and most others), shows zero chunks of concrete attached to them... they are stripped clean.

[@Tomcatt817]

When we look near the pool where the slab did not fail, why do we not even see the 2 pieces of rebar that we saw at column 72?

[@markmorden8857]

Excellent video. As a former forensic architect, I appreciate you sharing your knowledge and providing solid info for everyone watching. I am tired of watching home inspectors who think they undestand structural engineering and are making videos of "what happened." Yours is treat to watch.

[@markb.1259]

LOVE YOUR CONTENT JOSH!!! The west side of Champlain South didn't fall because it appears that west side was built on 24" by 24" columns... vice the skinnier column on the east side. Just my observation. Please keep bringing more to the table. You are THE ONLY person who's theories I can relate with and support!!!

[@Angelaius]

Always thought those coloums way to thin in the garage area

[@goyaliza]

You have to wonder if by adding that thirteen floor if there was any strengthening of the supports for that part of the building. Am I right in thinking that if you change the load at the top, you have to have support at the bottom that corresponds to what it's holding up?

[@lindap.p.1337]

Think it will also be less water intrusion. We"ll see!

[@TyphoonVstrom]

The columns did not fail people, they are still right there poking through the slabs. The columns were pulled/ knocked over as the structure pulled down around them in the higher levels.

[@GoSvideoEye]

Suspect the large sheer wall on the opposite side of the elevator bank and stairwell was a huge reason it remained erect even with all the vibrations and ripped out rebar as the portion more eastward collapsed.

[@_i_am_unceded]

Honor and Respect

[@dougwainer8768]

Excellent Presentation. Just makes me appreciate what it takes to design/build a safe building.

[@limrc1]

Crystal clear explanations with flawless logic. Thank You and looking forward to the next video!

[@stuartmalin661]

Brilliant forensics, talented observations, and exceptional explanations. Every minute worth wathcing!

[@CraigRasmussen1]

Of all the structural engineers who have presentations on KZbin, Jeff has done the best job by far in explaining possible causes for the collapse.

[@chuckcampbell3927]

What a blessing to listen to such an articulate mind. You have shed much light on an industry that is taken for granted. We are expected to have faith in every building that we walk into. You have certainly changed my perception forever. GOD bless 📖🛐🛫🛬👍🧠👍

[@leeboriack8054]

I pray managers and residents of highrises watch your videos that can be life saving. You are an excellent speaker and teacher.

[@pamelabrown9484]

This is like a master class in Architectural Engineering! Thankyou, for the time you have taken to educate us non engineers! You are a born teacher...

[@LeewardStudios]

Very informative. God forbid I ever have to respond to this type of emergency but it gives me an idea of what to look for, and document for future analysis. Thank you.

[@Brandonpowerstroke]

This is actually the only Channel that I do not fast forward and skip through. I work in construction and every second of these videos are interesting.

[@lyleparadise2764]

Your teaching skills are impeccable. Before watching your videos I knew absolutely nothing about structural engineering. But your clear and keeping it simple to understand explanations and illustrations are making me feel like a novice engineer. Thanks for doing this.

[@markgreen7701]

You do a GREAT job explaining all of this. Miles ahead of most other KZbinrs. Thanks!

[@aprilmorrill5735]

I am learning so much because you present the information in such a clear and concise manner. Thank you.

[@catherinejohns8449]

I must compliment you on your clear, well thought out explanations that laymen can understand. I've tried to watch others but they're rambling style that hasn't been organized before hand are so disappointing I can't watch their entire video. Your videos are clear and concise and easily understood. Thank you for your professionalism.

[@hermanwillekes6273]

From an old wood butcher, thanks for a great explanation. Spent a few years of my career working on high rises, so I have a vested interest. Again thanks.

[@TheStampinMom]

Thank you so much for sharing. You are clear, concise, and very easy to follow.

[@yodaiam1000]

I realized earlier the interaction between the top steel and punching shear was likely an issue. I do a lot of concrete design including shear and punching shear. The top bars do not contribute directly to punching shear resistance. What they do is give you bending resistance. As long as you have the minimum bending steel on the top, the calculation for punching is independent of the total amount of top steel. However, what the top steel actually does do is increase the size of the "hammer head" over the column. With less than minimum top steel, the punching resistance does start to decrease and it will decrease rapidly. Below minimum levels, the reduced top bars do affect punching shear since the size of the "hammer head" is reduced. Even if you ignore the delamination, you still have an issue with punching. Heavily reinforced top bars, makes the angle of the cracked concrete shallower. It is the shallow angle that gives you more shear resistance. Without the top reinforcing (or corroding and delaminating top bars), the angle becomes vertical and the punching shear capacity starts to become insignificant. You are not shearing the bars. The bars control the angle of the crack which affects the punching. The punching shear crack is approximately parallel to a compression strut in the concrete. By extending the crack out, you extend the compression strut out and that is what gives you punching resistance. Shear in concrete is actually about the compression and tension in the concrete and not a shear stress. All shear stress can be resolved into pure compression and tension stresses. A shear stress is just looking at compression and tension in a different orientation. Once concrete cracks, the tension in the concrete dramatically reduces and you are left fundamentally with a compression strut that supports the slab shear.

[@princejesterful]

Thank you for the detailed explanation!

[@heathernewman5272]

As someone with no experience in this area, I now have a good understanding of punching shear. Thank you.

[@c.rincon7145]

I had no idea columns could punch shear, due to delamination of upper slab. I was under the impression, once conrete hardened around rebar, it was there for life.

[@TyphoonVstrom]

The myth of reinfirced concrete construction. Every structure has a finite life, much shorter if exposed to corrosion of it's internal structure.

[@gordonrichardson2972]

The continued water intrusion actually dissolved and removed some of the concrete around the rebar. All the stalagtites seen in the parking area were remnants of that cement.

[@stargazer7644]

It is - until the rebar rusts.

[@tkdally]

Thanks for these videos Josh, you do an amazing job explaining complicated concepts to the lay person. I'd take a class of yours any day!

[@suzyqc7607able]

My Heart sinks, It's so sad- Thank you for explaining it in overly simplified terms.. I have two bachelor's degrees from Miami U. & am just an RN - no engineer. It makes my heart ache for everyone there.. I've long since gone- Back up here to Tennessee - .. Surfside, You are in my prayers & thoughts every day.

[@kathieh7012]

I've been watching you, Josh, since your 1st video after the Champlain Towers South collapsed. I have no building experience except with my kids & Legos. But your presentations are really helping me to understand building concepts & what the possibilities are that either the design, the actual materials used or how they were used to build this tower may have been flawed. Thanks so much for doing such an awesome job clarifying everything! We sooo appreciate you--keep up the great work!! 🤗

[@mariannerogers8003]

Your videos are the best presentations explaining the collapse. I’ve never been interested in engineering design but your videos are so good that I have to keep watching!

[@ItsAlwaysRusty]

Always such a thoughtful and logical presentation. You make the subjects so easy to understand.

[@VisoMoraine]

The cracked cores would not necessarily indicate spalled missing concrete. I see this a lot when the field techs performing the core (not the brightest bulbs) are too lazy to anchor bolt the core machine to the slab and, instead, attempt to hold it by hand. The core bit, which is a long cylindrical bit gets out of plumb as it goes down. Now its rotating but at a slight angle perpendicular to the slab plane. The core bit puts lateral force on the core and shears it in two before getting it extracted. I've had to video them taking the core and then stop the tech to show him how far his bit is leaning over.

[@wordcarr8750]

Didn't the video presenter imply that *if* these cores were simply broken as they were taken, the breaks would 'line up' and appear as a thin line when core is pressed together? i.e. to me, that indicates *these breaks* couldn't, alone, be caused by the way the cores were taken.

[@r19surf]

The diamonds passing thru the steel could create a torque causing the concrete in the core to fail. Would have been beneficial to have a photograph or notes of what was inside the core hole.

[@drofwarcnwahs2108]

@@r19surf No. The torque is not significant. You can see in one of the last core samples presented that he shows a piece of rebar that is cleanly cut with no impact to the concrete around it. I've had coring done at a workplace and have seen the cores. In each case the rebar is cleanly cut with no impact to the surrounding concrete just as what we see in that one core sample towards the end.

[@shellyperryman995]

Josh u are so good at what u do and explaining things, I have learned so much. I can't wait for your next video. Thank you

[@LesFelts]

Josh, your investigation, diagnosis and explanation makes my brain happy. Thanks!

[@zimman20]

Always look forward to Josh's videos and explanations in layman's terms. I thought those broken core samples were suspect, I've seen core samples before and they were solid thru...the way these were broken seemed indicative of what was happening out of sight. Thanks for your time and expertise explaining these points, sir - Kudos!

[@destdan2]

These are great videos and can’t wait for next one. I would like to see what you would or could have been done to fix this had it been identified earlier. Thank you again.

[@KristaMae]

Although should have, could have, would have are typicality fluffy in nature, I, too, would like to hear what could have been done at significant moments in the past. What was "known" before the core samples? One thing I know: going forward, I want to waterproof everything.

[@tarawerner4271]

@@KristaMae and drainage at the correct grade to the correct location. Seems all so simple that it makes this tragedy infuriating along with horrifically sad.

[@vkng_drag0n982]

I used to interpret for Deaf students in engineering and drafting programs and I learned so much. This refresh my memories and your explanations are very easy to understand. Thanks!

[@MelbaOzzie]

Again, a very informative video. I think there are a lot of apartment dwellers, not just in Florida, who are feeling very nervous right about now. Thank you for your efforts.

[@lonnymo]

Good review. From the point of view of a construction special inspector with over 20 years experience, I see all the things you have pointed plus a couple more that may have contributed: 1. The top mat is too low and it is possible the bottom mat is too high leaving a thinner reinforced slab section. There may also be cold joints present from interruptions during concrete placement. The lack of top reinforcing as required by the structural plans is a likely result of relying on the contractor to do the job correctly. Inspection is a critical part of getting the building built correctly. The punching shear failure is very obvious in many of the photos. Now apply this scenario to to a seismic zone like we are in here in Seattle and you see it is critical to proper construction of the slab. It is a very unfortunate example of how building codes are written in blood. I look forward to your future reviews as more information comes to light.

[@paulmccormick5042]

Love the presentation style. Simple, clear and respectful.

[@sababylon]

Your videos on the collapse are very informative! You explain everything clearly, and the diagrams help.

[@jimblack5596]

Your videos are excellent! Rational, honest, fact based. Bravo!

[@k.y.6148]

Very, very interesting and informative as always.

[@515nathaniel]

I can't watch any other KZbinrs on the Surfside collapse after I found your channel! It's so detailed and well-informed.

[@rscbmr1023]

I learn so much listening/watching to your video's. You are a great teacher even for those of us who have only learned about anything engineering from "talking shop" with her son.

[@scofab]

Another fantastic job, well done.

[@W1se0ldg33zer]

It really looked like that whole pad area was just total crumbled concrete with a little bit of rusty rebar holding it somewhat together. 'Don't nobody breathe!'

[@SeedlingNL]

Imagine someone standing in the garage, slamming a fist against a pillar because he was mad about something, a crack creeps up the pillar where he hit it and the whole ceiling comes crashing down...

[@-._.-KRiS-._.-]

@@SeedlingNL What would that be called, "Hulking Yourself"?

[@SeedlingNL]

@@-._.-KRiS-._.- I watched Wandering Earth a day before, it's a scene in that movie:)

[@dpg0jod]

Excellent series of videos. The best on KZbin for Champlain South.

[@postie48]

Great presentation (as are the other videos in the series). I regard my self as a "technical viewer'- a PE (Civil) for about 50 years but focused on Highways in early years, and contract and project management later. I did however do (as a very young engineer) some work on structures after Ronan Point collapse and much later on housing blocks in Far East that were severely suffering from issues around corroded reinforcement due to salt water leakage inside buildings (used for WC flushing, fire fighting and washdown of public spaces), too thin walls/slabs, missing cement/rebar in concrete, and concrete made with salt water, and some questionable overall structural decisions. But my later role was contract and project management not structural engineering - although I did 'comment' on a few worrying bits! Anyway, I consider a likely failure mode at Champlain is collapse of pool deck due to punching shear caused by delamination, possible with overloading due to planters etc., this slab collapse then pulled out columns under building due to beams linking pool decks and building columns - so a pancaking of the building. BUT looking at some of the structural drawings and looking at various concrete strengths and how they changed for slabs and columns I couldn't work out how the columns/slab nodes could be constructed while maintaining the different specified concrete strengths WITHOUT leaving a construction joint running either in column in the plane of the underside of the slab, or in the slab on the line(s) of the column face(s). If anyone can help me here I would be grateful. At first glance of rebar drawings (actually I looked quite carefully) the rebar out of the columns bending into the slabs looked to have minimal lap lengths, and I was surprised that none of the column bars didn't 'turn' (either at 45 degrees or at 90 degrees to the level of the top slab rebar. As shown in the video the lack of shear reinforcement around the column heads are surprising. (as clear from remaining columns and drawings). The overall impression is of 'light' levels of rebar although residential building do have light floor loadings. Also looking at the drawings - and the photographs - the lack of 'beam strips' in the slabs appears clear, and if these had been present would have added a redundancy and robustness to the structure (But I have never been involved in flat slab design/construction so have no credentials in this area). For a robust structure in a strong wind environment (i.e. hurricane/typhoon) the columns look slender (although are generally very short i.e. limited story height) and the grid looks sparse, and the overall (i.e. whole building complex) limited shear wall/or L shaped columns appears surprising - but maybe that is just my 20/20 rearward looking vision. But it is a hurricane/typhoon and having been on upper floors of 20/30 story buildings during high winds I respect those forces! After the failure of Ronan Point in 1968 (which was built from precast panels) we added, for future similar buildings, additional rebar to some panels to provide column/beam strips and we added more steel strengthening at wall/slab joints especially at column/beam strip nodes. There was some retrofitting as well. To repeat thank you for the professionalism of the videos and the quality of the engineering knowledge is very clear. From myself and I am sure from many engineers worldwide my sympathy is extended to the families of the victims and my regrets in what appears to be a failure of my profession.

[@ernestrollins383]

After this lesson on the cores, and how little rebar was actually effective in holding the building together, it is alarming to see pieces of that very important rebar being removed in the core sample.

[@TyphoonVstrom]

Buildings are designed with many times over minimum strength requirements, specifically to allow for aging of materials, damage of various members etc. One or two pieces of steel should not cause an issue. The real issue is that the entire top layer of steel was no longer supporting the slab, as it was detached.A slab can cope with a few piece of compromised rebar, but not more than half of it not doing anything.

[@ernestrollins383]

Exactly, but my point is with the top layer useless you are relying on the 6 points in the bottom layer supporting the load, and when you cut a couple of those with the core drill, that compromises what little support you had right there.

[@stargazer7644]

The rebar removed is completely insignificant.

[@kathleenwilliam]

Another great video and another piece in the puzzle. I hope that at the end of this series, you'll do a summary video, bringing all the various pieces together until we can see the larger story of a building that shoud have held but didn't. Thank again, Josh.

[@vzburns1]

I really appreciate these videos. You are making things interesting, and informative without talking over our (my) head or dumbing it down to the point it doesn't really teach anything. Thank you and I look forward to the coming videos.

[@manuelkong10]

I feel SO lucky/blessed to have found someone with SUCH intellect and abilities. It is ALWAYS a blessing to have an opportunity to listen to someone of great knowledge and educational skills. You have made this understandable AND Interesting. You are precise in your language and speech and I appreciate it. I HANG on your words and Really attempt to Learn from your lessons. Again, a Great opportunity thank you thank you thank you

[@edzachary8657]

As always I was really happy to see one of your videos come out because of all the people that are out there giving their opinion I have way more respect and trust in what you say, and again I know I learned a lot from you, great job as always explaining something really technical in terms that we can understand, great work I look forward to your videos

[@michaelcrossley5661]

I always look forward to your new videos. Great work!

[@dlrho397]

Thank you for your very clear explanations!!! You leave your viewers with a good understanding of the topics you cover!

[@dustincanale184]

great job man, I am a Smaller General Contractor in AZ and really appreciate the way you explain things.

[@SJKile]

I have to say, I’m no engineer, but…..You’re pretty damned good at what you do & how you explain it. This building was doomed at the time it was built. Which brings up my question, shouldn’t every high rise apartment building, now, be scrutinized from the original plans to reveal defects. Thanks for all your insight. I’m fascinated by all this stuff I never knew anything about.

[@kalidilerious]

I do not know a thing about Engineering. But his two week theory of a high rise building collapsing in 7 seconds because of a 25ft X 15ft pool and (get this) PLANTER BOXES didn't add up. No REAL engineer just comes up with that crap. He is just saying this now because other people in the comment section (with NO engineering background) sort of told him like yeah this building was NEVER built to code for a collapse to happen the way it did.

[@newyorkval1478]

You still don't know.

[@loopthetube]

@@kalidilerious just wait till you hear about the palm trees. I had a tree grow and totally ruin my slab once. Soooo....

[@SandrA-hr5zk]

@@kalidilerious ohh, you mean because he's not jumping on every bandwagon someone says? And the pool deck represents the entire patio area behind the building, not just the pool in the corner. If you look at any picture, there is absolutely nothing wrong with the pool. It didn't even crack or lose water.

[@jeffreyswanson3798]

I have 45 years experience as a builder my father was a bridge-builder way before me and his father was a bridge-builder we have never had a failure like this before but it is time for the future way of building Public Safety lives matter yes it may be more expensive but what matters is zero loss of life

[@whiteeaglewarrior]

Love these presentations. We had a swimming pool complex built with low grade concrete and other materials. The builder was caught and flung in jail and the building kept an eye on. It was demolished in late 90s. This tragedy is so sad, but lives will be saved further down line as people start to get a keener eye for these hidden issues....thanks to channels like this and a couple of others. Estate agents wont be able to flog pretty death traps as the dream anymore

[@Rattsu]

You are very good at explaining how and why things matter. Thank you for taking the time to share your knowledge.

[@jimanonymous113]

I think rebar atop the column helps with the moment of the slab above and adjacent to the columns but rebar to resist punch-thru shear should be on the bottom of the slab above the column. Otherwise the column will just push the steel out through the top, as it did. That steel never got a chance to resist shear. The slab would have had to shear it before the column could punch thru if the steel had been near the bottom of the slab, assuming no extreme spalling on the bottom that would allow the rebar to slide out of the slab and bend down along the column as the slab slid down the column.

[@RobMancusoJr]

As other commenters have said, you really are a great teacher. I was a total novice to these concepts prior to discovering your channel, but have learned a lot and keep learning!! Awesome content man, can’t wait for the next one 👏👏👏

[@davidnielsen4490]

Thank you Josh for this very informative video. Your explanation shows how important small details must be followed.

[@morenofranco9235]

It is all SO boring... UNTIL you start seeing the failures. Then the story starts to unfold. In some sort of cascading nightmare. And I get sucked into having to watch with morbid fascination, until reaching the *"WTF were they thinking?"* moment. Great analysis. Thank you.

[@xbjrrtc]

You are the best! I really appreciate these videos!

[@sarahb.6475]

The drawing you showed of the cracked core with all the material missing around the bar reminds me of a balance board!! Because I bought a balance board from amazon. And it looks just like the upper part of the drawing: an object trying to balance on top of a circular bar. But building columns should NOT be trying to balance like that!!

[@chocolatechip12]

Thank you very much for your explanation. I was wondering how the columns could punch through so cleanly. The way you explain is so easy to understand, even for someone like me, who couldn't even get my elementary school art projects to stand up right.

[@willgriffin3490]

Great video! Clear, concise and to the point! Looking forward to more on this event.

[@richardrodriguez1742]

I called this on the second day, I saw the plans & my first comment was where are the column caps in the plans, dropped panels 6" thicker than the slab usually 8'X8' around the column, they prevent punch through + extra reinforcement.

[@billj5645]

Column capitols and drop panels are not always necessary, IF the slab is designed properly. Nobody in an expensive condo wants to be bumping their head on a drop panel as they walk around. But you are free to do your own punching shear calculations- I think you will find that the original building was not designed properly.

[@DR-nh6oo]

Been pondering why I find these videos to be, apart from informative, very relaxing, I think that it helps to know there is an order behind the chaos amongst the laws of physics, and hopefully knowledge like this will help to ensure the bastards don’t get away with it if they have cut corners for profit.

[@vegasbabyy]

Another great video!! When I knew what the overburden was called before you said it having learned in your previous videos says so much!! Excellent teacher!!

[@trucid2]

Up til now we saw the building was in poor shape but no smoking gun for why it collapsed. This delamination looks like the smoking gun.

[@bigiron5508]

But the mayor called it a act of god... so freakin sad

[@kiwidiesel]

I would have to agree with you there, many years ago I worked for a demolition contractor and had the pleasure of jack hammering concrete structures with excavators and by hand. We also did core holes in existing buildings for the addition of more services, pipes etc through concrete walls and floors. The moment i saw those core samples I was shocked at how they had deteriorated. A good core sample doesn't fracture like the first couple were shown to and should always look at least like the last couple of pics where the drill will cut the rebar like a knife and have to fracturing around the rebar. It looked as though neither the top mat or lower mat was attached to the concrete slab in any way by the crumbling around those rebar pieces.