 It is the afternoon of the 23rd of April 1987 and rescue workers and emergency responders are clawing through the rubble of an under construction apartment block that has collapsed. Is rapid, unexpected deconstruction would raise serious questions about its rather not so common construction method? The death toll would be 28, making it one of the worst construction death tolls in modern US history. The disaster has been a rather often requested subject on this channel. So let's get into it. My name is John and welcome to Plainly Difficult. Today we're looking at the La Ambiance Plaza Collapse. Background Now today's subject is a concrete building failure, but it is all the construction method I don't think I've covered before on this channel. So I'm going to start with this for the video. After that we'll go into the background of the building. But when I'm talking about this building method, I am talking about it in the specific case of the La Ambiance Plaza. So the building's technique is the lift slab method. It does have some advantages, but also some drawbacks. You see, instead of doing concrete pouring in situ during a building construction, the lift slab method does all of its castings on the ground, one on top of one another. After being poured and cured, each slab is then hydraulically jacked up to its final place. The big advantage of this is that you get all the tricky casting on the ground, which reduces the need for formwork and reshaws to hold freshly poured floors in place. Great. Surely as that whole process has caused many issues with buildings I've covered on this channel before. So once each floor has been jacked up to the required height, it is attached to the column. Don't worry, we'll go into this in a little bit more detail shortly. The downside is that lifting heavy stuff is risky and requires a team of specialist workers. The lifting of each slab is a complex affair, requiring a hydraulic lifting jack on each column that is to receive a slab. Each jack has its own control gear, and in the case of the La Ambiance Plaza, each was impendently movable. During lifting of any floor, the jacks could raise at 1 ½ inch increments for a speed of 1.5 meters per hour, or for the Imperial thinkers, 5 feet per hour. Which when you think about it is actually rather fast for lifting very heavy concrete slabs. So the slabs were what's known as pre-stressed. That is, as Michael Collins explains in his book Pre-Stressed Concrete Structures, pre-stressed concrete is a type of reinforced concrete, in which the steel reinforcement has been tensioned against the concrete. This tensioning operation results in a self-equilibrating system of internal stresses. So this drawing may explain it a little easier. The left is non-pre-stressed concrete, and the pre-stressed concrete is on the right. The images below are what happens when they are placed under load. Not only are the slabs pre-stressed, but the stressing is done post-concrete pour. In fact, the tensioning cables are pulled tight after the slab has hardened. Anywho, the La Ambiance used pre-stressed slabs, which were ultimately supported on columns, which were steel beams. Instead of concrete supports. Those always seem to show up in my videos. Each slab was 7 inches thick, or 178mm. They also had a thing called a shear head, which worked to transfer vertical loads from the slab to the column. They were also used in the lifting operations as well. So for lifting, the slabs, the shear head has an attachment built into it called the lifting angle. The lifting rod extension, which is attached to the threaded jack rod, runs through this, and is secured in place by a lifting nut. During attachment to the column, a team of workers put the shear head into use. Once the floor was at the correct elevation, a wedge was placed between the slab and a weld block. Once the load was transferred to the wedge, the wedges intact welded into place. From there, the wedges were more substantially welded. Seal blocks were added and welded in place, and all of this was sealed in with concrete eventually. That is what they do for a permanent parking, but slabs could be temporarily parked awaiting lifting further. This involved the wedges just being left tack welded. It was enough to keep a slab in place for the short term. The La Ambiance Plaza was set to be a 16-storey, high luxury apartment complex along the Washington Avenue in Bridgeport, Connecticut. The sum $16 million project was set to be a nice little addition to the city's late 1980s skyline. The plaza consisted of two offset rectangular towers, each 112 feet by 62 feet. They were known on the plans as the East and West Tower. The two towers were essentially separate buildings, as such they were constructed independently, and the whole site had a three-level carpark below it. The first columns were erected in October 1986, and the building continued at a good pace, with a total of 24 columns in the east tower and 25 in the west. By April 1987, the project was around halfway complete. All of the slabs had been poured and were just awaiting lifting to their final places. But sadly, the project wouldn't last the end of the month. The Collapse So it's time to grab your bingo card and let's start the disaster. So it's the 23rd of April 1987. For a useless reference, this was about one month before the release of Ken Koyabashi's sizzle album, which is the album I've been listening to as I write this script. Anyway, city pop aside, the La Ambiance Plaza is over halfway done, and structural work is being undertaken on both the east and west towers. On both towers, a number of concrete slabs have been temporarily parked, awaiting further jacking up. This was on the ninth floor. The west tower, by around 11am, had three floors parked temporarily, being held up by the hydraulic jacks, awaiting the temporary spot weld to the wedge blocks at the sheer head of the slabs. Work not pertaining to the building structure was being undertaken at the same time, things like first fix electrical wiring. At around 11.30am, a loud bang and creak rang out from the west tower, near the elevator shaft. Witnesses saw the slab around the columns E3 and E4 begin to crack. The top slab crashed into the one below. After a short moment, the load overwhelmed it and continued to collapse. Within just a few seconds, the west tower had succumbed to a progressive failure. Some columns were bent in the collapse. Debury was thrown from the west tower and crashed into the east, causing it to also fail. After just five seconds from the initial collapse, both east and west towers were now on the ground. Many workers were buried in the rubble. Immediately after the collapse, rescuers frantically started digging through a mess of concrete and steel. Soon after emergency workers attended the scene, helping to extract any survivors. After just a few days of salvaging and exhuming, 28 people were confirmed to have died in the collapse, marking it one of the bloodiest construction disasters in US recent history and the worst involving a lift slab project. But of course, the pile of rubble and the many dead meant that the cause had to be discovered, and this would result in the National Bureau of Standards getting into our story. The investigation So the gang in charge of investigating the collapse was the NBS. They arrived on scene the day after the disaster and immediately began searching for the cause. The investigation would take 48 witness statements, some of which were somewhat conflicting. Understandable when the whole event began and finished in just a matter of seconds. In addition to the statements, every inch of the structure was inspected. The witness accounts and evidence of deformed sheer heads hinted at slab deflection. This led investigators to look at situations that would cause the slab to become off the level. A number of possible failure modes were explored, including subsidence of one of the columns, but evidence did not support the theory. But something about the work being undertaken at a certain part of the west tower pointed towards the probable cause. You see around column E3 and E4, welding work was being undertaken to temporarily park the slabs. Investigators assumed that in order to complete this, the elevation would need to be adjusted and this opened the door for a potential deflection. This is all pretty normal, but columns E4 and E3 were the most heavily loaded that day. And as the jack was used to adjust the slabs, one of the sheer heads lifting angles deformed, which in turn allowed their lifting nut to slip off. Thus removing the slab support at the lift jack and as the welding work to secure the slab hadn't been completed, there was nothing to link the column to the floor. Lift slab constructed buildings are very vulnerable to lateral loads whilst under construction, at least until the sheer walls are poured. So once the slab was unsupported, the house of cards would just collapse. The disaster would raise serious concerns about the building method, with it being temporarily banned in Connecticut. So financially the disaster would cost a rather large amount of money, $41 million to be precise, with just under two years after the disaster being paid out. The speediness of the payout was done via mediation, avoiding the usually decades long fight of litigation. The site would be cleared and the Renaissance plaza apartments would be built, along with a seven foot high memorial to the disaster. So on the disaster scale, I'm going to put it as a four, as there was some death, but the disaster was fairly localized to, well, just the building site. And with my bingo card, this is what I've crossed off. So what did you get? Now this video's facts and figures were based off the NBS report, and the link is in the description. I'd also recommend reading Why Buildings Fall Down by Mattis Levy and Mario Salvadori. It's one of my favorite books and I've even bought it for other people, including my brother-in-law. Read it, it's good and it covers a whole cornucopia of building's unkeeping upness. This is a plainly full production. All videos on the channel are creative, common and attribution-share-alike licensed. I'm currently recording this script in the gooch time between Christmas and New Years, and I feel absolutely really rough. It's also currently very windy and miserable here in South London. This also gives you an opportunity to see how long it takes from script recording to final video release. It takes a few weeks per video. Anyway, I have Instagram and YouTube, or second YouTube channel, so check them out if you want to see other bits and pieces of what I get up to. I'd also like to have a very warm thank you to my Patreon and YouTube members for your financial support, and the rest of you for tuning in every week. And all that's left to say is thank you for watching, and Mr Music, play us out please.