Hi, I'm Rauf Aliev.

Check out this cool story on the Veritasium channel about a critical flaw discovered in the Citicorp skyscraper after the building was finished.

The incident occurred 46 years ago, in 1978, just a year after its inauguration, when structural engineer Bill LeMessur discovered a critical error in the design of one of New York’s most ambitious skyscrapers — Citicorp. The 59-story building was constructed on stilts that were positioned at the center of each side rather than at the corners, as is typical, to preserve an old church that stood at one corner. This unusual decision required a unique system of diagonal braces (chevrons) that redistributed the loads. However, it was later revealed that due to the substitution of welded joints with bolted ones, under diagonal wind — initially not accounted for in the calculations — the building could completely collapse from a hurricane with just 110 km/h winds, creating a real threat to the city with a shocking probability of 1/16. Had it fallen, the surrounding skyscrapers would have toppled like a house of cards.

Recognizing the scale of the threat, LeMessur clandestinely began overnight repair operations named Project Serene, literally sealing every vulnerable joint in the structure with metal ‘patches’ while office workers went about their day undisturbed. Neither the employees nor the citizens were aware of the danger in order to avoid panic. A covert evacuation plan for ten city blocks was even developed. Despite the patches, everything depended on a tuned mass damper — a massive 400-ton concrete slab installed on the roof of the building, which oscillated out of phase to dampen swaying movements. It was so critical that if the power went out, the system would stop working, and the building could almost certainly collapse in strong winds. The patches did help.

For a long time, it was believed that a Princeton student named Diana Hartley brought attention to the building’s problem. However, in 2011, architect Lee DeCarolus, who was a freshman architecture student at the New Jersey Institute of Technology at the time, claimed that he was the one who called William LeMessur in the spring of 1978. His interest in the building’s unique design and a remark from his professor, who doubted the project’s reliability, prompted his call. But, there still are doubts about who the real hero was. Well, it’s clear as mud.

I watched a cool documentary about the Hoover Dam. We visited it 10 years ago when we were traveling through California, and the impressions are hard to put into words. Just now, I finally got around to finding out how it was built, and four days ago Animagraffs released an hour-long video with “x-ray style” 3D animation that details every aspect of the construction. Friends, it’s like the pyramids of Egypt, only projected onto the early 20th century and tech levels of that time.

First off — the gigantic volume of concrete. The dam contains enough to fill ~5850 average American homes. That’s roughly the amount needed to build a road from San Francisco to New York.

Next, you can’t just pour all the concrete in one go. If the concrete had been poured as a single solid block, the heat generated during curing would have dissipated over about 125 years, which would likely have caused fatal structural cracks. A complex system of pipes with chilled water was embedded in layers of concrete to artificially cool it down. I didn’t even know such a problem existed. Thanks to this design, the dam itself can physically last up to 10,000 years and will become one of the last visible and recognizable monuments of humanity on the planet. (Incidentally, the largest dam built by beavers is in Alberta, Canada — twice as wide as the Hoover Dam — and it’s visible from space. Just something that came to mind).

Extensive “grouting” was conducted — injecting cement slurry under pressure deep into the rocky foundation (up to 150 feet for the main “grout curtain”) to fill cracks and, essentially, “weld” the dam to the canyon, preventing leaks and uplifting pressure from water.

The dam was not poured monolithically. It was constructed from interlinked vertical columns built in 5-foot “lifts” (layers) to manage the heat release, control cracking due to expansion/compression, and ensure structural integrity. The video beautifully demonstrates this.

Four massive tunnels (17m/56feet in diameter, about 3/4 mile long each) were bored just to divert the Colorado River. Instead of abandoning them, they were later plugged and incorporated into the permanent structure as part of the intakes (penstocks) and emergency spillways.

“High Scalers”: Workers known as “high scalers” were lowered hundreds of feet down the sheer canyon walls in simple bosun’s chairs, using jackhammers and dynamite to remove loose rock.

Giant aerial trams: An extensive network of cableways crossed the canyon, including one with a capacity of 150 tons, which was used to move massive equipment (even trucks) and concrete buckets.

Special heavy-duty equipment: For the project, special trucks were developed with doubled load capacity, greater power, and frames made of lightweight aluminum alloys.

The emergency spillways have a total capacity of 400,000 cubic feet per second – about four times greater than the average flow of Niagara Falls during peak season.

In the spillways, massive, 100-foot tall hollow drum gates are used, which float at the water level in their chambers, automatically rising or falling to manage floodwaters. Quite an interesting engineering structure — no motors and designed to handle severe loads.

Despite its scale, the dam was officially opened in 1936, significantly ahead of schedule (construction began in 1931). Revenue from power generation had fully paid for the construction cost by 1987.

Link at the bottom, or Google Animagraffs Hoover