More : [..] While Millennium Tower, with its concrete frame, is the third-heaviest building to be constructed in San Francisco — coming in at 686 million pound

More : [..] While Millennium Tower, with its concrete frame, is the third-heaviest building to be constructed in San Francisco — coming in at 686 million pounds — no other tall buildings in the vicinity had drilled down to bedrock before Millennium Tower was built. (One Market Plaza and 101 California are the only buildings that are heavier.*) So when an engineer recommended concrete piles that only went down to a clay layer, the developers didn't question it. Subsequent construction, including that of the lighter-weight Salesforce Tower, did go down to bedrock with their foundations.[..] [..] Interestingly, Salesforce, which is slightly taller than Millennium Tower, weighs about half as much due to the nature of its construction. YouTuber Grady Hillhouse, whose channel is called Practical Engineering, suggests that all of this is kind of an experiment — with such tall buildings still relatively new to San Francisco, and, "Our ability to predict how soils will behave under new and extreme conditions isn't perfect, especially when those soils are far below the surface."[..]