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Rooftop swimming pools and spas offer breathtaking views, writes Rob Holmer, but they also tend to come with an array of unique challenges. Here, he discusses how knowing the forces at work on elevated concrete shells will help when it comes to getting things just right.

Elevated Engineering

I had a college professor who was fond of saying, "There are only two types of concrete in this world: The first is concrete that is cracked, and the second is concrete that is going to crack." That's a good laugh line, but the tough thing about it is that it's also true. This is why the engineering design procedures for all reinforced concrete (pursuant to ACI 318 and 350, which are the key American Concrete Institute standards for concrete structures) allow for