My good friend Phil called this week looking for geotechnical engineering support on another one of his interesting tank projects. Phil is an international expert in tank design, maintenance, and operation, and the projects that we’ve worked together have all been career highlights for me.
Storage tanks have the most interesting foundation issues of all structures. On the one hand, a structure completely full of liquid is orders of magnitude heavier than a structure full of air and people and office furniture. Only steel mills have higher loadings. On the other hand, though, the structures are extraordinarily flexible, at least in many ways, and can often accommodate settlements that the heavy loads induce. So deflection management, rather than avoidance, is the design goal. I’ve been inside tanks in Mississippi that look like skateparks on the inside, and where the interior support columns have threaded rods sticking through the roof so the columns can be lengthened with a torque wrench while the bottom plate and foundations settle.
These particular tanks, the ones that Phil called about, are being retrofitted to allow higher interior pressure. Even a couple of lb/in2 inside a tank 200 feet in diameter creates a huge force. The bottom plate is flexible, which is a good thing, but in these tanks the shells and annular rings are not anchored to the ringwall foundations. So adding pressure to the tanks will inflate them like a very stiff, very expensive balloon and lift the shell off the foundation. The engineering task is to design reliable, economical anchorage to resist uplift forces around the tank shell caused by increasing the tanks interior operating pressure. It’s not a large project, but the people that I get to work with and the practical solutions that I get to develop make these small projects some of the most rewarding.