I have passed by these things many times as I drive through St. Louis. I have seen structures exactly like this in Atlanta and at least one other large city between Indiana and Florida. There are always two. One has a stair leading up to the top of the framework, but the other does not. They appear to be in use as the property is clean and lit at night. The stairs are lit, too. There are no visible reasons for the framework, which is perched above what appears to be a large tank below. I can see no pipes or cables, other than those that stabilize the right angles on the framework.
What the heck is this?
I have no idea, so I'm asking you.