Four-Day Planet by H. Beam Piper

The ceiling on Main City Level is two hundred feet high; in order to permit free circulation of air and avoid traffic jams, nothing is built higher than a hundred and fifty feet except the square buildings, two hundred yards apart, which rest on foundations on the Bottom Level and extend up to support the roof. The _Times_ has one of these pillar-buildings, and we have the whole thing to ourselves. In a city built for a quarter of a million, twenty thousand people don't have to crowd very closely on one another. Naturally, we don't have a top landing stage, but except for the buttresses at the corners and solid central column, the whole street floor is open.

Tom hadn't said anything after we left the stacks of wax and the men guarding them. We came up a vehicle shaft a few blocks up Broadway, and he brought the jeep down and floated it in through one of the archways. As usual, the place was cluttered with equipment we hadn't gotten around to repairing or installing, merchandise we'd taken in exchange for advertising, and vehicles, our own and everybody else's. A couple of mechanics were tinkering on one of them. I decided, for the oomptieth time, to do something about cleaning it up. Say in another two or three hundred hours, when the ships would all be in port and work would be slack, and I could hire a couple of good men to help.