The footprint of the building when viewed in plan resembles the shape of the letter U.
Note that this also includes “C” shaped buildings (which look like a “U” when viewed from another direction).
Fairmont Chateau Laurier Hotel in Ottawa, Canada has a U-shaped plan (Map data ©2013 Google, DigitalGlobe)
City Hall in Toronto, Canada has two half-circle office towers, curving around a circular council chamber; the towers can be classified as U-shaped building plans (Map data ©2013 Google, DigitalGlobe)
A building with U-shaped plan (left photo: C. Scawthorn, right: Map data ©2013 Google, ZENRIN)