Curved, with an opening in plan [PLFCO] - jcgomezz/Building_taxonomy GitHub Wiki

The building footprint is a round shape; circular, elliptical, or ovoid, and it has an interior open courtyard (whether roofed or not).

Beijing National Stadium (also known as the Bird's Nest) has a circular plan shape with an opening (Map data ©2013 Google, DigitalGlobe)