For the past nine summers, the courtyard at P.S.1 in Queens has been the scene of an art-world bacchanalia known as Warm Up. From each of the five boroughs and beyond, young hipsters converge on the fortified-concrete enclosure that constitutes the museum’s courtyard, wherein they lounge around in the imported sand of an artificial beach and mingle to the music of a DJ hired for the occasion.

There is, however, a more serious component to all of this. Each year, the curators of P.S.1, in tandem with those of the Museum of Modern Art, stage a competition for up-and-coming architectural firms to transform the courtyard in the most daring ways imaginable.The slipshod impermanence of the resulting structures encourages unpredictable forms that seem to defy gravity.

This year’s winning entry, by a New York-based firm called OBRA (its two principals are Pablo Castro and Jennifer Lee) opens to the public today. The architects describe their installation – titled “Beatfuse!” – thus: “No two New Yorkers are alike. Everyone moves to a different beat. When the Warm Up DJs match tunes, beats fuse. Form follows tension and air is suffused with mist and light, as all dance under a penumbra of moire.”

For a young architectural firm, this plum commission is seen as an important career boost, and everyone in the architectural community is paying attention. The point of the exercise is a worthy one: To create something of an impressive size and form to occupy the aberrant triangle of P.S.1’s courtyard, and to transform the space beyond all recognition.

OBRA has gone about the task by conceiving the courtyard as a recreated Roman bath. In imitation of that ancient building type, the three main chambers that the courtyard comprises are called respectively a caldarium, a tepidarium, and a frigidarium, according to the shifting warmth, from quite hot to quite cool, of each.

The temperatures are moderated by the water-filled tidal pools in each chamber, as well as by mist-producing devices and various forms of insulation.The largest area, given over to the tepidarium, consists of a sequence of interlocking shells, made of plywood and covered in a translucent skin of polypropylene mesh. The vault-like armature of the roof suggests its distant roots in such feats of Roman engineering as the Baths of Diocletian, even as it remains insistently contemporary in its use of aberrantly curving forms.

That this irregular curvature is an essential component of contemporary architectural thought and vision is clear from the competition’s four other entries, which are now exhibited at MoMA.

While OBRA sought a totalizing, immersive experience, Contemporary Architecture Practice came up with something called “Catalyst,” far simpler in conception and far more complex in

form. Rejecting the rough-cut exposed plywood in which OBRA seems to revel, this firm sought to fill the space with spindly, interlocking pieces of amoeboid wood covered in an oil-based enamel finish. The resulting structure looks like a cross between a perforated tent and white sand dunes.

The Los Angeles-based firm Gnuform created “Purple Haze,” a structure named after a Jimi Hendrix’s classic song of 40 years past. It contains four horizontal layers, each different from the others. To judge from the illustration at MoMA, its interior resembles that of a disco. Since it was never built, we shall never know how it would have felt to sit on its swirling green seating as misters pour down purple rain from above.

The structure conceived by the Boston-based firm Howeler + Yoon’s was to have consisted of a lattice of chaotic strings. Imagined by the designers as a jungle gym for adults, its stark white exterior seems like a cross between multiplying cells and lengths of cheese whiz.

Finally there is “Flowers,” an installation by Kivi Sotamaa of Columbus, Ohio. This is divided into four parts, each bearing a name: Blaze, Party Girl, Love Potion and Golden Beauty. These are divided into “campuses” by lengths of torqued steel that intentionally invoke the minimalist sculptures of Richard Serra and that unfurl like flowers, hence, the title.

As for OBRA’s winning design, the ultimate success of “Beatfuse!” cannot be known until all the beau monde shows up. If you fancy yourself of their number, be warned that there should be long lines to crash what is widely held to be one of the best parties in town. For that same reason, we cannot know how well the cheap plywood structure will bear up beneath the crush of people. If previous years are any indication, it could wear down fast.

P.S.1’s WarmUp 2006 series takes place every Saturday from July 1 until September 2 (22-25 Jackson Avenue at 46th Avenue in Queens, 718-784-2084). “Young Architects Program” until September 24 at MoMA (11 W. 53rd Street, between Fifth and Sixth Avenues, 212-708-9400).