The Miami-based architecture firm that designed the Infinity towers, troubled 33 Tehama, and the mostly hideous Trinity Place has just unveiled renderings for a curious new residential tower — with a cube at the top that appears to float above the rest of the building.

The Chronicle's architecture guy John King just picked up the story of 620 Folsom after SFYimby wrote about it last week, following the submission of the renderings and a few scant details to Planning from developer Align Real Estate. The proposed 62-story tower will certainly stand out on the skyline if it gets built — though King is no fan, and calls the design and height "far out of kilter with its surroundings."

The building is a mostly conventional, no-frills square-box tower with one defining feature, a five-story, 108-foot-high cube hovering, seemingly detached, over the rest of the building. This cube — or as the developer calls it, "a subtle glowing lantern" — will feature four floors of residences as well as a roof level mechanical floor. And the void over which it hovers is actually a glass-walled "amenity floor" with a fitness center, two lounges, and shared work spaces. As the developer describes it, "the building’s cubic form reflects a design that is both unique and expressive of its time, but also complementary to its historic Bay context." I dunno about that.

King rightfully points out that "renderings aim to deceive" and that this magically invisible amenity floor won't be so invisible in real life. And we can assume that the finished building, should it be approved as is, won't be quite so sleek and pretty when it is rendered in steel and glass instead of on a computer.

And how does a purely glass-enclosed floor support the weight of a five-story cube? Planning Director Rich Hillis tells the Chronicle, "In the rendering it looks pretty neat. I’m not sure how you actually build it, though." (A simplified floor plan suggests that the glass walls will surround a fairly conventional steel support structure, but who knows how you make that structure disappear like it does in the rendering.)

The architect, Arquitectonica, designed the attractive Infinity towers nearby in SoMa. But they're also responsible for 33 Tehama, whose residents have been displaced by a massive flood near the roof, as well as the behemoth Trinity Place at 8th and Market — the scale and monotony of which is well disguised in the firm's photos.

The building as proposed takes advantage of the state's density-bonus law, which allows you to build taller if you include 15% affordable units — something San Francisco already requires of all developments.

The plan is for 826 rental units, 691 of which will be market-rate and 135 of which will be affordable. The unit mix will include 118 studios, 118 one-bedrooms, 472 two-bedrooms, and 118 three-bedrooms, with a mix of each on each floor.

The floor plans suggest virtually no retail space on the ground floor facing Folsom Street, with just a small carve-out for a "cafe."

The development would replace a a low-slung, three-story brick building currently on the property — two doors down from The FlyTrap restaurant, near the Bay Bridge approach.

As SFYimby points out, this tall tower won't be alone in the part of town — an SOM-designed, 491-foot tower at 95 Hawthorne is also in the entitlements phase.

Will Planning let this slide through, given San Francisco's urgent mandate from the state to build more housing in the next decade? We'll see!