But just when you think it's safe to put your Jane Jacobs back on the shelf, along comes a design from a developer who must have skipped the chapter on neighborhood placemaking. The latest proposal from PMC, the Philadelphia apartment giant, would set a pair of skyscrapers atop two massive garages along the Schuylkill waterfront in the Logan Square neighborhood. Though the compound has intermittent openings for retail, most of the perimeter would be sheathed in a screen made from perforated metal panels. In certain places, those unrelenting metal walls would rise five stories above the sidewalk, taller than some nearby rowhouses.