High-rise construction in Philadelphia comes in waves. The last big crest a decade ago brought in a handful of pricey condo towers, mainly clustered in established neighborhoods around Rittenhouse and Washington Squares. This time, the tide is rolling westward, from Center City out toward the universities, and it's looking like a tsunami.

Five apartment towers are or will be going up along Market and Chestnut Streets, between 20th and 38th, one glassy slab after another. The total grows to seven if you count two clever retrofits where developers have piled extra floors on top of existing buildings, turning height-challenged mid-rises into full-fledged high-rises.

This stretch - from the tattered western edge of Center City to the University City Science Center - has long been an ill-defined territory, not uniformly academic, commercial, or residential. The arrival of a couple thousand residents can't help but make these blocks feel more lived-in, and the bustle should advance the goal of knitting together the two sides of the Schuylkill.

Although this isn't the only place in the city where high-rises are planned, the cluster tells us a lot about how Philadelphia is being reshaped by the new economy, with its focus on health care, university research, and higher ed. The seven projects are being positioned as adjuncts to the Drexel, Penn, and Science Center campuses, which are expanding with more employees, grad students, and researchers.

The architecture (but not the architects) has changed significantly from the condo-boom years. During that high-octane period, developers were convinced affluent empty-nesters from the suburbs would head downtown in droves for lavish designs that were often described as estates in the sky. More than two dozen condo towers were proposed at one point. But the number of buyers fell short, and only a few projects were completed.

This time around, developers have grown up and sobered up. They now see the typical high-rise resident as a twentysomething with a good-paying job at a hospital or tech start-up.

That explains why all the new towers are planned as rentals, with an emphasis on smaller units for singles. Developers have lowered their sights in another way: The high-rises are all between 25 and 34 stories, well below the dizzying, 40- to 60-story giants that were proposed a decade ago.

Two of the projects, the Grove and Chestnut Square, are really high-end dormitories that can accommodate 850 residents apiece. The Grove, a joint design by Erdy McHenry and Pelli Clarke Pelli, will be pitched to a slightly older group of graduate students. It includes the kind of amenities your parents never imagined in their dorm, such as a rooftop pool that extends toward the horizon and Center City's skyline.

At the opposite end of the spectrum are the three Center City buildings, which have fancier interiors and more two-bedroom units. Perhaps as a sign of the new restraint, they've been given addresses rather than fanciful names: 1919 Market, 2040 Market, and 2116 Chestnut.

Because the new towers are all rentals, their occupants will be a transient bunch, the kind of people who cycle in and out of university and hospital jobs.

Unfortunately, the architecture reflects that. Several of the designs have a generic, placeless quality, like long-stay hotels. They are mostly plain, rectangular monoliths that rely on asymmetrical window placement and shifts in materials to energize their facades. It's hard to imagine such buildings turning the area into a real neighborhood.

At least the developers have learned some important lessons in urbanism over the last few years. All the buildings have been carefully sited to present their best face to the street. They include space for retail and restaurants, and for the most part, they take care to camouflage the parking. That's progress.

Here's a quick tour of what's coming.

2116 Chestnut, nearly complete, by the John Buck Co. Chicago. Tallest of the group at 34 stories, its slick, glass facade has about as much excitement as a blank sheet of paper.

2040 Market, leasing, by PMC Property Group/Varenhorst Architects. Varenhorst cantilevered eight additional stories onto the former AAA office building, a five-story structure, repeating the same mid-'60s ribbon windows in the new floors. The approach could serve as a model for reclaiming other obsolete mid-rises, like the Police Administration Building.

Chestnut Square, 32d and Chestnut Streets, nearly complete, by American Campus Communities/Robert A.M. Stern Architects. The architects wrap a private dorm around Drexel University's Mandell Theater. The eastern corner is marked by a 19-story tower.

The Grove, 30th and Chestnut Streets, under construction, by Brandywine Realty Trust and Campus Crest/Erdy McHenry and Pelli Clarke Pelli. This 33-story tower will occupy the site between the former post office and a parking garage. Besides the usual Erdy McHenry flair evident in the facade, the stylish ground-floor spaces will be an important force in encouraging people to cross the river.

1919 Market, breaking ground late this year, by Brandywine Realty Trust/Barton Architects. The 27-story apartment tower will occupy the huge corner site that was set aside two decades ago for a twin of the Blue Cross building.

3601 Market, breaking ground late this year, by University City Science Center/BLT Architects. The 26-story tower is envisioned as a domestic annex to the science center's Market Street lab spaces.

Episcopal Cathedral tower, 38th and Chestnut Streets, seeking approvals, by Radnor Property Group/BLT Architects. The developers, who would build on the site of two historic brownstones, are expected to reveal the details of the design to the Historical Commission on Tuesday.