As Minneapolis’ IDS Center enters its fifth decade, the glass curtain wall suffers the inescapable effects of time. What does it take to maintain this iconic glass tower in a way that preserves its character and beauty?

By Todd Grover, AIA

At daybreak, the Minneapolis skyline boasts a modern marvel: When the sun first crests the horizon, it illuminates the IDS Center against the dark morning sky, the light shimmering on the faceted curtain wall in a way that never fails to inspire awe. The tower’s dynamic glass, together with its height and distinctive form, has made the IDS the symbol of downtown Minneapolis and of Modern design in Minnesota since it opened in 1972.

But while the design is aging extremely well, the building is now 43 years old, and older curtain-wall buildings present a different set of maintenance and preservation challenges than do the masonry and brick buildings that came before. Indeed, a lack of proper curtain-wall management, or an ill-conceived repair, can result in the need for larger interventions that can significantly alter architectural character—an alarming prospect for a building as important as the IDS.

Fortunately, the outlook for retention of the building’s aesthetic integrity is bright. The behind-the-scenes story of the curtain wall’s stewardship includes one stormy, hair-raising chapter, but the narrative centers on the building’s good fortune to have had remarkable continuity in the management team overseeing its maintenance, through several supportive ownerships.

The story begins with the design.

In the late 1960s, Investors Diversified Services (IDS), Inc. commissioned architects Philip Johnson and John Burgee to create a reimagined glass skyscraper to represent the company. Johnson, who had grown tired of the mirrored glass-box towers prevalent in East Coast cities, called the opportunity “an architect’s dream.” In his own work, he was transitioning from the formal orthodoxies of the International Style—which he had long championed—to a more expressive application of Modern design elements.

With the IDS, Johnson experimented with a new curtain wall expression. Instead of making the window mullions flush with the glass, as was commonly done, he designed them to project from the glass. The deep mullions then divided smaller panes (30 inches wide instead of the typical 60), creating what Johnson described as a “network of lines—more the aspect of a birdcage than of a glass box.” For the glass, Johnson tested a range of types and colors before choosing a cobalt glass with a 20-percent daylight transmittance, allowing the highest amount of daylight to enter the building while still having a reflectivity that produced an abundance of shifting light on the building’s surface.

Johnson also broke from the standard-issue box skyscraper by articulating the curtain wall at the corners, creating steps—which he called “zogs”—where the deep horizontal lines of the mullions intersected at 90 degrees. The corners produce a fractured array of reflections and lend a scale to the building through subtle angularity. These and other refined yet expressive design elements coalesced to create what Johnson later proclaimed was “our best building.”

Johnson wasn’t the only one to hold the building in such high esteem. From the start, the building’s owners and management firm, now called Accesso Services, recognized they possessed something special, right down to the intricate details of the design. Says Accesso operations manager Thom Cowhey, who started working at the IDS Center six months after it opened: “I’ve always been told by all of the contractors who worked on the building that this curtain wall is exceptional compared to some of the curtain walls in newer buildings. It was designed to allow water to weep out underneath the glass at different locations.”

Not that the curtain wall doesn’t require its fair share of monitoring and repair. The system has to endure extreme temperature swings in the Minnesota climate. Glass breaks and seals fail, and when they do, the windows have to be replaced. In fact, the glass at the IDS is in a constant, incremental state of replacement.

To ensure uniform appearance and quality, the Accesso team places large orders, anticipating need. “We replace roughly 50 to 70 pieces of glass every year,” says Deb Kolar, general manager of operations, who’s been with the office for 20 years. “The manufacturer makes the glass twice a year, and when they do we place our order.”

One staggering exception to this steady rate of attrition was caused by a violent storm on June 19, 1979. Cowhey was working that night, and from the top floor he could see an approaching “black curtain” of clouds punctuated by “bright blue explosions” as trees downed power lines and transformers overloaded across the city. Winds tore through the Crystal Court and battered the tower with pea-gravel ballast from the roof of Midwest Plaza (now McGladrey Plaza). “It looked like somebody had machine-gunned the mid-section of the building on the south side,” says Cowhey. “That was a four-year, $4 million insurance claim to replace 12,000 windows.”

The original glass manufacturer, Libbey-Owens-Ford, was able to provide matching panes, but a few years later the company stopped production of the glass. In the early 1980s, Cowhey and his team sought out other manufacturers, but none could accurately reproduce the panels. A close examination of the building today reveals that some panes from this era have a slightly amber tone that differs from the original blue.

Fortunately, in the mid-1980s, the management team established a relationship with Brin Northwestern Glass, a distributor that was able to source glass that replicated the color, light transmittance, and reflectivity of the glass specified in Johnson’s original design. The firm has served the IDS ever since.

Like all good design, the deep mullions provide more than just an aesthetic benefit; they also support the window-washing rig. And at the IDS, window cleaning is more than simple housekeeping—it’s a big part of maintaining the specialized construction of the curtain wall. “By diligently removing the grit from the windows,” says Kolar, “the weep system of the building does not clog.” This task is so critical that the IDS Center recently purchased a second window-washing rig to increase the frequency of cleaning.

What’s more, the mullions’ added heft helps the building better withstand the winds and temperature changes of Minnesota’s climate. In fact, it may be the main reason why recent wind-load and leakage tests revealed that the IDS curtain wall is performing extremely well.

In contrast, the most vulnerable part of the system may be the wet sealant. “Part of the puzzle of doing anything with the curtain wall is to make sure we’re caulking the right locations,” says Cowhey. “We do. That’s part of the process.” But larger repairs, like those needed for the failing weep system on the mechanical (unglazed) levels, are sometimes more difficult to plan.

“We’re learning that they put this building together like a puzzle, in a sequence,” says Cowhey. “They started at one point and worked their way around.” The complex assembly, which is still not fully understood, complicates repairs that call for taking the curtain wall apart; there is the risk of not being able to put it back together the way it was. The Accesso team is therefore taking a methodical approach to its plans for repairing the mechanical-level weeps.

Downtown workers strolling through the Crystal Court on a busy weekday, absorbing its lively atmosphere, aren’t thinking about the ongoing stewardship effort that’s required to preserve the IDS Center for future generations. They don’t have to, because the building is in the hands of committed owners and managers. And some of those hands have had a very long association with the building.

“Thom is our resident historian,” says Kolar. “He has so many stories, and the facts that come with those stories are invaluable. When we have someone new coming onboard, we encourage him or her to spend time with Thom walking around the property.” She jokes that with all of the great patent attorneys in the IDS, she wants to develop and patent a technology that enables her to download her colleague’s brain.

“Over the years I’ve always told the different owners of the property that part of my job is to instill in new staff that feeling of allegiance to the property—to its history and status as an icon,” says Cowhey. “It’s not a hard thing to do, because people naturally connect to it. It becomes a second home.”