- By Michael Fickes
- March 1st, 2008
“Before we start a project, we poll students about the kinds of buildings they want,” said Lorena Zellmer, project architect with SmithGroup in Washington, DC. “Today, students want lots of windows, transparency, visual connections to the community. They want to see, and they want to be seen.”
A number of college and university officials are listening to students and changing their traditional campus architectural requirements. Instead of solid, massive buildings with few windows, new campus buildings are opening up with glass curtain wall systems. “As a type of architecture, curtain wall systems are sought after on campus today,” added Zellmer.
A curtain wall is a type of building façade or skin that attaches to the building frame and does not support any of the building’s structural loads. “Curtain walls are usually glazed,” said Stefan Schwarzkopf, an associate in the Washington, DC, offices of Gensler, a national architectural firm based in San Francisco. “But other materials can be used in curtain walls as well. These include metal panels, thin stone panels, and panels made of other solid materials.”
Dramatically different from brick or stone facades, curtain walls raise six design issues for reconsideration: indoor environmental quality, thermal performance, moisture resistance, noise reduction, smoke and fire control, and, in today’s world, blast resistance.
Indoor Environmental Quality and Thermal Performance
Brick and stone buildings and curtain wall buildings manage indoor environmental quality — including air quality — and thermal performance quite differently.
Load-bearing building facades have few windows and require substantial amounts of artificial light. By contrast, a curtain wall allows natural sunlight to wash over the interior spaces of a building, making people more comfortable while reducing the need for expensive electricity to power artificial lighting systems. Curtain wall systems can take advantage of dimming technology that senses the amount of natural light and adjusts artificial lighting up or down as required.
While curtain walls offer huge expanses of glass and admit natural light, they typically do not provide operable windows, a feature many brick and stone façade buildings do offer. That’s important today in light of the sustainable design movement toward giving individuals more control over their personal environments. “Curtain walls generally have not offered operable windows in the past,” noted Schwarzkopf. “But in fact, curtain walls can provide casement-style windows hinged top and bottom and cranked open and closed.
“Whether you would want operable windows or not, though, depends on factors like a building’s location. In a downtown environment, operable windows might degrade indoor air quality. On the other hand, a building in a rural or suburban setting might benefit.”
In terms of thermal performance, it is probably easier to heat and cool a brick or stone building. But the thermal performance of glass has improved in recent years. “Coatings help,” Schwarzkopf said. “The type of coating is influenced by the climate. In cold northern climates, you want coatings that control heat loss through the glass. Another option for keeping heat in is triple glazing — three pieces of glass with two airspaces. In Los Angeles, you would be more concerned with controlling heat gain from the sun by reflecting radiation.”
Another technique that boosts thermal performance is to add spandrels, or horizontal curtain wall panels, to the system in between floors, from just above the ceiling line of one floor to just below the floor line of the next floor up. These spandrels can be made of glass panels with opaque fritting, or metal or stone panels. “Since you can’t see through these spandrels, you can back them up with insulation,” Schwarzkopf said.
Masonry walls resist moisture with solid walls, well-sealed window frames, roof eaves, and gutters.
Curtain walls, on the other hand, are made of glass panels bound together by an aluminum frame and primary and secondary gaskets. While moisture can’t get through a pane of glass, it can get in through a poor glass-frame connection.
“Curtain walls use primary and second gasket seals to control moisture,” Schwarzkopf said. “There is a primary outside gasket and a secondary gasket inside. The outside seal keeps most of the water out. But a little bit of water does get through. Curtain walls have built-in drainage systems that get rid of that moisture. When all of the components of the system work together correctly, nothing gets through to the inside.”
Glass transmits exterior noise much more efficiently than thick solid-mass walls like brick or pre-cast concrete, which will effectively soak up exterior sounds and noises, continued Schwarzkopf. Curtain walls are not just glass, however. Curtain wall units typically include two pieces of one-quarter-inch glass separated by a half-inch of airspace.
A clear or patterned laminate affixed to the glass wall panels will soak up even more sound. “Laminates muffle sound because they are made of soft materials that naturally dampen and absorb sound,” Schwarzkopf said. “Hard glass surfaces transmit sound energy.”
Smoke and Fire
Curtain walls attach to a building behind or inside the protective glass skin. The connection is made to the structural members of the building — the exterior columns and cross members.
When a curtain wall hangs on a building, air can pass up and down the height of the structure. Left unattended, the air passage will transmit smoke and fire from one floor to the next to the next. “The curtain wall and floor slab must be sealed together on each floor to prevent this,” Schwarzkopf said.
“Today, we’re also seeing curtain walls being made blast resistant with the addition of a laminate that allows the glass to break into tiny pieces without separating from the frame,” added SmithGroup’s Zellmer. “Most importantly, it prevents the glass from breaking into flying shards of glass.”
Flying glass shards have been found to be responsible for many deaths and injuries after an explosion.
Three Kinds of Curtain Wall Systems
Curtain walls come in three varieties, for small, medium, and large buildings.
Small building curtain walls come in off-the-shelf systems. The design specifies a particular manufacturer’s system, and the contractor buys the parts and assembles the curtain wall at the site.
Customized curtain walls fit out buildings of four to 10 stories. Generally, the work involves customizing the aluminum frame. “Since aluminum can be extruded, you can create many different shapes of dies,” Schwarzkopf said. “Of course, custom curtain walls go up in price.”
For larger buildings, 10 stories and up, manufacturers make individual glazing units that contractors attach to the building at the site to prepared connections on the structure. “The panels just click into place,” Schwarzkopf said. “The prefabricated units are excellent quality because they are made in a controlled environment.
One Student Center, Two Curtain Walls
At Indiana University–Purdue University Indianapolis (IUPUI) in Indianapolis, SmithGroup recently completed a 262,000-sq.-ft. student center. The facility has two curtain walls. One is a five-story system enclosing an atrium. Steel columns and steel beams support the wall.
The second curtain wall is a three-story system shaped like a cube in the front corner of the building. Attached to the concrete floor slabs from the third to the fifth story, the system appears to hover in mid-air. The system encloses the student activities center, which houses offices for various student clubs.
Adding interest to the design are the different configurations of the glass units. “The atrium curtain wall is expressed with vertical mullions,” Zellmer said. “The cube has a horizontal expression.”
Both systems were constructed at the site from the mullions or “sticks” and pre-cut glass panels and then hung on the building.
Re-Skinning an Entire Building
Not long ago, Gensler renovated a 100,000-sq.-ft. building for Towson University, part of the University of Maryland system in Towson. The assignment was to re-clad an aging 1960s-era building and design a new 30,000-sq.-ft. addition. The building is an important university landmark, as it is located at the southern gateway to the campus.
“The framework of the original structure supports the new building skin,” Schwarzkopf said. “We used glass curtain wall in some areas and in other areas we used more opaque materials — in those cases we matched the aesthetics of other buildings on campus.
Curtain walls are opening up college and university campuses that used to be defined by brick and stone buildings. It’s a new look that responds to the desires of a new generation of students that prefer group learning and collaboration to individual study. Those students want to see and be seen. Curtain wall building façades can help satisfy those desires.