Curtain-wall designs

Consider anchorage conditions, workmanship and future maintenance while planning
By Derek McCowan, Mark Brown and Michael Louis
COMMERCIAL, RETAIL, FABRICATION

Second in a series

Glass-and-metal curtain walls can provide an attractive, durable and cost-effective cladding solution, but in many buildings, these wall systems are plagued with problems ranging from air and water leakage to falling trim covers. The first article on page 82 of April Glass Magazine discussed some of the most common curtain-wall design problems that afflict owners, architects and developers. This article focuses on additional design and coordination issues. The final article will cover fabrication and construction, as well as items relating to laboratory and field testing.

Think about maintenance
All curtain walls will eventually require maintenance, including window washing, replacement of seals and insulating glass, and so on. It is important to recognize this during the design of the building.

Access: Future access to the curtain wall must be considered to ensure it is practical. If re-engagement of a single loose gasket will require disassembly of 30 sunshade features or temporary shoring off a low roof below, there is a problem. At one building we investigated, the repair and maintenance contractor had to provide cables that penetrate horizontal projections or “porches” at a niche in the curtain wall at great expense to the owner. The “house rig” provided per the original design documents could not access the niche. See photo, right.
Holes that were drilled into an aluminum projection to allow installation of support cables for a swing-stage.
Coordination between curtain-wall design and the design of access for maintenance is critical. At a recent curtain-wall investigation, the owner asked us to determine the cause(s) of falling snap-on mullion covers. We found that the vertical guide track for the swing staging, a necessary safety feature, caused the swing stage platform to contact the protruding horizontal trim covers. This contact occasionally caused a snap-on cover to become disengaged and fall several stories to the ground, endangering pedestrians below as well as the building. The addition of fasteners to the snap-on mullions and re-training of maintenance crews to avoid excessive contact with protruding mullions helped to reduce the risk.

Re-glazing: Inside glazing for curtain-wall assemblies—installing glass from the interior of the building—typically reduces glass replacement costs because no exterior swing stage access is required. However, inside glazing is typically only an option for vision lites. Interior access to spandrel lites is typically blocked by structural elements, mechanical equipment and/or interior finishes after initial construction.

Replacement of glass from the exterior can be costly enough; if replacement of a lite will require cutting and redesigning a structural mullion, there is a problem with the design of the wall system (See diagram on next page). If addressed during the design development phase, however, these problems can often be avoided.

For a successful “cradle-to-grave” approach to curtain-wall design, the design team must understand that the design and construction phases only constitute a small portion of the life of the wall. Serviceability, access, and repair and maintenance should be necessary considerations.

Beware of ‘shadow boxes’
Shadow boxes are spandrel features that usually incorporate transparent glass. Inboard of the glass is an opaque layer that is often formed by insulation or a metal panel mounted to the mullion frame. Monolithic glass is often provided in lieu of insulating glass because of the other curtain-wall insulation that is provided in these areas. Architects often like to include shadow boxes with dark backups into curtain-wall systems with non-reflective vision glass, rather than opaque spandrel glass, because shadow boxes can more closely mimic the appearance of the non-reflective vision glass, providing a more uniform appearance to the curtain wall. When used with lighter backups, shadow boxes add an element of depth to the curtain-wall design.

The top, bottom, back and sides of the insulation or back pan are typically sealed to the surrounding framing to prevent the migration of warm moist air from within the building into the shadow box. Common shadow box problems include the following:

Imperfect seals permit moist interior air to reach the shadow box area and condense on the inboard face of the spandrel glass. Such condensation can lead to staining of the spandrel glass and leakage to the interior surfaces below. It is almost inevitable that some imperfect workmanship, such as small air leaks into the shadow box, will occur because of the large number of joints in the framing, gaskets, insulation facers and other materials that must be sealed.

In addition, the shadow box becomes hot from solar radiation, causing the organic seals to degrade and off-gas volatiles that stain the interior face of the glass. When combined with condensation that often occurs around the perimeter of the spandrel glass, streaking and “picture framing” occurs. Since the inner portions of the shadow box are not readily accessible for cleaning, the stains cannot be removed without significant effort and cost.

To address shadow box problems, we recommend the following options:

Provide a hard and durable ceramic coating, known as frit, on the interior side of the spandrel glass in lieu of a shadow box. Frit on the inboard face of an insulating glass unit with a non-reflective outer pane can often create an appearance sufficiently close to that of the vision areas and is not prone to the condensation and streaking problems outlined above.

Where an appearance of greater depth is desired, provide a metal back-pan with welded or soldered joints and integral flanges that are glazed into the spandrel glazing pocket inboard of the IG unit. This solution is generally found to be more costly than the option outlined above.
Curtain wall anchors that penetrate through membrane sill flashing.
Curtain-wall anchors
Curtain-wall anchors are an integral part of any curtain-wall installation, and, as noted in our first article, they can pose problems with perimeter flashings if not designed correctly. Most curtain-wall designers and installers consider the weatherproofing system and the anchors as independent elements, even though these systems often meet at the top and bottom of the wall. We have investigated leakage that resulted from a high-quality through wall flashing system that was notched around each sill anchor, rendering the flashing ineffective.

We also have worked on a number of projects where the curtain-wall anchors were not designed specifically for the application, or were designed for the typical conditions only and other atypical conditions were not considered. Sometimes a manufacturer ’s standard anchors are provided at the job site, though the anchors do not work at some or all job-site conditions and an alternate anchor detail is “designed” in the field by a glazier or ironworker. No owner should have to rely on a mechanic ’s estimation of an adequate anchorage condition. We have worked on projects where haphazard anchorage conditions have resulted in catastrophic failure.

McCowan is a senior engineer at the Waltham, Mass., office of  Simpson Gumpertz & Heger Inc., 781/907-9000, dbmccowan@sgh.com; Brown is a senior project manager in the New York office, 212/271-2000, mabrown@sgh.com; and Louis a principal in the Waltham office, 781/907-9407, mjlouis@sgh.com.