Rigid Requirements: Commercial Fenestration Faces Design
by Ken Brenden
If windows for residential and light commercial applications seem subject to a lot of requirements, consider the challenges before the designers of heavy commercial and architectural fenestration. Take curtainwall as an
Live Loads Take Precedence
The principal static force acting upon a curtainwall is, of course, gravity. Because of the relatively lightweight of materials used in curtainwalls, this dead load holds secondary significance. The dynamic, or live loads, such as wind, thermal expansion and seismic are primary concerns to the entire building team. Joint and anchorage design become the key to accommodating building movements caused by these loads.
• Wind Loading. Lateral wind forces are the primary live loads on all windows. In tall buildings, winds also contribute to the movement of the wall itself, affecting joint seals and wall anchorage. Furthermore, the nature and intensity of such loads are affected by the height and geometry of the building as well as by surroundings. Designers must also consider negative wind loading (suction forces) acting on the wall, which can be augmented by internal HVAC pressures.
• Thermal Expansion. Architects often specify aluminum products for commercial and architectural applications because it’s strong and lightweight. Since aluminum has a relatively high coefficient of expansion, there is the potential for wide daily and seasonal fluctuations in the metal’s surface temperature (which can cover a range of as much as 150° to 200°F). This can induce stresses from thermal expansion—typically ¼ - to 5⁄16 inches in a 10-foot framing member. A contiguous lite of glass will expand by less than half that amount. This disparity causes relative movement that must be accommodated without causing undue stress on glass, curtainwall joints, anchors, joint seals or structural
• Seismic Loads and Inter-story Drift. While both wind and seismic loads can cause inter-story drift or racking (relative horizontal movement between adjacent stories), seismic action is by far the greater concern in several areas of the United States, such as along the West Coast. The designer must calculate maximum deformations likely to occur, striking a balance with respect to seismic design based on function, cost and probability of damage.
Design Considerations Must Accommodate Movement
• Tolerances. Because curtainwall construction involves covering a field-constructed skeleton with a factory-made skin, the designer must consider how the curtainwall system connects to many other parts of the building. Many of the trades must work to address this because significant deviations generally can’t be corrected in the field. All of these factors converge to require careful attention to manufacturing and installation variances, with the resulting tolerance stack-ups still capable of providing the clearances necessary to accommodate building movement due to live loads.
• Wall Anchorage. Anchors must, of course, have the hardness, yield and tensile strength to bear the weight of the wall itself. They must also withstand the forces imposed by dynamic loads, while accommodating fabrication and construction tolerances and allowing for thermal movement.
• Joint Sealant. Whether the joints are “working joints,” designed to accommodate movement, or “non-working joints,” secured by fasteners, some kind of seal is usually required. It is particularly important that the size of the sealant joint take into account the maximum thermal expansion and contraction, as well as building movements that will affect the joint.
• Deflection of Glass-Supporting Frame Members. The more the frame deflects under the load, the more stress is placed on the glass and the greater the likelihood of breakage.
Considering the entire wall as an interactive system has long been the standard procedure in commercial and architectural applications, which involve building movement under dynamic loading and the interactions of multiple building elements that extend beyond the glazed unit itself.
the author Ken Brenden serves as codes and industry affairs manager for the American Architectural Manufacturers Association in Schaumburg, Ill. Mr. Brenden’s opinions are solely his own and not necessarily those of USGlass magazine.
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