Curtain Wall Design Loads
Butler Plaza, a Gold LEED building in Jacksonville, Florida, is an innovative example of a curtain wall used in an energy efficient design. Photo by Keymark.
Key Curtain Wall Design Loads to Consider
- AAMA 508-14, Voluntary Test Method and Specification for Pressure Equalized Rain Screen Wall Cladding Systems
- AAMA TIR-A1-15, Sound Control for Fenestration Products
- AAMA 1801-13, Voluntary Specification for the Acoustical Rating of Exterior Windows, Doors, Skylights and Glazed Wall Sections
- AAMA 1503, Voluntary Test Method for Thermal Transmittance and Condensation Resistance of Windows, Doors and Glazed Wall Sections
- AAMA 507, Standard Practice for Determining the Thermal, Performance Characteristics of Fenestration Systems in Commercial Buildings
Curtain wall, whether installed piece-by-piece in a stick system or in prefabricated sections as a unitized system, is a complex, integrated assembly spanning multiple floors. In the design of metal curtain wall, there are typically five matters of chief concern: structural integrity, weather-tightness, sound control, energy efficiency and provision for movement. Successful curtain wall design and installation depends on understanding the various load requirements.
As with all types of fenestration, lateral wind forces are the primary live loads affecting curtain wall structural integrity. The different design wind loads at different building heights and at areas adjacent to the building corners must be taken into account, as well as both positive (inward acting) and negative (outward acting) loads. Appropriate design wind loads for most situations are typically derived from ASCE/SEI 7, Minimum Design Loads for Buildings and Other Structures.
A related consideration is the deflection of glass-supporting frame members, typically limited to L/175 (where L is the span length), to avoid excessive stress on the glass that could lead to breakage.
A third consideration is providing for movement due to differential thermal expansion or contraction between aluminum and glass. This disparity must be accommodated without causing undue stress on glass, joints and anchors, or reducing glass “bite.”
Two methods have been developed for preventing water penetration through curtain wall. One is the “internal drainage” system, wherein minor leakage can be prevented by providing flashing, collection devices and ample drainage outlets within the wall itself. The other is the more sophisticated “pressure equalization” method, based on the “rain screen principle,” which requires a ventilated outer wall surface, backed by drained air spaces in which pressures are maintained equal to those outside the wall.
In terms of sound transmission, the weak links in most walls are the glazed areas. Insulating glazing units with laminated glass generally improve sound attenuation. Where a high degree of sound insulation is required, air infiltration and resonance of rigidly supported glass lites should also be minimized.
Although metal and glass have high heat conductance, improving thermal performance can be accomplished by minimizing the proportion of metal framing members exposed to the outdoors, eliminating thermal short circuits by means of thermal breaks, using high-performance IGU, and providing adequate insulation in spandrel areas.
In addition, the large glazed areas allow natural light to penetrate deeper within the building to save energy by supplanting electric lighting, while minimizing solar heat gain through the use of low-emissivity coatings. In addition, multiple studies have shown that, by connecting people to their environment, fenestration with views improves attitudes, increases productivity, and provides ventilation to improve indoor air quality.
The terms “tolerance” and “clearance” are often confused. A tolerance is a permissible deviation from a specified or nominal characteristic; in general, tighter tolerances equal higher costs. A clearance is the space purposely provided between adjacent parts, to allow for movements, anticipated size variations, or to provide installation working space. Both are critical, because covering a field-constructed skeleton with a factory-made skin, involving the work of numerous trades, introduces numerous sources for variation.
Allowances for dimensional tolerances and proper clearances are of particular importance in the detailing of glazing frames, where ample edge clearance and sufficient bite are prime factors affecting glass performance. Glass-holding members must also provide openings that are within certain tolerances for squareness (expressed as the difference between the measured lengths of the diagonals, which should not exceed 1/8-inch), corner offset, and bow.
Industry-recommended curtain wall system tolerances are available for framing and trim dimensions, and for alignment of framing members (end-to-end, variation from plane and joint gap width).
To aid in curtain wall design and specification, AAMA recently released AAMA CWM-19, Curtain Wall Manual. The guide addresses many aspects of curtain wall design, specification, testing and installation. New content includes detailed recommended fabrication and installation tolerances and barrier interface guidelines. A major feature is a comprehensive Guide Specification, formatted to include mandatory language, optional language and detailed technical commentary, which allows architects and specifiers to customize project-specific performance and testing requirements. Learn more at aamanet.org/store.