By Amy Roberts
Successful selection, specification, and construction of curtain wall systems requires an understanding of project-based goals and site conditions, and a commitment to teamwork. Supporting performance-based specifications, it helps to first define the terminology and then the methodology.
A curtain wall system—relieved of the required strength necessary to support the weight of upper floors, as was the case for vintage construction methods—imposes a relatively minimal dead load and has become primarily a filtering envelope for the building. In this role, it controls the passage of heat, light, water, air, and sound, and has opened many options for architectural esthetic expression.
In its basic form, a glass and metal curtain wall consists of a lightweight metal, usually aluminum, gridwork assembled as individual pieces on site (stick-built system) or as part of factory preassembled panels (unitized system), and a combination of transparent and opaque infill panels (spandrels). The grid is typically attached at discrete points to the floor-slab edges, hanging like a curtain on the building.
Glass forms the most popular choice for the vision area, whether fixed or partially operable to admit ventilation. Insulating glazing units (IGUs) improve thermal performance with double or triple panes of glass, usually tempered or laminated, tinted, or with low-emissivity (low-e) coatings, separated by a space filled with desiccated air or with an inert gas(es), or at the cutting edge, a vacuum. Spandrel glass, which is darkened or opaque, typically is located between the areas of vision glass aligned with the floor edge of a building. Spandrel glass is the area of glass panels that conceals structural building components such as columns, floors, HVAC systems, electrical wiring, plumbing, etc. Unlike vision glass, which is meant to be transparent, spandrel glass is designed to be opaque to help hide features between the floors of a building, including vents, wires, slab ends, and mechanical equipment. Other common infills include stone veneer, metal panels, louvres, composites, plastics, and operable windows or vents.
When designing metal-framed curtain walls, there are generally five key design considerations:
- Structural integrity
- Provision for movement
- Energy efficiency
- Sound control
Although made of lightweight materials, a curtain wall must withstand lateral live loads imposed primarily by the wind, transferring these loads along the load path from the curtain wall, to the main building structure, through anchoring devices attached to floor or columnar elements. Quantifying and designing to accommodate these loads is a routine procedure for structural engineers, albeit aggravated due to the height and geometry of the building, as well as by surrounding structures or natural topography. In addition, there are loads imposed by building sway induced by wind, seismic forces, and by thermal expansion and contraction.
It is strongly recommended design loads (in Pa) specific to all relevant areas of the building be stated in the specifications. Special requirements for snow or ice loads on projecting wall elements, such as sunshades, are often applicable. Appropriate design wind loads for most situations may be selected by the architect from ASCE/SEI 7, Minimum Design Loads for Buildings and Other Structures by the American Society of Civil Engineers and the Structural Engineering Institute, or in Canada, the National Building Code of Canada (NBC).
Compliance shall be demonstrated by testing in accordance with American Architectural Manufacturers Association (AAMA) 501.4 “Recommended Static Test Method for Evaluating Curtain wall and Storefront Systems Subjected to Wind and Seismic Induced Inter-Story Drifts.” CWM-19 (page 47 – 220.127.116.11.6) says:
“Determination of seismic movements and load(s) is the sole responsibility of the building’s Engineer of Record, considering code interpretation issues and/or prescriptive requirements not included in Contract Documents. The curtain wall manufacturer is not responsible for determination of these movements or loads, and AAMA strongly recommends that design criteria specific to all relevant areas of the building be provided by the Specifier.”