FGIA: The many facets of curtainwall construction
By Amy Roberts
“Proper and timely communication among project team members is essential for successful curtain wall installation.”
By Amy Roberts
Curtainwall design, fabrication and installation is many-faceted – an echo of the many options it offers for creative expression. Glass and metal curtain walls are highly engineered and built to close tolerances. The result is a gridwork of lightweight metal, assembled either from individual pieces in the field or as part of factory-preassembled panels and fitted with some combination of transparent and opaque infill panels. Installation requires the placement of these precision-built parts on a skeleton built to much greater dimensional tolerances, involving the work of numerous trades. Thus, proper and timely communication among the members of the project team – architect, specifier, wall manufacturer, general contractor, installation contractor and the glazier – is essential for a successful installation. There are generally five matters of chief concern for the design and erection of metal curtainwall.
Structural integrity and wind loading: A curtain wall must withstand wind loads, transferring them through anchoring devices to the floor or columnar elements of the building framework. Appropriate design wind loads for most situations are determined from ASCE/SEI 7, “Minimum Design Loads for Buildings and Other Structures,” or, in Canada, the National Building Code of Canada. Special requirements for snow or ice loads on projecting wall elements are often applicable. In Canada, aluminum work should be according to CAN/CSA S157, with deflection under loads limited to the industry consensus maximum of L/175 of an unsupported span length (L) of up to 4.115 metres, or, for longer spans, a maximum of L/240 plus 6.35 millimetres. Glass design should be according to CAN/CGSB-12.20.
Provision for movement: Seasonal heating can induce thermal expansion of the aluminum framing members that is roughly twice that of contiguous sheets of glass. This relative movement must be accommodated without causing undue stress on the glass, joints and anchors, or without excessively reducing the frame’s “bite” on the glass.
Weather-tightness: Weather-tightness means protection against both water leakage and excessive air infiltration. Two methods have been developed for preventing leakage through the wall. One simply provides for minor leakage to be drained to the outside. The more sophisticated pressure equalization method, based on the rainscreen principle tends to be preferred in Canada. Industry recommendations limit air infiltration through the wall to 0.3 litres per second per meter squared of fixed wall area when tested at an air pressure difference of 75 Pascales.
Energy Efficiency: Although aluminum framing has inherently low resistance to heat flow, improved thermal performance is usually achieved by using high-performance insulating glass. In Canada, energy performance targets are determined according to the National Energy Code for Buildings, or ASHRAE 90.1 criteria. The glass area of the curtain wall must have an overall maximum thermal transmittance U-factor stipulated by applicable codes based on project location. The resistance of curtainwall framing to condensation is particularly important in a cold climate. In Canada, using the test and calculation method defined in CSA-A440.2 yields the Temperature Index (I).
Sound control: Using laminated insulating glass, in addition to minimizing air infiltration,
generally improves sound attenuation, which is of particular interest near airports and in metro areas.
Most of these concerns are addressed by several industry-consensus guide manuals, which offer recommended tolerances, model specifications and testing methods for confirming design performance levels. In North America, the comprehensive resource is the AAMA CWM-19, Curtain Wall Manual, which covers various aspects of curtain wall design, specification, testing, and installation, as well as the typical responsibilities of all players in the design-through-installation sequence. Other publications specific to the Canadian market reference traditional Canadian versus international standards for specific materials and testing.
Amy Roberts, FGIA director of Canadian and technical glass operations