Dec. 30, 2024
Above: Aaron Blom presenting during GlassBuild America
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Editor&#;s Note: The following is based on the education session &#;Curtain Walls | Design Considerations, Tolerances and Testing,&#; presented by Blom at GlassBuild America, held Oct. 18-20, , in Las Vegas. Find more news and coverage from the show.
In its basic form, a curtain wall system consists of insulating glass and extruded aluminum framing either installed piece by piece in the field (stock lengths), or installed using pre-assembled, pre-glazed framing and glass panels (unitized). Hybrid versions include various system options and fabrication and installation techniques that can be used to achieve specific design aesthetics as well as maximize fabrication and installation efficiency.
Regardless of the installation methods used, curtain wall systems must address five primary design considerations: structural integrity, movement capability, weathertightness, energy efficiency and sound control.
Structural integrity
As with all types of fenestration, wind load is an important structural consideration for curtain wall systems. The more the frame deflects due to the design wind load, the more stress is placed on the assembly and the greater the likelihood of system failure and/or glass breakage. The engineering calculation used to determine maximum vertical framing deflection is L/175 for spans up to 13 feet 6 inches and L/240 + ¼-inch for spans that are greater, where L is the vertical span length. Other structural performance considerations include live load, dead load, seismic and inter-story drift.
Provision for movement
There are multiple factors to consider when designing a curtain wall system to accommodate expected movement, including thermal expansion and contraction, movement due to wind load and gravitational forces, and movement caused by deformation or displacement of the building. Movement must be accommodated to limit the stress on the glass, framing and anchors, and without excessively reducing the frame&#;s &#;bite&#; or capture of the glass.
Weathertightness
Weathertightness means protection against both air and water leakage.
Water penetration. There are two methods for preventing water leakage through a curtain wall system. One is the &#;internal drainage&#; system utilizing weep holes in the pressure plate and face cap. The other is achieved through "pressure equalization" of the system. Basically, a pressure-equalized design ensures there is not a significant negative pressure pulling water into the system from the exterior.
Air infiltration. For obvious reasons, excessive air infiltration is less critical than water penetration but can compromise the energy efficiency of the building envelope. Industry standards limit air infiltration to 0.3 liters per second air leakage per square meter of fixed wall area (0.06 cubic feet per minute per square foot of area) when tested at an air pressure difference of 1.57 pound-force per square foot.
Energy efficiency
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Curtain wall systems must meet an overall maximum thermal transmittance or U-Factor (W/m2&#;K or Btu/hr&#;ft2&#;°F) stipulated by applicable codes based on project location and other project-specific thermal performance requirements. Improved thermal performance can be accomplished in multiple ways, including using thermally broken framing materials, using high-performance insulating glass and adequately insulating large spandrel areas.
Sound control
Sound Transmission Class (STC) and Outdoor/Indoor Transmission Class (OITC) are the standard methods for rating sound attenuation of glass and window systems. Using laminated insulating glass, using insulating glass with an increased airspace and minimizing air infiltration will generally improve sound transmission&#;of particular concern near airports and in metropolitan areas.
Performance and testing requirements
Due to the wide range of forces that act upon a building in a particular location, physical testing for air infiltration, water penetration and structural performance (including frame deflection limits) is often the only reliable means of verifying performance in field conditions. This testing may be conducted on stand-alone mock-ups or on completed installations, also called in-place mock-ups.
Curtain wall systems, no matter the scale, are complex projects that require careful planning. Five key curtain wall design load considerations are structural integrity, weather-tightness, sound control, energy efficiency, and provision for movement.
Steven Saffell of Glass Magazine gives an in-depth rundown on what these design loans mean, and how to account for them:
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 the 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 &#;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 IG units, 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. Tolerance is a permissible deviation from a specified or nominal characteristic; in general, tighter tolerances equal higher costs. 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).
Keep these guidelines in mind when designing and planning a curtain wall. Get more information about our SCW series or request a quote to get started.
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