Good Curtain-Wall Design Goes Below the Surface
More goes into curtain-wall design than meets the eye.
Ken Brenden, AAMA Codes and Industry Affairs Man
A well-designed expanse of curtain wall with imaginative contours and attractive glazing is impressive to behold. But aesthetics are just the tip of the iceberg. These are complex, integrated systems of framing members, sash, glazing, spandrel panels, fasteners, and sealants-each of which are subject to an array of design trade offs to accommodate a wide variety of structural and environmental forces specific to the building’s purpose and location. Some considerations are traditional and obvious, while some are moving to the forefront.
“Nuts and Bolts”
The obvious concerns include structural adequacy and energy efficiency. In a curtain wall, which bears none of the building’s structural loads and uses materials with relatively light weight, handling structural (or “dead”) loads imposes few demands beyond ensuring that the horizontal members can support the weight of large glazing units and spandrels without excessive twisting or deflection. The requirements of stiffness rather than strength usually govern.
|Energy efficiency plays a major role in curtain-wall design and green-building guidelines.|
For energy efficiency, well-known solutions apply-insulating-glass units with inert gas fills, high-performance spacers, and reflective or low-emissivity coatings. Though aluminum offers a high strength-to-weight ratio, the high thermal conductivity inherent with any metal must be controlled using technology such as thermal barriers or separators. These same solutions also curtail condensation while enhancing acoustic performance by attenuating outside noise from traffic or airports.
Practicing architects, specifiers, or engineers routinely deal with these fundamentals. However, curtain-wall applications bring into play a number of subtleties that may require additional attention.
Of primary concern are the dynamic or “live” loads that induce movement in the structure. In dealing with these factors, architectural philosophy has slowly evolved from one of brute-force prevention of motion to one of accommodation.
- Wind loading-Designing for lateral wind forces is a routine procedure, although defining their nature and magnitude is not a trivial task for high-rise structures due to the effects of building geometry, surrounding structures, and natural topography. A key consideration that sometimes gets less attention than it should is negative wind loading (suction forces) acting on the wall. On high-rise buildings, these negative pressures usually reach a maximum near exterior corners and the top of the structure, where they may be more than twice that of any positive load.
- Thermal expansion-Aluminum’s relatively high coefficient of expansion, combined with the potentially wide daily and seasonal fluctuations in the metal’s surface temperature (which can cover a broad range from 0 to 180 F) can induce stresses from thermal expansion. Actual movement is approximately 1/8 to 3/16 in. in a 10-ft. framing member. A contiguous sheet 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, frame joints, anchors, joint seals, or structural elements.
- Seismic loads and inter-story drift-While both wind and seismic loads can cause inter-story drift (relative horizontal movement between adjacent stories), seismic action is by far the greater concern in at-risk areas. True to the principle of accommodating motion, the design approach is one of preventing building collapse while accepting some damage as a result of different earthquake magnitudes. 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.
To provide for the inevitable movement from all of these various sources, the designer must take into account proper tolerances, adequate anchorage, sufficient framing member strength to prevent excessive glass deflection, and use of suitable sealants.
- Tolerances-Because curtain wall construction typically involves covering a field-constructed skeleton with a factory-made skin, the designer must consider how the system connects to many other parts of the building. For example, glass-holding members must be erected so they provide openings within acceptable tolerances for squareness, corner offset, and bow. It is generally recommended that the difference between the lengths of the diagonals not exceed 1/8 in.
Because significant deviations are generally not correctable by field cutting, fitting, shimming, or patching, it is advisable to pay careful attention to manufacturing and installation variances. The resulting tolerance stack-ups should still be capable of providing the clearances necessary to permit movement due to live loads.
- Wall anchorage-Anchors must have the hardness, yield, and tensile strength to bear the weight of the curtain wall itself, as well as withstand the forces imposed by dynamic loads, while taking into account fabrication and construction tolerances and allowing for thermal movement. Locking devices must be employed to prevent loosening or turning out due to thermal expansion-and-contraction cycles, building movements, or wind-induced vibration. The metal type and coating must be carefully chosen to prevent dissimilar material reaction and corrosion, especially in harsh environments such as coastal locations.
- 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. Care must be taken in sealant selection, as compounds vary considerably in the amount of extension and compression movement they can withstand.
This is particularly important when structural-sealant glazing systems are used. This popular technique allows installation of glass with no visible means of support, resulting in a clean, uncluttered exterior surface. The sealant serves double duty as a weather seal and as a means to transfer structural loads from the glazing infill to its perimeter support.
The most critical properties of a sealant are its adhesive and cohesive strength, i.e., the amount of extension and compression movement they can withstand before failure. Also important are its recovery ability after deformation, compatibility with other compounds it may contact, and durability under the effects of weathering.
- Deflection of glass-supporting frame members-The more the frame deflects under the load, the more stress is placed on the glass, increasing the likelihood of breakage. The typical design convention limits frame member deflection to a maximum of 1/175 of the unsupported (clear) span length (L) of 13 ft. 6 in. or less when subjected to design loads. Certain sealants may require a lower deflection ratio than L/175, as will the need to limit movement to avoid damage to interior finishes or disengagement of applied exterior snap covers or trim. Lower deflections usually require the use of heavier frame cross-sections or reinforcements, which have visual impact. Refer to AAMA TIR-All for more information on recommended deflection limits.
|References for Curtain-Wall Design
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On the Horizon
Design priorities change over time, literally with the weather and societal conditions. Those more recently in vogue are borne of responses to issues that read like current newspaper headlines, and it behooves designers to stay up to date on current concerns and the means to address them.
- Impact resistance-Intensive research into hurricane damage has revealed that impact from windborne debris is a bigger factor in causing damage than the force of the wind alone. Impact-resistant designs, featuring heavier framing and laminated glass, may be qualified for Atlantic and Gulf coastal exposures, including defined Wind Borne Debris regions and High Velocity Hurricane Zones (HVHZ), through testing using ASTM E 1886 and E 1996, AAMA 506, or Miami-Dade (FL) protocols TAS 201, 202, or 203, as applicable.
- Blast resistance-In response to the recent increased focus on protecting against bomb blasts and terrorism schemes, American Architectural Manufacturers Association (AAMA), Schaumburg, IL, has collaborated with several government and private-sector organizations to produce the first standard for blast hazard mitigation (AAMA 510-06), which has led to a blast-resistance rating and certification program offered by Architectural Testing Inc., York, PA. The standard provides performance definitions and testing protocols that allow a system to be qualified for a specific level of blast resistance-the primary goal being to maintain building envelope integrity while minimizing flying glass fragments.
- Green guidelines-Environmental compatibility and long-term sustainability have moved to the forefront of design concerns in the commercial marketplace. The U.S. Green Building Council, Washington, has promulgated its LEED rating system that scores points for the use of environmentally friendly, energy-efficient designs and materials. Other green guidelines, notably the Green Building Initiative’s “Green Globes” program for commercial structures, have been or are being developed. Fortunately, being comprised primarily of glass and aluminum-which are highly recyclable, use abundant natural resources, and are environmentally inert-most curtain-wall systems have good green credentials to start with, and well-planned design techniques can rise to the green challenge for particular projects.