by sadia_badhon | November 8, 2019 12:34 pm
by Steven Saffell
Metal curtain walls are a ubiquitous feature of most cityscapes, but their ultimate expression and aesthetic is as varied as the creativity of architects employing virtually unlimited design options. Additionally, a curtain wall’s dual mission of providing structural integrity to accommodate both static and dynamic forces, while serving as a two-way filter controlling the through-flow of heat, light, and air, makes system design, specification, and installation a complicated process.
A volunteer task group of industry-representative curtain wall manufacturers has published guidelines to aid this process, marking the culmination of a multiyear effort. Representing the latest knowledge and process consensus as evolved over the past 20 years, the guidelines— published under the title American Architectural Manufacturers Association (AAMA), Curtain Wall Manual (CWM)—are intended for manufacturers, architects, specifiers, contractors, and testing agencies. The new manual addresses many aspects of curtain wall design, specification, testing, and installation with information presented in eight sections, primarily dealing with design-related topics including material and system performance criteria, architectural detailing guidelines, and building tolerances. It also addresses design validation through testing and installation issues from the perspective of many technical stakeholders, including architects, specifiers, manufacturers, installers, glaziers, and general contractors (GCs).
A guide specification (section 5 of AAMA CWM), of particular use to architects and specifiers, features new content in the form of detailed fabrication and installation tolerances, as well as air barrier interface guidelines. Intended to cover the complete exterior metal curtain wall, the guide can be referenced as a performance standard.
Key design considerations
Experience dictates that in the design of metal curtain wall there are generally four matters of chief concern:
While it is important to consider all possible forces acting on the curtain wall system, wind loads are one of the most challenging.
The guide specification requires the curtain wall system to be designed to withstand specified allowable stress design (ASD) wind loads. Given the nature of curtain walls, dead loads are of minor concern. As with all types of fenestration, the lateral wind forces are the primary live loads affecting curtain wall. It is strongly recommended design loads (in psf or Pa) specific to all relevant areas of the building be provided by the specifier. Special requirements for snow or ice loads on projecting wall elements, such as sun shades, may be applicable.
Provision is made here for specifying design wind loads at different building heights and at areas adjacent to the building corners. The architect selects actual load limits for positive and negative loads at ground level, at various defined heights (stories) above ground, and at corners. Appropriate design wind loads for most situations may be selected by the architect from the American Society of Civil Engineers/Structural Engineering Institute (ASCE/SEI) 7, Minimum Design Loads for Buildings and Other Structures. For other situations, such as unusual building shapes or sites with extreme wind conditions, boundary layer wind tunnel testing, and/or special engineering studies, may be needed to determine the required design wind load.
Compliance can be demonstrated by calculations performed in accordance with accepted engineering standards established by the Aluminum Association (AA), American Institute of Steel Construction (AISC), American Iron and Steel Institute (AISI), or Copper Development Association (CDA) for the materials being used.
Provision for movement
The laws of physics are constantly acting on curtain walls so they must be designed with movement in mind.
Temperature differences must be considered in curtain wall design, as they relate to differential expansion and contraction of various materials. Aluminum is a common material choice for curtain walls due to its strength and light weight. However, its relatively high coefficient of expansion, in comparison to glass, means the potentially wide daily and seasonal fluctuations in the metal’s surface temperature can induce stresses from differential thermal expansion that can cause stress on glass, joints, and anchors, or reduce glass ‘bite.’
AAMA CWM guide specification in section 5 recommends curtain wall systems design should provide for such expansion and contraction of component materials due to an exterior metal surface temperature range of –17 C (1 F) to 82 C (180 F) without causing buckling, undue stress on glass or structural elements, failure of joint seals, damaging loads on fasteners, reduction of performance, or other detrimental effects. This range comes directly from the AAMA CWM specifications and prior documents. While local conditions may require more stringent design criteria, this is the anticipated starting point. More stringent requirements would need to be determined by the local design professional and stated in the building specifications.
Controlling water and air movement at the building envelope is important to the long-term integrity of the structure and the comfort of its occupants.
Two methods have been developed for preventing leakage through the curtain wall system. One is referred to as the ‘internal drainage’ or ‘secondary defense’ system wherein minor leakage can be prevented from penetrating by providing within the wall itself a system of flashing and collection devices, with ample drainage outlets to the outdoor face of the wall. The other is the more sophisticated ‘pressure equalization’ method, based on the ‘rainscreen principle,’ which requires the provision of a ventilated outer wall surface, backed by drained air spaces in which pressures are maintained equal to those outside the wall.
The specifier may optionally specify pressure-equalized rainscreen wall cladding (PRWC) systems meeting the requirements of AAMA 508, Voluntary Test Method and Specification for Pressure Equalized Rain Screen Wall Cladding (Panel) Systems, when tested in accordance with ASTM E331, Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference. The static air pressure difference used in the test is set at 20 percent of the specified maximum inward acting ASD wind load pressure, but not less than 300 Pa (6 psf) nor more than 720 Pa (15). No uncontrolled water penetration may occur when the curtain wall is tested to the required specification.
The guide specification requires that air infiltration through the curtain wall should not exceed 0.3 L/s•m2 (0.06 cfm/sf) of fixed wall area and the permissible allowance specified for operable windows or doors when tested in accordance with ASTM E283, Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen, at a static air pressure difference of 300 Pa.
With increased attention being placed on the impact of large buildings on the environment by policies like those recently introduced by the mayor of New York, energy efficiency of curtain wall systems will continue to grow in importance.
The fixed light area of the curtain wall, including glass and metal framing, should have a condensation resistance factor (CRF), not less than the one selected by the architect based on climate zone when tested in accordance with AAMA 1503, Voluntary Test Method for Thermal Transmittance and Condensation Resistance of Windows, Doors, and Glazed Wall Sections.
The fixed lite area of the curtain wall must have an overall thermal transmittance U-Factor (W/m2•K [BTU/hr-sf-F]) not exceeding that specified by the architect. It is important to note the selected U-factor may be defined by applicable codes based on project location or desired environmental design credits, such as offered by the Leadership in Energy and Environmental Design (LEED). U-factors should be tested per AAMA 1503, or simulated per AAMA 507, Standard Practice for Determining the Thermal Performance Characteristics of Fenestration Systems in Commercial Buildings, or (optionally) applicable National Fenestration Ratings Council (NFRC) testing, modeling, and validation protocols.
A primary feature of AAMA CWM is more attention is paid to the issue of tolerances and clearances. Since they may be closely related, the two terms are often confused. However, they have different meanings. A tolerance is a permissible amount of deviation from a specified or nominal characteristic—in general, tighter tolerances equal higher costs. A clearance is a space or distance purposely provided between adjacent parts, either to allow for movements or anticipated size variations, to offer working space or for other reasons. Both are critical. Since curtain wall construction involves covering a field-constructed skeleton with a factory-made skin, the designer must consider how the curtain wall system connects to many other parts of the building, thus involving the work of numerous trades.
The included guide specification in section 5 of AAMA CWM sets forth recommended tolerances for both component and system tolerances based on recognized industry standards. Component tolerances reference AA standards or defer to those permitted by the manufacturer’s quality assurance (QA) documentation, whichever is more stringent.
The recognition of normal dimensional tolerances and the provision of proper clearances are of critical importance in several aspects of metal curtain wall design, including detailing of glazing frames, where ample edge clearance and sufficient ‘bite’ are prime factors affecting glass performance.
Lack of attention to these matters often necessitates changes and adjustments in the field, not only delaying the work, but usually resulting in unnecessary costs and sometimes, impairing the appearance of the wall.
It is also the architect’s responsibility to control, by details and specifications, and to alert supervision in the field, the tolerances permitted in the basic building structure. In the case of a reinforced concrete building frame or precast/prestressed concrete, the maximum tolerances permitted should be those listed in the American Concrete Institute (ACI) 117, Dimensional Tolerances of the Building Structure. Alternately, tolerances for structural-steel building frames should be specified per AISC 303-16, Code of Standard Practice.
Specified clearances should be based on the assumption this construction will be as far out of position in the wrong direction as is allowed. In short, one needs to assume the worst-case scenario based on the tolerances provided and design accordingly. In no case should the nominal clearance shown on architectural details be less than 50 mm (2 in.).
Overall curtain wall system tolerances (excluding installation and substrates)
These cover factory-assembled framing and trim and alignment of framing members.
Alignment of framing and trim members
Maximum offset from true alignment between two identical exposed members, abutting end to end in line within an assembly, is recommended at 1.5 mm (60 mils). The maximum variation from co-planar alignment between two exposed members at corner joinery within an assembly is two degrees. Maximum variation in nominal joint width between exposed members within an assembly is 1.5 mm, not including purposeful gaps for drainage, expansion, movement, or installation tolerances.
Recommended allowable maximum deviations during installation from the nominal position of curtain wall framing are called out for level, plumb, and true characteristics, as well as straightness and variation from plane. Tolerances for the installation of panels, column covers, windows, and doors can be optionally specified as well by the architect or design professional. How tight these tolerances must be is a function of the design and the manufacturer’s requirements.
Air barrier interface design guidelines
The second major addition to the new AAMA CWM is a detailed discussion on the application of air barriers. The guide specification includes a provision to specify in detail accessories, fastening devices, perimeter sheets, sealants, adhesive, and/or tapes used in air barrier installation. Specifications should also clearly indicate the coordination of the work of various involved building trades.
AAMA CWM also provides guidance on the use of vapor permeable and non-permeable air barriers and their placement that is relative to wall insulation and interfacing with framing.
Guide specification offers roadmap
Formatted to be compatible with the three-part Construction Specifications Institute (CSI) MasterFormat, the guide specification (see section 5 of AAMA CWM) covers performance and testing requirements, and fabrication and installation methods, referencing a broad range of accepted industry standards.
In writing the specifications for any one project, an architect or specifier must delete inapplicable paragraphs or add paragraphs to meet special requirements of the design. Beyond the recommended mandatory requirements, optional requirements can be selected for project-specific specifications.
The general layout of the guide specification’s three parts is as follows.
Part 1: General
This part contains descriptions of product, scope of the system, and a list of work to be included in the specification. It further references additional items that should be specified, such as:
Performance and testing requirements
During the design and development stage, all curtain wall systems should be tested for the leakage of air infiltration, water penetration, and for structural performance (including frame deflection limits) at the wind loads applicable for the building site. This is one of the most important parts of curtain wall specifications. Testing is the only way in which certain capabilities of a wall, such as resistance to air leakage or water penetration, can be determined. The sequence of testing should be specified so the effect of exposure to test conditions on other performance parameters can be accurately assessed (for example, repeat water penetration resistance tests after subjecting the specimen to design loads). Any modifications to the design resulting from the testing must be communicated to all interested parties and fully documented to ensure it is incorporated fully into the design.
Particularly for custom designs, a preconstruction mockup test should be scheduled well in advance of the final production schedule for a building, affording ample opportunity to make corrections relatively easily and less expensive. If a mockup is deemed necessary, the guide specification provides optional language for specifying mockup testing including what parts of the system are to be represented and where the mockup is to be erected. Compliance with ASTM E2099, Standard Practice for the Specification and Evaluation of Pre-Construction Laboratory Mockups of Exterior Wall Systems, for procedures and documentation required for laboratory mockups should also be required.
Deflection of glass-supporting frame members
The more the frame deflects under the load, the more stress is placed on the glass and the greater the likelihood of breakage. The typical design convention is to limit frame member deflection to maximum of L/175 of the unsupported span length (L) of up to 4 m (14 ft) in length, maximum L/240 + 6.4 mm (250 mils) at spans over 4 m; or 2L/175 at unsupported cantilevers, except when a plastered surface or dry wall is subjected to bending, the deflection shall not exceed L/360 of the span.
Uniform structural loads and testing requirements
Uniform load structural tests of the curtain wall system (in addition to deflection tests) should be conducted per ASTM E330, Standard Test Method for Structural Performance of Exterior Windows, Doors, Skylights, and Curtain Walls by Uniform Static Air Pressure Difference, subjecting the system to inward and outward acting uniform loads equal to 1.5 times the inward and outward acting design wind pressures.
Other specific mandatory requirements in this section deal with:
Part 2: Product
The product section describes the framing material (metals, finishes, and protective coatings), glazing materials, system configuration, and components. Fabrication requirements also should be specified to include requirements for the desired degree of factory assembly and/or glazing, mechanical fastening, and sealing of joints. It is important for all to understand the anticipated level of factory assembly and/or glazing to avoid unexpected delays in the field or costly charge backs. Additionally, special conditions such as drywall pass through glazed units need to be carefully detailed in the specifications and construction documents.
Part 2 of the guide specification lists third-party specifications like ASTM, AISC, American Iron and Steel Institute (AISI), to which different components (such as sheet and plate, extrusions, and welding rods) must conform, for aluminum, carbon steel, stainless steel, and (optionally) special alloys.
Other specific mandatory requirements in this section deal with the following.
To include elements that comprise the specified system and related work to be performed, including any or all of the metal framing, anchoring devices, glazing, roofing elements, siding panels, or operable building windows.
Fasteners should be of sufficient hardness, yield, and tensile strength to bear the weight of the wall, withstand imposed dynamic forces, and also must accommodate the fabrication and construction tolerances and thermal movement.
Protective coating for metals
The guide specification lists third-party standards and specifications (federal, ASTM, AAMA) that should be referenced for finishes or integral coatings on steel or aluminum.
Frame design to enable glazing from exterior or interior
This is the architect’s choice, but glazing from the interior is preferred.
Sealing and glazing materials
Sealing must accommodate thermal movement, inter-story differential movement, concrete creep, column foreshortening, and seismic movement.
Referenced to drawings and bills of material, properly finished welding (per American Welding Society [AWS] guidelines), alignment, and fit evaluation criteria.
Metal work joints
Requiring careful matching of assembled components to provide continuity of line and design with accurately fitted and rigidly secured joints.
This is preferred to the extent possible.
Protection of metals
From the finish of the metals to the isolation from other materials that could have an unintended reaction, the designer must consider the long-term environmental impacts on the metal and associated materials. Protection against galvanic action must be provided wherever dissimilar metals are in contact.
Use of sealing materials in fabrication
It is important to comply with sealing and flashing manufacturer recommendations and allow for approved tolerances between sealed components.
General fabrication requirements
To include overall alignment and fit of the installed curtain wall, including trim and accessories, when observed at a prescribed distance and within tolerance stack-up limitations.
Part 3: Execution
This part covers requirements for matching the curtain wall to the building structure, such as erection tolerances and clearances, plus requirements related to field installation, such as welding, sealing, glazing, insulating, protecting, cleaning, and other onsite work.
To guide the wall contractor, the guide specification covers the provision of building perimeter offset lines on each floor and benchmarks to be scribed on each floor on a designated column.
Among the installation-related requirements in the guide specification are:
Language for a variety of optional project-specific requirements are included in the guide specification.
Among these is guidance for the inclusion of:
Dealing with installation
While careful attention to tolerances and clearances of both the erection of the building skeleton substrate and in the fabrication and installation of the curtain wall system contribute to ease of installation, AAMA CWM offers additional points concerning the architect’s and specifier’s roles.
For best results, design and installation considerations cannot be limited to their respective silos. AAMA CWM features an in-depth discussion of metal curtain wall installation procedures. Some architects or specifiers may consider installation to be only of minor concern, deferring to the contractor for correct placement. Too often the architect is not made aware of problems encountered, or likely to occur in the field, or may not have been advised as to how the wall should be detailed to facilitate its installation. However, without a clear knowledge of how the wall is to be installed, what problems may arise in the field and how to avoid them, there are likely to be serious deficiencies in wall design. To avoid such situations, AAMA CWM enumerates nine specific installation-related architect/specifier responsibilities, the basics of which come down to clear and open communication.
Teamwork is required and each member of the team—architect, specifier, manufacturer, contractor, and curtain wall subcontractor—must understand the needs and the problems of the others.
Curtain wall systems can be complex, but with teamwork and a plan like what is outlined in AAMA CWM, it can be simplified and managed.
Steven Saffell is the American Architectural Manufacturers Association (AAMA) technical director, overseeing the standards, product certification, and codes and regulatory affairs aspects of the association. With more than 32 years of experience, Saffell’s background is a tapestry of architectural firm work, modular design, and residential and commercial fenestration experience. Saffell also spent three years teaching as an adjunct professor. He is experienced in managing technical teams, including employee development, operational strategy, and financial management. He can be reached at email@example.com.
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