May 7, 2015
by Dean Lewis
Curtain walls are often the focal point of aesthetic design for a multi-story building. Behind the attractive façade are the pedestrian, yet arguably more important, functional components that ensure safety and reliability—fasteners that transfer loads both imposed and experienced by the assembly to the building’s structural framework.
Being key elements of the load path, all fasteners should be analyzed to ensure their material, thickness, and quantity are adequate. In doing so, designers must quantify both dead and live loads on the wall system—which translate to bending, shear, bearing, and pull-out loads on the connections.
Fasteners must have the hardness, along with the yield, tensile, and shear strength to appropriately withstand these loads without compromising the wall’s integrity and weathertightness. Different anchorage configurations are also required for the various substrates typically encountered in commercial or institutional-type structures, featuring concrete, masonry, or steel frame.
Numerous organizations have developed specifications, performance testing methods, and design guides for wall cladding fasteners, including:
Compiling, applying, and in some cases reconciling the data from these various sources to produce a viable fastener specification is a formidable task.
That task is made easier by the American Architectural Manufacturers Association (AAMA) Technical Information Report (TIR)-A9-14, Design Guide for Metal Cladding Fasteners, which provides the data necessary to select fasteners for anchoring the curtain wall to the building structure. Metals used in fasteners covered by TIR-A9 include various types of carbon steel and stainless steel alloys. It should be noted aluminum fasteners are not recommended for curtain wall anchoring systems.
A complete specification must include fastener size and type, as well as material, calculated minimum mechanical properties (e.g. bending, shear, bearing and pull-out loads, thickness), and type of protective coating required. Fastener quality and corrosion resistance also should be addressed by specifiers.
“Fastener Specification Checklist” adequately defines the parameters for a given application including proposed anchorage arrangement, which defines the number and location of the fasteners, as well as determination of the forces acting on the attachments and the maximum design load per attachment.
A recommendation of attachment devices to resist the determined forces should also be included as the number and spacing must be sufficient to meet the loads. Additionally, fastener length should be sufficient to penetrate the substrate to a depth designed to meet applicable building codes, manufacturer recommendations, and structural calculations.
Confirmation the curtain wall frame can resist the forces at the attachment points is required. The installation of fasteners or fastening systems must limit distortion of any framing member and must not in any way impede the action of operable components. It is also necessary to specify the required anchorage and/or fastener quantity, spacing, type, material, strength, embedment, edge distances, and other parameters as appropriate.
The mode of potential failure for the material and fasteners along the load path should be considered. This includes the fastener itself, as well as the material being fastened to the substrate, which can fail by bending, buckling, or pull-out (i.e. the force required to pull the fastener out of the base material), and/or pullover (i.e. the force to pull the material over the head of the fastener). TIR-A9 tables give allowable pull-out values for different aluminum thicknesses of various alloy designations (e.g. 3003-H14, 5005-H34, 6061-T6, 6063-T5, 6063-T6, and 6005A-T61) for UNC (i.e. threads designated as ‘coarse’ under the United Thread Standard [UTS]) and spaced threads for different screw diameters.
The basic information needed to analyze the anchorage includes the following elements.
This includes properties of the frame profiles, parts, and anchors and specifications for the surrounding wall conditions at the product perimeter. For metals, the thickness, alloy, temper, and strength are needed. Equivalent information for concrete and masonry substrates must be provided.
Project-specific design pressure
The proposed anchorage system must provide a load resistance equal to or greater than the project-specific design pressure. Of primary concern are the dynamic, or ‘live,’ that cause building movement, such as those imposed by wind, thermal expansion, and (in appropriate location-dependent cases) seismic activity, impact of hurricane wind-borne debris, or potential for blast waves. Wind loading is determined based on the expected maximum wind velocity that is likely to be experienced at the building location, as typically derived from the current, or code-cited, American Society of Civil Engineers/Structural Engineering Institute (ASCE/SEI) 7, Minimum Design Loads for Buildings and Other Structures.
A contributing factor will be the weight of the glass of the type and thickness required to resist the wind loading, as determined by ASTM E1300, Standard Practice for Determining Load Resistance of Glass in Buildings. A rule of thumb is to use a glass type and thickness that just exceeds the required strength of the glazing.
Design load-bearing capacities
Design load-bearing capacity of fasteners for the surrounding wall substrate material should be considered. With respect to loads, a fastener system is primarily exposed to either tension, shear, or a combination of the two, depending on the configuration.
Load-bearing analysis equations presented in TIR-A9 are the basis for determination of the values used in the document’s load tables, which provide allowable tension, shear, and bearing loads for a range of different fastener sizes, carbon steel, and stainless steel alloys. A total of 21 fastener sizes are covered, ranging from #6-32 through 1-8.
The tensile strength or pull value of a connection is the amount of force required to pull a fastener apart by securing one end and pulling up on the other end. The shear strength of a connection is the force required to pull the base material in one direction and the top material in the other direction (a shearing motion) until failure.
AAMA TIR-A9 walks through the derivation of equations used to calculate these and other parameters when different substrate material and thickness and various fastener types and sizes are used, as well as different hole thread configurations (e.g. UNC and SAE spaced threads).
Since a fastener’s load capacity depends on its size and the material of which it is made, the easiest way to evaluate the loaded performance of various size fasteners and fastener metals is from a load table, such as those published in this revised TIR. Additionally, many manufacturers publish load tables specific to their own fasteners.
TIR-A9 also provides:
|Fastener Specification Checklist|
To aid in the fastener selection process, American Architectural Manufacturers Association (AAMA) Technical Information Report (TIR)-A9-14, Design Guide for Metal Cladding Fasteners, provides the following suggested fastener specification checklist.
A safety factor (SF) is used in the allowable strength design (ASD) method. This method was used to determine the allowable values presented in TIR-A9, which have been determined after a study of several industry standards. There is also another design approach—the load and resistance factor design (LRFD) method—in which the combined use of a load factor ‘m’ (greater than 1) and a resistance factor ‘j’ (less than 1) is the equivalent of using a safety factor. That is, SF= m/j. Load factors are given in the governing building code. Resistance factors, also termed strength-reduction or capacity factors, are given in the specification for the structural material/components being connected.
For fasteners of 6.35 mm (0.25 in.) or less in diameter, a SF of 3.0 is used to generate allowable values. This value is used in both the North American Specification for Cold-formed Steel Structures (2007 and 2001) and the 2010 Specification for Aluminum Structures for this size range of tapping screws.
For fastener diameters exceeding 6.35 mm, but are 25 mm (1 in.) or less, a SF of 2.5 is used. This is more conservative than the safety factors recommended by others, especially for the 12.7 to 25 mm (0.5 to 1 in.) range, by the cold-formed steel specification, Specification for Cold-formed Stainless Steel (ASCE-8), and the AISC Specification for Structural Steel Buildings ranging between 2 and 2.42.
Protection from corrosion, hydrogen embrittlement
It is essential fasteners have adequate protection against corrosion to prevent eventual failures due to moisture from rain and condensation. Additionally, a defense must also be provided against galvanic corrosion, which occurs when dissimilar metals are in contact in the presence of moisture, especially in harsh environments such as seacoast locations.
Stress corrosion is the effect of corrosion on a metal under stress. When metals are under constant or cycling stress (the latter of which causes metal fatigue), the effect of corrosion can be much more severe than when metals are not stressed. Stress corrosion failures can occur shortly after the load is applied, but may not occur for months or years later. Such failures occur without warning and can be dangerous.
To forestall corrosion, carbon steel fasteners should be plated or coated with zinc, cadmium, nickel, or chromium in accordance with the specifications in TIR-A9 Section 4, “Protection against Corrosion and Hydrogen Embrittlement.”
The specifier and purchaser must be aware of these matters and make the best compromise, all factors considered, in the selection of the fasteners. Designs for curtain wall anchoring systems must account for the stresses for which fasteners must be selected and the coatings to be employed to eliminate problems due to galvanic action and stress corrosion.
Various ASTM standards govern coatings or platings in terms of class, service condition, and type, which dictate acceptance testing such as duration of salt-spray application. Class designation relates to the coating thickness, while service condition indicates the severity of the exposure for which the coating is intended (ranging in four steps from mild or indoor to very severe). The type indicates the composition of the coating, such as whether chromate or phosphate layers are added.
Stainless steel offers better corrosion protection than plated carbon steel, but can present difficulties in terms of matching finishes to the framing.
Hydrogen embrittlement is a condition of low ductility in metals resulting from the absorption of hydrogen, which typically occurs during the manufacturing process (particularly electroplating), although it also can occur through in-service corrosion. Bolts and screws—with a hardness of C35 or greater on the Rockwell C scale—are particularly subject to embrittlement. Hydrogen embrittlement can cause unpredictable and potentially disastrous failure, especially of a fastener under tensile load. Tests are available to assess whether hydrogen embrittlement is present
in a batch of fasteners.
Beyond developing a thorough specification, it is essential the fastener purchaser has a means for determining the fasteners that are received meet the specifications.
ASTM standards provide chemical and mechanical requirements for the steels used in fasteners, as well as quality control procedures for shipment lot testing, source inspection, alloy control, heat control, permeability, manufacturer’s identification, and material identification. Additionally, the end user should consider adding a requirement that fasteners be produced under a recognized quality assurance (QA) program, such as that governed by the quality management system standard International Organization for Standardization (ISO) 9001, Quality Management Systems–Requirements.
|AAMA curtain wall design references|
|Additional American Architectural Manufacturers Association (AAMA) curtain wall design references for design professionals and specifiers include:
Dean Lewis currently serves as American Architectural Manufacturers Association’s (AAMA’s) educational and technical information manager, bringing his knowledge of technical training to advance the Fenestration Masters professional certification program. Lewis began his career in the fenestration industry at PPG Industries with positions in project engineering, product design, and sales and customer technical support. He has served on committees of ANSI, ASTM, and ASHRAE. Further experience includes teaching in the industrial and military sectors, and 35 years of managing technical training, publishing, and certification. Lewis can be contacted by e-mail at firstname.lastname@example.org.
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