Holding it All Together: Designing, specifying, and installing connectors

August 8, 2017

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Photo © BigStockPhoto

by Larry W. Williams, APR
A building’s framing system performs the essential task of providing strength and stability to the structure, offering a path to transfer loads from gravity, uplift, and seismic forces to the foundation. However, even properly designed and built framing depends not just on the strength of these components, but also on the performance of the connections holding the elements together. These connections are even more critical in coastal and high-seismic zones where structures must resist the movement, stress, and loading from natural events such as earthquakes and high-velocity winds.

Critical connections occur throughout the structure, but the most important are found in four crucial areas for the majority of buildings:

Connectors, clips, or framing hardware are used 
to either make or support cold-formed framing connections, with the most common types being rigid (i.e. fixed) and deflection (i.e. movement) connectors.

Rigid connectors
Rigid connectors and supports are used in various ways to secure cold-formed framing members to each other or to 
the building structure. Common types of rigid and fixed connections include:

The types of connectors used in these locations include cold-rolled channel (CRC) clips, hangers, pre-punched clips, holdowns, and hurricane ties.

Until recently, it was common practice on cold-formed steel framing projects for installers to fabricate these types of connectors in the field using whatever scrap or stud material available. In the past few years, however, the shift to pre-engineered and premanufactured connectors has been gaining momentum for a number of important reasons.

First, design professionals are increasingly producing high-performance engineering that demands all the elements used in a building perform as expected. Pre-engineered connectors are designed and tested for ultimate and allowable load capacities, which can then be used by the designer to ensure calculations are based on how the connector actually performs.

As a growing number of these pre-engineered, premanufactured connectors gain code listings, building officials increasingly expect to see a third-party verification proving that the connector is manufactured according to the applicable standards. In 2014, the Steel Framing Industry Association (SFIA) rolled out a certification program providing independent third-party verification the associated connectors meet quality guidelines, building codes, and AISI ASTM standards.

Certification includes unannounced audits and onsite inspections of certified companies, along with field and laboratory tests to ensure the certified steel products meet standard tolerances for shape, material thickness, and corrosion protection. Having design tables based on testing is essential to predictable and safe design, but a third-party certification the products have been manufactured or fabricated as designed provides an additional level of certainty.

A third factor fueling the move away from field-fabricated connectors is contractor recognition of the fact making these parts onsite consumes valuable hours that can eat away any perceived cost savings from fabricating a connector out of scrap. Performance may also suffer, as these hand-made connectors lack the pre-punched holes that help installers properly place the fasteners in the locations ensuring the connector achieves the intended 
design load.

At first blush, a field-fabricated connector would seem to be the clear economic choice because the material used would otherwise be discarded as waste. However, contractors are also recognizing this savings is quickly offset by inefficiencies from having to cut, bend, and fashion custom-made connectors for each location where one is needed. In large projects, this can consume many valuable hours. Performance further suffers, as these hand-made connectors lack the pre-punched holes that help installers properly place the fasteners in the locations ensuring the connector achieves the intended design load. There are some instances where a field fabricator could make sense, but they are almost always for temporary bracing where the element will be replaced with permanent bracing.

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Deflection clips are used to attach exterior curtain-wall studs to the building structure and provide for both vertical and lateral movement.
Images courtesy ClarkDietrich

Deflection connectors
Deflection or movement connections are designed to permit movement of the primary structure of the building without imposing any axial loading onto the exterior curtain wall or interior head of wall. Movement of the primary structure may result 
from thermal expansion and contraction, seismic disturbances, foundation settling, or normal head-of-wall compression. Deflection connectors are used in an array of framing conditions, including exterior walls, bypass, head of wall or infill, and interior head of wall.

When designing for a deflection connector, the two primary considerations are deflection and standoff distances.

Deflection distances
Deflection distances are the measure of the maximum vertical distance the primary structure is anticipated to move due to the imposition or removal of 
‘live loading.’

Standoff distances
Standoff distances are the space between the secondary and primary frames. The former is the curtain wall assembly made of cold-formed steel framing, while the latter is the main building structural support that usually consists of red iron or concrete. The standoff distance allows for the framer to install the cold-formed framing in a ‘true’ line, while the structure of the building may vary either ‘in or out’ or out-of-plumb from floor to floor.

Corrosion protection
As with other cold-formed steel framing products, connectors are protected from corrosion by a coating metallurgically bonded to the steel substrate. There are various coatings available, with the minimum being a G60 zinc coating as found in Table 1 in ASTM C955, Standard Specification for Cold-formed Steel Structural Framing Members. Epoxy-based or semi-metallic coatings that supplement the zinc coating or provide equivalent protection are also available for severe environments. Code-compliant and marketed under a trade name, they can be an economical choice.

Certain environments have high potential for corrosion. These include areas exposed to ocean salt air, fire retardants, fumes, fertilizers, preservative-treated wood, de-icing salts, and dissimilar metals. For these locations, it is possible to augment the level of protection by increasing the coating thickness. 
In such circumstances, G90 is the most commonly specified next step up; in rare, extremely harsh conditions, a G120 coating could be justified, but 
the cost implications should also be weighed.

It is common to see corrosion in outdoor applications, but this does not mean the load capacity has been affected or failure is imminent. (White rust is the light film that forms on the surface of a hot-dip galvanized connector as 
a natural result of the contact of the zinc with oxygen, called oxidation.’ This is harmless. In outdoor applications, it is not unusual to see some corrosion of a connector [i.e. red or brown rust] on the edges of a connector. Despite this, the connector is still being protected against corrosion by the ‘galvanic,’ or sacrificial action 
of the zinc; the connector is still able to perform as designed. If significant corrosion is suspected, the connector should be inspected by a qualified engineer or inspector. If the area in question takes on a black appearance, then enough zinc has been lost and the steel substrate has begun deteriorating. In normal conditions, this is a process that can take decades.) When significant corrosion is apparent or suspected, the framing members, fasteners, and connectors should be inspected by a qualified engineer or inspector. Replacement of affected components 
may be appropriate.

Some wood-preservative and fire-retardant chemicals and retentions also pose increased corrosion potential, and are more corrosive to steel connectors and fasteners than others. Connector manufacturers can provide specific guidance on the interactions of their products with the wide variety of wood treatments.

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The steel thickness of a connector, clip, support, or hanger is referenced in terms of gauge or mils. The mil thickness measures the uncoated base metal material, and is a key contributor to the strength of the product.

General instructions for the designer
To ensure proper selection and installation of 
pre-engineered/premanufactured connector products, design professionals should follow 
certain general instructions. The recommendations in the following section are in addition to the specific design and installation instructions and notes provided for each particular product, all of which should be consulted before and during the design process.

1. The term ‘designer’ is intended to mean a licensed/certified building design professional, a licensed professional engineer, or a licensed architect.

2. Allowable loads are determined per American Iron and Steel Institute (AISI) S100, North American Specification for the Design of Cold-formed Steel Structural Members, unless otherwise specified. Other code agencies may use different methodologies of their own.

3. The allowable load is typically limited to an average test load at 3-mm (1/8-in.) deflection, or 
an average or lowest test value (nominal load) divided by 
a safety factor or the calculation value. The safety factor is prescribed by Section F1 of AISI S100.

4. Allowable simultaneous loads in more than one direction on a single connector must be evaluated as follows:

Design Uplift/Allowable Uplift + Design Lateral Parallel to Track/
Allowable Lateral Parallel to Track + Design Lateral Perpendicular to Track/Allowable Lateral Perpendicular to Track ≤ 1.0

The three terms in the unity equation are due to the three possible directions to generate force on a connector. The number of terms that must be considered for simultaneous loading is at the sole discretion of the designer; it depends on his or her method of calculating wind forces and the utilization of the connector within the structural system.

5. All connected members and related elements shall be designed by the designer.

6. Unless otherwise noted, member strength is not considered in the loads given and, therefore, one should reduce allowable loads when member strength is limiting.

7. The average ultimate breaking strength for some models is listed under ‘nominal tension load.’

8. The dimensions of the supporting member must be verified as sufficient to receive the specified fasteners, and develop the top flange bearing length.

9. Most of the allowable loads provided by manufacturers of pre-engineered, premanufactured connectors are based on the traditional allowable stress design (ASD) methodology. A method for using load and resistance factor design (LRFD) for cold-formed steel is also included in AISI S100. When designing with LRFD, the nominal connector strength multiplied by the resistance factor must be used.

10. All steel-to-steel connector screws must comply with ASTM C1513, Standard Specification for Steel Tapping Screws for Cold-formed Steel Framing Connections.

11. Screw strength shall be calculated in accordance with AISI S100 Section E4 or be based on the manufacturer’s design capacity, which is determined from testing.

12. Local and/or regional building codes may demand meeting special conditions. Building codes often require special inspection of anchors installed in concrete and masonry. To achieve compliance with these requirements, it is necessary to contact the local and/or regional building authority. Except where mandated by code, some pre-engineered, premanufactured products do not require special inspection.

13. When connectors are attached to two cold-formed steel members of different thicknesses, the designer shall use the thinner of the two members for selecting allowable loads.

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It is critical to specify the right fastener to ensure the proper performance of the connections in light-gauge (cold-formed) steel. The most common and widely used connection methods are screw connections, powder-actuated fastener connections, and welded connections.
Photo courtesy Steel Framing Industry Association

General instructions for the installer
These general instructions are in addition to the specific installation instructions and notes provided for each particular product, all of which should be consulted prior 
to and during installation of any manufacturer’s pre-engineered, premanufactured product.

1. All specified fasteners must be installed according to 
the instructions in the manufacturer’s catalog. Incorrect fastener quantity, size, type, material, or finish may cause the connection to fail.

2. Holes for 13-mm (½-in.) diameter or greater bolts 
shall be no more than a maximum of 1.5 mm (1/16 in.) larger than the bolt diameter per AISI S100 Table E3a, Appendix A.

3. All specified fasteners must be installed before loading 
the connection.

4. Some hardened fasteners may have premature failure if exposed to moisture. The fasteners are recommended to be used specifically for dry interior applications.

5. Proper safety equipment must be used.

6. When installing a joist into a connector with a seat, it shall bear completely on the seat. The gap between the end of the joist and the connector or header shall not exceed 3 mm (1/8 in.) per International Code Council Evaluation Service (ICC-ES) Acceptance Criteria (AC) 261, Connectors Used with Cold-formed Steel Structural Members, and ASTM D1761, Standard Test Methods for Mechanical Fasteners in Wood, unless otherwise noted.

7. For holdowns, anchor bolt nuts should be finger-tight plus a third- to half-turn with a hand wrench. Care should be taken to avoid over-torqueing the nut or using impact wrenches, as this may preload the holdown.

8. Holdowns and tension ties may be raised off the track as dictated by field conditions to accommodate an anchor mislocated no more than 38 mm (1 ½ in.). The holdown shall be raised off the bottom track at least 76 mm (3 in.) for every 6.4 mm (¼ in.) the anchor is offset from the model’s centerline. Anchor bolt slope shall be no greater than 1:12 (or five degrees). The designer should be contacted if the holdown anchor is offset more than 38 mm or raised more than 457 mm (18 in.). Raised holdown height is measured from the top of the concrete to the top of the holdown bearing plate.

9. All screws shall be installed in accordance with the screw manufacturer’s recommendations. They must penetrate and protrude through the attached materials a minimum of three full exposed threads per AISI S200 General Provisions Section D1.3.

10. Since welding galvanized steel may produce harmful fumes, proper procedures and safety precautions must be followed. Welding should be in accordance with American Welding Society (AWS) standards. The manufacturer’s installation guidelines should be consulted to verify the specific connector in use can be welded.

11. Temporary lateral support for members may be required during installation.

Considerations for specifying clips and connectors
There are a few factors to consider when specifying clips, but most importantly are the clip itself and the manufacturer producing the product. As previously mentioned, connectors are considered structural elements, and play a vital role in the design of modern buildings by helping ensure buildings are safe even when subjected to high winds and seismic forces. Therefore, it is necessary to specify for quality assurance (QA).

Quality assurance refers to the procedures for obtaining 
data indicating performance, properties and other attributes meeting the requirements. This includes manufacturer qualifications, engineering responsibility and qualifications, product test reports, and evaluation/research reports.

A good rule of thumb when specifying is only selecting clips from manufacturers in good standing of a trade association, as products are certified under an independent third-party inspection program administered by an International Accreditation Service (IAS)-accredited agency per ICC ES AC98, Accreditation Criteria for Inspection Agencies. These standards include requirements governing the materials used to produce the connectors, protection against corrosion, manufacturing tolerances, and marking requirements. Marking requirements are especially important, as they ensure the installer can easily identify and use the proper connection. For the engineer or architect, the markings provide an accurate way to verify in the field whether the proper connector is used.

For engineering responsibility, one should specify shop drawings, design calculations, and other structural data be prepared by a qualified professional engineer registered in the state of the project and experienced in cold-formed metal framing design. Product test reports should be required to be performed or witnessed by a qualified testing agency.

Adhering to the following guidelines and standards helps ensure the technical specifications direct contractors to use clips and connectors that have code-approved technical reviews from such IAS-accredited agencies like ICC-ES, Intertek, and International Association of Plumbing and Mechanical Officials (IAPMO), thus avoiding use of unproven field-fabricated clips.

Materials standards
Material standards to follow include:

Product standards
For the benefits laid out in this article, one should strongly consider requiring compliance with SFIA’s Manufacturing Compliance Certification Program 
for Connectors.

Design standards
In addition to the aforementioned AISI S100, another important design standard is AISI S200, North American Standard for Cold-formed Steel Framing–General Provisions.

Installation standards
Installation standards with which to comply include both AISI S200 and another standard, ASTM C1007, Standard Specification for Installation of Loadbearing (Transverse and Axial) Steel Studs and Related Accessories.

Test method standards
For test method standards, contractors should follow ASTM A90, Standard Test Method for Weight [Mass] of Coating on Iron and Steel Articles with Zinc or Zinc-alloy Coating.

Conclusion
With a building project, having the right connectors 
is everything. Everyone first thinks about the quality of the framing members, but many forget no matter how good the framing, there will definitely be problems with the overall structure and weather resistance of the construction if proper attention is not paid to the element holding everything together.

Larry W. Williams, APR, is the executive director of the Steel Framing Industry Association (SFIA), an organization dedicated to expanding the market for cold-formed steel through promotion, advocacy, education, and providing a positive environment for innovation. Williams has more than 35 years of experience in management, marketing, and strategic communications, including 25 years in the steel and construction markets. He has also served as general manager of market development and sustainability for the World Steel Association (Brussels), president of the Steel Framing Alliance, and founder/executive director of the Light Gauge Steel Engineers Association (now Cold-formed Steel Engineers Institute [CFSEI]). He can be reached via e-mail by contacting williams@steelframingassociation.org[5].

 

Endnotes:
  1. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2017/08/bigstock-173473160.jpg
  2. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2017/08/CD_DriftClip-Application-Rendering_1-19-171-e1501682943257.jpg
  3. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2017/08/Clip-2_20x241.jpg
  4. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2017/08/DSC_43991.jpg
  5. williams@steelframingassociation.org: mailto:williams@steelframingassociation.org

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