A2P1033

Cladding Attachment Methods with Continuous Insulation and Building Enclosure Systems

January 17, 2023

Introduction

Self-drilling fasteners are versatile devices used to attach many different materials. They are particularly useful when pre-drilling holes is impossible or traditional fasteners are challenging to use. These fasteners create their own hole as they are driven into the material, making them ideal for quickly and securely fastening materials in various settings.

Although the construction industry has used self-drilling fasteners for decades, their performance and reliability in various materials can be taken for granted. While testing has proven their effectiveness with steel, the same cannot be said for other materials.

There are varying degrees of effectiveness when it comes to cladding attachment to continuous insulation systems. This article will cover the typical cladding attachment methods for each z-girt material type, including steel, fiber-reinforced (FRP), and GreenGirt CMH™, a composite metal hybrid material.

Steel Z-Girts

Traditional steel Z-girts have, for years, utilized self-drilling fasteners for strong, effective cladding attachment. Pull-out, shear, and pullover technical data are readily available from most fastener manufacturers for different gauges of steel. Designing attachment methods allows for consistency and a history of testing. The same principles hold true for attaching to steel studs as a substrate.

It is essential to use the correct type of fastener when attaching the cladding to steel Z-girts. The fastener must be able to handle the load and securely attach the cladding to the steel Z-girts.

Generic Fiber-Reinforced Polymer (FRP) Z-Girts

Fiber-reinforced polymer (FRP) industry data does not recommend screws as a best practice for structural attachment. Potential liabilities include the following:¹

Singularity points for stress are created, which weakens the section

Discontinuity in the fibers weakens the section
The discontinuity in the matrix is prone to crack propagation and fracture
Torque resistance of the screw into FRP deteriorates with time
Ability for load retention in case of fire is reduced
Reduced fastener pull-out values and retention performance
Pull-out values can markedly decrease at high temperatures

Best Practices for FRP Cladding Attachment

The best practice for cladding attachment in FRP remains a bolted connection as far as structural integrity,² as this is an established, conventional attachment method. Self-drilling fastener manufacturers do not presently provide data for fastening into FRP.

Published values for self-drilling fasteners used in FRP material must account for all potential liabilities. Test pull-out values up to and including 190°F to match service temperatures in wall cavities. Pull-out, torque, and shear values into FRP can also weaken over time due to the relaxation of the FRP material. Published data should include not only the initial value but also how that value decreases over time and after repeated temperature and loading cycles.

Greater care also needs to be taken when installing fasteners into FRP compared to steel. Due to the liabilities mentioned above, fasteners should be 2 to 3 times the diameter away from the edge of the material, and the minimum clear distance between fasteners should be five times the diameter.³ This limitation reduces the capability to fasten into or through the FRP. Multiple fasteners rarely fit into the same 1 5/8” wide stud, and different fasteners cannot achieve larger pull-out values for cladding attachment.

Potential Issues with Fasteners in FRP

Material Weakness and Safety Concerns

Fastening into FRP may cause issues in the FRP material itself. For example, drilling holes in composites can decrease the strength of the material by up to 60% at the hole due to creating discontinuous fibers and stress concentrations.¹ Use adequate safety factors for pull-out, shear, and pullover values, and also for the material itself, considering the holes in it.

Increased Safety Factors Required

Four safety factors are typically used for fasteners’ pull-out and shear values in steel. However, steel is a uniform, consistent material, whereas FRP is not. If published values for self-drilling fasteners into FRP do not account for potential liabilities, use safety factors in addition to the typical four. Some FRP values for pull-out decrease by 50 percent or more over time or with typical service temperatures. So, safety factors in the 8 to 10 range may be necessary based on these liabilities alone.

Attachment to Solid Substrates

There is less liability in attaching a fastener through the composite and into a solid substrate, as torque and pull-out are not an issue. Calculate material pull-through for thin flange material, small fastener head bearing surfaces, or unevenly installed products. Provide testing data and include adequate safety factors.

Best Practices for Cladding Attachment

To ensure best practices and a safe, effective cladding attachment, use time-tested and proven materials with thorough, published data available. Installing self-drilling screws into FRP as a joining method introduces numerous damage mechanisms, which makes the load-carrying capacity deteriorate over time. This potentially creates an unsafe design and makes the load-carrying capacity of the fastener unpredictable over time, deviating from initial design intentions and creating opportunities for trailing liability as buildings continue to age.

GreenGirt CMH™ Z-Girts

GreenGirt CMH Z-girts, utilized in GreenGirt CMH continuous insulation and SMARTci building enclosure systems, receive fasteners into its integral composite metal hybrid material, including its continuous metal insert. Therefore, it uses time-proven self-drilling fasteners for strong and effective cladding attachment. Values for pull-out, shear, etc., are the same, if not better, than with its steel counterpart, as the attachment through the FRP into the continuous steel also strengthens the profile. When designing the fasteners for attaching to GreenGirt CMH, use the same published values as those for 16 ga. steel. GreenGirt CMH features a 16 ga., 50 KSI, G90 galvanized steel insert.

As with steel, the GreenGirt CMH cladding connection will retain its torque and pull-out capacity over time. As a result, it will not seriously degrade, whether at room temperature or exposed to harsh environmental conditions. The same applies to attaching to the substrate, which uses an integral continuous metal insert for attachment.

Conclusion

There are many things to consider when choosing the best cladding attachment system for your project. In this article, we covered the typical cladding attachment methods for each z-girt material type, including steel, fiber-reinforced polymer (FRP), and GreenGirt CMH. If you have any questions about which material is right for your project, please don’t hesitate to contact us. If you’re looking for a high-performance best practice cladding attachment system, check out our GreenGirt CMH™ z-girts, GreenGirt CMH continuous insulation, and SMARTci® building enclosure systems – they offer the best of both worlds in terms of strength, durability, and thermal efficiency.

Sources:

Flynn, Susan Keen, and O’Leary, Melissa. “University R&D Advances Novel Ideas to Ground-Breaking Applications,” American Composites Manufacturers Association. “Composites Manufacturing,” July/August 2017, Pages 14-16

“Guidelines and Recommended Practices for Fiber-Reinforced-Polymer (FRP) Architectural Products.” ACMA: American Composites Manufacturers Associations. 2016.

Duthinh, Dat. “Connection of Fiber Reinforced Polymer (FRP) Structural Members: A Review of the State of the Art,” National Institute of Standards and Technology 6532, August 2000.

“Prospect for New Guidance in The Design Of FRP,” European Commission, 2016

“Pre-Standard for Load and Resistance Factor Design (LRFD) for Pultruded Fiber Reinforced Polymer (FRP) Structure,” American Society of Civil Engineers. November 2010
“Structural Plastic Design Manual,” American Society of Civil Engineers. ASCE Manuals Reports on Engineering Practice No. 63, ASCE 1984.