Introduction
Self-drilling fasteners are versatile fastening devices that can be used to attach many different materials together. Self-drilling fasteners are generally used when it is impossible to pre-drill holes or where it would be challenging to use traditional fasteners. They are designed to create their own hole as they are driven into the material, making them ideal for quickly and securely fastening materials in various settings.
While self-drilling fasteners have been used in the construction industry for decades, their performance and reliability in various materials can be taken for granted. Their use with steel has been proven over time and with testing, but the same cannot be said of 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 composite metal hybrid (CMH).
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 is readily available from most fastener manufacturers for different gauges of steel. In designing attachment methods, this 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
Discontinuity in the fibers weakens the sectionThe 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.
Any values published for self-drilling fasteners being used in an FRP material should take all the potential liabilities into account. Service temperatures in wall cavities can approach 190⁰ Fahrenheit, so testing of pull-out should be done up to and including 190⁰. 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 can limit the capability of fastening into or through the FRP, as multiple fasteners rarely can be used into the same 1 5/8” wide stud, and various fasteners cannot be used to achieve larger pull-out values for cladding attachment.
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 Adequate safety factors must be used not only for the pull-out, shear, and pullover values but also for the material itself, considering these holes in the material.
Safety factors of four are typically used on fasteners’ pull-out and shear values into steel. However, steel is a uniform, consistent material, whereas FRP is not. If published values for self-drilling fasteners into FRP are not taking potential liabilities into account, safety factors on top of the typical four should be used. With some FRP values for pull-out decreasing by 50 percent or more over time and/or with typical service temperatures, safety factors in the 8 to 10 range may have to be used based on those two liabilities alone.
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. However, calculations must be made for material pull-through if the flange material is thin, if the fastener head bearing surface is small, or if the product is unevenly installed. Testing data should be provided, and adequate safety factors should be included.
To ensure best practices and a safe, effective cladding attachment, it is best to use materials that are time-tested and proven and of which there is 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.
Composite Metal Hybrid (CMH™) Z-Girts
Composite metal hybrid (CMH™) receives fasteners into its integral 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 used for attaching into CMH, the same published values as is used for 16 Ga. steel can be utilized for CMH, as it has a 16 Ga., 50 KSI, G90 galvanized steel insert.
As with steel, the 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 is true for attaching to the substrate, as an integral continuous metal insert is used for attachment into the substrate.
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 composite metal hybrid (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® composite metal hybrid (CMH™) z-girts and SMARTci™ continuous insulation systems – they offer the best of both worlds in terms of strength, durability, and thermal efficiency.
© 2023 Advanced Architectural Products
Sources:
1. 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
2. “Pre-Standard for Load and Resistance Factor Design (LRFD) for Pultruded Fiber Reinforced Polymer (FRP) Structure,” American Society of Civil Engineers. November 2010
3. “Structural Plastic Design Manual,” American Society of Civil Engineers. ASCE Manuals Reports on Engineering Practice No. 63, ASCE 1984.
