Rainscreens are an integral part of building enclosure systems. With a focus on water and moisture protection, rainscreens prevent damage to insulation that occurs from moisture over time. To ensure that rainscreens can effectively protect from moisture damage, they must be installed properly, specifically with the correct sub-framing system and fasteners, to ensure permanent structural integrity. While some architects have previously turned to fiber-reinforced polymer (FRP) products for rainscreen systems, the lack of metal in the system’s design leads to “unavoidable failure” in fastener connectivity, based on guidance in the “American Society of Civil Engineers’ Design Guide for FRP Composite Connections.” The best solution for permanent fastener retention is a steel-to-steel connection provided by steel or composite metal hybrid Z-girts.
What is ‘unavoidable failure’?
Unavoidable failure is defined as “connection-related failures that occur in FRP components when they are fastened like steel.” This leads to rapid stiffness and strength degradation over time. FRP cannot be fastened in the same way as steel and CMH Z-girts because they lack the metal component that fasteners require for a strong, permanent connection. This is crucial for rainscreens because durability, structural integrity, and building health are impacted by how fasteners are connected to the building’s structure. To ensure a permanent attachment, rainscreen fasteners must be connected to metal, either from steel Z-girts or the metal flanges of CMH Z-girts.
The causes of ‘unavoidable failure’
There are three major causes of “unavoidable failure” in FRP fastener retention: steel-like connections, self-tapping screws, and drilling holes. These practices lead to irreversible damage to the FRP rainscreen over time, decreasing the building’s health and leading to expensive repairs to the building’s facade.
Steel-like connection in FRP
Treating FRP like steel Z-girt products is a major cause of “unavoidable failure.” Connecting fasteners to FRP in the same way that fasteners are connected to steel will cause rapid stiffness and strength degradation of pultruded members. The FRP Z-girts will crack and become brittle over time, causing significant damage to the building’s health and requiring major repairs to the building’s facade.
Self-tapping screws
Self-tapping screws are not recommended for pultruded FRP as an appropriate fastener connection. These connections will not last long and will decrease the durability of the FRP rainscreen system. According to ASCE, FRP products should only be fastened in the following three ways: (1) bolted, (2) adhesively bonded, and (3) combined (bolted and bonded). Any other method of fastening into FRP will cause unavoidable damage to the rainscreen sub-framing system.
Drilling holes
According to Composites Manufacturing Magazine, drilling holes in FRP creates failure initiation points and weakened joints. These holes sacrifice up to 60 percent of the load-carrying capacity. Once the hole is drilled into FRP, the fibers become discontinuous and create stress concentrations around the hole, causing delamination and weakening of the FRP joint. These failure initiation points cannot be fixed without replacing the entire system since the rainscreen will continue to degrade around the drilled holes.
Solutions to fastener retention: Steel and CMH Z-girts
To successfully provide permanent fastener retention to building facades, rainscreen systems that utilize steel or CMH Z-girts are fundamental. These two systems are the most advantageous methods for permanently fastening rainscreen systems to building structures, ensuring that “unavoidable failure” will not happen over the building’s lifetime. Using metal in both systems, which establishes a steel-to-steel connection with the rainscreen and the structure, reinforces that the system will not degrade or crack over time. Not only are steel and CMH Z-girts successful at establishing a permanent fastener connection, but both systems enhance the durability and structural integrity of buildings because of the presence of metal in both systems.
While both steel and CMH sub-framing systems for rainscreens provide permanent fastener retention and durability, CMH excels in strength and thermal efficiency. CMH is designed with fiberglass and metal to meet high structural load demands while effectively preventing thermal bridging. CMH is the ultimate blend of the benefits of steel and fiberglass Z-girts, developing the best possible solution for permanent fastener retention, structural integrity, and thermal efficiency.
Conclusion
Fasteners require metal to provide the ultimate steel-to-steel connection that promotes permanent structural integrity on rainscreen systems. Steel and CMH Z-girts are effective rainscreen sub-framing systems that provide permanent metal connections that will not degrade over time. CMH combines the durability of steel with the thermal efficiency of fiberglass to achieve permanent structural integrity and thermal efficiency all in one system.
Resources
Mosallam, Ayman. Design Guide for FRP Composite Connections. ASCE Manuals and Reports on Engineering Practice No. 102, 2011.
Krause, Gary M., Krause, Matthew, Walsh, Sean, Kukkonen, Zach (2023). Fastener Failure in FRP vs. Permanent Fastening in Steel and GreenGirt CMH® (Composite Metal Hybrid) for Construction Framing
ASCE Design Guide for FRP Composite Connections, pages 19, 371-372
Composites Manufacturing Magazine, Major Factors Affecting the Long-Term Performance of Existing Pultruded Structural Systems, July/August 2017, page 15
Zach Kukkonen is a design engineer at Advanced Architectural Products. He earned his BA in Journalism from the University of Wisconsin in 2007 and his BS in Civil Engineering from Michigan Technological University in 2012. He has spent 12 years in the composite sub-framing and window industries, specializing in structural analysis, product application, and thermal analysis. Kukkonen has been involved in all facets of composites—from testing and structural analysis to pultrusion and installation.
Kevin Mlutkowski joined Advanced Architectural Products in 2023, bringing more than 20 years of experience in expanding market opportunities within the fields of structural engineering, fire protection, and green building. He holds a BS in Technical Communications with a minor in Chemistry from Lawrence Technological University, as well as an MBA from Oakland University. In his current role, Kevin leads efforts to expand the markets and strengthen the brands for GreenGirt CMH continuous insulation (c.i.) systems and SMARTci building enclosure systems.