The below is a collaboration from myself and John Wheaton of Wheaton & Sprague Engineering, Inc. We hope you enjoy as we are #Elevating and #Creating Structure.
I remember learning about fastener strength in my statics classes, but I wasn’t exactly enthralled with the topic at the time. It’s such a prominent aspect of façade engineering that I felt I needed to better understand the importance myself. As a result, I hopped on google, did a little research on different types of fastener failure, and sent it all over to John so he could review and educate me.
Below are my initial thoughts, with John’s expertise thrown in, making this a thorough but quick review on the important topic.
So without further ado..
When a metal roof or wall panel is screwed to a structural framing member (like galvanized steel hat channel), that screw is subjected to tensile loads associated with both positive and negative wind pressures. These negative pressures essentially cause suction that pull the exterior cladding away from the wall, thereby putting stress on the screw holding it to the structure (Image #1 below).
That stress can cause the assembly to fail in a few ways (Image #2 below).
One failure mode in thin gaged panels such as gaged roofing or gaged siding is called “pullover.”
In pullover failure, the idea is that the fastener is stronger than the material you’re screwing into the wall, which can cause the cladding material to pullover the fastener head without the screw failing.
Pullover is really what I describe as a “punchout shear” failure through the area of resistance which is the circumference of the screw head x the thickness of the material.
To combat it, many wall panel systems are attached using extruded clips that are thicker than the wall panel itself (Image #3 below).
True, or larger than typical screw heads to increase bearing surface because what’s most important is to get more bearing and broader holding (more circumferential dimension) under the screw and on the panel.
Which thereby increases the negative pressures required to pull the material over the fastener head. Combined with the proper on center fastener spacing, the likelihood of failure occurring is decreased.
Proper engineering analysis uses a design methodology to keep the allowable pullover strength greater than the design strength pullover value.
The type of structural framing is also critical as it must be designed so that fastener pullout failure doesn’t occur either. This is when the fastener (as the name suggests) pulls out from the structural framing all together. This is determined by the tensile strength of the structural framing, and can make all the difference in keeping fasteners attached and panels on the wall.
It is really based on a couple things. It is based on the allowable tensile strength of the fastener, which controls in thicker material. In typical thin gage girts and purlins, the controlling factor is the withdrawal or tear out between the threaded grip and the material.
This is typically the controlling item in panel systems attached to girts or clips and clips attached to 16 or 18 ga studs. There’s quantitative ways to calculate it and also empirical data from manufacturers to provide tabulated values based on testing.
Engineering calculations will specify a fastener, spacing, and structural framing type that dictate the rules for façade panel attachment. Installers must follow those rules to ensure panels stay on the wall.
If you don’t understand, then ASK. Public safety depends on it.
Thanks for reading along- what’d we miss?