Load Specs- understanding manufacturer’s fastener spec
Hello again GBAer’s
I’m having trouble interpreting a load spec for a Simpson fastener (SDWS221000DB). Looking at a table that addresses my specific application (furring strip on 6″ of exterior board foam insulation, 16″ studs), the heading of the table reads:
“Maximum Allowable Cladding Weight to be Supported (psf)”
This seems fairly clear, except doesn’t ‘psf’ refer to a _pressure_? But this wouldn’t make much sense.
Further confusing, the table states that a weight of ’30 psf’ requires a maximum fastener spacing of 24″, but a relatively heavy siding like fiber cement has a weight of about 3 pounds per square foot.
So should I interpret this as meaning that I can hang 320 pounds (30 pounds * 16″ * 96″ / 144 ) off each furring strip?
Thanks in advance for any input!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Gerald,
The situation is confusing because, when it comes to discussing furring strips and siding, there are two different values that are measured in pounds per square foot (psf).
The first relevant value is the weight of the siding. For example, wood siding plus furring strips can be assumed to have a weight of about 1.5 psf, while fiber-cement siding plus furring strips can be assumed to have a weight of about 3.0 psf.
The second relevant value is the design wind pressure that the wall assembly is designed to resist. Information on design wind pressures can be found in Table R302.2(2) of the International Residential Code. Most homes in the U.S. are designed to resist wind speeds up to 90 mph. However, in coastal areas, the design wind speed is much higher; it is 130 mph in some coastal areas and 150 mph in south Florida.
If your design wind speed is 90 mph, and your home is located in a typical suburban or wooded location, your design wind load is 19.5 psf. If your design wind speed is 90 mph, and your home is located in open, flat terrain, your design wind load is 29.1 psf.
All of this, and more, is explained in this article: Fastening Furring Strips to a Foam-Sheathed Wall.
-- Martin Holladay
Wow. Great. Once again, I'm really glad I asked. Makes sense. I'll read the article right now.
Thanks!
Hi, The Simpson catalog page for the SDWS and foam is here http://embed.widencdn.net/pdf/plus/ssttoolbox/vk6kxkteny/C-F-2017-p292-305and314-315.pdf?u=gwur7r _ It does not address the wind loading issue directly other than a footnote on the allowable out-of-plane loading for each fastener.
The FastenMaster tables end at 4" foam. If you are using 6" foam - then it needs to be designed. It looks like the Simpson screw with a diameter of 0.220" is stiffer than the FastenMaster at 0.191" so fewer screws are needed to keep the assembly defection low, Simpson indicates 0.015" deflection
Thanks Tim- that's very helpful.