When Simpson Strong-Tie set out to re-engineer their Strong-Drive® WSV Subfloor screw, the plan was to bring a code-evaluated, no-squeak, no-glue all screw replacement for sheathed horizontal wood-frame diaphragms to the market. We knew that these collated fasteners, which were designed for fast subfloor installation using the auto-feed Quik Drive® system, were going to be an ideal replacement for 8d and 10d nails. The only thing left to do was to prove it…
To start, the WSV Subfloor screws are the first fasteners to be dual-evaluated per ICC-ES AC233 and ICC-ES AC120. Tests of single fasteners and full-scale diaphragms, conducted at Simpson’s Tye Gilb Lab, were used to prove that the WSV Subfloor screws have lateral, shear and withdrawal characteristics that exceed those of 10d common nails. ICC-ES Evaluation Report ESR-1472 provides design values for Simpson Strong-Tie WSNTL and WSV screws used in the following applications:
- In engineered horizontal wood structural panel (WSP) diaphragms
- In engineered diagonally sheathed lumber diaphragms
- For prescriptive attachment of WSP to wood framing
- For prescriptive connections described in ESR-1472
In addition to the work presented in ESR-1472, Simpson Strong-Tie also provides engineering letters that address the Simpson Strong-Tie® Strong-Drive® WSV SUBFLOOR Screw as Alternates to 8d & 10d Subfloor Nail and Simpson Strong-Tie® Strong-Drive® WSVF SUBFLOOR Screws – Corrosion Resistance. Closer inspection of the engineering letter dealing with screw alternatives to subfloor nails provides a list of 1-to-1 replacements for a specified nail size given a subfloor sheathing thickness. Footnote #2 of the engineering letter states the following:
Applies to 2021, 2018, and 2015 IBC Section 2306.2 Wood Framed Diaphragms and 2021 SDPWS Tables 4.2A, 4.2B, 4.2C and 4.2D; 2021, 2018, and 2015 IRC Table R602.3(1). WSV134 not to be used in diaphragms designed per SDPWS Tables 4.2B, 4.2D, or connections in accordance with IRC Table R602.3(1).
The only thing left to do is to take the values for allowable shear found in Table 3 of ESR-1472 and compare those values to shear capacities reported for nailed diaphragms in the 2021 SDPWS. As a starting point for direct comparison, it’s important to recognize that the values published in ESR-1472 Table 3 are allowable strengths (ASD) and should be compared to service-level (unfactored) applied loads in design or ASD-level capacities when demonstrating equivalency.
Let’s consider a situation where 3/8″ structural I panels are attached to 3″-wide framing members at adjoining panel edges and boundaries with WSV134S screws in place of 8d nails. It’s also relevant to point out that section 4.1.1.1 of ESR-1472 allows a 40% increase in the values published in Table 3 for wind design. The allowable shears for the panel described and attached with WSV screws are the following:
Seismic: 300, 400, 600, or 675 plf
Wind: 420, 560, 840, or 945 plf
Next, we need to calculate the ASD allowable shear capacity for the nailed diaphragm from the nominal unit shear capacities provided in Table 4A of the 2021 SDPWS. Pay attention here, because these nominal values are neither ASD nor LRFD; so even though it’s tempting to make direct comparisons, we can’t do that. Footnote 1 takes us to section 4.1.4 of the SDPWS where we can see that for seismic design, the ASD allowable shear capacity is determined by dividing the nominal shear capacity by the ASD reduction factor of 2.8. Similarly, for wind design, the ASD reduction factor of 2.0 should be used.
The equivalent nailed condition from Table 4.2A, which uses 8d nails at the same spacing and configuration, produces nominal unit shears of 840, 1120, 1680, or 1890 plf. Now to turn these into ASD allowable shear capacities, we need to divide by 2.8 for seismic or 2.0 for wind, which would result in the following values:
Seismic: 300, 400, 600, or 675 plf
Wind: 420, 560, 840, or 945 plf
So as you can see, we have to factor the values from each table in order to compare them to one another. This process has produced identical values, demonstrating that WSNTL and WSV screws can be used as a 1:1 replacement for nails in engineered horizontal wood structural panel diaphragms.
