Installation & Fastening of Wood Structural Panel Wall Bracing
Installation & Fastening of Wood Structural Panel Wall Bracing
In light of the recent collapse of a condominium building under construction in the Briar Creek area of Raleigh, NC, a closer examination of the design/installation of sheathing for braced wall lines may be needed. There are quite a few unknowns about the stage of the construction process and how installation was implemented in the case of this collapse. However, it certainly provides an excellent example of the importance of the application of connectors because it is likely that the fastening/connections in the structure played a prominent role in the collapse. An understanding of how fastener size and installation method can affect the strength of shear walls (a.k.a., braced wall panels) is critical to providing proper wall bracing.
What effects can installation methods and fastener sizes have on the strength of wood structural panel (WSP) wall bracing?
A variety of nail sizes and spacing can be used to attach WSPs to the framing members to provide lateral resistance bracing. The construction documents for each project should clearly indicate the diameter and length of the nails to use for each shear wall, as well as the spacing of the fasteners around the perimeter and in the field of the panels (e.g., 8d [2-½" x 0.131"] @ 6" o.c. max. at all panel edges and 8d nails @ 12" o.c. max. at all intermediate studs).
Specifying only the penny weight of the nail is not sufficient because there are usually several different diameters and lengths for a given size. For instance, a 6d nail could be a 6d common (2" x 0.113"), a 6d box (2" x 0.099"), or a 6d sinker (1-7/8" x 0.092"). It is typical for a gun nail with any of these sizes to be called a 6d nail. For a building constructed in accordance with the International Residential Code (IRC), changing from a 6d common (2" x 0.113") nail to a 6d box (2" x 0.099") nail would require the fastener spacing to decrease from 6" o.c. along panel edges and 12" o.c. in the field to 3" o.c. along panel edges and 6" o.c. in the field. If the framer uses the 6d box (2" x 0.099") nail without reducing the fastener spacing appropriately, the building may not have sufficient lateral resistance to withstand the design wind forces.
Unlike the prescriptive fastening provisions of the IRC (e.g., 6d common [2" x 0.113"] at 6" o.c. edge and 12" o.c. field), the Special Design Provisions for Wind and Seismic (SDPWS) allows for either common or galvanized box nails to be used to fasten WSP sheathing with no change to the fastener spacing or design values. A 6d common nail has a shank diameter of 0.113", while a 6d galvanized box nail has a diameter of only 0.099". Similarly, an 8d common nail has a shank diameter of 0.131", compared to an 8d galvanized box nail that has a diameter of only 0.113". Compared to the corresponding common nail diameter, there is about a 12 percent and 14 percent reduction in the shank diameter (0.99/0.113 and 0.113/0.131) of 6d and 8d galvanized box nails, respectively.
Using the National Design Specification (NDS) yield limit equations for dowel-type fasteners, the lateral load resistance for 7/16" OSB sheathing fastened to an SPF framing member drops by 19 percent and 22 percent when 6d and 8d galvanized box nails are used instead of 6d and 8d common nails, respectively.
To help resolve this discrepancy, data from industry shear wall tests conducted by the SBC Research Institute (SBCRI) was examined to provide a better understanding of the sheathing-to-framing fastener performance.
Wall Bracing Tests
SBCRI* performed 49 tests of segmented shear walls sheathed with 3/8", 7/16", or 15/32" Sheathing Category OSB fastened with either 8d common (2-½" x 0.131") or 8d box (2-3/8" x 0.113") nails. The wall framing consisted of SPF studs spaced 16" o.c. because this is a common construction in light-frame buildings. The nails were spaced precisely at 6" o.c. along the panel edges and 12" o.c. in the field because SBCRI staff marked the fastener locations on the panel. The nails were installed using a minimum of a 3/8" edge distance that was chalk-lined on the panels so each nail had SDPWS, Wood Frame Construction Manual (WFCM), IRC and IBC code-compliant installation. Obviously, this type of test lab framing is more precise than field framing. Photos of the six different setups used to conduct the shear wall tests are shown in Figures 2 through 4.
Figures 5 and 6 provide a histogram that shows the distribution of the data for each of the nail sizes. Table 1 shows the variation in the shear wall capacity for the two different nail types used in the segmented shear wall tests using near perfectly installed connectors. As can be seen from Figures 5 and 6, there is a significant amount of variability in the shear resistance capacity of the tested WSP walls.
The SBCRI testing minimized the variation in the results by carefully controlling the tested materials, construction, and boundary conditions for each shear wall test set-up. SBCRI staff always chalk-lined the panel edges to ensure the sheathing fasteners were placed a minimum of 3/8" from the panel edge per SDPWS. Any shiners (when present) were removed and a new fastener was installed. The studs were always straight and spaced precisely. In other words, these walls represented an ideal case from a construction practice perspective. It is expected that the variability in the SBCRI tests represent quality control issues with the OSB sheathing, wood framing, and nail specifications. Greater variability than is shown in Figures 5 and 6 will likely exist in real-world applications using common OSB field construction practices for shear walls.
This SBCRI testing shows that, on average, there is about a 20 percent decrease in the lateral load resistance when 8d galvanized box (2-3/8" x 0.113") nails are used instead of 8d common (2-½" x 0.131") nails. Notice that this difference is very similar to the 23 percent decrease between the design values for 6d common (2" x 0.113") nails vs. 8d common (2-½" x 0.131") nails in 3/8" OSB given in SDPWS.
Of the 29 SBCRI shear wall tests with the 8d box (2-3/8" x 0.113") nails, only one test met the published SDPWS nominal unit shear capacity of 672 plf (see Figure 5).
As currently written, Table 4.3A in SDPWS shows that a 3/8" WSP shear wall with 8d galvanized box (2-3/8" x 0.113") nails and a 3/8" WSP shear wall with 8d common (2-½” x 0.131”) nails both have the same nominal unit shear capacity of 672 plf (730 plf multiplied by 0.92 for DF to SPF reduction).
However, as shown in Figure 5, the nominal unit shear capacity for the 8d galvanized box (2-3/8" x 0.113") nail has a median value between 500 and 525 plf, about 150 plf less than the design value given in SDPWS.
Since a 6d common (2" x 0.113") nail has the same diameter as the 8d galvanized box nail (2-3/8" x 0.113"), the test results in Figure 5 were compared to the SDPWS design value for 6d common nails. Table 4.3A in SDPWS gives a nominal unit shear capacity of 515 plf (560 plf times 0.92 for DF to SPF reduction) for a 3/8" WSP shear wall with 6d common (2" x 0.113") nails. This is comparable to the median shear capacity for the shear walls with 8d galvanized box (2-3/8" x 0.113") nails.
It seems that the 8d galvanized box (2-3/8" x 0.113") nail and the 6d common (2" x 0.113") nail should have the same design value as the only difference between the two fasteners is the 3/8" greater fastener penetration of the 8d galvanized box nail. However, the provisions of SDPWS inaptly state otherwise.
It is clear that the design values used for fastening systems in OSB shear walls need to be seriously reviewed and updated. The concern is that there is a high degree of variability in both nails and OSB that may cause unintended WSP design value consequences that are typically unknown and unappreciated by the professional engineering and building design community, and further, this result occurs under ideal laboratory construction conditions. This clearly should be an issue of serious importance to APA-The Engineered Wood Association and to the SDPWS and WFCM ANSI standards developer, the American Wood Council (AWC).**
The lack of a reduction in strength for the smaller nail diameter of box nails is a position that APA has recognized. In the publication entitled “Shear Wall Test Results Comparing 8d Common and 8d Box Nails” (TT-087B), APA states the following regarding the performance of 8d box and 8d common nails:
Furthermore, this APA publication states:
The referenced APA publication finally concludes that:
APA’s viewpoint is that they have not observed a difference in the shear wall performance with 8d box versus 8d common nails. Hence, the same shear wall design value can be used for either nail type.
Although the results of the SBCRI shear wall tests have been provided to APA, there have been no changes made to SDPWS. APA’s position is based on testing of common and box nails with a maximum fastener spacing of 4" o.c. around the panel edges and 6" o.c. in the field (APA Research Report T2004-14***).
SBCRI tested shear walls with the same fastener spacing (4" o.c. around the panel edges and 6" o.c. in the field) as the APA shear wall tests to verify the results. The bar chart in Figure 7 shows the results of the APA and SBCRI tests. The results are similar except for the 8d galvanized box nails. The 8d hot-dipped galvanized box nails used by SBCRI resulted in an average decrease of 12 percent compared to the 8d common nails.
The SBCRI test results suggest that the nominal unit shear capacity of the wall sheathing is highly dependent on the fastener type and fastener itself, when all other factors are controlled. This includes but is not limited to:
1. Fastener edge distance
2. Fastener steel yield and ultimate strengths
3. Fastener specifications, diameter tolerances, and gun nail glues
4. Specific fastener installation instructions from the WSP and/or fastener supplier or both, including number of allowable shiners
5. Field installed fastener inspections by building officials
6. Unexpected new fasteners replacing existing fastener types, while being called similar names
Currently, the primary nail used in the field is the 8d galvanized box (2-3/8" x 0.113") nail. In SBCRI’s experience, gun-driven 8d common (2-½" x 0.131") nails can only be obtained by special order from a nail/nail gun supplier. Also, in the normal field environment, there is little or no quality control on the required minimum edge distance for fasteners, and no guidance is available on the number of allowable shiners.
This means that the majority of the WSP shear walls constructed have a shear capacity where there is a high degree of shear wall design value variability. This is due at least in part to all the items listed above, and there may be more that come to light with further testing that is more realistic with respect to actual field application conditions. All of this may lead to unintended WSP design value consequences that are typically unknown and unappreciated by the professional engineering, building design and specification community.
There is the very real potential that actual design values are significantly below the nominal unit shear capacities divided by a factor of safety of 2 as given in SDPWS, WFCM, the IBC and the IRC. These inaccuracies reduce the expected factor of safety for structures using WSPs or, in other words, take advantage of the building system effect. Without clear installation information and realistic design values, a designer’s ability to provide adequate lateral load resistance in IRC- or IBC-compliant structures could easily have unintended consequences attached to unknown and unappreciated application conditions that would affect the design value used or result in a much lower than expected overall building factor of safety. Having accurate design value knowledge and design values will allow for much better engineering judgments to be made and increase the value of engineering, engineers and innovative engineered design.
*This testing was funded by Qualtim, Inc. Qualtim has granted SBCA/SBCRI the exclusive right to use this data to thereby improve the SBC industry’s knowledge regarding shear wall performance, to enhance the design and use of wall panels and provide a foundation for innovation within the engineered wall panel marketplace.
**Since August 2011, when SBCA and SBCRI were certain their testing was consistent and repeatable, they have provided their findings to the market. This includes sending data and conclusions to interest groups such as APA, AWC, ICC-ES, and ICC. Since neither test data, nor an analytical response correcting these findings has ever been received, SBCA and SBCRI believe the testing/analysis presented here is precise, accurate and legitimate. This information has been provided as a public service to the professional engineering and specification community in order to be fully transparent. The goal is to provide facts, backed by empirical test data, so that wiser engineered design decisions can be made. For all past correspondence regarding OSB shear wall (a.k.a., braced wall panel) performance, click here.