In 1998, APA conducted a dowel bearing strength analysis of the required depth of nail penetration for wood structural panel connections in shear walls and diaphragms. The results of that analysis showed that the lateral connection strength, calculated per the 1997 National Design Specification (NDSi) for Wood Construction, was not affected by going from 1-5/8 inch to 1-1/2 inch for 10d common nails and from 1-1/2 inch to 1-3/8 inch for 8d common nails. Based on that work, new minimum nail penetration requirements were placed in the shear wall and diaphragm tables in the 2000 International Building Code (IBC).
The analysis conducted by APA is covered fully in Nail Penetration and Framing Specific Gravity Factors for Shear Walls and Diaphragms. The same analysis procedure is repeated in this paper but the 2001 NDS provisions are followed. The results from the 2001 NDS show that even smaller penetrations will still provide full lateral connection strength, making the recommendations in the IBC conservative.
APA Nail Penetration Guideline Background
Before the 1991 National Design Specification (NDS), nail lateral load design values were based on a simple empirical equation. The empirical equation predicted the design load of a single-nail connection which was defined as the load at which the fastener deformation (joint slip) was 0.015 inch. According to the Wood Handbookii, the empirical equation was intended to apply to connections in which the thickness of the side member was at least half the penetration of the nail in the main member. In many cases, a joint which contains wood structural panels may have a side member that is less than was assumed when the original empirical equation was developed. According to the Wood Handbook, the recommended minimum penetration was implemented “to maintain a sufficient ratio between ultimate load and the load at 0.015 inch deformation.” Eleven nail diameters (11 x D) penetration was required for lateral connections of Douglas fir-larch or southern pine. Other wood species had other (greater) required minimum penetrations. Since the basis of the shear wall and diaphragm tables are Douglas fir-larch and southern pine lumber, 11 x D penetration was applied to the tables. The diameter for a 10d common nail is 0.148 inch; thus the required penetration for full capacity of the connection was taken as: 11 x 0.148 inch = 1.628 inches which rounds to 1-5/8 inches.
Changes in the NDS
The 1991 NDS made significant changes to the method by which connections are designed. These changes were based on work conducted by the U.S. Forest Products Laboratory. New equations were introduced which accounted for the bearing strength of the side and main member, and the yield strength of the nail. Another significant change was that the minimum penetration was slightly increased to 12 nail diameters (12 x D) regardless of the species of lumber. The 12 x D minimum penetration was implemented as a conservative simplified approach (the previous version of the NDS had various minimum penetration depths for different lumber species). Unfortunately, the change caused a conflict with the minimum penetration (11 x D) specified in the shear wall and diaphragm tables. The 1997 version of the NDS made no further changes in the method with which connections are designed. For nailed connections, four yield modes were checked. The design value for a single nail is based on the minimum of the four yield modes. If the penetration is less than 12 x D, but greater than 6 x D, then a depth of penetration factor, Cd, was used. Now the 2001 NDS has slightly changed calculations so that all six yield mode equations are used and the depth of penetration factor is removed. The reason for no longer using the depth of penetration factor is that the dowel bearing length of the member receiving the nail is accounted for in the yield equations. It should also be noted that Tables 11Q and R of the NDS footnote that 10D penetration is required, however the provisions of the NDS have no such requirement and the table footnote appears to be provided for conservatism. For consistency, the 2001 NDS yield equations as found in the provisions, not the tablulated values, are used in this analysis.
Because APA has no control over quality of workmanship or the conditions under which engineered wood products are used, it cannot accept responsibility of product performance or designs as actually constructed. Consult your local jurisdiction or design professional to assure compliance with code, construction, and performance requirements.
American Forest & Paper Association. National Design Specification for Wood Construction. AF&PA. Washington, D.C.
Wood Handbook: Wood as an Engineering Material. 1987. Agriculture Handbook 72. U.S. Department of Agriculture. Washington, DC.
Aune, P. and Patton-Mallory, M. 1986. Lateral Load-Bearing Capacity of Nailed Joints Based on the Yield Theory: Theoretical Development. Res. Pap. FPL 469. USDA Forest Service, Forest Products Laboratory. Madison, WI. iv