This week’s post was written by Bob Leichti, Manager of Engineering for Fastening Systems. Prior to joining Simpson Strong-Tie in 2012, Bob was an Engineering Manager covering structural fasteners, hand tools, regulatory compliance and code reports for a major manufacturer of power tools and equipment. Prior to that, Bob was a Professor in the Department of Wood Science and Engineering at Oregon State University. He received his B.S. and M.S. from the University of Illinois, and his M.S. and Ph.D. from Auburn University.
When test results don’t make sense, we start by eliminating causes of the problem. When our withdrawal test values came up low, we checked the load cell calibration, the specific gravity of the wood, the nail dimensions, even the units – everything was correct. So why were the nail withdrawal values so low? More wood, more nails, more tests – same results. Ultimately, we concluded that the withdrawal resistance of stainless-steel, smooth-shank nails is not well described by the withdrawal function in the 2012 NDS, section 11.2.3, equation 11.2-3.
Withdrawal resistance for smooth-shank nails can be either calculated using the 2012 NDS nail withdrawal function or one can refer to the 2012 NDS Table 11.2C, which provides withdrawal design values for smooth-shank nails (common and box nails and common wire staples). The design function produces reference allowable withdrawal resistance in pounds of force per inch of penetration into the main member, and several research papers have shown that total withdrawal resistance increases linearly with penetration depth. In 2012, an equation was introduced for ring-shank post-frame nails. See 2012 NDS, section 11.2.3, equation 11.2-4.