Contemporary Issues Facing Nail Fasteners
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CONTEMPORARY ISSUES FACING NAIL FASTENERS B.L. Wills D.A. Bender S.G. Winistorfer1 INTRODUCTION Nails have been used for hundreds of years for a variety of purposes, but it was not until the 19th century that nails were mass produced for building construction. In the U.S., the first allowable nail design values were published in 1944 by the National Lumber Manufacturer Association, NLMA. Significant changes in nail fasteners have occurred since the 1930’s and 1940’s, such as modifications in nail manufacturing processes, quality of steel used for nails, and the introduction of new nail types. Historically, when new nail fasteners were introduced, the allowable design values were estimated from similar nails, together with conservative engineering judgement. Today, there is a critical need to update the database on nail performance over a broad range of sizes (6d to 90d) and types (e.g. threaded nails). This need is intensified by the introduction of European yield theory into the 1991 National Design Specification for Wood Construction (NDS). European yield theory requires test data on wood and nails not previously required. The introduction of European yield theory to the 1991 NDS has prompted many engineers and scientists to more closely examine needs for nail fastener performance data, new nail classification system, and new manufacturing and test standards. For example, our current pennyweight classification system does not specify consistent nail diameter or steel strength -- both of which are now needed for design. European yield theory has the potential to improve international harmonization of design codes and standards; however, several discrepancies already exist between different countries with respect to test methods and analysis procedures. This paper is intended to focus attention on problems facing manufacturers, designers and users of nail fasteners, and to recommend approaches for solving these problems. Threaded nail fasteners are emphasized in this paper due to their extensive use in agricultural structures, together with the fact that relatively little performance data are available for these types of fasteners. Nail Manufacturing The use of nails dates back to the days of ancient Egypt, Greece and Rome. It was not until the mid-1800's that the nail manufacturing industry was revolutionized by the development of the automated wire-nail production machine for mass production. In this process, the wire is produced in the steel mills by drawing rod stock (approx. 0.280 inch diameter) through a series of dies to the required nail diameter. Then, the steel wire is compressed to form the nail head and pinched to form the point. Once the nail is formed, it may go through other processes such as mechanical deformation or surface coating application, depending on the end-use requirements. Today, there are approximately 5,000 different types and sizes of nails commercially available around the world, although only approximately 2900 types of nails are used for wood construction applications (Ehlbeck, 1979). These nails not only differ by shank 1The authors are respectively, Graduate Instructor and Associate Professor, Agricultural Engineering Department, Texas A&M University, College Station TX 77843-21 17; and Research Engineer, U.S. Forest Products Laboratory, One Gifford Pinchot Drive, Madison, WI 53705-2398. Wills et al. -- 2 diameter size, length, head type and point type, but also by metal or metal alloy material, shank surface coating and shank deformation. Shank Properties -- Nail shanks can be plain (smooth), threaded or barbed. Common and box nails are plain-shanked. The box nail is usually the same length as the common nail, but with a slightly smaller wire diameter (USDA, 1987). The threaded-shank nail, commonly used in the post-frame and pallet industries, has a mechanically deformed shank with either annular or helical threads as shown in Figure 1. An annularly threaded nail, commonly called a ring-shank nail, has multiple ring-like threads rolled around the shank in planes perpendicular to the nail axis. After the rolling process, the annularly threaded nail will have a smaller root diameter than the original wire diameter. Helical threads are continuous multiple helix depressions rolled into the nail shank with resulting expansion approximately equal to the depression (ASTM, 1992a). The thread of a helically threaded nail runs approximately two-thirds of the nail length, similar to a thread of a wood screw. Since the cross-sectional area before and after the helical nail is formed can be assumed to be the same (equal depression and expansion), the design diameter will be the same as the wire diameter. A barbed-shank nail covers all mechanically, deformed shanks not qualifying as a annularly or helically threaded-shank (ASTM, 1992a). Manufacturing Standards -- Presently, Federal Specification FF-N-105B (1977) is the only standard the nail manufacturers must follow. The nail criteria established by FF-N-105B is that the steel wire shall be of "good commercial" quality and satisfy a certain minimum cold bend angle criteria depending on the carbon content and the hardening process, except for mechanically deformed-shank nails which does not require a cold bend test. In addition to the above requirements, unlike steel wire, aluminum alloy wire must have a minimum ultimate tensile strength. The size and shape requirements in FF-N-105B include pennyweight (d), nail length, diameter or width of head, and wire diameter prior to deformation. The National Design Specification for Wood Construction (NDS) also requires all threaded nails to be of high carbon steel and heat-treated and tempered (NFPA, 1991). The current state of standardization in the nail manufacturing industry permits significant differences between plain-shank and deformed-shank nails. Since FF-N-105B does not differentiate between the size and shape of plain-shank nails and mechanically, deformed-shank nails, the wire diameter of the plain-shank nail varies from 6 to 33% of the threaded-shank nail for the same pennyweight designation (NFPA, 1986) as summarized in Table 1. For example, a 16d plain-shank nail has a wire diameter of 0.162 inches compared to 0.148 inches for a threaded-shank nail. With smaller nails, 8d to 20d, the plain-shank nail diameter is only 6 to 8 % larger than the threaded-shank nail diameter. In larger diameter nails (20d to 60d), the plain- shank nail diameter continues to increase with each increased pennyweight designation (0.192 to 0.263 inches). However, the diameter of the threaded-shank nail remains constant over the same range of pennyweights (0.177 inches). These differences in wire diameter result in lower allowable design values for threaded nails as compared to plain-shank nails. Nail Design Methodology The need for standard design values for wood construction dates back to 1933 when the U.S. Department of Agriculture, Forest Products Laboratory (FPL) in cooperation with the National Lumber Manufacturers Association (NLMA) published the U.S. Department of Agriculture Miscellaneous Publication 185 - "Guide to the grading of structural timbers and the determining of working stress." Over the last forty-nine years, the NDS has gone through several name changes and revisions but the scope has remained the same -- to provide accurate wood design criteria and information to the wood design engineer. Wills et al. -- 3 Approximately two-thirds of the 1991 NDS addresses mechanical fasteners -- nails, spikes, wood and lag screws, bolts and timber connectors (Winistorfer, 1992). A joint fastener can be placed under pure withdrawal, pure lateral (shear) loading or any combination of the two. The NDS only addresses pure withdrawal and pure lateral loading for nail design values. Withdrawal loads place axial forces on the fastener and tries to withdraw the fastener from the wood. Lateral loading places a perpendicular force through the nail axis trying to shear the nail through its cross-section. Withdrawal Design Values -- The empirical equation used to determine the allowable withdrawal design values for nails and spikes has changed only slightly since the 1944 edition. The empirical equation is based on research conducted at the Forest Products Laboratory in 193 1 and revised and updated in 1958 and 1965 to determine the ultimate withdrawal resistance of nails embedded in 54 American softwoods and hardwoods, ranging in specific gravity of 0.32 to 0.74 (FPL, 1965). Based on this research, the original ultimate withdrawal equation is: (1) where: = ultimate withdrawal value per inch of penetration in member holding nail Wu point, lbs 6900 = empirical constant G = specific gravity of member holding nail point (oven dry weight and volume) D = wire diameter of nail, inches The allowable withdrawal design equation was derived from the ultimate withdrawal Equation (1) by dividing the empirical constant by a factor of 5 to account for duration of load, safety and experience, and arriving at the allowable withdrawal load, Wa: (2) During the 1970’s, the U.S. Forest Products Laboratory adopted a policy which required all specific gravity data to be based on oven dry weight and volume at 12% moisture instead of oven dry weight and oven dry volume. Converting the original allowable withdrawal design equation to account for the change in specific gravity gives today’s allowable withdrawal equation for nails and spikes: (USDA, 1987) (3) where: 1570 = empirical constant G = specific gravity based on oven dry weight and volume at 12% moisture D = wire diameter of nail, inches The only two parameters used to determine the allowable withdrawal values are the wood specific gravity and the nail wire diameter. The wood strength, or holding capacity, is directly correlated to the specific gravity of the wood and wire diameter gives an index to the amount of surface contact between the nail and the surrounding wood. As the nail is driven, either by hand or pneumatically, into the wood, the wood fibers are forced outward.