B.L. Wills D.A. Bender S.G. Winistorfer 1

Transcription

1 CONTEMPORARY ISSUES FACING NAIL FASTENERS B.L. Wills D.A. Bender S.G. Winistorfer 1 INTRODUCTION Nails have been used for hundreds of years for a variety of purposes, but it was not until the 19 th 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 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 1 The authors are respectively, Graduate Instructor and Associate Professor, Agricultural Engineering Department, Texas A&M University, College Station TX ; and Research Engineer, U.S. Forest Products Laboratory, One Gifford Pinchot Drive, Madison, WI

2 Wills et al 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 inches compared to 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 plainshank nail diameter continues to increase with each increased pennyweight designation (0.192 to 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 "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.

3 Wills et al 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 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: where: W u = ultimate withdrawal value per inch of penetration in member holding nail 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, W a : 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) 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. This causes the nail to be wedged between the adjacent wood fibers, giving the nail withdrawal resistance properties. (1) (2) (3)

4 Wills et al Any broken wood fibers caused by nail driving will provide insignificant nail holding capacity (Stern, 1978) NDS Lateral Design Values -- From conception through the 1986 NDS, relatively little change occurred in the allowable lateral nail design equations. The empirical equation for allowable lateral design values was based on research conducted at the U.S. Forest Products Laboratory during the 1930 s (AFPA, 1993) as follows. (4) where: Z K D = allowable lateral load at in. deformation, lbs = empirical constant, based on specific gravity of wood = 2040 Group I G = 0.62 to 0.75 = 1650 Group II G = 0.49 to 0.55 = 1350 Group III G = 0.42 to 0.49 = 1080 Group IV G = 0.31 to 0.41 = nail shank diameter, in Similar to withdrawal design values, the allowable lateral design values for nails are based ultimate strength data, adjusted for duration of load, safety and experience, by a factor of 1/5 for softwoods and 1/9 for hardwoods. The proportional limit in this early research was defined as inch of joint deformation as illustrated in Figure 2. The inch proportional limit was determined by methods specified in American Society for Testing Materials (ASTM) D1761 which specifies a plain-shank, 8d common wire nail. All other types and sizes of nails are assumed to have the proportional limit occur at approximately the same deformation. There has been little research test the validity of this assumption. Another limitation of the empirical equation is its failure to consider different nail-shank geometries, e.g. threaded nails. In 1962, the NDS first recognized the use of threaded hardened-steel nails and established their allowable lateral design values. Problems with Design Methodology -- Through field experience, the threaded nail is believed to provide increased strength over the common nail (Geisthardt, et al., 1991). However, code writing agencies do not have a comprehensive database on threaded nail strengths to base such a conclusion. Also, the limited research preformed in threaded nails shows that discrepancies exist in the thread geometry which significantly affects the strength. Since the original nail data were collected using common nails, and there has not been a comprehensive data collection for threaded nails, the threaded nails were assigned the same allowable design values as common nails of the same pennyweight (NFPA, 1986). As previously mentioned, the wire diameter of threaded-shank nails are slightly smaller than the wire diameter of the plain-shank nail for the same pennyweight designation (Table 1). Since the wire diameter has a direct influence on the allowable design values, the larger diameter of the plain-shank nail is assumed to directly offset the increased strength related to the deformation of the threaded nail. Also, the larger threaded nail (greater than 20d) diameters do not continuously increase as those of common wire nails. Therefore, a ratio of common-to-threaded nail diameter was used to assign the allowable design values for threaded nails. For example, 30d to 60d threaded nails were given the same values as the 20d common nail and the 70d to 90d threaded nails were given the same value as the 40d common nail. Figures 3 and 4 depict withdrawal and lateral design values for smooth and threaded nails, respectively. The NDS does recognize the superior performance of threaded nails compared to smooth nails under conditions of variable wood

5 Wills et al moisture content. During cyclic moisture conditions, wood fibers expand and contract losing the majority of the holding power for plain-shank nails, resulting in a 75% reduction in allowable withdrawal design value (NFPA, 1991). When using a threaded nail, the wood fibers remain locked into the nail threads and no decrease in allowable design value is specified. Besides the differences between threaded-shank nails and plain-shank nails, limited research shows significant differences within each type of threaded-shank nail. For example, thread angle and geometry vary widely among threaded nails (thread angle is defined in Figure 1). A low-thread-angle (less than 20 ) performs similarly to annularly threaded nails. When driven, the wood fibers are wedged between the helical threads which slightly increases Withdrawal resistance. When driving medium-thread-angle (45 to 65 ) nails, the nail will rotate or thread itself into the wood fibers reducing the amount of wood fiber damaged, similar to the principle of wood and lag screws. A medium-thread-angle provides the greatest increase in withdrawal resistance over the plain-shank nail. A large-thread-angle (greater than 65 ) nail, like the medium-thread-angle nail, will rotate into the wood, except with the longer helix the nail has the tendency to back out under axial forces. Thread geometry, the depth of depression and expansion of metal, also has an impact on the withdrawal resistance. White and Gale (1990) and Ehlbeck (1976) showed that thread expansion, the difference between crest diameter and wire diameter, varies from to inches representing approximately 123 % difference in withdrawal resistance. The optimum helically threaded-shank nail should have a medium-leadangle with a thread expansion distance of inches (Ehlbeck, 1979). Similar experiences occur in the manufacturing of annularly threaded-shank nails. During an informal survey of nail manufacturers, Winistorfer (1992) found the root diameter to vary between 70 to 85% of the wire diameter. The root diameter has a major influence when calculating the allowable lateral design values with the threaded portion in the shear plane. Other Limited Research -- The threaded-shank nail is favored in demanding applications, such as in post-frame construction, due to the superior withdrawal resistance. Limited research shows the ultimate withdrawal resistance of a in. diameter threaded spike is approximately twice the resistance of a larger in. diameter plain-shank spike when driven into dry lumber (Ehlbeck, 1973). Other studies display similar results for withdrawal resistance (Stern, 1963; Ehlbeck, 1976; Quackenbush, 1977). Stern (1977) conducted a study using 8d, 12d, 16d, 40d and 60d helically threaded nails. He observed exponential increases in ultimate lateral strength with each increase in wire diameter. With the smaller diameter nails (8d to 16d), the ratio of ultimate-to-allowable lateral load was 7:1 and the 40d and 60d nails had ratios of 8.6:1 and 9.3:1, respectively. Since a safety factor of 5 generally is considered adequate (AFPA, 1993), the helically threaded nail allowable values appear to be conservative. Hoadley (1977), like Stern, observed ultimate lateral values of annularly threaded nails with and 0,300-inch shank diameter and compared them to lag screws with similar diameters (0.243 and inch). Hoadley observed the annularly threaded nails carried 28% more lateral load than the corresponding lag screws. Since Hoadley and Stern did not observe the lateral load at inch deformation, these values cannot be directly used for design. Although, the results do attest to the increased strength provided by threaded nails. Recommendations -- Since threaded nails were first recognized in the 1962 edition of the NDS, there has been little research to fully understand the withdrawal and lateral performance. Since Federal Specification FF-N-105B does not have specific requirements for threaded nails, each manufacturer fabricates their nail threads slightly differently to extend die and roller life and to reduce manufacturing and maintenance costs. Limited research has shown commercially available threaded nails vary significantly in thread angle, thread expansion and root diameter -- each of which has a significant impact on strength (Ehlbeck, 1976; Ehlbeck

6 Wills et al ; White and Gales, 1990; Winistorfer, 1992). Research is needed on threaded nails to characterize the effects of these variables on the lateral and withdrawal strength. Then, a minimum standard threaded nail geometry, including the maximum thread angle and minimum thread expansion and root diameter, is needed before the NDS can recognize their apparent superior strength. EUROPEAN YIELD THEORY The 1991 NDS edition contained major changes in the allowable lateral design values. The equations are now based on the European yield theory developed by Johansen (1949). Johansen used theoretical models to predict the shear transfer through a bolted connection of wood members. The models have been expanded to include all dowel connections. The yield model describes the yield strength of a single dowel-fastened connection. The NDS defines this yield point as the 5% offset yield load. The 5% offset yield load is found by drawing a line parallel to the initial stiffness of the joint connection offset by 5% of the fastener diameter as shown in Figure 2. This yield point falls between the proportional limit and the ultimate strength. The offset value is then reduced by a conversion factor to arrive at the new allowable design value. The 5 % offset is a better prediction of the joint performance than the assumption of the proportional limit at inch deformation since the offset value describes the individual specimen, and is not generically assumed at inch deformation. For example in Figure 5, a stiff connection (A) having a near vertical initial slope has a higher lateral load capacity at inch deformation than a less stiff connection (B). However, if the 5% load is used, Connection B would have a higher lateral load capacity. In 1986, Aune and Patton-Mallow (1986a and 1986b) applied the yield theory to nailed connections. They determined three different yielding modes; 1) wood yielding (crushing) in either side or main member, 2) wood yielding in both members and with nail yielding in one member, and 3) wood and nail yielding in both members (Figure 6). The design equations derived from the yield models considers the dowel bearing strength, connection geometry and bending yield strength of the nail. Nail Bending Yield Strength The European yield theory has caused additional research needs to surface. Among the most critical needs, the nail bending yield strength, a mechanical property, needs to be established. Loferski and McLain (1991) performed tests to arrive at the bending yield values found in the 1991 NDS. The nails were subjected to a simple-supported center-point loading condition which was a modified version of the Nordtest method, a common test method used throughout the European community. The load-deformation curve was plotted and the 5 % offset yield load was applied to determine the nail yielding value. The bending yield strength was observed to increase with decreasing wire diameter. This was expected since the smaller diameter wire is drawn through more forming dies and therefore experiences more strain hardening than the larger diameter nails. The common nails used in this study ranged in size from 6d to 20d. The bending yield strength for larger diameter nails (40d to 90d) were extrapolated from the test data. However, this extrapolation is not advisable since less strain hardening occurs in the larger diameter nails. Another limitation of this study was that threaded nails were not considered. Hence, the allowable yield strength for threaded nails was assumed to be 30% higher than common nails due to the hardened-steel (NFPA, 1991). Using regression analysis, Loferski and McLain (1991) determined the following empirical equation to model the bending yield strength of nails based on wire diameter.

7 Wills et al F yb = D (5) where: F yb D = yield stress at 5% offset, ksi = nail shank diameter, in The coefficient of determination was extremely low (R 2 = 0.22) due to inconsistences in the quality of steel used in nails. Currently, Federal Specification FF-N-105B (1977) does not set a minimum quality standard for the steel -- it only specifies a "good commercial quality steel". Therefore, nail manufacturers use steel ranging from 0.10% to 0.44% carbon content ( grade). A better approach for predicting yield strength would be to use the grade of steel or a mechanical property of the wire such as the wire tensile strength. The tensile strength is determined by the steel quality and is known when the steel is purchased. Pennyweight Classification System With the advent of European yield theory, the inadequacies of the current pennyweight system are obvious. There is no established relationship between pennyweight and nail diameter among common and threaded nails and the quality of the steel. For example, the wire diameter of a 16d common nail is inches while the wire diameter of a 16d threaded nail is inches, representing a 8.6% difference. Loferski and McLain (1991) recommended the pennyweight system should not be used to specify nails for engineering purposes; rather, the nail diameter should be explicitly specified. Expanding on Loferski and McLain (1991) suggestion, the nail classification also should give an indication of nail bending strength, such as the grade of steel or wire tensile strength. The following are three possible example nail classification systems which would satisfy the above requirements. The first example of a possible nail classification system would be to require the manufacturers to label each container with the nail bending yield strength, wire diameter and nail length instead of the pennyweight designation. Another example would be to create nail grades based on bending yield strength, similar to lumber grades. The manufacturer would label the containers with the nail diameter, nail length and the nail grade. For example, the current 16d nail would be specified as a 0.162A-3.5 inch nail for a nail with a high bending yield strength, inch diameter and 3.5 inches long. Both of these examples would require extensive education of contractors and designers, and would be difficult to monitor and enforce. Another classification system, which would be easier to implement with contractors and designers, would require nail manufacturers to use a minimum grade steel or a minimum wire tensile strength for the production of nails, similar to the requirements for aluminum alloy wire nails. Then the designer could specify the wire diameter and would be guaranteed a minimum joint performance. This system could cause repercussions for nail manufacturers who are currently using steel below this minimum standard. By upgrading the quality of steel, the manufacturer would have the increased steel costs as well as reduced machine life and increased required maintenance. In summary, a new nail classification system is needed which specifies the nail bending strength, diameter and the length. HARMONIZATION OF INTERNATIONAL CODES The European yield theory offers the opportunity to unite fastener design methodologies throughout the world. Currently Australia, Canada, Europe, Japan, New Zealand, and the Unites States have adopted all or a portion of the European yield theory into their respective codes

8 Wills et al (Pollock, 1992). However, there are subtle, but significant, differences between the countries' definitions of yield point and test methods to determine dowel bearing strength and nail bending yield strength properties. Some countries use the 5% offset yield to define yield point, while others use the proportional limit, and still others define the yield point as the ultimate strength (Pollock, 1992). Dowel Bearing Strength Test Method Currently in the U.S., there is no standard method for measuring dowel bearing strength. Aune and Patton-Mallory (1986a) used the Nordtest method (common in Europe) in their experiments. The Nordtest method requires a nail to be driven into a pre-drilled hole and supported on both sides as shown in Figure 7. A compression load is applied parallel to grain and the load-deformation curve is recorded. the 5 % offset yield load is used as the measure of dowel bearing strength. This procedure has been criticized because of the possible stress concentrations at the wood edges caused by nail yielding. Wilkinson (1991) determined the dowel bearing strength values used in the 1991 NDS by an experimental apparatus that applies a uniform load across the entire fastener length (Figure 8). With this method, the wood is cut into two pieces, then reclamped and pre-drilled to 50% of the nail diameter. The nail is driven and the clamps and on-half of the wood are removed. The intent is to produce a half-hole with surface characteristics caused by a driven nail. Finally, a uniform load is applied and the load-deformation curve is plotted. Again, the 5 % offset yield load is used to measure the dowel bearing strength. An ASTM Standard is currently under review for this test method. Nail Bending Yield Strength Test Method Similar differences exist with nail bending yield strength tests. Some countries use a slight modification of the single-point loading test apparatus as used by Loferski and Mclain (1991). Other countries (including the U.S.) favor the use of variations of two-point loading test methods. SUMMARY This paper focuses on problems facing manufacturers, designers and users of nail fasteners, and recommends 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. 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 accurate values for nail bending strength and dowel bearing strength of wood. Neither of these properties were required in the design methodology preceding the 1991 NDS. In addition, the current pennyweight nail classification system does not specify these properties. 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.

9 Wills et al LITERATURE CITED American Forest and Paper Association (AFPA) Commentary on the national design specification for wood construction. AFPA. Washington, D.C. American Society for Testing and Materials (ASTM). 1992a. Standard terminology of nails for use with wood and wood-base materials. Standard F547-77, ASTM, Philadelphia, PA. American Society for Testing and Materials (ASTM). 1992b. Standard test methods for mechanical fasteners in wood. Standard D , ASTM. Philadelphia, PA. Aune, P. and M. Patton-Mallory. 1986a. Lateral Load-bearing capacity of nailed joints based on the yield theory: experimental verification. Research Paper FPL 470, U.S. Forest Products Laboratory, Madison, WI. Aune, P., and M. Patton-Mallory. 1986b. Lateral load-bearing capacity of nailed joints based on the yield theory: theoretical development. Research Paper FPL 469, U.S. Forest Products Laboratory, Madison, WI. Ehlbeck, J A new effective spike made in West Germany. In Proc. 27 th Annual Forest Products Research Society Conference. pp Ehlbeck, J Withdrawal resistance of threaded nails in wood used for building construction in Germany. Proc. 30 th Annual Forest Products Research Conference. 15 July. Ehlbeck, J Nailed joints in wood structures. Bulletin No. 166, Wood Research Laboratory, Virginia Polytechnic Institute, Blacksburg, VA. Federal Specification FF-N-105B Wire, cut and wrought nails, staples and spikes. The International Staple, Nail, and Tool Association. Forest Products Laboratory Nail-withdrawal resistance of American woods. Research Note FPL-093, U.S. Forest Products Laboratory, Madison, WI. Geisthardt, A.C., C. Siegal and L. Shirek Research and development: an industry perspective. ASAE Technical Paper, American Society of Agricultural Engineers, St. Joseph, MI. Hoadley, B Comparison of lag screw and threaded nails in a typical structural joint. Forest Products Journal 27(12): Johnansen, K. W Theory of timber connections. Publications of the International Association for Bridge and Structural Engineering. Zurich, Switzerland. Loferski, J.R., and T.E. McLain Static and impact flexural properties of common wire nails. Journal of Testing and Evaluation 19(4): National Forest Products Association (NFPA) National design specification for wood construction. NFPA, Washington, D.C. National Forest Products Association (NFPA) National design specification for wood construction. NFPA, Washington, D.C. Pollock, D.G Compatibility of North American standards with international standards: fastener and connections. Proc. Forest Products Research Society Conference on Engineered Wood Structures. Quackenbush, J.E Withdrawal resistance of various types of nails: fluted-shank Norwegian nails verse similar-size American nails. Proc. 31 th Annual Forest Products Research Society Conference. Soltis, L.A European yield model for wood connections. Proc. 9 th Structural Congress of the American Society of Civil Engineers. pp Stern, E.G Withdrawal resistance of plain shank and threaded nails of 2 1/2" length driven by hand-hammer vs. single powersert automatic nailer. Bulletin No. 50, Wood Research Laboratory, Virginia Polytechnic Institute, Blacksburg, VA.

10 Wills et al Stern, E.G Recent research on performance of mechanical fasteners for wood at Virginia Polytechnic Institute and State University. Proc. 31 th Annual Forest Products Research Society Conference. pp , 26-32, Stern, E.G Mechanical fastening of wood, a review of the state of art. Proceeding No. P Forest Products Research Society. USDA, Wood handbook: wood as an engineer material. Agricultural Handbook No. 72, U.S. Department of Agriculture, Washington, D.C. White, M.S. and T.L. Gales Quality of variations in helical threaded nails. Forest Products Journal 40(11/12): Wilkinson, T.L Dowel bearing strength. Research Paper Rp-505, U.S. Forest Products Laboratory, Madison, WI. Winistorfer, S.G. 1992b. NDS nail design method comparison. Frame Building News (July/August):32-35, 38-41, 44.

11 Wills et al Table 1 - Compares the wire diameter difference between a common shank nail and deformed shank nail (NFPA, 1991a).

12 Wills et al Figure 1 - Defines the helical threaded-shank and annular threaded-shank nail. Notice with the helical threads, the thread expansion equals the thread depressions therefore the effective wire diameter is unchanged. However with the annular threads, the effective diameter (root diameter) is less than the wire diameter.

13 Wills et al Figure 2. Compares the 1986 NDS proportional limit (defined as the load at inches ofdeformation) and the 1991 NDS 5% offset yield. (Winistorfer, 1992b).

14 Wills et al Figure 3 - Compares the withdrawal design value (lbs/in ofpenetration) ofdifferent pennyweight designations based on specific gravity of0.55 (NFPA, 1986a).

15 Wills et al Figure 4 - Compares lateral design values (lbs/in of penetration) based on the pennyweight designation and species group II (NFPA, 1986a).

16 Wills et al Figure 5. Compares the difference between the proportional limit at inches and the 5 percent offset yield. Notice the 5 percent offset yield could better predict the individual nailed-joint performance.

17 Wills et al Figure 6. The three possible yield modes of nail connections as defined by Aune and Patton-Mallory (1986a and 1986b).

18 Wills et al Figure 7. Wood embedment test apparatus used by Aune and Patton-Mallory (1986a) to determine wood embedding strength. This test method follows the Nordtest method.

19 Wills et al Figure 8. Test apparatus used by Wilkinson (1991) to obtain the dowel bearing strength of wood. This test method is currently under consideration for an ASTM standard. Paper Paper presented at 1993 International winter meeting of the American Society of Agricultural Engineers; 1993 December 14-17; Chicago. St. Joseph, MI: American Society of Agricultural Engineers; p. Printed on recycled paper