PULL-OUT RESISTANCE OF SELF-TAPPING WOOD SCREWS WITH CONTINUOUS THREAD

Transcription

1 PULL-OUT RESISTANCE OF SELF-TAPPING WOOD SCREWS WITH CONTINUOUS THREAD by MAIK GEHLOFF Dipl.-Ing. (FH), University of Applied Sciences, Eberswalde, Germany, A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE STUDIES (Forestry) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) July 11 Maik Gehloff, 11

2 ABSTRACT Over the past centuries the use of timber in structures has seen waves of decline and rediscovery. Timber structures have evolved from empirical structures using timber within its natural boundaries in terms of shape, size and length to modern day engineering design approach using computers and sophisticated numerical models; this has led to the need of high performance connections in such structures. With the help of mechanical fasteners the envelope was pushed time and time again, creating ever stronger connections. However, the capacity of such connections is not only governed by the mechanical properties of the connectors, but also by the mechanical properties of the connecting wood members. Researchers have been developing different methods of reinforcing the inherent weakness of wood, namely the low strength in tension and compression perpendicular to the grain, as well as the low capacity in longitudinal shear. This thesis examines experimentally the pull-out resistance of self-tapping wood screws with continuous thread, a new type of fastener that can be used as a fastener, but also as reinforcement considering Canadian major wood species. Utilizing its high withdrawal capacity and high tensile strength, this type of connector can potentially be used to transfer internal forces in the wood along the length-axis of the screw instead of loading the wood in its weak directions. The results show that self-tapping wood screws (STSs) have a high resistance to pull-out and are an economical alternative to other reinforcement methods. Besides the superior capacities of STSs in withdrawal and tensile strength to other methods, they are also very easy to install since no pre-drilling of holes is required and thus, give an economical solution to many challenges in engineered timber construction. ii

3 TABLE OF CONTENTS ABSTRACT... ii TABLE OF CONTENTS... iii LIST OF TABLES... iv LIST OF FIGURES... v ACKNOWLEDGEMENTS... xiii 1. INTRODUCTION History Self-tapping wood screws EXPERIMENTAL DESIGN / EQUATIONS Parameter considerations Test setup European code equations Equivalency calculations RESULTS / DISCUSSION Results Discussion CONCLUSIONS AND RECOMMENDATIONS Conclusions..... Recommendations... 9 BIBLIOGRAPHY... 1 APPENDIX SUPPLEMENTAL MATERIAL... iii

4 LIST OF TABLES Table 1: Effective embedment depths... 1 Table : Average densities... Table 3: Withdrawal test results for 9... Table : Withdrawal test results for... 9 Table : Withdrawal test results for Table : Comparison of test results to code equation predictions for STS iv

5 LIST OF FIGURES Figure 1: Development of wood design since Figure : Knitted glass and aramid fibre fabric (spiral)... Figure 3: Transversally reinforced carbon fibre loop... Figure : Typical load-displacement curve... Figure : Beam splice using glued-in rods... Figure : Screw thread in accordance with DIN Figure 7: Self-tapping wood screws... 9 Figure : Self-tapping wood screw drill-tips... 9 Figure 9: Shank cutter on partially threaded screw... 9 Figure 1: STS as embedment and splitting reinforcement Figure 11: Test setup for 9º tests... 1 Figure 1: Test setup for º tests... 1 Figure 13: Test setup for 3º tests... 1 Figure 1: Transducer to measure deflection Figure 1: Effective embedment depth... 1 Figure 1: Average withdrawal resistance for mm 9º... 7 Figure 17: Average withdrawal resistance for mm 9º... 7 Figure 1: Average withdrawal resistance for 1 mm 9º... Figure 19: Average withdrawal resistance for mm º... 3 Figure : Average withdrawal resistance for mm º... 3 Figure 1: Average withdrawal resistance for 1 mm º Figure : Average withdrawal resistance for mm 3º Figure 3: Average withdrawal resistance for mm 3º v

6 Figure : Average withdrawal resistance for 1 mm 3º... 3 Figure : Typical screw failure... 3 Figure : Typical load deformation plot... 3 Figure 7: Wood density distribution... 3 Figure : Comparison of mm results with DIN 1: Figure 9: Comparison of mm results with EC Figure 3: Comparison of mm results with DIN 1:- Figure 31: Comparison of mm results with EC Figure 3: Comparison of mm results with DIN 1: Figure 33: Comparison of mm results with EC Figure 3: Comparison of mm results with DIN 1: Figure 3: Comparison of mm results with EC 9... Figure 3: Comparison of mm results with DIN 1:- 3 Figure 37: Comparison of mm results with EC 3 Figure 3: Comparison of mm results with DIN 1: Figure 39: Comparison of mm results with EC 3... Figure : Comparison of 1 mm results with DIN 1: Figure 1: Comparison of 1 mm results with EC 9... Figure : Comparison of 1 mm results with DIN 1:- Figure 3: Comparison of 1 mm results with EC Figure : Comparison of 1 mm results with DIN 1: Figure : Comparison of 1 mm results with EC Figure : Comparison of mm results with DIN 1: Figure 7: Comparison of mm results with EC vi

7 Figure : Comparison of mm results with DIN 1:- Figure 9: Comparison of mm results with EC Figure : Comparison of mm results with DIN 1: Figure 1: Comparison of mm results with EC Figure : Comparison of mm results with DIN 1: Figure 3: Comparison of mm results with EC 9... Figure : Comparison of mm results with DIN 1:- 3 Figure : Comparison of mm results with EC 3 Figure : Comparison of mm results with DIN 1: Figure 7: Comparison of mm results with EC 3... Figure : Comparison of 1 mm results with DIN 1: Figure 9: Comparison of 1 mm results with EC 9... Figure : Comparison of 1 mm results with DIN 1:- Figure 1: Comparison of 1 mm results with EC Figure : Comparison of 1 mm results with DIN 1: Figure 3: Comparison of 1 mm results with EC 7 Figure : Load Deformation (mm, d, 9, Douglas-fir)... Figure : Load Deformation (mm, d, 9, S-P-F)... Figure : Load Deformation (mm, d, 9, Hemlock)... Figure 7: Load Deformation (mm, d,, Douglas-fir)... Figure : Load Deformation (mm, d,, S-P-F)... Figure 9: Load Deformation (mm, d,, Hemlock)... Figure 7: Load Deformation (mm, d, 3, Douglas-fir)... 7 Figure 71: Load Deformation (mm, d, 3, S-P-F)... 7 vii

8 Figure 7: Load Deformation (mm, d, 3, Hemlock)... Figure 73: Load Deformation (mm, 1d, 9, Douglas-fir)... Figure 7: Load Deformation (mm, 1d, 9, S-P-F)... 9 Figure 7: Load Deformation (mm, 1d, 9, Hemlock)... 9 Figure 7: Load Deformation (mm, 1d,, Douglas-fir)... 7 Figure 77: Load Deformation (mm, 1d,, S-P-F)... 7 Figure 7: Load Deformation (mm, 1d,, Hemlock) Figure 79: Load Deformation (mm, 1d, 3, Douglas-fir) Figure : Load Deformation (mm, 1d, 3, S-P-F)... 7 Figure 1: Load Deformation (mm, 1d, 3, Hemlock)... 7 Figure : Load Deformation (mm, 1d, 9, Douglas-fir) Figure 3: Load Deformation (mm, 1d, 9, S-P-F) Figure : Load Deformation (mm, 1d, 9, Hemlock)... 7 Figure : Load Deformation (mm, 1d,, Douglas-fir)... 7 Figure : Load Deformation (mm, 1d,, S-P-F)... 7 Figure 7: Load Deformation (mm, 1d,, Hemlock)... 7 Figure : Load Deformation (mm, 1d, 3, Douglas-fir)... 7 Figure 9: Load Deformation (mm, 1d, 3, S-P-F)... 7 Figure 9: Load Deformation (mm, 1d, 3, Hemlock) Figure 91: Load Deformation (mm, 1d, 9, Douglas-fir) Figure 9: Load Deformation (mm, 1d, 9, S-P-F)... 7 Figure 93: Load Deformation (mm, 1d, 9, Hemlock)... 7 Figure 9: Load Deformation (mm, 1d,, Douglas-fir) Figure 9: Load Deformation (mm, 1d,, S-P-F) viii

9 Figure 9: Load Deformation (mm, 1d,, Hemlock)... Figure 97: Load Deformation (mm, 1d, 3, Douglas-fir)... Figure 9: Load Deformation (mm, 1d, 3, S-P-F)... 1 Figure 99: Load Deformation (mm, 1d, 3, Hemlock)... 1 Figure 1: Load Deformation (mm, d, 9, Douglas-fir)... Figure 11: Load Deformation (mm, d, 9, S-P-F)... Figure 1: Load Deformation (mm, d, 9, Hemlock)... 3 Figure 13: Load Deformation (mm, d,, Douglas-fir)... 3 Figure 1: Load Deformation (mm, d,, S-P-F)... Figure 1: Load Deformation (mm, d,, Hemlock)... Figure 1: Load Deformation (mm, d, 3, Douglas-fir)... Figure 17: Load Deformation (mm, d, 3, S-P-F)... Figure 1: Load Deformation (mm, d, 3, Hemlock)... Figure 19: Load Deformation (mm, 1d, 9, Douglas-fir)... Figure 11: Load Deformation (mm, 1d, 9, S-P-F)... 7 Figure 111: Load Deformation (mm, 1d, 9, Hemlock)... 7 Figure 11: Load Deformation (mm, 1d,, Douglas-fir)... Figure 113: Load Deformation (mm, 1d,, S-P-F)... Figure 11: Load Deformation (mm, 1d,, Hemlock)... 9 Figure 11: Load Deformation (mm, 1d, 3, Douglas-fir)... 9 Figure 11: Load Deformation (mm, 1d, 3, S-P-F)... 9 Figure 117: Load Deformation (mm, 1d, 3, Hemlock)... 9 Figure 11: Load Deformation (mm, 1d, 9, Douglas-fir) Figure 119: Load Deformation (mm, 1d, 9, S-P-F) ix

10 Figure 1: Load Deformation (mm, 1d, 9, Hemlock)... 9 Figure 11: Load Deformation (mm, 1d,, Douglas-fir)... 9 Figure 1: Load Deformation (mm, 1d,, S-P-F) Figure 13: Load Deformation (mm, 1d,, Hemlock) Figure 1: Load Deformation (mm, 1d, 3, Douglas-fir)... 9 Figure 1: Load Deformation (mm, 1d, 3, S-P-F)... 9 Figure 1: Load Deformation (mm, 1d, 3, Hemlock)... 9 Figure 17: Load Deformation (mm, 1d, 9, Douglas-fir)... 9 Figure 1: Load Deformation (mm, 1d, 9, S-P-F)... 9 Figure 19: Load Deformation (mm, 1d, 9, Hemlock)... 9 Figure 13: Load Deformation (mm, 1d,, Douglas-fir) Figure 131: Load Deformation (mm, 1d,, S-P-F) Figure 13: Load Deformation (mm, 1d,, Hemlock)... 9 Figure 133: Load Deformation (mm, 1d, 3, Douglas-fir)... 9 Figure 13: Load Deformation (mm, 1d, 3, S-P-F) Figure 13: Load Deformation (mm, 1d, 3, Hemlock) Figure 13: Load Deformation (1mm, d, 9, Douglas-fir)... 1 Figure 137: Load Deformation (1mm, d, 9, S-P-F)... 1 Figure 13: Load Deformation (1mm, d, 9, Hemlock) Figure 139: Load Deformation (1mm, d,, Douglas-fir) Figure 1: Load Deformation (1mm, d,, S-P-F)... 1 Figure 11: Load Deformation (1mm, d,, Hemlock)... 1 Figure 1: Load Deformation (1mm, d, 3, Douglas-fir) Figure 13: Load Deformation (1mm, d, 3, S-P-F) x

11 Figure 1: Load Deformation (1mm, d, 3, Hemlock)... 1 Figure 1: Load Deformation (1mm, 1d, 9, Douglas-fir)... 1 Figure 1: Load Deformation (1mm, 1d, 9, S-P-F)... 1 Figure 17: Load Deformation (1mm, 1d, 9, Hemlock)... 1 Figure 1: Load Deformation (1mm, 1d,, Douglas-fir)... 1 Figure 19: Load Deformation (1mm, 1d,, S-P-F)... 1 Figure 1: Load Deformation (1mm, 1d,, Hemlock) Figure 11: Load Deformation (1mm, 1d, 3, Douglas-fir) Figure 1: Load Deformation (1mm, 1d, 3, S-P-F)... 1 Figure 13: Load Deformation (1mm, 1d, 3, Hemlock)... 1 Figure 1: Load Deformation (1mm, 1d, 9, Douglas-fir) Figure 1: Load Deformation (1mm, 1d, 9, S-P-F) Figure 1: Load Deformation (1mm, 1d, 9, Hemlock) Figure 17: Load Deformation (1mm, 1d,, Douglas-fir) Figure 1: Load Deformation (1mm, 1d,, S-P-F) Figure 19: Load Deformation (1mm, 1d,, Hemlock) Figure 1: Load Deformation (1mm, 1d, 3, Douglas-fir) Figure 11: Load Deformation (1mm, 1d, 3, S-P-F) Figure 1: Load Deformation (1mm, 1d, 3, Hemlock) Figure 13: Load Deformation (1mm, 1d, 9, Douglas-fir) Figure 1: Load Deformation (1mm, 1d, 9, S-P-F) Figure 1: Load Deformation (1mm, 1d, 9, Hemlock) Figure 1: Load Deformation (1mm, 1d,, Douglas-fir) Figure 17: Load Deformation (1mm, 1d,, S-P-F) xi

12 Figure 1: Load Deformation (1mm, 1d,, Hemlock) Figure 19: Load Deformation (1mm, 1d, 3, Douglas-fir) Figure 17: Load Deformation (1mm, 1d, 3, S-P-F) Figure 171: Load Deformation (1mm, 1d, 3, Hemlock) xii

13 ACKNOWLEDGEMENTS I would like to thank Dr. Frank Lam for his guidance throughout this project and Maximilian Closen as well as the TEAM technicians for the help provided during testing. I would also like to express my gratitude to Natural Sciences and Engineering Research Council of Canada for the financial support in this research project. The cooperation received from Adolf Würth GmbH & Co. KG ( for the supply of the ASSY screws and Hans Hundegger - Maschinenbau GmbH ( for the use of the Hundegger K milling machine is also appreciated. xiii

14 1. INTRODUCTION 1.1. History Wood is one of the oldest and most common building materials. In the past, if wood was not readily available or available in limited quantities then other materials like loam, stone, bamboo etc. were used in buildings. Wood was primarily used in its natural form and shape, only the development of tools enabled builders to shape wood into desired components and allowed wood to be usable beyond its natural limitations, like length for example. The desire and move toward planned structures was the cradle of timber design and planned timber construction. Until late in the 19 th century a timber structure was a mere testimony to craftsmanship and was carried out without structural analysis. These structures were built by empirical considerations and experience only. The first structures fully analyzed by engineers were bridges and trestles. They were designed either as timber arches or timber trusses to reach spans of m and up to m. Illustrated in Figure 1 is the evolution of timber structure design over a period of about year (years rounded to decade). Shown are the results of the great craftsmanship of Swiss carpenter Ulrich Grubenmann (Killer, 199) using only sawn wood and few mechanical fasteners in 17. The American Thompson S. Brown designed the RW Bridge about 1 years later utilizing structural analysis of arched trusses as it was available at that time by incorporating mechanical fasteners (Zimmer, ). Howard Hughes (Herzog et al, 3) conquered yet another new challenge when put in charge of building an airplane, a task that would have been impossible to undertake without the use of adhesives. In modern day design of timber structures the computer has become an indispensable tool to allow 1

15 the completion of each task for complex structures such as the shell roof structure for the World Exposition EXPO in Hannover Germany (Herzog et al, 3) (Herzog, ). Figure 1: Development of wood design since 17 With the industrial revolution and the technological developments in the second half of the 1 th century came the mass production of iron and later, steel and concrete. The development of these materials and their superior mechanical properties compared to wood almost led to the complete disappearance of wood as building material in commercial and high rise construction. Only the limited availability of iron and steel after the Second World War and the targeted development in timber construction facilitated the revival of larger wood structures. Some key developments like that of glue laminated timber by Otto Hetzer (Müller, ) at the beginning of the th century along with the development of connections and connectors further helped that revival.

16 It became apparent in recent years that the topic of connections in timber construction and design is a crucial one. Researchers around the world are targeting some inherent weaknesses of wood and moreover, connections in timber structures and revising current design rules and codes accordingly. These activities also led to the development of new connections and connection systems with superior structural properties. With these innovative connection systems, safer and more economical timber structures can be built since connection design is critical and often governing in timber engineering. Timber connections with dowel-type fasteners are typically designed based on the European-Yield model or Johansen theory (Johansen, 199) and its ductile failure modes. Splitting failure becomes more and more severe due to the relatively low tension perpendicular to grain strength of the wooden connection members with an increase in the number of fasteners and the fastener diameter. Defects like cracks, specifically endcracks in the wood, further reduce the resistance of a connection. Such cracks occur not only during drying and seasoning, but can also occur in glue laminated beams in indoor application during the service life of the beam. Furthermore, be it intended or nonintended by design, if moments were imposed onto the connection, splitting failure will occur. In the recent past more and more research is being conducted on the topic of reinforcement of such connections to increase their resistances and capacity. With the inherent low tension perpendicular to grain strength of wood and defects like cracks being a big concern for the splitting of connections, a lot of focus was set to reinforce dowel-type connections with the goal of minimizing splitting. The types of reinforcements are ranging from glued in rods over reinforcement with glued and 3

17 screwed on plywood and glued on high tensile fibres at the connection to more recently developed self-tapping wood screws (STS) with continuous threads. Figure and Figure 3 show specimens that were reinforced using different types of high tensile fibres as well as a different method on applying the reinforcement. Although some reinforcement against splitting of the wood is provided, this method is used primarily to reinforce the embedment strength of the wood (Haller et al, ). The load displacement curve in Figure shows the effectiveness of the reinforcement. Figure : Knitted glass and aramid fibre fabric (spiral) Figure 3: Transversally reinforced carbon fibre loop Figure : Typical load-displacement curve

18 Madsen () summarizes the research work done on glued-in steel rods and its broad applicability to multiple common problems in timber engineering. These applications reach from knee-joints, beam splices and moment connections to reinforcing the bearing strength as well as tension perpendicular to grain strength. An example of a beam splice using glued-in rods is shown in Figure. On the other hand Hockey (1999) and Blaß et al. () used commonly available and well know truss plates to reinforce bolted connections in both embedment strength as well as splitting perpendicular to the grain. Findings show an increase in ultimate capacity of the connection as well as a change in failure mode from brittle failure to a much more desirable ductile failure. The method of using truss plates as reinforcement offers an economical approach to the problem. First of all, the truss plates are comparably inexpensive as they are widely used in the manufacturing of conventional -by trusses. Another advantage of using truss plates is the fact that it eliminates the need for predrilled holes in the timber member, as they are required for glued in rods, but also is easier and cleaner to apply than fibre reinforces plastics that require epoxy resins for their application.

19 Figure : Beam splice using glued-in rods Self-tapping wood screws with continuous threads have also proven to not only to be an effective but also an economical method of reinforcing connections with dowel-type fasteners (Bejtka Blaß, ). When compared to glued in rods and glued on fibre reinforcements for example, self-tapping wood screws are relatively inexpensive, fast and easy to install. The self-tapping wood screws, as their name implies, do not require any pre-drilling, similar to the afore mentioned truss plates, and can be installed virtually invisibly. This is another big advantage of such self-tapping wood screws especially when they are used in retrofit applications to restore and reinforce existing connections. However, the use of self-tapping wood screws is not limited to reinforcing connections,

20 but to generally reinforce inherent weaknesses of timber beams like the tension perpendicular to grain and compression perpendicular to grain strengths. 1.. Self-tapping wood screws STSs provide economical means of assembling components, especially where materials must be joined together or reinforced. Thread forming and thread cutting are the two major types of self-tapping screws. The thread cutting screws remove the material physically from which they are drilled into and are typically used in timber connections. Thread forming STSs, however, plastically deform the material that they are driven into, providing a permanent thread. This distinguished mechanical and form giving bond with the wood, offers a great method of transferring tensile and compressive forces along the axis of the STS. The development of STSs occurred primarily in Europe where earlier, more common, wood screws are widely used. These more common screws used the same principle of transferring loads, but were limited in dimensions. These types of wood screws are standardized according to DIN 9, DIN 97 or DIN 71 with a thread type in accordance with DIN 799 for all of them. Screws with a thread type standardized in DIN 799 require a pilot hole; their thread length makes up about % of the length of the screw with a length limited to about 1 mm. The limited length meant that these screws could not be used to connect members with larger cross-section. The commonly used lagscrews in North America are of similar nature and have essentially the same restrictions and a comparable thread type like the screws in accordance with DIN 799 as shown in Figure. 7

21 Figure : Screw thread in accordance with DIN 799 With the emergence of ever larger cross-sectional glue laminated timbers, the need for new longer screws was becoming more and more apparent. The development of selftapping wood screws was filling that need with screw lengths of now up to 1, mm and diameters of up to 1 mm. STSs are manufactured as partially threaded screws and as screws with continuous thread depending on their application. A variety of different STSs are shown in Figure 7. To achieve such long, large diameter screws that can be installed without a pilot hole, the screws are hardened after the thread has been rolled onto them. The hardening of the screws is a highly secretive process; it is different for each manufacturer, which can increase the mechanical properties such as the yield strength, tensile and compressive strength, as well as the torsional strength of the screws. Selftapping wood screws are manufactured with a drill-tip (Figure ) and coated with a company specific lubricant to reduce the torque required to install the STS and to prevent splitting of the wood during installation. When partially threaded self-tapping wood screws are used, the introduction of a shank cutter as seen in Figure 9 helps to further reduce friction. Unlike the standardized conventional wood screws, self-tapping wood screws are not standardized and need a technical or general construction approval. In Germany the Deutsche Institut für Bautechnik DIBt provides the technical approvals for non-standardized construction materials like STSs.

22 Figure 7: Self-tapping wood screws Figure : Self-tapping wood screw drill-tips Figure 9: Shank cutter on partially threaded screw The use of self-tapping screws, which eliminated the need for a pilot hole, has increased considerably over the past decade in Europe. Initially, a conceivably big disadvantage of self-tapping wood screws compared to nails, was the fact that little experimental work 9

23 had been done on these new types of screws. Most mechanical properties in wood-wood or wood-steel joints were estimated mainly by giving self-tapping screws the same properties as nails with similar dimensions. Therefore, the strongest attribute, the withdrawal resistance, of a self-tapping screw was not taken into account. In shear connections only their dowel action was taken into consideration during design. Furthermore, self-tapping screws could only be applied in timber connections without a pilot hole where the density of timber was less than - kg/m 3 and if the shank diameter of the threaded part was less than - mm. Further research (Blaß & Bejtka, ) concerning lateral and withdrawal resistance of STSs in European species found that the lateral strength of self-tapping wood screws varied almost linearly with the specific gravity of the wood and the square root of the diameter of the screw. In addition, the withdrawal strength of self-tapping wood screws varies almost linearly with the embedment depth or, more specifically, the effective embedment length of the screw s thread. No practical difference was observed between radial and tangential withdrawal strengths. In this study, basic strength data on the withdrawal capacity of self-tapping screws with major Canadian wood species is evaluated. This basic information is needed for the development of design rules for these types of screws with Canadian species. The establishment of such design rules would allow engineers and builders to facilitate the full potential of these types of screws. This is of great interest to engineers in North America as they are facing similar technical issues as their European colleagues in that right now STSs can only be designed as lag-screws or nails. As previously mentioned, using the lag-screw or nail equivalent approach; the greatest benefit of the self-tapping 1

24 screws, the high withdrawal capacity, is entirely neglected. By establishing evaluation data the full benefit of the screws could be explored. The use of STSs is vast and many different kinds of applications have been tried and looked at by researchers (Blaß & Bejtka, ). In Europe these screws are used as reinforcements for longitudinal shear, tension perpendicular to the grain, and compression perpendicular to the grain. They are also used to reinforce embedment strength in dowel-type connections and, more recently, have been discovered as primary fasteners as well. The application as reinforcements is wide-spread and ranges from rehabilitation of old existing structures and members to reinforcement for connections, as shown in Figure 1. Figure 1: STS as embedment and splitting reinforcement Research done by Lam et al. () at the University of British Columbia (UBC) investigated the use of STS in moment resisting bolted connections as reinforcement 11

25 perpendicular to the grain. The study compared results of unreinforced specimens to specimens that were previously broken and rehabilitated using STS as well as specimens that were reinforced using self-tapping wood screws. The results revealed that the ultimate capacity as well as ductility of the connections could be improved using STS as reinforcement. It is worth noting that even the previously broken specimens that were rehabilitated using STS had significantly higher values of capacity than the unreinforced specimens. Additional work done by Lam et al. (1) and Gehloff et al. (1) at UBC looked at further increasing the capacity of such moment resisting bolted connections by using larger diameter bolts with reduced end and edge distances. The results confirmed that the reinforcement with self-tapping wood screws is a viable method to increase the capacity and ductility of bolted connections subjected to cyclic loading; even for connections with larger diameter bolts, which are more prone to splitting, and reduced end and edge distances. 1

26 . EXPERIMENTAL DESIGN / EQUATIONS.1. Parameter considerations The focus of this work is the pull out resistance or withdrawal resistance of self-tapping screws with continuous thread. Experiments are conducted by testing the pull out resistance at different angles to the grain in different wood species. The angles of the screw to the grain are 9 degrees (perpendicular), degrees and 3 degrees. The pull out resistance will be compared under the afore mentioned angles and different embedment depths in three different wood species. The chosen wood species are Douglas-fir and S-P-F glulam, as well as, Hemlock solid sawn timber to cover the most commonly used construction materials in Canadian heavy timber construction and density range. The screws used for the tests are Würth ASSY plus VG, where VG denotes continuous thread. The Würth ASSY plus VG screws are of mm, mm and 1 mm in diameter and are provided in various lengths of up to mm for the 1 mm diameter screws. Preliminary withdrawal tests were conducted by pulling the screws from the specimens. Results have shown that the tensile strength of the screw itself became the limiting factor at embedment depths of ~1d (d = diameter of the screw). The test setup was re-configured in a way that instead of pulling on the screws, the screws were pushed into the wood. Preliminary tests also revealed that the heads of the screws failed at the shoulder between the shaft of the screw and the head when the screws were tested by pulling them through the specimen. This stress concentration resulted from the grip device of the test setup which would not be present in real applications. When the screws 13

27 are used with steel side plates, the screws have to be counter-sunk into the steel plates to ensure proper contact of the screw head and shoulder with the steel plate to avoid stress concentrations and premature failure. Further preliminary tests proved that the new compression based setup was more suitable to get results at higher embedment depths of up to 1d before the screws failed in buckling. Based on these preliminary results, four different embedment depths were selected: d, 1d, 1d and 1d. The low embedment depth of d was selected to gather insights on the reinforcement of bolted connection with smaller edge distances. For each combination of screw diameter, embedment depth, angle to the grain and wood species, 1 replications were tested. With 1 different setups and 1 replicates for each setup, a total of 1 test were conducted and evaluated. Furthermore, the results are compared to predictions based on the German building code DIN 1:- and the Eurocode. Both building codes predict the withdrawal resistance of this type of screws and therefore their potential as reinforcement in tension perpendicular to grain in doweltype connections. Input parameters in the equations in the German code are the mean specific gravity of the wood, the angle to the grain, the screw diameter and the embedment depth. These parameters coincide with the studied parameters in the tests conducted for this study. 1

28 .. Test setup The machine used for the tests was the Sintech 3/D with a 1, kn load cell. Wooden blocks were used to create enough clearance above the machine test table for the screws. Steel rectangular tubing was used to reduce the span and therefore limit the deflection of the specimens. A transducer was used to measure the wood deflection near the tested screw as correction value. Figures 11to13 show the test setup for the 9, and 3 degree tests respectively. The transducer used to measure the deflection of the specimen is shown in Figure 1. Figure 11: Test setup for 9º tests 1

29 Figure 1: Test setup for º tests Figure 13: Test setup for 3º tests 1

30 Figure 1: Transducer to measure deflection As shown in Figures 11 to 13, the screws were driven all the way through the specimen to eliminate any resistance at the screw tip during testing, as well as to reduce the slenderness ratio. The reduced slenderness will help prevent buckling of the screws at higher loads and greater embedment depths. The embedment depth was controlled by the thickness of the specimens. All specimens were cut and planed to the thickness that equalled the embedment depth for the specific tests. Table 1 and Figure 1 show the effective embedment depth for the different screw inclinations. The embedment depths were chosen to remain constant since, otherwise, not enough material would have been left to properly install the screws and test the screws without breaking the wood first. In a situation where the screws are used to reinforce bolted connections the approach would be similar. The calculated minimum edge distances, even if reduced due to the presence of self-tapping wood screws as reinforcement for the bolts would, be a value that is 17

31 perpendicular (9 ) to the edges of the beam. Thus, the obtained withdrawal resistance values are applicable to such reinforcements even if it means that the values for different screw inclinations cannot be compared directly. Table 1: Effective embedment depths Embedment depth d 1d 1d 1d Effective embedment depth 9 3 d (1/sin ) d (1/sin 3 ) d ~. d d 1d (1/sin ) 1d (1/sin 3 ) 1d ~ 1.1 d d 1d (1/sin ) 1d (1/sin 3 ) 1d ~ 1.9 d d 1d (1/sin ) 1d (1/sin 3 ) 1d ~. d 3 d Figure 1: Effective embedment depth 1

32 To transfer the load from the load cell to the head of the self-tapping screw, a harden hexhead screw was used. Due to the hardness of the self-tapping screw the hex-head was introduced to avoid damage to the load cell. Testing was done in accordance with the German standard DIN EN 13, which specifies the speed of testing such that failure can be reached in 9 seconds ± 3 seconds. Multiple screws were placed in the specimens to speed up testing. The row spacing of d as well as the end, edge and screw spacing of 1d was followed as given in the standard..3. European code equations The German wood building code DIN 1:- defines the withdrawal resistance of screws by taking the tensile strength min. f u,k = N/mm of the screw, the axial capacity of the thread in wood and the head-pull through into account. Given that, head pull through of full threaded screws is neglected, the main parameters influencing withdrawal resistance are the penetration depth of the thread (l ef ) including the tip embedded in the wood, the diameter (d), the angle (α) and the characteristic value of the withdrawal resistance ( f 1. K ). Further influencing parameters are the apparent density, as well as the axial capacity of the threaded part embedded in the wood. Wood screws are separated into three different strength groups regarding their characteristic axial capacity parameter ( f 1. K ). For group one to three, the parameter is 1, K 1 k f where the value ( ) can vary from for group one, 7 for group two and for group three, respectively. Group one represents any wood screw other than screws that can be placed into either of the other two groups. Screws with threads in accordance with DIN

33 (Figure ) can be placed in group two without the need of further proof. Group three, on the other hand, represents a group for hardened screws that are proven to withstand a certain threshold capacity and generally require a general construction approval. Selftapping wood screws require a general construction approval and fall into group three unless stated otherwise in the approval of the particular screw. The characteristic value of the withdrawal resistance R ax, k is calculated by Equation 1. R min sin f1. K d lef cos 3 ax. K ; f. K d k [N] (1) where; 1, K 1 k f, with ρ k = characteristic density, kg/m 3 d = outside screw diameter, mm l ef = effective embedment depth including the tip, mm α = angle between screw length axis and wood grain, degree ( ) The second part of Equation 1 is used to calculate the head pull-through resistance of the screws, where f. K denotes the characteristic head pull-through parameter and dk the outside head diameter, or if a washer is used the outside diameter of the washer. In case of fully threaded self-tapping wood screws, however, the head pull-through resistance is not considered since the loads are transferred through the thread and the shaft of the screws and not the head.

34 Equation 1 can only be applied for angles α 9. Blaß and Bejtka () show that the equation holds true for angles (α) down to 3. In addition, Blaß et al. () found that the predicted withdrawal resistance R ax, k is very conservative. Thus, the characteristic axial capacity f 1. K could be increased by increasing the value ( ) up to 113 for 9 angles and up to 19 for angles less than 9. Contrary to the value of R ax, k presented in DIN 1:- where the screw tip is included in the effective embedment depth of the thread in the wood, Equation () of the Eurocode (EC) - considers the screw tip by subtracting one time the diameter (d) from the embedded length. Furthermore, a possible group effect is taken into consideration by an exponent for the number of fasteners (n) k R ax. K n d lef [N] () sin 1.cos 1. where; d = outside screw diameter, mm l ef = effective embedment depth excluding the tip, mm ρ k = characteristic density, kg/m 3 α = angle between screw length axis and wood grain, degree ( ) The findings by Blaß and Bejtka (3) show Equation over-predicted the withdrawal resistance R ax, k compared to their test results and, in conclusion, the parameter of 3. was lowered to. to match the test results. 1

35 .. Equivalency calculations Neither the Canadian timber building code CSA-O-9, nor the National Design Specifications (NDS) in the United States give indication for the calculation of the withdrawal resistance of wood screws similar to the self-tapping wood screws. As no technical approvals are in existence to date in North America for this type of screw, only equivalency calculations can be done. The only dowel type fasteners that are of similar type and are permitted to be loaded axial in withdrawal are nails and spikes, wood screws and lag-bolts or lag-screws. When looking at the Canadian CSA-O-9, paragraph states that nails and spikes may only be loaded in withdrawal for wind and earthquake design. The same is stated for wood screws in paragraph Consequently, an equivalency calculation is only given for wood screws as they have more similarity to self-tapping wood screws than nails and spikes. The only fastener permitted to be designed in withdrawal in accordance with CSA-O-9 that is similar in shape and function to the self-tapping wood screw is the lag-bolt or lag-screw. The NDS in the United States, however, permits the axial loading in withdrawal for all three types of fasteners, the nail and spike, wood screw as well as the lag-bolt or lag-screw; both terms are used interchangeably. The withdrawal resistance of lag-bolts in accordance with the Canadian CSA-O-9 is calculated with Equation 3 and CSA-O-9 Table as follows; P rw Y L n J [N/mm] (3) w t F e

36 where; ɸ =. Y w = y w (K D K SF K T ) y w = basic withdrawal resistance per millimeter of penetration, N/mm (Table 1...1) L t = length of penetration of threaded portion of lag-bolt in main member, mm n F = number of lag-bolts in the connection J e = end grain factor for lag-bolts;.7 in end grain, 1. in all other cases For wooden side members an appropriate washer has to be used to prevent the head of the lag-bolt from pulling through the side member. The withdrawal resistance of wood-screws in accordance with the Canadian CSA-O-9 is calculated with Equation and CSA-O-9 Tables A.1.1 and as follows; P rw Y L n [N/mm] () w p t F where; ɸ =. Y w = y w (K T K SF ) y w = basic withdrawal resistance per millimeter of threaded shank penetration in main member; = d. F G 1.77, N/mm G = mean relative density of main member (Table A.1.1) d f = nominal wood screw diameter, mm (Table ) L pt = threaded length penetration in the main member, mm n F = number of wood screws in the connection For a joint with three members, the threaded length penetration shall be the maximum 3

37 threaded length within any member other than the head-side member. For wooden side members an appropriate washer has to be used to prevent the head of the wood screw from pulling through the side member. In accordance with the United States NDS, the withdrawal resistance is calculated by multiplying the reference withdrawal resistance W with all applicable adjustment factors as they can be found in Table in the NDS. The reference withdrawal resistance is calculated with Equations to 7 for lag-bolts, wood screws and nails and spikes respectively. These values are also tabulated in NDS Tables 11.A, 11.B and 11.C respectively. W 3/ 3/ 1 G D [lb/in] () W G D [lb/in] () W / 13 G D [lb/in] (7) The calculations for the withdrawal resistance in Canada as well as the United States are limited to the fastener being installed perpendicular to the grain, meaning an angle of 9 between the axis of the screw and the grain. No other installation angles are allowed; thus limiting design parameters to the diameter of the fastener, the penetration depth of the fastener, and the density of the wood member the fastener is driven into. A comparison of the calculated withdrawal resistances using the DIN and Eurocode code equations as well as the equivalency calculations is given in Chapter three. The results of various calculated predictions are also compared to the test results in an effort to evaluate their respective applicability.

38 3. RESULTS / DISCUSSION 3.1. Results The withdrawal test results are shown in Table 3 through Table for all 1 test series. Each test configuration was tested with 1 replicates bringing the total number of tests to 1 individual screws. The tables are formatted to group the results for all screws with the same angle between the axis of the screw relative to the grain. It should be mentioned that the results in Tables 3 to showing average values for the mm, mm and 1 mm screws respectively, are taken from all 1 test replicates and are not calculated based on the shown minimum and maximum values alone. To display the results a little more clearly and to be able to understand the effects of different parameters, the results are also shown in Figures 1 to. The figures are divided not only into the angle of screw installation with respect to the grain, but also split into single screw diameters in order to keep the graphs legible. The error bars show the minimum and maximum test values. The data below clearly shows the effect the individual parameters have on the results. With an increase in embedment depth, the withdrawal resistance also increased. The same can be said for the increase in screw diameter and the different densities of the wood samples. The effect of the density, however, can only really be discussed on the bases of the different wood species (Table ) as multiple screws were tested in the same specimen and local density variations like knots within the specimen were present. Table : Average densities Douglas-fir Spruce - Pine - Fir Hemlock Average density [kg/m 3 ] Standard Deviation [kg/m 3 ]

39 Table 3: Withdrawal test results for 9 Test configuration Withdrawal Capacity [kn] Ø [mm] Embedment Angle [ ] Species Min Max Average STDV d 9 DF d 9 SPF d 9 H d 9 DF d 9 SPF d 9 H d 9 DF d 9 SPF d 9 H d 9 DF d 9 SPF d 9 H d 9 DF d 9 SPF d 9 H d 9 DF d 9 SPF d 9 H d 9 DF d 9 SPF d 9 H d 9 DF d 9 SPF d 9 H d 9 DF d 9 SPF d 9 H d 9 DF d 9 SPF d 9 H d 9 DF d 9 SPF d 9 H d 9 DF d 9 SPF d 9 H Note: DF = Douglas-fir, SPF = Spruce-Pine-Fir, H = Hemlock

40 Avarege withdrawal resistance [kn] Avarege withdrawal resistance [kn] Würth 9º angle Douglas-fir S-P-F Hem.-fir... d 1d 1d 1d Embedment depth Figure 1: Average withdrawal resistance for mm 9º Würth 9º angle Douglas-fir S-P-F Hem.-fir.. d 1d 1d 1d Embedment depth Figure 17: Average withdrawal resistance for mm 9º 7

41 Avarege withdrawal resistance [kn] Würth 1 9º angle Douglas-fir S-P-F Hem.-fir.. d 1d 1d 1d Embedment depth Figure 1: Average withdrawal resistance for 1 mm 9º Results shown in Figures 1 to 1 indicate a general trend that the average values for the withdrawal resistance following the same trend as the average densities. With an increased average density, the values for the average withdrawal resistance are also increased. The above stated effect of the embedment depth as well as the screw diameter is also clearly seen in the graphs. The permissible characteristic tensile strength of the screws as per the German general construction approval are 11.3 kn for the mm screws, 1.9 kn for the mm screws and. kn for the 1 mm screws. Granted the fact that the permissible characteristic values for the tensile strength of the steel are th percentile value and the shown withdrawal values are average values, it can still be seen that the embedment depth of 1d almost always cause the screw to fail not the wood in withdrawal. A closer look at this will be taken in the discussions section of this study.

42 Table : Withdrawal test results for Test configuration Withdrawal Capacity [kn] Ø [mm] Embedment Angle [ ] Species Min Max Average STDV d DF d SPF d H d DF d SPF d H d DF d SPF d H d DF d SPF d H d DF d SPF d H d DF d SPF d H d DF d SPF d H d DF d SPF d H d DF d SPF d H d DF d SPF d H d DF d SPF d H d DF d SPF d H Note: DF = Douglas-fir, SPF = Spruce-Pine-Fir, H = Hemlock 9

43 Avarege withdrawal resistance [kn] Avarege withdrawal resistance [kn] Würth º angle Douglas-fir S-P-F Hem.-fir... d 1d 1d 1d Embedment depth Figure 19: Average withdrawal resistance for mm º Würth º angle Douglas-fir S-P-F Hem.-fir.. d 1d 1d 1d Embedment depth Figure : Average withdrawal resistance for mm º 3

44 Avarege withdrawal resistance [kn] Würth 1 º angle Douglas-fir S-P-F Hem.-fir.. d 1d 1d 1d Embedment depth Figure 1: Average withdrawal resistance for 1 mm º The values for inclination angles of, as well as 3, are exhibiting similar trends as seen for the perpendicular to the grain installed screws. However, the effect of the increased effective embedment depth leads to screw failure in a few cases at a depth of 1d since the effective depth would be about 1d compared to the 9 degree tests (Table 1). This is even more apparent for angles of 3 degrees where almost all specimens at 1d already shown steel failure in the screw instead of withdrawal failure in the wood. The calculated embedment depth for perpendicular installed screws would be in the range of 1d to 1d depending on the density of the wood. Figure depicts typical screw failure as it is reached when the embedment depth exceeds the limit the steel of the screw can transfer into the wood. Figure shows a typical load displacement plot of the withdrawal test, all plots can be found in the appendix. 31

45 Table : Withdrawal test results for 3 Test configuration Withdrawal Capacity [kn] Ø [mm] Embedment Angle [ ] Species Min Max Average STDV d 3 DF d 3 SPF d 3 H d 3 DF d 3 SPF d 3 H d 3 DF d 3 SPF d 3 H d 3 DF d 3 SPF d 3 H d 3 DF d 3 SPF d 3 H d 3 DF d 3 SPF d 3 H d 3 DF d 3 SPF d 3 H d 3 DF d 3 SPF d 3 H d 3 DF d 3 SPF d 3 H d 3 DF d 3 SPF d 3 H d 3 DF d 3 SPF d 3 H d 3 DF d 3 SPF d 3 H Note: DF = Douglas-fir, SPF = Spruce-Pine-Fir, H = Hemlock 3

46 Avarege withdrawal resistance [kn] Avarege withdrawal resistance [kn] Würth 3º angle Douglas-fir S-P-F Hem.-fir... d 1d 1d 1d Embedment depth Figure : Average withdrawal resistance for mm 3º Würth 3º angle Douglas-fir S-P-F Hem.-fir.. d 1d 1d 1d Embedment depth Figure 3: Average withdrawal resistance for mm 3º 33

47 Avarege withdrawal resistance [kn] Würth 1 3º angle Douglas-fir S-P-F Hem.-fir.. d 1d 1d 1d Embedment depth Figure : Average withdrawal resistance for 1 mm 3º Figure : Typical screw failure 3

48 Load [kn] Würth mm 9 deg. - S-P-F Figure : Typical load deformation plot The densities and moisture content for all specimens have been recorded during the testing. A needle moisture meter was used to measure the moisture content, driving the needles about mm in to the specimen. The moisture content varied from 7.% to 13.% with most specimens having moisture contents between.% and 1.%. To establish the density of the specimens, the specimens were weighed at the ambient climate and their respective moisture contents and measured in all dimensions to establish their volume. The density was then calculated using the measured weight and volume of the specimens. Densities for all specimens, regardless of species, have been sorted in density groups with kg/m 3 increments and a normal distribution has then been fitted, as shown in Figure 7. Figure 7 shows that a normal distribution fits quite well to the recorded density values. 3