R I T DESIGN AND TESTING OF A HIDDEN DECK FASTENER FOR SYNTHETIC LUMBER MD F

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1 R I T Proceedings of KGCOE-MD23: Multi-Disciplinary Engineering Design Conference Kate Gleason College of Engineering Rochester Institute of Technology Rochester New York May, 23 MD23-28F DESIGN AND TESTING OF A HIDDEN DECK FASTENER FOR SYNTHETIC LUMBER Jared Dolatowski Mechanical Engineering MS/BS Mike Steger Mechanical Engineering BS Dan Willistein Mechanical Engineering BS ABSTRACT A hidden deck fastener for use in synthetic lumber has been designed, fabricated, and tested. The fastener design is the result of FEA modeling and experimental analysis. The fasteners are fabricated on a progressive die, out of 31 stainless steel. Testing has been performed and compared to typical decking screws to determine feasibility. Keywords: decking, hidden deck fasteners, synthetic lumber INTRODUCTION Tiger Claw Inc. is a small company based out of Connecticut that has developed into a leader in the deck building industry. Their primary product is the hidden deck fastener known as the Tiger Claw #1 (TC1). The TC1 is a simple device used to install the surface of a wooden deck without the use of visible nails or screws. The fastener is relatively inexpensive, easy to install, and very strong. The current product works extremely well in standard medium density wood decking, however, it is over-designed, and does not function well in synthetic lumbers Figure 1: Existing Design [1] The current TC1 design securely attaches decking boards by holding the edges of the boards, rather than screwing or nailing through the top. This leaves the fastener virtually invisible. The first step of installation is to hammer the fastener into the first deck board, and then secure it to the joist with a common decking screw. Once securely screwed down, the next board is hammered onto the remaining two spikes. To see how it is installed see Fig. 2. The owners of Tiger Claw Inc. initially drew the current design on a bar room napkin. It was designed to work well, be easy to install, and be very strong. However, the design was not optimized to reduce the amount of material used in the part. The existing TC1 fastener can be seen in Fig Rochester Institute of Technology

2 Proceedings of KGCOE-MD23: Multi-Disciplinary Engineering Design Conference Page 2 The resulting fastener must be strong, easy to install, and relatively inexpensive. The cost of retooling for part production was not within the scope of the project. NOMENCLATURE TC1 Tiger Claw 1, fastener for typical decking Tiger Claw 3, fastener for synthetic lumber CONCEPTUAL DESIGN Figure 2: Installation of the TC1 When compared to nails and screws, hidden deck fasteners: a) are virtually invisible. b) reduce the potential for wood rot (which can begin around the head of screws and nails). c) eliminate splits at ends of boards caused by screws and nails. d) give a deck a smooth professional appearance (Fig. 3). Figure 3: Picture Of A Deck Surface Using The TC1 e) reduce cupping because they hold deck boards from the edges. f) do not restrict decking when boards shrink so wood splitting is not a problem. g) do not damage the decking surface during installation. h) create a nail-free surface which is easier to maintain. i) do not corrode, so the deck surface isn't stained. j) extend the life of a deck, and reduces long-term ownership costs. The TC1 functions extremely well in standard decking materials, however, it does not in synthetic lumber. The TC1 pulls out of the synthetic lumber very easily and does not have the rigidity needed for installation. Tiger Claw Inc. would like to have a fastener that would work in synthetic lumber due to its increasing popularity in the deck building industry. The existing TC1 fastener is a well-designed product. It functions well with standard decking lumber, the material for which it was designed. The TC1 also functions well in less than ideal deck building situations. This means it can handle the added stresses of warped boards and off center hits of a hammer. However, the TC1 does have major weaknesses in synthetic lumbers. The design is weak because of its triangular spike and its overall thickness. The triangular spike of the TC1 makes installation easy, but an installed fastener can be removed from its secure position by hand. This situation occurs in standard lumber, but is more prominent in synthetic lumber. Synthetic lumber has an interesting mechanical property; once it is deformed it stays that way. The lumber does not form around the spike when it is installed, hence making the fastener extremely easy to pull out. In general, lumber has the tendency to shrink and expand depending on temperature and length of environmental exposure. Over time, this could have the effect of changing the position of the spike as shown in Fig. 4. The top row of pictures shows a triangular spike and a nail type spike fully inserted into the lumber, shown in red. If the triangular spike moves outward even a small amount, the entire length of the spike loses contact with the lumber. This diminishes the integrity of the deck, both in strength and in safety. However, if the nail type spike moves outward, there is still a large amount of surface area in contact with the lumber. This contact maintains the integrity of the deck. Figure 4: Triangular Spike vs. Nail Spike The design is virtually identical to the existing TC1, although its spikes are straight rather than triangular. This increases the amount of fastener surface area in contact with the wood (Fig. 4). The initial design concept can be found in Fig. 5. Paper Number MD23-28F

3 Proceedings of KGCOE-MD23: Multi-Disciplinary Engineering Design Conference Page 3 Figure 5: Model Of The The four main spikes are.5 inches (12.7 mm) long,.125 inches (3.175 mm) wide, and the thickness is dependent on application. The overall length of the fastener is 1.75 inches (44.45 mm). The fastener for the standard decking and the synthetic lumber would use 11 half-hard cold-rolled steel at a thickness of.6 inches (1.524 mm). The spikes on the bottom of the fastener were retained from the original TC1 fastener after experimentation with the fastener revealed that they help stabilize the fastener during installation. However, they were rounded to reduce the number of sharp edges in order to maximize die life. The installation procedure would be the same as the preexisting TC1 fastener as described earlier. A computer model of a deck was created in order to quickly test this concept for its installation ease. This model can be found in Fig 6. Analysis Figure 7: Model Of A Butt Joint With The Two FEA models were generated of the. The first model was a worst-case installation scenario in which the fastener was essentially driven into a rigid board. The second model was of a failure mode in which the prongs were driven upward, simulating warping of a deck board. The installation model is shown in Fig. 8 below. The boundary conditions were applied to simulate the use of an installation block with the force distributed on the flange of the fastener. The ends of the spikes were constrained in all directions to model the spikes digging into the board. The bottom of the fastener was not allowed to move downward in order to simulate the presence of a joist. It was found that the maximum stress was 339 ksi (2.34 Gpa). This value is an order of magnitude higher than the yield stress of the 11 steel used in the model. If the fastener geometry were optimized to handle this loading condition, the fastener would be extremely overdesigned. The yield strength of the lumber is on average an order of magnitude less than the yield strength of the metal. Therefore, the lumber would deform before this condition could arise. The FEA model was used in a qualitative sense to determine locations of high stress areas. Figure 6: Installation Model With In the event that a butt-end-joint needs to be constructed, the fastener would need to be installed backwards. This is done to keep the spikes away from the end of the board to prevent splitting. This concept is shown in Fig 7. The deck boards have been made transparent to make viewing easier. Flange Figure 8: Installation FEA Model Of The Copyright 23 by Rochester Institute of Technology

4 Proceedings of KGCOE-MD23: Multi-Disciplinary Engineering Design Conference Page 4 Due to the fact that the stresses found in the installation FEA model could not be used, experimental testing was employed to determine the optimum geometry. The locations of the high stress areas found in the FEA analysis were used to develop the initial prototype. Different length spikes, as well as different width spikes, were evaluated to find the optimum values. For all three varieties of decking, it was found that the length of the spike should be.5 inches (12.7 mm) long and.125 inches (3.175 mm) in width. The second FEA model was of a possible failure mode of the fastener using the optimized geometry found experimentally. This model is shown in Fig. 9. The model simulates the stresses induced due to the warping of the deck boards. The applied force is distributed over the entire length of the spikes as if it were fully installed into two deck boards. The boundary conditions simulate a rigid connection to the joist. It would require a load of 66 lbf (293.6 N) distributed over the spikes to cause failure in this mode. 1. The height of the fastener was decreased significantly in order to strike the decking board lower on the side 2. An elongated hole was added to account for the linear expansion that occurs in synthetic lumber. Some synthetic lumbers can expand up to.375 inches (9.525 mm) over the entire length of the board. 3. The spikes that were on the lower edge of the flange have been removed so the fastener can move horizontally, again to account for linear expansion effects. 4. Supporting braces were added to the secondary spikes to add rigidity to the fastener as a whole. 5. The material of the fastener was changed to 31 stainless steel to hold up for the extended life of synthetic lumber. This is not indicated in Fig. 1. Figure 1: Prototype Fastener Figure 9: Warping FEA Model Of The Interpretations of Findings Analysis and experimental testing proved that this fastener was physically strong enough to handle installation and strong warping forces. This fastener also has the visual appeal necessary for marketing value. The provides direct connection of the decking boards to the supporting joist below, through the use of a standard decking screw. This provides the necessary rigidity to overcome any force due to warping. FASTENER FOR PROTOTYPICAL TESTING Tiger Claw Inc. decided to use the basic idea of the design that was recommended to them. However, they did make a few changes to the design based on the experiences of their construction experts. Figure 1 indicates the changes made by Tiger Claw Inc. and represents the fastener design used for testing. The changes made are summarized below with their numbers corresponding to the numbers in Fig.1. PROTOTYPICAL TESTING Testing was completed to prove feasibility in synthetic lumber by destructively testing model deck sections. Feasibility testing included qualifying the new fastener in installation, upheaval, and shear modes with typical decking screws being used as controls. The test sections were constructed out of the two most popular synthetic lumbers, Trex and Fiberon. Small deck sections were constructed with 4 fasteners installed between three deck boards mounted on a joist frame. The sections measured 21 inches (533 mm) by 17.5 inches (445 mm). This geometry modeled a realistic deck system of fasteners and lumber. Figure 11 shows the deck section mounted for upheaval testing. In this mode, force is applied to the center deck board which is connected to the rest of the assembly with fasteners only. The outer boards are screwed to the joist frame on the outer edges modeling an otherwise rigid deck. Paper Number MD23-28F

5 Proceedings of KGCOE-MD23: Multi-Disciplinary Engineering Design Conference Page 5 Figure 11: Deck Section Mounted For Upheaval Test In testing the deck section it was found that the was not able to withstand the same force that a typical decking screw was. This can be seen in Fig. 12 and Fig. 13, which are the load versus time plots for the upheaval tests in Trex and Fiberon Upheaval Comparison - Fiberon Figure 14: Screw Hole Failure The third failure was a result of the lowered height of the fastener. The lower height caused the fastener to strike the synthetic lumber too low on its side. The result was that when load was applied, the lumber actually ruptured. This failure is pictured in Fig Figure 12: Upheaval Test In Fiberon Upheaval Comparison - Trex Figure 15: Material Rupture Failure Figure 16 shows the deck section mounted for shear testing, which includes a support bracket added for safety. The center deck board is tested in a similar fashion as in the upheaval mode; the only difference being the direction of force applied Figure 13: Upheaval Test In Trex There were three reasons for this difference. The first reason is that the material chosen was a fully annealed stainless steel. This material does not have the strength to withstand the higher loads. The second reason is that the elongated hole in the center of the fastener is so large that the screw actually pulls through it. This failure mode can be seen in Fig. 14. The stair step effect in Fig. 12 and Fig. 13 is due to this failure. Figure 16: Deck Section Mounted For Shear Test In testing the deck section it was found that the was able to withstand a similar force to that of the typical decking screw This can be seen in Fig. 17 and Fig. 18, which are the load versus time plots for the shear tests in Trex and Fiberon. Copyright 23 by Rochester Institute of Technology

6 Proceedings of KGCOE-MD23: Multi-Disciplinary Engineering Design Conference Page 6 25 Shear Comparison - Fiberon Figure 17: Shear Test In Fiberon Figure 19: Fastener Failure In Shear CONCLUSIONS/RECOMMENDATIONS Shear Comparison - Trex It is estimated that synthetic lumber accounts for 8% of the lumber used in deck building. The will effectively corner this market because it is one of the few commercially available fasteners that is physically strong and cost effective Figure 18: Shear Test In Trex The fastener was able to allow for approximately.25 inches (6.35 mm) of movement without damage to the fastener itself. This is a significant amount of movement and will help to limit the issues associated with the linear expansion of the lumber. Manufacturer recommendations are to use typical decking screws, which allow for only limited linear expansion. Failure of the fastener was at 2.25 inches (57.15 mm) of movement. At this time, the outside spikes were bent parallel to the fastener and were no longer able to hold any load. This failure is shown in Fig The fastener performed well in the shear feasibility tests, and worked adequately in the upheaval test. While the force handled by the in the upheaval mode is adequate, several design changes should be made to improve its performance. The material of the fastener should be changed to a hardened material, such as quarter-hard 34 stainless steel. The second design change recommended is to decrease the size of the hole used to securely fasten the fastener to the joist. This would help to alleviate the problem that could occur if the screw pulled through the fastener. The third design change is to increase the flange height of the fastener to eliminate rupturing that may occur in the actual synthetic lumber. ACKNOWLEDGMENTS Team 8F would like to thank Dave Hartmann and Dave Martel of Tiger Claw Inc. for their support during the design and testing process. Team 8F would also like to thank Dr. Alan Nye and Dr. Edward Hensel of the Rochester Institute of Technology for their continued support and advising. REFERENCES [1] Tiger Claw Inc., 2 April 23. Product Page. 21 April 23. < Paper Number MD23-28F