Foard Panel Surface Spline Joinery Standard INTRODUCTION

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Beverly Franklin
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1 Foard Panel Surface Spline Joinery Standard PAUL MALKO, TECHNICAL DIRECTOR FOARD PANEL, INC. INTRODUCTION The majority of Foard s in-plane SIP-to-SIP joinery is accomplished with double surface splines. See Figure 1. The in-plane shear capacity of this joint and fastening requirements are documented in Foard s EPS and NEO SIP evaluation report. 1 Some projects, however, cannot use the standard double surface spline joint because of previously installed interior finishes, the location of supporting beams, or other details. This bulletin discusses some of the alternative in-plane SIP joint types that lack 3rd party documentation and Foard's rationale for our assigned in-house design capacity. GENERAL DESIGN ELEMENTS NOTES In-plane shear (racking) is the primary load applied to spline joints. SIP spline joints have little stiffness or capacity for transverse bending or out-of-plane (thru-thickness) shear. The only out-of-plane capacity Foard claims for surface spline joinery is to help maintain basic, cosmetic alignment of the OSB skin against wood movement and other minor building movements over time. Many applications fall outside the exact configurations described within the evaluation report. This bulletin describes the methods, assumptions, and internal design standards used by Foard Panel s in-house designers for these cases. The project s licensed design professional is responsible for evaluating the applicability of the evaluation report and Foard s design methods to a particular project. 1 NTA Listing Report FRD , dated 3/9/17 PO Box Stow Dr. West Chesterfield, NH Page 1 of 5

2 This document may be viewed as one potential design analysis approach, but should not be regarded as the definitive, industry-standard design guide. SPLINE MATERIAL Foard s standard splines are 19/32 thick, 2-13/16 wide, C-D Exposure 1 (CDX), Species Group 2 plywood delivered in 8 lengths, (nominally 5/8 thick x 3 wide). 2 Foard uses this spline instead of the industry standard 7/16 thick x 3 wide OSB for two reasons. The thicker plywood splines provide more attachment options for porches, awnings, and other features at the splines joints. Also, the greater fastener retention of the 5/8 plywood reduces installation labor as pneumatically driven spline nails are less likely to bounce and require re-tightening with a hammer. 7/16 x 3 OSB splines were used during the original testing that underpins Foard's evaluation report. Under in-plane shear loading, the capacity of a surface spline joint is limited by deflection beyond practical limits. The largest source of this deflection is the rotation of the nail. When viewing a nail s single-shear capacity from the viewpoint of the NDS Chapter 11, the very thin main member of a SIP surface spline joint is clearly a key factor. 3 We argue that Foard s standard surface splines provide capacity at least equal to the 7/16 OSB spline. Foard doesn t have 3rd party testing or an evaluation of SIP joinery using Foard s standard spline. On the other hand, the properties of 5/8 plywood listed in the NDS are superior to those of 7/16 OSB (including dowel bearing strength, once thickness is considered). SPLINE FASTENERS Nails are used to fasten SIP skins to the splines. The nails specified in Foard s evaluation report are dia. x 2.5 long nails. These dimensions dovetail with the definition of an 8d common nail in ASTM F1667, the standard used by Chapter 11 of the NDS. 4 While the nail length may be excessive for use in surface splines, they are a standard size and the same nails can be used for structural splines and edge blocking. SIP assemblies are highly dependent on their fasteners for their in-plane shear capacity. This is true for both spline fasteners and perimeter blocking fasteners. The dominant failure mode for nailed spline assemblies is tear-out the nail at the edge of the SIP skin. Staples and screws have been investigated, but these fasteners experienced fastener failures before OSB SIP skin failures and provided less capacity than nails. 5 A word of caution: We have noticed that many brands of collated nails labeled 8d are of smaller diameter than the industry standards require. For this reason, Foard has abandoned the pennysize description on our drawings, listing instead the minimum nail diameter and minimum nail length. We recommend that the actual nail size be checked on the packaging, or measured directly, for all nails used for SIP joinery. When the ideal nails are unavailable, Foard recommends meeting the minimum diameter requirement, even if the result is using a longer nail. SIP CORE TYPE SIPs are far from the only structural materials where the strength-limiting considerations revolve around the connections. The limits of the fundamental material itself are rarely reached. This holds true 2 Plywood specifications as defined in APA Form No. F405M, dated American Wood Council NDS (National Design Specification for Wood Construction), 2015 Edition 4 ASTM F , Table 15, Type I, Style 10 Common Nails, 8d, Identifier NL CMS-07Z 5 Parametric Study of Structural Insulated Panels Under Monotonic and Cyclic Loading, Terentiuk & Memari, 2009, Pennsylvania State University PO Box Stow Dr. West Chesterfield, NH Page 2 of 5

3 for in-plane shear stress applied to Foard SIPs. For this reason, we argue that the foam core type has limited influence on a SIP's in-plane shear capacity. The in-plane shear properties listed in Foard s evaluation report are for EPS and NEO core SIPs only. We argue that the in-plane shear properties of XPS and PIR panels are substantially similar because the skin materials, fasteners, and joint geometry are the same. The primary contributions of the core to in-plane shear capacity is transferring shear load from one skin to another and stabilizing the OSB skin from out-of-plane buckling. When in-plane shear loads are transferred to the SIP from an underlying structure via SIP screws, it is assumed that all of the shear load is transferred from the screw shank to the interior skin. The flexibility of the screw shank prevents load transfer to the exterior skin. The SIP core experiences shear stress as it transfers this shear load between skins. SIP assemblies connected by double surface spline joints transfer in-plane across the joint by sharing the load equally between the SIP skins. See Figure 2. In cases where single surface spline joints are used, all of the in-plane shear load must be transferred between SIPs via the exterior skin. See Figure 3. In all but the most extreme cases, where the screw-fastened SIP is very small, the stiffness and shear strength of all four of Foard's core materials combines with the core's engaged area to form a shear capacity much higher than that of the spline joint. While an OSB skin is experiencing in-plane shear, the stabilizing stress required to prevent out-of-plane buckling is very small. The buckling-resistance stress is low enough to be well within Foard s lamination bond strength and the properties of all of Foard s foam cores. After dozens of field investigations and decades of experience, we have yet to find any evidence of out-of-plane skin buckling caused by in-plane shear stress. SIP THICKNESS The SIP industry consensus is that a SIP's foam core thickness has little impact on in-plane shear capacity. Testing has revealed failure modes that revolve only around fasteners, splines, and boundary effects, all independent of core thickness. This dovetails with Special Design Provisions for Wind Seismic, which is silent on stud/joist/rafter depth for shear walls and horizontal diaphragms. 6 6 American Wood Council Special Design Provisions for Wind and Seismic (SDPWS), 2015 Edition PO Box Stow Dr. West Chesterfield, NH Page 3 of 5

4 ALTERNATIVE SURFACE SPLINE TYPES SINGLE SURFACE SPLINE Foard s evaluation report lists in-plane shear strength design values for EPS and NEO SIP joinery with single surface splines connecting the exterior skins of adjacent SIPs. 7 See Figure 4. Generally, Foard uses these values for all core types and across all SIP thicknesses through 17 for the reasons described earlier. SIPs thicker than and cases where the design requires that a fabricated SIP be very small should be examined individually because the question of shear transfer between SIP skins arises. 6 WIDE DOUBLE SURFACE SPLINE Foard has developed a surface spline joint for increased in-plane shear strength. Shear wall testing has revealed that the controlling failure modes are Mode II-style spline nail rotation and OSB edge tear-out. 8,9 Adding more fasteners increases shear capacity. The 6 wide double surface spline doubles the number of nails from the default double surface spline joint, with all of the additional nails having a much larger edge distance. See Figure 5. In cases where the shear loading is being applied or resisted through the SIPs edge blocking, double thickness or 4x edge blocking, and increased fastening, is specified around the entire SIP shear assembly. While it could be argued that this would double the shear strength of the joint, we don t have enough testing to rule out the possibility that other failure modes may exist. For this reason, Foard s 7 NTA FRD , Table 12 8 Mode II per American Wood Council NDS (National Design Specification for Wood Construction), 2015 Edition, Appendix I 9 Terentiuk, 2009 PO Box Stow Dr. West Chesterfield, NH Page 4 of 5

5 internal design standard is to assume the 6 wide double spline joint has a design value of 175% of the capacity of the typical double surface spline. DOUBLE SURFACE SPLINES OVER INTERIOR FINISHES SIPs are often installed on top of a ceiling finish, T&G planking for example. The ceiling finish prevents access to the interior side of the SIP for fastening the interior surface spline. In many small, residential projects, the diaphragm loads are low enough for single surface splines to be adequate. For many larger projects, the contribution of the interior-side spline is critical. Whenever practical, the whole plane, or surface, of SIPs is pre-assembled on the ground, where both sides of the surface are accessible for fastening. When a surface of SIPs is too large to be lifted as one piece, the surface is broken into sections. Each section is assembled on the ground with the section s mating edges blocked with 2x KD lumber. The sections are craned into place individually and fastened to the underlying structure. The adjacent sections are fastened to each other with toe-screws, as shown in the right half of Figure 6. The nailed connection of the SIP to the 2x KD splines is at least equivalent to that of double surface splines. 10 Fully threaded wood screws (typically Simpson SDWC15600) are used to connect the two 2x KD splines together. Using fully threaded screws avoids the requirement for the screw head to bear on the 2x, which is buried in the SIP and is at an angle to the screw. The fully threaded screws have very small heads, so driving the screw deeply enough to fully engage with the far-side spline is easy, even if the screw head is buried in the SIP. Foard s default procedure is to specify the 2x to 2x connection to be at least equal shear capacity to that of the section s double surface spline joint. 11 This connection between sections creates an unavoidable thermal bridge. Foard s design team works with the installers to develop a section design that is practical to install while also minimizing the number of double 2x splines. 10 Terentiuk, Simpson SDWC15600 screws are dia. x 6 long. Foard uses the values from NDS-2012, Table 11R, t s=1.5, D=0.151, G=0.36 min., thus Z = 59 lb/screw. Therefore, screws 3 OC from each side = 8 screws/ft = 472 lb/ft which is greater than the double surface spline joint s capacity. PO Box Stow Dr. West Chesterfield, NH Page 5 of 5

ICC-ES Evaluation Report Reissued June 1, 2010 This report is subject to re-examination in one year.

ICC-ES Evaluation Report Reissued June 1, 2010 This report is subject to re-examination in one year. ICC-ES Evaluation Report ESR-2648 Reissued June 1, 2010 This report is subject to re-examination in one year. www.icc-es.org (800) 423-6587 (562) 699-0543 A Subsidiary of the International Code Council