When it comes to modernizing building codes and standards, ensuring the buildings in which we live and work are safe is a top priority for the American Wood Council (AWC). AWC experts work to achieve this through ongoing research and development of design standards and guidelines that demonstrate how wood products can safely and efficiently meet or exceed U.S. requirements.
With technological advances and material innovations regularly affecting the use of wood products, AWC is committed to ensuring these changes are accurately reflected in the codes and properly implemented by industry professionals. Now that the International Code Council’s 2018 International Building Code (IBC) is available for enforcement, AWC has updated its National Design Specification (NDS) for Wood Construction (http://awc.org/codes-standards/publications/nds-2018) accordingly.
The 2018 NDS was developed by AWC’s Wood Design Standards Committee and is referenced for wood design in the 2018 IBC. It has been approved as an American National Standard by the American National Standards Institute (ANSI). Significant additions to the 2018 NDS include new Roof Sheathing Ring Shank nails and fastener head pull-through design provisions to address increased wind loads in ASCE 7-16 Minimum Design Loads and Associated Criteria for Buildings and Other Structures.
More specifically, changes in the 2018 NDS include:
- allowance for incising factors for specific incising patterns and lumber sizes when obtained from the company providing the incising;
- inclusion of a volume factor for structural composite lumber tension parallel to grain values;
- inclusion of effective shear stiffness for cross-laminated timber;
- added equation for withdrawal design values for smooth shank stainless steel nails;
- new provisions for Roof Sheathing Ring Shank (RSRS) nails in accordance with ASTM F 1667;
- new design provisions for fastener head pull-through of fasteners with round heads;
- revision to method for calculation of lateral design values for threaded nails to be based on use of shank diameter instead of root diameter in accordance with changes in ASTM F 1575;
- revised timber rivet design value tables to limit maximum distance perpendicular to grain between outermost rows of fasteners; and
- revised terminology for Fire Design of Wood Members to clarify the difference between “char depth” and “effective char depth” used in structural calculations. Provisions for connections is also revised to more precisely describe the requirements for protection of the connection from fire exposure.
Structural composite lumber
NDS Chapter 8 on structural composite lumber (SCL) was revised to include a volume factor, Cv, for tension parallel to grain design values, Ft. A change was also made to clarify that dry service conditions are associated with conditions in which the moisture content of sawn lumber is less than 16 percent, as in most covered structures. These changes correlate with ASTM D 5456 Standard Specification for Evaluation of Structural Composite Lumber Products.
Revisions were made to cross-laminated timber (CLT) deflection provisions to include the term GAeff (effective shear stiffness of the CLT section). This change correlates with ANSI/APA PRG 320-2017 Standard for Performance-Rated Cross-Laminated Timber — to facilitate the calculation of apparent bending stiffness (EI)app consistent with properties as provided in PRG 320 (see Equation 10.4-1).
Fire Design of Wood Members
NDS Chapter 16 on Fire Design of Wood Members was revised to provide separate calculations of char depth based on nominal char rates for wood, achar, and effective char depth for use in structural calculations, aeff. Increased use of wood as a fire protective covering has made it important to provide provisions for calculation of the expected achar separate from aeff. Previous versions of the NDS have only provided aeff, which is increased 20 percent over achar to account for loss of strength and stiffness due to elevated temperatures in uncharred wood near the char front.
Revision of NDS connection design provisions were primarily in response to significant increases in Components and Cladding (C&C) roof wind pressures in ASCE 7-16 Minimum Design Loads and Associated Criteria for Buildings and Other Structures. Primary changes include:
• Added equation for stainless steel nail withdrawal strength: Stainless steel nails have lower withdrawal strength compared to carbon steel wire nails of the same diameter due to the reduced surface friction of stainless steel. The differences in withdrawal strength vary with the specific gravity of wood. When stainless steel nails are specified as an alternative to reference smooth shank carbon steel wire (bright or galvanized) nails in wood construction, including shear walls and diaphragms, these differences in withdrawal strength must be considered. For example, where smooth shank stainless steel nails are used for roof sheathing attachment, more nails, or nails of greater strength or diameter, may be required to provide equivalent withdrawal strength performance for wind uplift.
• Added design provisions for RSRS nails: RSRS nails, which have higher withdrawal design values than smooth shank nails, were recently added to ASTM F 1667 Standard Specification for Driven Fasteners: Nails, Spikes, and Staples. RSRS nails provide additional options for efficient attachment of wood structural panel sheathing. In many cases, specification of RSRS nails will produce a reduced roof sheathing attachment schedule than permissible by use of smooth shank nails, and enable the use of a single minimum fastener schedule for roof perimeter edge zones and interior zones (see Figure 1).
• Added new fastener head pull-through provisions: Analysis of fastener head pull-through data, used to set industry recommendations for wood structural panels, combined with historical data from tests of lumber and plywood, was analyzed to develop new fastener head pull-through provisions. Within the range of head diameters, thicknesses, and specific gravities in the NDS, the analysis found that head pull-through is related to the perimeter of the fastener head. For the design of roof sheathing fastening to resist wind uplift, the addition of head pull-through allows the controlling roof sheathing fastener spacing to be calculated from the lesser of the head pull-through design value or the fastener withdrawal design value from wood in accordance with the NDS. Previously, such design required use of a combination of design documents, including minimum prescribed spacing criteria for wood panels (see Table 12.2F).
• Revised provisions for specifying diameter for threaded nails: A change in ASTM F 1575 Standard Test Method for Determining Bending Yield Moment of Nails clarifies that nail bending yield strength, Fyb, is based on the nominal diameter, D, not on the root diameter, Dr. This change allows simplification of the calculation of nail moment resistance in the NDS yield equations for nails specified in F 1667 because D is provided for all nail types, but Dr is not always provided for deformed shank nails. As a result, the revised NDS provisions allow the use of D for deformed shank nails per F 1667 when calculating lateral design values in accordance with the yield limit equations.
It is also worth noting that NDS Supplement design values remain unchanged with only a few exceptions. New and revised grades of machine stress-rated lumber and machine evaluated lumber have been added. Redwood grades requiring “close grain” were removed due to general lack of availability for commercial use. Other revisions include the addition of Norway Spruce from Norway to foreign species dimension lumber, and the addition of shear-free moduli of elasticity for structural glued laminated softwood timber (glulam).
To make this code-referenced standard as user friendly as possible, AWC will also be developing supporting publications such as an NDS Commentary, revising technical reports and design aids, and conducting further research to ensure continued widespread acceptance of wood in construction.
The 2018 NDS and 2018 NDS Supplement: Design Values for Wood Construction are currently available online as free electronic downloads in PDF format (www.awc.org/codes-standards/publications/nds-2018). The NDS Commentary and other support documents will be available in the coming weeks as part of AWC’s “2018 Wood Design Package.” At that time, print versions of the 2018 NDS with Commentary and 2018 NDS Supplement, as well as printable PDFs, will be available for purchase.
In the meantime, AWC thanks the volunteers on our Wood Design Standards Committee for their work on this important publication. The 2018 NDS represents the state-of-the-art for design of wood members and connections.
Brad Douglas, P.E., is vice president of engineering at the American Wood Council (AWC; www.awc.org), which represents the interests of the North American wood products industry. On behalf of the industry it represents, AWC is committed to ensuring a resilient, safe, and sustainable built environment. To achieve these objectives, AWC contributes to the development of sound public policies, codes, and regulations that allow for the appropriate and responsible manufacture and use of wood products.