Equation 10.4-1: Calculation of apparent bending stiffness
Figure 1:Withdrawal strength for smooth shank nails
roof wind pressures in ASCE 7-16 Minimum Design Loads and As- sociated Criteria for Buildings and Other Structures. Primary changes include: • Added equation for stainless steel nail withdrawal strength: Stain- less 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 dia- phragms, 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 recent- ly 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 at- tachment schedule than permissible by use of smooth shank nails, and enable the use of a single minimum fastener schedule for roof perim- eter 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-
Structural composite lumber NDS Chapter 8 on structural composite lumber (SCL) was revised to include a volume factor, C v , for tension parallel to grain design values, F t . A change was also made to clarify that dry service conditions are as- sociated 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. Cross-laminated timber Revisions were made to cross-laminated timber (CLT) deflection provisions to include the term GA eff (effective shear stiffness of the CLT section). This change correlates with ANSI/APA PRG 320-2017 Standard for Performance-Rated Cross-Laminated Timber — to facili- tate 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, a char , and effective char depth for use in structural calcula- tions, a eff . Increased use of wood as a fire protective covering has made it important to provide provisions for calculation of the expected a char separate from a eff . Previous versions of the NDS have only provided a eff , which is increased 20 percent over a char to account for loss of strength and stiffness due to elevated temperatures in uncharred wood near the char front. Fastener Design Revision of NDS connection design provisions were primarily in re- sponse to significant increases in Components and Cladding (C&C)
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