What the 2018 IBC means for wood construction: Part 2


    Ongoing development in high-wind and high-seismic areas increases the number of buildings exposed to extreme conditions. The construction industry must play an active role in enhancing resiliency, especially in these high-risk areas. Fortunately, tools and resources available today better enable planning and mitigation of such conditions, which has led to continuous building code improvements and updated performance criteria to make communities safer and more resilient.

    With technological advances and material innovations regularly affecting the use of wood products, the American Wood Council (AWC) is committed to ensuring these changes are accurately reflected in code-referenced design standards and properly implemented by industry professionals. The first article in this two-part series (Civil + Structural Engineer, March 2018, page 33; https://csengineermag.com/article/2018-ibc-means-wood-construction-part) outlined changes made to the 2018 National Design Specification (NDS) for Wood Construction referenced in the International Code Council’s 2018 International Building Code (IBC). This article examines changes made to the 2018 Wood Frame Construction Manual (WFCM) for One- and Two-Family Dwellings standard, which is available online at www.awc.org/codes-standards/publications/wfcm-2018.

    The 2018 WFCM was developed by AWC’s Wood Design Standards Committee and is referenced in the IBC and the 2018 International Residential Code (IRC). The publication offers both practical guidance and technical detail, covering wood design and construction provisions for connections, wall systems, floor systems, and roof systems for one- and two-family dwellings, as well as a range of structural elements such as sawn lumber, structural glued laminated timber (glulam), wood structural sheathing, I-joists, and trusses.

    Framing members attached with properly designed fasteners and connectors help to resist forces associated with high wind and seismic events, as do diaphragms and shear walls made from wood structural panels properly attached to wall and roof framing. With this in mind, the 2018 WFCM equips designers and code officials with engineered and prescriptive construction methods that result in more resilient wood buildings better able to withstand forces associated with extreme natural disasters.

    Figure 1: Rake overhang limits – Lookout blocks
    Figure 2: Rake overhang limits – Outlookers

    Primary changes in the 2018 WFCM include:

    • Updated design load standard reference from ASCE 7-10 to ASCE 7-16 Minimum Design Loads and Associated Criteria for Buildings and Other Structures.
    • Revised “Basic Wind Speeds for One- and Two-Family Dwellings” figure based on three-second Gust Basic Wind Speeds for Risk Category II Buildings to coordinate with updated reference to ASCE 7-16.
    • Added requirements for identification and description of wood structural panels to recognize “Performance Category” consistent with the IBC. WFCM references to wood structural panel (WSP) thickness appear in several forms. References to WSP thickness were revised throughout the standard to consistently describe wood structural panels and more accurately describe thickness as a nominal value to coordinate with changes to Section and the codes. For example, 15/32-inch WSP becomes Nominal 15/32-inch WSP; 7/16-inch becomes Nominal 7/16-inch; and Minimum Panel Thickness (inches) becomes Minimum Nominal Panel Thickness (inches).
    • Revisions to coordinate with ASCE 7-16’s increased roof uplift requirements due to changes in localized roof wind design pressures and addition of lower wind speed categories (e.g. 90, 95, 100, and 105 mph).
    • Addition of roof rafter uplift tables to provide roof uplift requirements and maximum spans for roof rafters in accordance with ASCE 7-16 localized roof wind design pressures.
    • Incorporation of fastener criteria from AWC’s 2018 NDS to include provisions for roof sheathing ring shank (RSRS) nails and fastener head pull-through design values.
    • Clarification of terminology used for rake overhangs and limit of rake overhang lookout blocks to 9 inches based on increased roof uplift requirements due to localized roof wind design pressures (see Figures 1 and 2).
    • Revised “Shear Wall Assembly Allowable Unit Shear Capacities,” “Maximum Shear Wall Segment Aspect Ratios,” and “Sheathing Type Adjustments” tables to incorporate updated aspect ratio adjustments consistent with AWC’s 2015 Special Design Provisions for Wind and Seismic (SDPWS).
    • Revised “Roof Sheathing Attachment Requirements for Wind Loads” table to give common, box, and RSRS nail size and sheathing options by including requirements for uplift load per nail and fastener uplift capacity.

    Visit www.awc.org/codes-standards/publications/wfcm-2018 to download a free version of the 2018 WFCM in PDF format, see more details regarding changes, and view a recent webinar to obtain free education credits. A print version of the standard is expected to be available for purchase in the coming months.

    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.