All buildings are at risk of experiencing damage during high winds. Each structure embodies a unique set of characteristics, such as stiffness, strength, and shape, in their response to wind loads.
Wind Load vs. Wind Speed
Wind speed, while the primary factor (2018 International Building Code (IBC) 1609.3 presents national wind zone maps that cover your jurisdiction), is one of several factors that contribute to wind loads on a structure. Those factors include many variables that the building designer must consider, ranging from wind direction (2018 IBC Section 1609.4.1), building size/shape, combined wind gust and external or internal pressure coefficients, topographic effects, and surface roughness (2018 IBC Section 1609.4.2), among others.
It’s interesting to note Section 1609.4.2 recognizes three categories of ground surface roughness. Surface Roughness B, for example, describes urban and suburban areas, wooded areas, or other terrain with numerous closely-spaced obstructions having the size of single-family dwellings or larger.
Understanding the effects of high wind loads on building design is a key component of the IBC. The building code presents a minimum level of compliance required to protect public health and safety related to building construction and occupancy regardless of what building material is used. Buildings constructed of concrete, steel, wood, or masonry, will respond safely to high wind loads when built in accordance to IBC requirements.
Each building material represents unique characteristics in response to major wind effects. Consider wood, for example. Wood sheathing in the form of wood structural panels (plywood or oriented strand board), fiberboard, particleboard, and board sheathing, can be used as part of floor and roof diaphragms and as part of shear walls to resist and transfer wind loads to the building foundation.
Determining Wind Load
The 2018 International Building Code contains very specific language on the design wind load and approved methods to determine it (Section 1609.1)
For example, the determination of wind load “… shall be determined in accordance with Chapters 26 to 30 of (the 2016 edition of the) ASCE 7.” The 2018 IBC goes on to explain how Section 1609 or ASCE 7 are the basis for identifying the:
- Type of opening protection required
- Basic design wind speeds miles per hour (mph) (km/hr) determined from Figures 1906.3(1) through 1609.3(8) or ASCE 7
- Exposure category
Section 1609.1.1 then notes six exceptions to designing for the wind loads per ASCE 7 including the use of pre-engineered designs included in the 2014 Standard for Residential Construction in High-wind Regions (ICC 600), 2018 Wood Frame Construction Manual for One- and Two-Family Dwellings (AWC WFCM) and 2015 Standard for Cold-formed Steel Framing—Prescriptive Method for One- and Two-family Dwellings (AISI S230) that you are encouraged to explore in detail. Note that, while the 2018 WFCM is the only one of these standards that has been updated to the loads in ASCE 7-16, ICC has initiated the process to revise ICC 600, which will include updating wind loads per ASCE 7-16. Section 1609.1.1.1 details applicability of ICC 600 to buildings located only within Exposure B or C (Section 1609.4) and concludes with three topographic limitations to the use of the pre-engineered design standards.
2018 IBC References
Building engineers consider wood structural panels, when properly attached to wood framing members, as among the most solid and stable roof, floor, and wall systems available. To that end, the 2018 IBC references AWC’s Special Design Provisions for Wind and Seismic (AWC SDPWS) standard for the design and construction of wood-frame diaphragms and wood-frame shear walls. Additional code requirements for wood-frame diaphragms and shear walls constructed with wood structural panels attached with staples are provided in 2018 IBC Section 2305.
Section 2308 of the 2018 IBC describes the minimum standards for Conventional Light-Frame Construction, including wind speed limitations and exceptions in 2018 IBC Section 2308.2.4. When wind speeds exceed those limitations, the building must conform to code provisions set forth in 2018 IBC Section 1609 Wind Loads or in the AWC WFCM or ICC 600 as specified in the exception to IBC Section 2308.2.4.
Wind uplift must also be considered. For example, Section 2308.7.5 provides prescriptive provisions for securing rafter and truss ties to the wall and what connectors are code compliant across various roof spans, overhangs, and wind speeds in low wind speed areas. The 2018 IBC also prescribes uplift capacities for hold-down devices for braced and alternate braced wall construction along with required connection ratings (Table 2308.7.5) in low wind speed areas.. For high wind speed areas, roof uplift is required to be designed per the 2018 IBC Section 1609 Wind Loads or per the AWC WFCM or ICC 600 as specified in the exception to IBC Section 2308.2.4.
The “rulebook” for wood construction officially referenced by the 2018 IBC is the National Design Specification for Wood Construction, published by the American Wood Council. The 2018 edition has significant additions that address increased wind loads in American Society of Civil Engineers 7-16, Minimum Design Loads and Associated Criteria for Buildings and Other Structures.
When designed to code, wood is an effective building material in wind-resistive design. Those wind-resistive characteristics include:
- Ability to carry substantially greater maximum loads for short duration than for long period of times as is typically the case in major wind events.
- Redundant and multiple load paths for wind resistance due to repetitive framing attached with numerous fasteners and connectors.
- Diaphragms, roofs, and shear walls of exceptional wind resistance when wood structural panels are properly attached to wood floor, roof, and wall framing.
Each structure, with its own unique characteristics and site conditions, reacts differently to wind loads. Code officials understand the complex and interrelated issues that must be considered to ensure life safety while observing code compliance. History has demonstrated code-compliant wood construction performs well during high wind events.
The views and opinions expressed in this article are those of the Think Wood and do not necessarily reflect those of the International Code Council, or Hanley Wood.