2012 International Residential Code – Wind Design

Wind Design Criteria

The 2012 IRC provides a basic wind speed map (Figure R301.2(4)A) which shows the “nominal design 3-second gust wind speeds in miles per hour at 33 feet above ground for Exposure C category.” This basic wind design speed value should also be listed in Table R301.2(1) which gives the climatic and geographic design criteria provided by each jurisdiction. These values can be modified to “fastest mile wind speeds” using Table R301.2.1.3 where needed.

Components & Cladding

The loads for components & cladding should be determined using Table R301.2(2), Table R301.2(3), and Figure R301.2(7). Table R301.2(2) provides both the positive and negative pressures (force acting toward and the force acting away) on each surface given the “effective wind area”, the building zone, and the basic wind speed previously determined. The building zone is determined using Figure R301.2(7). Table R301.2(3) is used to modify these force values for different mean roof heights (MRH) and exposure categories.

Asphalt Shingles

As opposed to the previous analysis, asphalt shingles are required to be designed for wind in accordance with R905.2.4.

Load Path

This section also states that the uplift forces calculated to be acting on the roof assemblies in R802.11.1 need to have a continuous load path all the way to the foundation.

Limitations & Exceptions

Figure R301.2(4)B shows regions where wind design is required. Additionally, any areas with a basic wind speed of 110 mph or more will require alternative methods and not those provided in the IRC. The IRC says there are 5 alternatives in these cases:

-Use the Wood Frame Construction Manual (WFCM)

-Use the ICC 600 Standard for Residential Construction in High-Wind Regions

-Use ASCE 7

-Use AISI S230 for cold-formed steel framing

-Use the International Building Code (IBC)

Concrete foundation and retaining walls designed according to R404, exterior concrete wall construction according to R611, and structural insulated panel (SIP) walls constructed according to R613 shall apply even in these special wind regions.

Openings in Windborne Debris Regions

Figure R301.2(4)C shows areas that are considered windborne debris regions and categorizes them as zone 1, 2 or 3. In these zones, glazing is required to meet certain protection standards. Properly sized, fitted, anchored, and attached wood structural panels can be utilized for opening protection in lieu of meeting these glazed opening requirements.

Exposure Categories

Usually, in wind load/speed analysis, the site is divided into a number of different quadrants or sectors from which wind can approach the structure. For each one of these approach sectors, an exposure category is assigned from A through D with A being terrain and development as in large city centers, B being other urban or suburban areas or areas with numerous obstructions, C being relatively open terrain, and D being flat open areas such as shorelines by open water. These categories are discussed in more detail in the code.

Topographic Wind Effects

If a jurisdiction in Table R301.2(1) has noted “YES” for topographic effects then they have “local historical data documenting structural damage to buildings caused by wind speed-up.” When a structure is located in an area with this historic issue and is oriented relative to certain qualifying topographic features, the basic wind speed must be increased based on the Simplified Topographic Wind Speed-Up Method of R301.2.1.5.1 or using ASCE 7.

(Go here to view International Code Council (ICC) codes such as the IRC and IBC for free.)


Sources:

International Code Council. 2012 International Residential Code: For One-and Two-family Dwellings. Country Club Hills, IL: International Code Council, 2011. Print.

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