Wind Loads, Part II: Component Wind Load Calculations

The first article in this series discussed some basic aspects of the 2003 International Building Code's requirements for wind load design. This article discusses several issues of note that arose for this author in the course of doing preliminary component wind load calculations for a project in the Midwest US that falls under the requirements of the 2003 IBC.

Caveat
This author is not a structural engineer, and the information provided in this series of articles is not intended as definitive instruction for those who need to make such calculations. For those who do, proper training or consultation with a qualified professional is advised.

Design Method
Using the IBC Simplified wind load method, for Components and cladding, Section 1609.6.1.2, appears on the surface as straightforward. Numbers are read from three separate tables and then multiplied together to obtain the design wind pressure for a given component:

• Adjustment factor for building height and exposure (Table 1609.6.2.1(4) accounts for differences in building height and in a building's exposure to winds. As one would expect, adjustment factors are greater for taller buildings, and for buildings less protected from the wind by surrounding topography.
• The importance factor (Table 1604.5) adjusts calculated wind loads according to the type of building occupancy. Buildings whose occupancies warrant a greater degree of survivability or protection, such as emergency centers, hospitals, high occupancy public buildings, and the like, are assigned a greater importance factor than buildings assumed to present a lower degree of hazard to occupants. For example the importance factor for a hospital is 1.15, whereas the importance factor for a residence is 1.
• The net design wind pressure table (Table 1609.6.2.1(2)) tabulates design pressures for a baseline building--one that is 30-feet tall, in Exposure B, with an Importance Factor of 1.

By reading from the net design wind pressure table, and adjusting for height, exposure, and importance, a final design pressure for the component in question can be determined. However reading from the net design wind pressure table requires several additional pieces of information that deserve additional comment.

Wind speed
Reading from the net design wind pressure table requires entering the table with a basic wind speed for your project location. For starters, basic wind speed can be read from Figure 1609 of the IBC which provides wind speed contour maps for the continental United States and Alaska. Indicated wind speeds range from a low of 85 miles per hour for major portions of West Coast states, to a high of 150 miles per hour at the southern tip of Florida. Much of the middle of the continental US is assigned a basic wind speed of 90 mph.

Before proceeding with the information taken from the basic wind speed map, the designer should also check with local building departments. Depending on local conditions, a building department may require use of a different wind speed than indicated in this map. This is also true for portions of the wind speed map indicated as "Special Wind Regions". Topographic features in these areas, such as mountains, river valleys, or gorges, can result in wind speeds significantly higher than indicated on the wind speed map, and must be assigned locally.

Use caution if you have reason to compare wind speeds from the 2003 IBC with other codes or standards to make sure you are making an apples-to-apples comparision. The IBC uses a wind speed scale called 3-second gust wind speeds. Some older standards, for example the 1997 Uniform Building Code, measure wind speed according to a different scale called fastest mile. If you do need to compare between such scales, you can use Table 1609.3.1 in the IBC to convert between the two.

For example, under the 1997 UBC, the City of Seattle requires a basic wind speed of 80 mph (fastest mile speed). According to the IBC conversion table, this is comparable to a 100 mph basic wind speed using the 3-second gust scale. Interestingly, in comparison to the 1997 UBC wind speed map, the 2003 IBC map appears to assign lower wind speeds to much of the central region of the continental US, even when speeds have been adjusted for comparison between the two scales.

Equivalent Area
The net design wind pressure table also requires entering the table with a factor termed effective area. For those who are not familiar with this factor (as this author was not), explanation is warranted. In many cases, the effective area of a component is simply the surface area of a cladding member, or tributary area of a framing member exposed to the force of the wind. When calculating component wind loads, the design pressure varies depending on the size of the effective area under consideration.

In what may at first seem to be a counter-intuitive result, design pressures for components with larger effective areas are smaller, and conversely, design pressures for smaller components are larger. For example, assuming a 100 mph basic wind speed acting on a component with an effective area of 10 square feet, the net design pressure is -19.5 pounds per square inch. If the component's effective area increases to 500 square feet, the net design pressure drops to -14.9 psf, a reduction of almost 25%. The rationale behind this reduction in pressure is that for larger areas, the pressure spikes caused by a wind gust do not act simultaneously on all parts of the component. While at any given moment some localized areas on the component will experience maximum pressures, others will not, resulting in an overall lower average pressure across the component's surface. For components with smaller effective areas, it is more likely that the entire surface of the component may simultaneously experience a maximum intensity pressure spike, and therefore, a higher design pressure is warranted.

In some cases, determining the effective area for a given building component can become a more difficult problem than just measuring its area. One complication is that the IBC increases the effective area of components that are relatively long and narrow in their proportions (see Section 1609.2 Defintions, Effective Area). Additionally, in some cases, the appropriate choice of effective area may not be immediately obvious. For example, in the case of reinforced masonry wall, is the effective area the total surface area of the wall, only the area supported by a vertically reinforced masonry cell, or perhaps the area spanning between window or other openings? This author found the ASCE's Guide to the Use of the Wind Load Provisions of ASCE 7-02, referenced below, helpful in clarifying such areas.

Next: Applying component wind load calculations to the specification of doors and windows.

More Information
For those needing to perform component wind load calculations using the IBC's simplified method, the code itself provides sufficient information to complete such calculations. For more detailed guidance, ASCE's Guide to the Use of the Wind Load Provisions of ASCE 7-02 provides a comprehensive set of examples of wind load calculations for various scenarios.