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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.

Wind Loads, Part I: The International Building Code

Recently, this author had to become familiar with provisions in the 2003 International Building Code for calculating wind loads on components of the building exterior such as windows, curtainwall, roofing, etc. This article is the first in a series that outlines that code's "simplified method" for calculating such component wind loads and discusses some noteworthy ramifications of this method. This article is not intended solely or primarily for structural engineers--many others in the building profession, such as architects, specifiers, fabricators, and others may at times have need to evaluate such loads.

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.

Wind Loads in IBC 2003
Wind loads are covered in Section 1609 of the International Building Code (IBC). This section starts off by stating that building wind loads should be determined according to the ASCE 7 standard. Separately, in Chapter 35 References, the code provides a specific reference to ASCE 7 as ASCE 7-02 Minimum Design Loads for Buildings and Other Structures.

So what is ASCE 7? ASCE is the American Society of Civil Engineers, an organization, over 150 years old, that represents the interests of civil engineers within the United States and internationally. Among its activities, ASCE develops voluntary standards which may be adopted by regulatory groups or model building codes such as the IBC. ASCE 7 is one such standard.

Through the consensus of the engineering profession, ASCE 7 has come to be accepted as one of a few definitive standards for the determination of building structural loads. Through its reference in the IBC, and the adoption of the IBC by various local building departments or other jurisdictions, designing to the ASCE 7-02 standard may then become a regulatory requirement.

It is also important to note the "-02" designation on the IBC's reference to this standard, signifying the version of this standard. There are significant differences between versions, and using a different version of the standard, such as ASCE 7-98, would not be compliant with the requirements of the 2003 IBC.

Following the reference to ASCE 7, the IBC lists several exceptions to this standard, one of which is the code's own Section 1609.6 Simplified wind load method. It is this simplified method which is the subject of the remainder of this article.

IBC Simplifed wind load method
The IBC's simplified wind load method is also derivied from ASCE 7, but offers two potential advantages. First, all the information needed to perform wind load calculations using this method are included in the IBC code book itself. (The full ASCE 7 standard is not included in the IBC code, and designers working to that standard must obtain it separately.) Second, as the name implies, this method simplifies the process of determining wind loads. In a typical scenario, a wind load calculaton using the simplified method requires little more than looking up figures in several tables and performing a straightforward multiplication.

The simplified wind load method is not suitable for all projects. This method can only be applied to:

• Buildings with a mean roof height not exceeding 60 feet, nor the length or width of the building;
• Buildings that are fully enclosed. For example, an airplane hanger which can at times be substantially open on one side cannot use the simplified method.
• Buildings not situated on the upper half of an isolated, steep hill or slope. (This restriction is more precisely described in paragraph 1609.6.1.)

Buildings that do not fall within these limitations must refer to the referenced ASCE 7 standard for determination of wind loads.

Primary Structure vs Secondary Structure
Wind load calculations are also divided into two sections depending on the part of the structure being evaluated. Calculation of wind loads acting on the building's primary structure are covered under requirements for Main windforce-resisting systems. Calculation of wind loads acting on secondary components or cladding elements are covered under requirements for Components and cladding. This remainder of the articles in this series look only at methods for such secondary elements, not primary structure.

Next: Component Wind Load Calculations

More Information
The ASCE 7-02 standard is available in both printed and cd-rom formats.
Guide to the Use of the Wind Load Provisions of ASCE 7-02 is ASCE's own companion handbook to the ASCE 7-02 standard.

Plastic Bridge Faring Well

One-year-old plastic bridge exceeds expectations, Structural Engineer, January 2004, reports on the current health of the nation's first all plastic bridge, the New Jersey Pine Barrens Mullica River Bridge, first covered on this site last November.

According to the article:

• After more than one year in service, the one-lane, 56-foot long bridge is so far exceeding expecations.
• The structural plastic material is a combination of polystyrene and polyethylene, recycled from items such as milk cartons, soda bottles, and foam cups.
• The bridge is rated to carry a 36-ton load and cost $75,000 to build. Costs to replace the original wood structure were estimated at $350,000.

Also according to the article, the Federal Highway Administration currently has sponsored more than 40 projects involving experimental plastic materials and sees an "incredible" market potential for these materials.

Structural Recycled Plastic

Plastic Fantastic, Metropolis May 2003, reports on a bridge made entirely of recycled plastic. The fifty-six foot long, single-lane bridge provides access over the Mullica River in the New Jersey Pine Barrens for hikers, forest rangers, and fire fighting vehicles. The recycled plastic, a mix of polystyrene and polyethylene, was developed by Rutgers University School of Engineering professors Tom Nosker and Richard Refree. Reported benefits include:

• Natural resistance to decay, without the need to treat with potentitally toxic chemicals
• 50-year lifespan
• Recyclability at the end of its useful lifespan

More information about this project and its development partners can be found at Rutgers Focus, March 10, 2003. The patented material is manufactured for commercial applications by Polywood Inc.

18 - Windows and Doors Links

This article contains external links to resources on the Web relevant to Chapter 18 Windows and Doors.

American Architectural Manufacturers Association

Standards for window, door, skylight and curtainwall design.

Fortifiber Moisture Control Flashing Systems Library

This flashing manufacturer's web page includes links to well-illustrated examples of window and door flashing recommendations.

Hurricane Grade Windows Video and Standards

By EFCO Windows, including downloadable copies of Dade County Hurricane Code standards, and a video that--while promotional--also provides an interesting demonstration of "Large Missile" testing on windows.

NFRC Suggested Links

A useful collection of links related to windows, glazing, and energy performance, from the National Fenestration Rating Council.

Windows and Daylighting Group, Lawrence Berkely National Laboratory

Information areas include glazing materials, software, advanced systems, window properties, daylighting, residential performance, and commercial performance.

Window & Door Manufacturers Association

Trade association representing U.S. and Canadian window and door manufacturers, and publisher of technical guides and standards