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.