The Simplified Wind Load Method has gone through some changes in the International Code Council’s 2003 Intern ational Building Code. (See the new scope of the provision below.) The goal in developing the simplified method in the code was to devise a method that would align the solut ion to the complexity of the problem, but not to penalize all designs. For a given simple diaphragm building, a user will get the same answer from the simplified method in the code as would be determined by performing the necessary calculations for the low-rise analytical method in ASCE 7.

According to ICC Senior Engineer John Henry, P.E., "The reasons the procedure is simplified in comparison to the analytical procedure is that it is a projected area method and not a normal force method. Design pressures only act horizontally and vertically on the projected area; there are no internal pressures. Design pressures are easily determined from tables, and the design pressures are applied to the building zones in the transverse and longitudinal directions according to figures given in the code." IBC Section 1609.6.1 Scope. The procedures in Section 1609.6 shall be permitted to be used for determining and applying wind pressures in the design of enclosed building with flat, gabled, and hipped roofs and having a mean roof height not exceeding the least horizontal dimension or 60 feet, whichever is less, subject to the limitations of Sections 1609.6.1.1 and 1609.6.1.2. If a building qualifies only under Section 1609.6.1.2 for design of its components and cladding, then its main wind force-resisting system shall be designed in accordance with Section 1609.1.1.

Exception: The provisions of Section 1609.6 shall not apply to buildings sited on the upper half of an isolated hill or escarpment meeting all of the following conditions:

• The hill or escarpment is 60 feet or higher if located in Exposure B or 30 feet or higher if located in Exposure C.
• The minimum average slope of the hill exceeds 10 percent.
• The hill or escarpment is unobstructed upwind by other such topographic features for a distance from the high point of 50 times the height of the hill or 1 mile, whichever is less.

Checklist for using the simplified wind design approach In order to use the simplified wind design methods in 1609.6, the following criteria must be satisfied:

• The building should not qualify as open or partially enclosed, as defined in Section 1609.2.
• The mean roof height should be less than or equal to 60 feet and should not exceed the least horizontal dimension of the building.
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• The building should have an approximately symmetrical cross section in each direction.
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• Roof slopes should not exceed 45 degrees.
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• The building should not have any expansion joints or structural separations.
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• The building should transmit the wind loads through the floor and ro o f diaphragms to the vertical lateral-force- resisting elements.
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• The building should not be slender or have a fundamental frequency less than 1 HZ.
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• The building should have a flat, gabled, or hipped roof.
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• The building should not have response characteristics, making it subject to across wind loading, vortex shedding, and instability due to galloping or flutter; and the building should not have a site location for which channeling effects or buffeting in the wake of upwind obstru ctions warrant special consideration.
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• If the building sits on the upper half of an isolated hill or escarpment, at least one of the answers to the following questions should be "no": - If Exposure B: Is H, as shown in Figure 1609.6, 60 feet or greater? - If Exposure C: Is H, as shown in Figure 1609.6, 30 feet or greater? - Does the slope of the hill exceed 10 percent? - Is the hill or escarpment unobstructed upwind by topographic features for a distance from the high point of the hill or escarpment for a distance equal to 1 mile or 50H, whichever is less?

Answers to FAQs

Q: Structural calculations submitted to our jurisdiction have been basing the wind design on Section 1609.6, simplified wind load method. I want to make sure that each structure qualifies to use this simplified approach. How do I know if the building can use the simplified wind load method?

A: The limitations that define whether or not a building can use the simplified wind load method are found in Sections 1609.6.1 and 1609.6.1.1, with some definitions in 1609.6.1.2. The checklist (at right) can be used to determine if the calculations can be based upon the simplified procedure of Section 1609.6.

Q: What is an "isolated hill or escarpment," as referred to in the Exception to Section 1609.6.1?

A: A hill can be defined as a land surface characterized by strong relief in any horizontal direction, and an escarpment can be defined as a cliff or steep slope generally separating two levels or gently sloping areas.