Understanding Structural Design Criteria for Pre-Engineered Metal Buildings (PEMBs): Wind Loads

Wind load is another important design consideration for pre-engineered metal buildings (PEMBs). Many people know that designing for a specific wind speed is important. But, there are other key factors to consider when designing a building. Wind speed, wind exposure and the enclosure type of the building all play a critical role.

Wind Speed

Basic design wind speed, V, is part of the equation used to determine the wind loads. The IBC maps and includes basic wind speed values. The Risk Category of the building affects the design wind speed. Lower Risk Category buildings have a lower design wind speed than higher Risk Category buildings. The IBC 2020 provides the below charts, and the values represent the 3-second gust wind speed in miles per hour.

Basic Design Wind Speed for Buildings with a Risk Category II

Basic Design Wind Speed for Buildings with a Risk Category III

Basic Design Wind Speed for Buildings with a Risk Category IV

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Wind Exposure Category

Wind Exposure Category is also considered, the Wind Exposure Category is based upon the Surface Roughness or Terrain Category around the building.

Per 1609.4.2 of IBC 2020 Surface Roughness Categories are defined as:

Surface Roughness B

Urban and suburban areas, wooded areas or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger.

Surface Roughness C

Open terrain with scattered obstructions having heights generally less than 30 feet. This category includes flat open country and grasslands.

Surface Roughness D

Flat, unobstructed areas and water surfaces. This category includes smooth mud flats, salt flats and unbroken ice.

The Wind Exposure Category mirrors the Surface Roughness Condition:

Most buildings we design in Western New York are in Exposure Categories B and C. If we’re unsure, we classify the Wind Exposure Category as C.

Enclosure Category

The enclosure category of the building is more complicated and can be confusing. The three main categories of building enclosure are Enclosed, Partially Enclosed, and Open.

Enclosed Buildings

The building has no permanent openings exterior. All openings are made to handle the same wind loads as the main structure.

Partially-Enclosed Buildings

Have one or more openings on the exterior of the building that will remain permanently open.

Open Buildings

Buildings with at least 80% of the walls open.

An Open building is simple to define. But, the difference between Enclosed and Partially-Enclosed can be more complex.

We built several data center buildings. They are classified as Partially-Enclosed. From the exterior, these buildings appear to be fully enclosed. There are no “openings” in the exterior façade. These buildings have louvers on most of the outside. They let cool air in to chill the computers inside. These louvers let air into the building, so these buildings are Partially-Enclosed.

This matters because the Enclosure Category affects the wind pressure the building needs to withstand. In open buildings, pressure is low because wind can pass through without obstruction. An enclosed building has a moderate level of pressure created by external pressure and suction forced.

Partially-enclosed buildings experience the most pressure. This happens because they create a ballooning effect. Wind gets into the building but has no way to get out. Besides exterior pressure and suction, internal pressure and suction also act on the building.

When designing a PEMB, consider wind speed, wind exposure category, and wind enclosure. The Engineer of Record should provide this information. But, builders need to grasp these loads and their impact on PEMBs.