Course: GFM 103 Exterior Environmental Management

Reading and Audio-Visual Assignments

(2/2)
Building Envelopes: Multiple Interactions From Differing Forces (including Wind)

The following Building Envelope Systems are covered in separate chapters (Whole Building Design Guide – Building Envelope Design Guide):

- Below Grade construction
- Exterior Walls, both structural (providing support for the building) and nonstructural (supported by the building structure)
- Fenestration, both windows and metal/glass curtain walls
- Roofs, both low- and steep-slope
- Atria.

This guide demonstrates that the design of the envelope is very complex and many factors have to be evaluated and balanced to ensure the desired levels of thermal, acoustic and visual comfort together with safety, accessibility and aesthetic excellence. As can be seen from the list of functions of the building envelope detailed in the Function and Performance section, the envelope plays a role in almost every building function, either directly or indirectly in its relationship to other building systems.

This Section provides an overview of the nature and occurrence of high winds (including wind/building interactions), discusses what wind can do to office buildings, and discusses priorities, costs, and benefits of potential risk reduction measures.

The Nature and Occurrence of High Winds

A variety of windstorm types occur in different areas of the U.S. The characteristics of the type of storms that can impact the site should be considered by the design team. For a discussion of the primary storm types see below.

Straight-line wind. This type of wind event is the most common. The wind is considered, in general, to blow in a straight line. Straight-line wind speeds range from very low to very high. High winds associated with intense low pressure can last for upward of a day at a given location. Straight-line winds occur throughout the U.S. and its possessions

Down-slope wind. Wind flowing down the slope of mountains is referred to as down-slope wind. Down-slope winds with very high wind speeds frequently occur in Alaska and Colorado. In the continental U.S., mountainous areas are referred to as "special wind regions" (see Figure 1). Neither the American Society of Civil Engineers (ASCE) Minimum Design Loads for Buildings and Other Structures, ASCE 7 or model building codes provide guidance on wind speeds in special wind regions. If the local building department has not established the basic speed, use of regional climatic data and consultation with a wind engineer or meteorologist is advised.

Thunderstorm. This type of storm can rapidly form and produce high wind speeds. Approximately 10,000 severe thunderstorms occur in the U.S. each year, typically in the spring and summer. They are most common in the Southeast and Midwest. Besides producing high winds, they often create heavy rain. Hail and tornadoes are also sometimes produced. Thunderstorms commonly move through an area quite rapidly, often causing high winds for only a few minutes at a given location. However, thunderstorms can also stall and become virtually stationary.

Downburst. Also known as microburst, it is a powerful downdraft associated with a thunderstorm. When the downdraft reaches the ground, it spreads out horizontally and may form one or more horizontal vortex rings around the downdraft. The outflow is typically 6,000 to 12,000 feet across and the vortex ring may rise 2,000 feet above the ground. The life-cycle of a downburst is usually between 15 to 20 minutes. Observations suggest that approximately 5 percent of all thunderstorms produce a downburst, which

can result in significant damage in a localized area.

Northeaster (nor'easter). This type of storm is cold and violent and occurs along the northeastern coast of the U.S. These storms blow in from the Northeast and may last for several days.

Hurricane. This is a system of spiraling winds converging with increasing speed toward the storm's center (the eye of the hurricane). Hurricanes form over warm oceans. The diameter of the storm varies between 50 and 600 miles. A hurricane's forward movement (translational speed) can vary between approximately 5 to 25 miles per hour (mph). Besides being capable of delivering extremely strong winds for several hours, many hurricanes also bring very heavy rainfall. Hurricanes also occasionally spawn tornadoes. The Saffir-Simpson Hurricane Scale rates the intensity of hurricanes. The five-step scale ranges from Category I (the weakest) to Category V (the strongest).

Of all the storm types, hurricanes have the greatest potential for devastating a very large geographical area and, hence, affect great numbers of people. The terms "hurricanes, tropical cyclones, and typhoons" are synonymous for the same type of storm. See Figure 1 for hurricane-prone regions.

Tornado. This is a violently rotating column of air extending from the base of a thunderstorm to the ground. The Fujita scale categorizes tornado severity based on observed damage. The six-step scale ranges from F0 (light damage) to F5 (incredible damage). Weak tornadoes (F0 and F1) are most common, but strong tornadoes (F2 and F3) frequently occur. Violent tornadoes (F4 and F5) are rare. Tornado path widths are typically less than 1,000 feet; however, widths of approximately 1 mile have been reported. Wind speed rapidly decreases with increased distance from the center of a tornado. An office building on the periphery of a strong or violent tornado could be subjected to moderate to high wind speeds, depending upon the distance from the core of the tornado. However, even though the wind speed might not be great, an office building on the periphery could still be impacted by many large pieces of windborne debris. Tornadoes are responsible for the greatest number of wind-related deaths each year in the U.S. Figure 2 shows frequency of occurrence for 1950 to 1998.