Title: Reading and Audio-Visual Assignments
Course: GFM 103 Exterior Environmental Management > Section: GFM 103 Exterior Environmental Management

Reading and Audio-Visual Assignments

(1/2)
Wind Building Interactions

Wind/Building Interactions

When wind interacts with a building, both positive and negative (i.e., suction) pressures occur simultaneously (see Figure 3). (Note: negative pressures are less than ambient pressure, and positive pressures are greater than ambient pressure.) An office building must have sufficient strength to resist the applied loads in order to prevent wind-induced building failure. The magnitude of the pressures is a function of the following primary factors:

Exposure. The characteristics of the ground roughness and surface irregularities in the vicinity of a building influence the wind loading.

Basic wind speed. ASCE 7 defines the basic wind speed as the wind speed with a 50-year mean recurrence interval (2 percent annual probability), measured at 33 feet above grade in Exposure C (flat open terrain). If the building is located in Exposure B or D, rather than C, an adjustment for the actual exposure is made in the ASCE 7 calculation procedure.

.Topography. Abrupt changes in topography, such as isolated hills, ridges, and escarpments, cause wind speed-up; therefore, an office building located near a ridge would receive higher wind loads than an office building located on relatively flat land. ASCE 7 provides a procedure to account for topographic influences.

Building height. Wind speed increases with height above the ground. Therefore, the taller the office building, the greater the speed and, hence, the greater the wind loads. ASCE 7 provides a procedure to account for building height.

Internal pressure (i.e., building pressurization/ depressurization). Wind striking a building can cause either an increase in the pressure within the building (i.e., positive pressure), or it can cause a decrease in the pressure (i.e., negative pressure). Internal pressure changes occur because of the porosity of the building envelope. Porosity is caused by openings around doors and window frames, and by air infiltration through walls that are not absolutely airtight. A door or window left in the open position also contributes to porosity.

Aerodynamic pressure. Because of building aerodynamics (i.e., the interaction between the wind and the building), the highest uplift loads occur at roof corners. The roof perimeter has a somewhat lower load, followed by the field of the roof. Exterior walls typically have lower loads than the field of the roof. The ends of walls have higher suction loads than the portion of wall between the ends. However, when the wall is loaded with positive pressure, the entire wall is uniformly loaded. Figure 6 illustrates these aerodynamic influences. The negative values shown in Figure 6 indicate suction pressure acting upward from the roof surface and outward from the wall surface. Positive values indicate positive pressure acting inward on the wall surface.