Volume 35, Number 6, June 2000
The Quest Continues
determining wind loads on buildings, part two
by Craig H. Wagner, P.E.
Hopefully, after reading last months column (see May USGlass, page 18), youre slightly less confused about how to determine wind loads on buildings. This months column discusses the other factors that must be considered when tackling this important topic.
It is important to account for how the wind speed will be affected by the terrain and setting for the building site. The basic wind speed for your geographical area on the map is based on flat, open terrain. The same wind passing through your neighborhood, wooded area or city center will be affected by numerous obstructions of various size and shape. Wind speed also varies with height, but the degree to which this varies depends on the terrain and obstructions that exist at ground level. The design provisions provide criteria for classifying the surrounding setting and provide adjustment factors based on building height accordingly. This is known as the exposure classification.
Then there are the special adjustment factors. Is the building located within a hurricane prone region? The probability of the design wind speed being exceeded in these areas is greater (the newer three-second wind speed mapping already considers this). Is the building located on a hill? Wind can speed-up on hills. Is it a two-dimensional hill like a ridge or is it a three-dimensional hill like a knoll? Use of ASCE 7-95 and 7-98 includes wind speed-up factors to account for these conditions.
Next you need to know what the building will be used for. Importance factors are established for use in the equations to account for the type of building being designed. Chicken barns dont get designed to the same factor of safety as essential facilities like police stations, fire stations and, hospitals. Of course, if the farmer wants to spend the money, he can specify that his chickens have the same level of protection.
Variation in Surface Pressures
While we like to design with uniform design pressures, the reality is there is nothing uniform about the actual wind pressures on a buildings surface. During exposure to wind, the surface areas of a building are subjected to extreme variations in pressure that are changing constantly. At any given instant, the pressure on the surface of a building, if visible, would look like a mountain range with extreme peaks and valleys. At a given location the pressure can be extreme at one moment and seconds later be practically nothing. To address this, pressure coefficients were derived that involve time and area averaging to provide design pressures for varying area size on the building surface. It is also known that pressures are significantly greater where airflow is disrupted, such as at corners and roof overhangs. Wind pressures at the corner of a building can be two times the pressures in the mid-areas of the wall. To account for this, the surface area is mapped into zones with different sets of coefficients derived for each zone.
Lets say we want the design pressure for a window. You need to know the size of the window and the location on the wall (height and distance to corner or roof eave). Some use of these coefficients reveals that a larger window may have a lower design pressure than a smaller window at the same location. Remember that the actual pressures are peaks and valleys throughout the surface area.
Pressure coefficients are further derived for positive and negative pressures. Positive pressures act toward the surface. This is easy to comprehendwind blows against the building. Negative pressure is a little more difficult for some to accept. What propels a sailboat through the water? Negative wind pressure. When wind blows against a building, the windward side feels positive pressure. The wind flowing around the sides and collapsing around the back (leeward) side develops negative surface pressures. Because wind can come from any direction, we must consider both positive and negative wind pressures. The coefficients provided in the design provisions are developed from measurements of pressure from wind in all directions.
Openings in the building envelope create ports for airflow and the development of internal pressures. The magnitude of internal pressure depends upon the ratio of open area on one side to the open area on the remainder of the building envelope. As with external pressure, internal pressure can be positive or negative depending on the location of the opening relative to the wind direction. The design provisions provide criteria to classify the potential for internal pressure based on openness and assigns pressure coefficients accordingly.
The basic velocity pressure for the building is established considering its geographical location, exposure and use. The exterior pressure coefficients are established based on the component size and location on the building. The internal pressure coefficients are determined based on the potential for wind being blown into or sucked out of the building. The product of all these factors give the positive and negative design pressures for the component.
It could even be simplified to the point that anyone can pick up the code and determine the required wind pressure without calculations, coefficients and layers of adjustment factors for special conditions. But, how conservative are we willing to be in the interest of simplification? The objective in engineering is to provide an adequate level of safety to public and property with the most efficient and economical use of resources. Given the enormous complexities involved, I think it is pretty simple. But then, everythings relative to me and Im an engineer.
Authors note: In part one of my column I stated that the wind load criteria in the new national building code is based on fastest mile wind speed. But, in its final published form the wind load section in IBC 2000 has been replaced with three-second gust speed.
Craig H. Wagner, P.E. is a licensed professional engineer and senior project engineer for Architectural Testing Inc., in York, Pa. Fenestration Focus appears monthly with rotating columnists.
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