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Windloads:
We've all seen the damage done by hurricanes, tropical storms, thunderstorms and high winds. Building owners, architects, engineers, builders, building officials, code officials and insurers are all aware of the need to build strong, wind-resistant structures.
In the 1997 edition of the Uniform Building Code (UBC), the wind load provisions are, for the most part, the same technical provisions incorporated into an earlier code, the 1991 UBC. These provisions occurred as a result of the work of the High Winds/Hurricane Design Code Development Committee of the International Conference of Building Officials (ICBO). In addition, the Structural Engineering Associations of Washington and California contributed to this work; producing a practical and logical set of code guidelines.
These provisions agreed with the prevailing national standard of the time – American Society of Civil Engineering's ASCE 7-88, Minimum Design Loads for Buildings & Other Structures – except that they are simpler as the result of certain assumptions made about the buildings under consideration.
Wind loads are dynamic in nature, but are treated in the code as quasi-static lateral inward and/or outward acting building forces.
The magnitude of the wind-induced pressure or suction depends upon the complex interrelationship of: wind velocity; air mass density; structural geometry including dimensions, stiffness, orientation, and location; and the surrounding ground surface conditions.
The International Building Code (IBC) has now been adopted in most areas of the United States. It sets up certain standards of wind resistance. All US building codes use the engineering standard published by the American Society of Civil Engineers, ASCE7 "Minimum Design Loads for Buildings and Other Structures" as the basis for wind load design and calculations.
A model building code developed by the International Code Council. Most of the U.S. has adopted this building code (some areas with minor, locally adopted variations).
Per the International Building Code wind speed map, the highest listed wind speed for all 50 states in the USA is 150 mph.
The wind load provisions in this code specifically calls out the use of ASCE7 in wind load calculations.
Both the International Building Code and ASCE7 include a wind speed map. The wind speed map is based on data compiled by the National Weather Service (NWS) from information gathered at airfields around the United States.
ASCE7: The American Society of Civil Engineers design standard: "Minimum Design Loads for Buildings and Other Structures". Section 6 deals with wind loads. ASCE7 is the basis for wind load calculations used by all major building codes.
Design Pressure: The measurement of wind resistance in both positive and negative (suction) forces. Design Pressures are usually expressed in both positive (PSF+) and negative (PSF-) values. Also known as design load.
Three-Second Gust: The National Weather Service measurement of wind speed. The data for this measurement is taken from measuring devices set 33' above the ground at airfields across the USA. This data is then compiled into wind maps found in both ASCE7 and the International Building Code. The wind speed maps are based on a yearly 2% probability of occurrence (50-year average peak wind).
Mean Roof Height (MRH) is the height above grade level of the midpoint of a roof. Mean Roof Height is used as part of design pressure calculations in both ASCE7 and the International Building Code, and is calculated by averaging the roof eave and ridge heights.
WIND VELOCITY: The actual measured speed of airflow, wind velocity is usually expressed in MPH.
WIND LOAD: Pressures placed on a structure or component during a severe weather event. Wind Load is both a positive and negative force depending on the direction of the wind in relation to the orientation of the structure. Structural components that are part of the building need to be designed and anchored to withstand both the positive pressure, and the suction.
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Most window, door, roofing, and other component manufacturers provide windload calculations for their respective components. These can be obtained by contacting the manufacturers. However, the windload calculations on building in Florida require that the windload calculations be prepared by either an Architect or Engineer. The calculations for the components may be requested by the building official when applying for a permit. Verify with the building official when submitting for a permit for a project that will be constructed by the Owner.
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