Code Master - Wind Design Overview 2009 Ibc - Asce 7-05

DETERMINATION OF WIND LOADS

The 2009 International Building COOs (IBC) wind proVisions found in IBC section 1609 have adopted the provisions of ASCE 7-05 , MInimum DeSJgn Loads for BuiJdmgs and Other Structures, by reference. These provisions, which appear in Chapter 6 of ASCE 7, can be difficolt to understand, This Codemaster provides step.by·step instructions explaming how to determine the deslgn wind loads First, Steps 1 through 4 address how to determine charactenslics about the slNcture's location and configuration that direcl1y affect the magnitude of wind loads. Next Step 5 addresses determination of Importance Factor which is a step common to all wind design methods. Next, Step 6 clearly outlines the different methods that may be used to determine the design wind loads lor the structure. Finally, a stepby-step format is presented to provide a clear explanabon for the three most I commonly used design methods

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DETERMINE BASIC WINO SPEED (3·5ECONO GUST)

IBC Figure 1609 (ASCE 7 Figure 6-1) shown below presents basic wind speeds for the contiguous United States, Alaska, Hawaii and other U.S territories. The basic wind speeds shown reflect the peak gust Wind speed recorded within an averaging time of approximately 3 seconds at 33 feet above ground lor Exposure Category C (see Step 2). The special wind regionS (as shown by gray shading on the map) are required by IBC section 1609.3 (ASCE 7 Section 6.5.4) to have their basic wind speed detennined in accordance With the local jUrisdiction requirements and ASCE 7 section 6.54 Even if not located in a special wind region, it is a good idea to confinnthe basic wind speed with the local jurisdicbon in which the structure wiJI be built.

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location Hawaii Puerto Rico Guam

V (mph)

location

V (mph)

105 145 170

Virgin Islands

145 125

American Samoa

Notes 1, Unear interpolation between wind contours is permitted. 2. Islands and costal areas outslde the last contour shall use the last wind speed contour of the costal area 3. MoontalflOUS terrain. gorges, ocean promontories, and special wtnd regions shaH be examined for unusual WInd conditions. STEP:

2

DETERMINE EXPOSURE CATEGORY

(8

C. OR D)

Three Exposure Categones (B, C, and 0) are defined in terms of the extent and types of Surface Roughness that are Upwind of the site Surface Roughness Categories B, C, and 0 are a classification system established to reflect the ;characteristics of ground surface irregularities. The more obstrucbons (e.g. trees, structures, fences. etc.) there are on the site upwind of the building, the more the effects of WInd forces are reduced due to friction. The Exposure Category (B, C, or 0) needs to be determined based on the Surface Roughness Category (IBC section 1609.4.2, ASCE 7 Section 6.5.6.2), l'Iflidl is a function of the topography, vegetation, and constructed lad lties. and the Upwind distance oyer which the Surface Roughness prevails The following table may be used to determine the Exposure Category lor the site (IBC Section 1609,4, ASCE 7 Section 6.5.6.3). EXP.

DESCRIPTION

B

Surface Roughness B is dlaractenzed by urban and suburban areas, wooded areas. or other terrain with numerous doseIy spaced obstructions haVIng the size of single-family dwellings or larger, Use of Exposure Category B is limited to those areas lor which tarratn representative of Surface Roughness B prevails in the upwind directlon for a distance of at least 2600 feet or 20 limes the height of the building or other structure, whichevef is greater,l

c

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o

Surface Roughness 0 is characterized by flat, unobstructed areas and watll!' surfaces outside hurricane prone regionS including smooth mud flats. salt flats, and unbrolo:en ice. Exposure 0 applies where Surface Roughness 0 prevails in the upwind direction fOf a distance greatll!' than 5000 fI or 20 times the building height, whichevef is greater. Exposure 0 extends into downwind areas of Surface Roughness B Of C lor a distance of 600 fI or 20 limes the height of the building or structure, whichever is greater

Exposure C applies for all cases where Exposure B or D does

'For buildings whose mean rooftlelQhlllles$ than or equal klJO ll. the upwn:! dislance may' be reduced tp 1500 tl.

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'M

• Mid and .Northern AtIanlic

western GuW Of Mexico I

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140 140 150

The term "mean roof heighr in ASCE 7 Section 6.2 is defined as the average of the roof eave height and the height to the highest poinl on the roof surface, except tha~ for roof angles of less than or equal 1010 degrees, the mean roof height is equal to the roof eave height Eave height is defined in ASCE 7 Section 6.2.

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