ASCE 7-05 & 02 Wind load calculation:

Basics in ASCE 7-05 & 02

Basic wind speed:

Basic wind speed in U.S. is shown in Figure 6.1. The basic wind speed is basic on statistical data collected from airport across U.S.  The wind speeds correspond to 3-second gust speeds at 33 ft above ground for exposure C category.  The wind speeds represent 50-year return period.

Exposure to wind:

Change from ASCE 7-98 to 02 & 05. Exposure A is deleted. A distinction was made surface roughness categories and exposure categories.  Interpolation between exposure categories is permitted by rational analysis.  Wind is to be evaluated in the worst case of the two 45-degree sectors either side of the wind direction.

Surface roughness categories:

• Surface roughness B: Urban and suburban areas, wooded areas or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger.
• Surface roughness C: Open terrain with scattered obstructions having heights generally less than 30 ft. including flat open country, grasslands, and all water surfaces in hurricane-prone regions.
• Surface roughness D: Flat, unobstructed areas and water surfaces outside hurricane-prone regions including smooth mud flats, salt flats, and unbroken ice.

  Exposure categories:

• Exposure B: Surface roughness B in upwind direction for 2630 ft or 10 times the height of the buildings.  For buildings with mean roof height 30 ft or less, the upwind distance is reduced to 1500 ft.
• Exposure C: apply to all cases not in B or D.
• Exposure D: Surface roughness D in upwind direction for 500 ft or 10 times the height of the buildings.  Expose D shall extend inland from the shoreline for a distance of 600 ft or 10 times the height of the building.

  Note: This also allow the building to be designed in different exposure in different direction.  A building located at edge of city or sea shore may be designed for exposure category in one direction and other exposure in other direction.

  Important factor, I for wind load

Building classifications

Building enclosure and openings and protection:

Open building: A building having each wall at least 80% open.

Partially enclosed building: building that meet the following conditions;

1. A_o > 1.10A_oi
2. A_o > 4 ft² or 0.1 A_g and A_oi/A_gi £ 0.2

A_o = total area of openings in a wall the received positive pressure

A_g = the gross area of wall in the same wall of A_o.

A_oi = the sum of area in the building envelop not including A_o.

A_gi = the sum of gross area of building envelop not including A_g.

  Examples of partially enclosed buildings: Airplane hangars.  One side of the building has door opening that larger than other 3 sides.

Enclosed buildings: buildings not qualified for open or partially enclosed.

Wind-Borne Debris:

Buildings in standard, essential and substantial hazard categories located in wind-borne debris regions shall be protected with impact-resistant covering or glazings (windows) at (1) below 60 ft (2) at 30 ft from roof with aggregate surface within 1500 ft of the building.

Low-rise building:

Building 60 ft or lower and height does not exceed least horizontal dimension.  

Minimum design wind load: 10 lb/ft² for vertical projection of building or components and cladding.

Method of wind calculation:

ASCE 7-02 provides two methods for wind load calculation: a simplified procedure and an analytical procedure. The simplified procedure is for building with simple diaphragm, roof slope less than 10 degree, mean roof height less than 30 ft, regular shape rigid building, no expansion joints, flat terrain and not subjected to special wind condition.  The analytical procedure is for all buildings and non-building structures.  Each procedure has two categories: wind for main wind force-resisting system and wind for component and claddings.  

Method 1: Simplified Procedure

Simplified procedure tablets wind pressures for exposure B at 30 ft in Figure 6-2 of ASCE 7-02.  An adjustment factor is provided for different exposure and height.  The wind pressure is the wind pressure in the table multiplied by adjustment factor and important factor.

Method 2: Analytical procedure

Apply to all buildings and other structures.