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Wind load calculation

ASCE 7-10 Wind load calculation:

Basics in ASCE 7-10

ASCE 7-10 change wind load calculaltion from service design wind load to strength design wind load..

ACSCE 7-10 covers wind load from chapter 26 to 31 as follows:

  • Chapter 26 - General requirements - It includes three wind maps for categories I, II, III and IV. for basic wind speed, V, wind directionality factor, Kd, exposure categories and constants, topographic effects (Kzt), gust response factor, G, enclosure classification, and internal coefficient (GCpi), etc.

  • Chapter 27: Wind load on Buildings - MWFRS (directional procedure) - Part 1: Wind load on building for all height - it includes velocity pressure equation, design wind pressure equation , parapmeter, Kz, Kh, Cp, CNPart 2: Wind load on simple diaphragm with h = or <160 ft - includes wind pressure tables for exposure B, C, and D.

  • Chapter 28 - Wind load on Buildings - MWRFS (Envelope Procedure) - Part 1: Wind pressure on low rise building with windward, leeward, and side walls pressure.  Part 2: Simplified design wind pressure tables for low-rise enclosed buildings.

  • Chapter 29 - Wind loads on other structures and building appurtenances - MWFRS - Wind pressure calculations for signs, chimneys, tanks, roof top equipment, lattice frameworks, trussed towers.

  • Chapter 30 - Components and Cladding (C & C) - Part 1: Low-rise buildings.  Part 2: Low-rise building (simplified). Part 3: Building with h > 60 ft.  Part 4: building with h = or < 160 ft. Part 5: Opening buildings.  Part 6: building appurtenances, rooftop structures and equipments.

  • Chapter 31 - Wind tunnel procedure

Chapter 26

Basic wind speed: (for strength design)

ASCE 7-10 removes important factor Iw from wind load equation from ASCE 7-05 and replaces it with three different wind maps.

Basic wind speed in U.S. is shown in wind hazard map:

Figure 26.5-1A. - Basic wind speeds for Occupancy Category II building and other structures

Figure 26.5-1B - Basic wind speeds for Occupancy Category III and IV buildings and Other Structures

Figure 26.5-1C - Basic wind speeds for Occupancy Category III and IV buildings and Other Structures

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.

Wind hazard map can be found on IBC website Chapter 16, section 1609.

Exposure to wind:

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.

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. Ao > 1.10Aoi
  2. Ao > 4 ft2 or 0.1 Ag and Aoi/Agi £ 0.2

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

Ag = the gross area of wall in the same wall of Ao.

Aoi = the sum of area in the building envelop not including Ao.

Agi = the sum of gross area of building envelop not including Ag.

  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.