WIND LOADS: ASCE 7 PROVISIONS Prepared by: Marcia C. Belcher, PE Uplift on Roof Wind at a Corner Wind Around a Corner Effects of Exposure on Wind Velocity A Beginner's Guide to ASCE 7-05 Typical Wind Pressure On A Structure A Beginner's Guide to ASCE 7-05 Permitted Design Methods We will study this approach Method 1—Simplified Procedure (ASCE 7-05 Section 6.4) Low rise buildings w/mean roof height not to exceed 60 feet. Method 2—Analytical Procedure (ASCE 7-05 Section 6.5) Most common procedure. Applicable to all structure types h Method 3—Wind Tunnel Procedure (ASCE 7-05 6.6) Mean Roof Height Simplified Method Tables are provided by ASCE 7 for wind pressures for exposure B at 30 ft. An adjustment factor is provided for different exposures and heights (hmax = 60 ft.) The total wind pressure is the wind pressure from the table multiplied by adjustment factors and importance factor. Simplified Method Main wind force-resisting system: The design wind pressure shall be determined as: Ps = I Ps30 Where: = Adjustment factor for building height and exposure I = Importance factor Ps30 = Simplified design wind pressure for exposure B at h = 30 ft (from tables) Application of Wind Pressures Wind pressures are applied to the vertical and horizontal projections of the structure. +Sign indicates pressure ON surface - Sign indicates pressure AWAY from surface (suction) Zones on the Longitudinal Wall Wind pressures on the longitudinal wall are calculated for each ―zone‖ of a structure as follows: Leeward Roof Windward Roof Overhang Roof Wall Load Case #1: Wind on Longitudinal Wall Zones on the End Wall Wind pressures on the transverse wall are calculated for each “zone” of a structure as follows: Load Case #2: Wind on End Wall Zone Definitions Calculating the length of end zone X=2a: The length, “a” shall be smaller 10% of least wall length or 0.4h, BUT not less than 8% of least horizontal dimension or 6 ft. Exposure Categories Exposure A – Deleted in ASCE 7-02 and later Extremely sheltered. Large city centers with tall buildings. Exposure B Urban and suburban areas, wooded areas, areas with many closely spaced obstructions. Exposure C Open terrain with scatter obstructions. Airports, areas that are generally flat open country. Exposure D Flat, unobstructed areas and water surfaces outside hurricane prone regions. This category includes smooth mud flats, salt flats, and unbroken ice that extend 5,000 ft or 20 times the building height in the upwind direction. Exposure B Exposure B and C Exposure C Exposure C Exposure D A Beginner's Guide to ASCE 7-05 Importance Factor Importance Factor for Wind Example: Wind Pressure Determination Using Simplified Method Wind blows on a small office building in a suburb of Chicago. The terrain is flat and unobstructed. The building is has a rectangular footprint with dimensions of 50’x100’. The eve height is 30 feet and the roof has an angle of 20 degrees. Determine wind load for main wind force resisting system using the Simplified Method for Load Case #1. Draw the pressure distribution on a typical interior section thru the building.
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