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Wind Loads: The ASCE 7 Provisions CE 694R – Fall 2007 T. Bart Quimby, P.E., Ph.D. Quimby & Associates A Beginner's Guide to ASCE 7-05 Permitted Design Methods Method 1—Simplified Procedure (ASCE 7-05 Section 6.4) Low rise buildings. This is an outgrowth of work done for/by the metal building industry. Method 2—Analytical Procedure (ASCE 7-05 Section 6.5) The typically used procedure. This is the main focus of this presentation. Method 3—Wind Tunnel Procedure (ASCE 7-05 6.6) See ASCE 7-05 6.1.2 A Beginner's Guide to ASCE 7-05 Important Definitions Basic Wind Speed Building open, enclosed, partially enclosed Low-Rise Building See ASCE 7-05 6.2 A Beginner's Guide to ASCE 7-05 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. See ASCE 7-05 6.5.6 & C6.5.6 (See images!) A Beginner's Guide to ASCE 7-05 Determining Exposure Wind Direction & Sectors (ASCE 7-05 6.5.6.1) the exposure of the building or structure shall be determined for the two upwind sectors extending 45 o either side of the selected wind direction. the exposure resulting in the highest wind loads shall be used to represent the winds from that direction. A Beginner's Guide to ASCE 7-05 ASCE 7-05 Wind Pressures The basic form of the pressure equation: p = qGC Where p = a wind pressure on a surface q = velocity pressure. This is the pressure due to a moving fluid on a flat plate G = gust factor. The gust factor accounts for dynamic interaction between the flowing air and the structure C = pressure coefficient. The pressure coefficient accounts for varying pressure across a surface. A Beginner's Guide to ASCE 7-05 Velocity Pressure, q q z =Velocity Pressure = 0.00256K z K zt K d V 2 I (lb/ft 2 ) Constant 0.00256 V = Basic wind speed in mph I = Importance Factor (i.e. different MRI) K z = Exposure Coefficient K zt = Topographical Factor K d = Wind Directionality Factor Evaluated at an elevation z: q z = 0.00256V 2 IK z K zt K d Evaluated at the building mean roof elevation, h: q h = 0.00256V 2 I K h K ht K d See ASCE 7-05 6.5.10 A Beginner's Guide to ASCE 7-05 The Velocity Coefficient Based on the average density of air at sea level. P' 1 2 DV 2 ' 1 2 [ 0.0765 32.2 ][ 5280 3600 ] 2 V 2 '0.00256V 2 See ASCE 7-05 C6.5.10 A Beginner's Guide to ASCE 7-05 Basic Wind Speed, V Obtained from Wind Speed maps in ASCE 7- 05 Figure 6-1. Determined by localized research using approved probabilistic methods. “The basic wind speed shall be increased where records or experience indicate that the wind speeds are higher than those reflected in Fig. 6-1.” (ASCE 7-05 6.5.4.1) See ASCE 7-05 6.5.4 A Beginner's Guide to ASCE 7-05 The Importance Factor, I Category I: I = .87 MRI is 25 years Category II: I = 1.00 MRI is 50 years Category III & IV: I = 1.15 MRI is 100 years Building Categories are listed in ASCE 7-05 Table 1-1. See ASCE 7-05 6.5.5, Table 6-1 and Commentary 6.5.5 A Beginner's Guide to ASCE 7-05