Wind load calculation ASCE 7-05 & 02

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.

  Main wind force-resisting system:

The design wind pressure shall be determined as

P_s = l I P_s30

l = Adjustment factor for building height and exposure from Figure 6.2 of ASCE 7-02.

I = Important factor

P_s30 = simplified design wind pressure for exposure B at h = 30 ft and I = 1 from Figure 6-2.

Notes:

1. Figure 6.2 includes Ps30 for basic wind speed from 85 to 170 mph and roof angle from 0 t0 45  degree.  Examples for basic wind speed of 90 mph from 15 to 45 degree are shown below.

2. Wind shall be calculated for horizontal pressure for zone A, B, C, D; vertical pressure for zone E, F, G, H; and overhang pressure, E_ch, G_ch  as follows:

3. Pressures shown are net pressure from windward, leeward and interior pressure and shall be applied to the horizontal and vertical projections of the building.
4. There are two cases shown in Figure 6.2: Case 1: transverse direction, and Case 2: longitudinal direction.  Both cases shall be checked for roof slope, 25° < q £ 45°
5. The length of end zone X=2a.  The length, a shall be 10% of least dimension or 0.4h, whichever is smaller, but not less than 8% of least horizontal dimension or 6 ft. 
6. For flat roof, the length of E/F and G/H shall be 1/2 length of the building.

Component and cladding:

The design wind pressure shall be determined as

P_net = l I P_net30

l = Adjustment factor for building height and exposure from Figure 6.3 of ASCE 7-02.

I = Important factor

P_net30 = simplified design wind pressure for exposure B at h = 30 ft and I = 1 from Figure 6-3.

In general, wind pressures for components and cladding are higher than those in main wind force resisting system.  Wind pressures show in Figure 6-3 are broken down to effective areas of 10, 50, and 200 square foot to account for localized effect.

  Notes:

Figure 6-3 shows Pnet30 for flat roof, hip roof, and gable roof.  Wind pressures on roof are divided to three zones: (1) interior zone, (2) end zone and (3) corner zone. Wind pressures on walls are divided to two zones: (4) interior zone and (5) end zone.  Example of wind zones for gable roof with roof angle from 7 to 45 degree is shown in the figure below.  The width of end zone is “a” instead of “2a”. Wind pressures for wall, roof, and roof overhang are shown in several tables.

Example

Example 1: Wind load on a small office building in suburb using simplified procedure

Design data:

Eve height: 30 ft

Length of building: L = 100 ft

Width of building, B = 50 ft

Roof angle: 20 degree

Location: suburb of Chicago

Topographic feature: flat land

Requirement: Determine wind load for main wind force resisting system

Solution:

1. Determine basic wind speed from Figure 6.1., V = 90 mph

2. Determine Building cateorgy from Table 1.1, and important factor from Table 6.1, I =1

3. Determine Exposure category from Section 6.5.6, Exposure B

4. Determine wind pressure, Ps30 and adjustment coefficient, l from Figure 6-2

Mean roof height: h = 30 ft + (B/2)tan (20 degree) = 39.1 ft

Use 40 ft for adjustment factor

Wind pressure Ps30

Horizontal pressure:

Zone A: P_s30A = 17.8 psf

Zone B: P_s30B = -4.7 psf

Zone C: P_s30C = 11.9 psf

Zone D: P_s30D = -2.6 psf

Vertical pressure:

Zone E: P_s30E = -15.4 psf

Zone F: P_s30F = -10.7 psf

Zone G: P_s30G = -10.7 psf

Zone H: P_s30H = -8.1 psf

Adjustment factor: l = 1.09

Design wind pressure:

Horizontal pressure:

Zone A: P_A = P_s30A I l= 19.4 psf

Zone B: P_B = P_s30B I l= -5.1 psf

Zone C: P_C = P_s30C I l=13 psf

Zone D: P_D = P_s30D I l= -2.8 psf

Vertical pressure:

Zone E: P_E = P_s30E I l= -16.8 psf

Zone F: P_F = P_s30F I l= -11.7 psf

Zone G: P_G = P_s30G I l= -11.7 psf

Zone H: P_H = P_s30H I l= -8.8 psf

Length of end zone, X = 2 a

a is smaller of 0.4h = 15.64 ft, 0.1B=5 ft, but not less than 3 ft

X = 10 ft