Wind load calculation ASCE 7-02

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 2: Analytical procedure

Apply to all buildings and other structures.

Velocity pressure:

Velocity pressure is calculated as

            q_z = 0.00256 K_z K_zt K_d V² I (lb/ft²)

where V is basic wind speed, I is important factor, K_d is wind directionality factor, K_zt is topographic factor, and K_z is velocity pressure exposure coefficient.

Velocity pressure exposure factors are listed Table 6-3 of ASCE 7-02 or can be calculated as

K_z = 2.01 (z/z_g)^2/a.

z is height above ground, z shall not be less than 15 ft. except that z shall not be less than 30 ft for exposure B for low rise building and for component and cladding.

a and z_g are taken as follows:

Topographic Factor,

K_zt = (1+K₁+K₂+K₃)² 

where K₁, K₂, K₃ are determined from Figure 6-4 of ASCE 7-02 based on hill, ridge or escarpment.

Wind load for main wind force resisting system

Rigid building of all height:

The design wind pressure shall be calculated as

P = q G C_p – q_i (GC_pi)

Where

q = q_z  for windward walls evaluated at height z above ground.

q = q_h  for Leeward walls, side walls, and roof evaluated at mean roof height h above ground.

G = 0.85 is gust response factor or may be calculated by Eq. 6-4.

C_p is external pressure coefficient from Figure 6.6 to 6.8 of ASCE 7-02.

• Figure 6.6 is for gable, hip roof, monoslope roof, and mansard roof

• Figure 6.7 is for dome roof

• Figure 6.8 is for arched roof

GC_pi is internal pressure coefficient from Figure 6.5 of ASCE 7-02.

q_i is internal pressure evaluated as follows:

• Enclosed building: q_i = q_h evaluated at mean roof height for windward, leeward, and side walls, and roof.

• Partial enclosed building: q_i = q_h for negative internal pressure, q_i = q_z for positive internal pressure at height z at the level of highest opening.

Note: The internal pressure shall be applied simultaneously on windward and leeward walls and both positive and negative pressures need to be considered.  Therefore, it cancels each other for enclosed building except for roof.  For partially enclosed building, internal pressure shall be added to leeward wall at the height of opening.

Wall pressure coefficient Cp for Gable, Hip roof (from figure 6.6 of ASCE 7-02):

Low-rise building.

The design wind pressure shall be calculated as

P = q_h[ (GC_pf )– q_i (GC_pi)]

Where

q_h  is velocity pressure at mean roof height h above ground.

GC_pf is external pressure coefficient from Figure 6.10 of ASCE 7-02.

GC_pi is internal pressure coefficient from Figure 6.5 of ASCE 7-02.

Note: For wind pressures at edges and corners of walls and roof are higher than interior zone.  Wind pressure at each zone needs to be calculated seperatly.

External pressure coefficient GC_pf (from Figure 6-10 of ASCE 7-02)

Parapets

The design wind pressure shall be calculated as

P_p = q_p GC_pn

Where

q_p  is velocity pressure at top of parapet.

GC_pn is combined net pressure coefficient, +1.8 for windward, -1.1 for leeward.

Design wind load with eccentricities

Wind load design cases:

Case 1: Full wind loads in two perpendicular directions considered separately.

Case 2: 75% wind loads in two perpendicular directions with 15% eccentricity considered separately.

Case 3: 75% wind loads in two perpendicular directions simultaneously.

Case 4: 56.3% (75%x75%) of wind load in two perpendicular directions with 15% eccentricity simultaneously.

Wind load for component and cladding.

Building 60 ft or lower

The design wind pressure shall be calculated as

P = q_h[ (GC_p )– q_i (GC_pi)]

Where

q_h  is velocity pressure at mean roof height h above ground.

GC_p is external pressure coefficient from Figure 6.11 to 6.16 of ASCE 7-02.

GC_pi is internal pressure coefficient from Figure 6.5 of ASCE 7-02.

Building higher than 60 ft

The design wind pressure shall be calculated as

P = q (GC_p) – q_i (GC_pi)

Where

q = q_z  for windward walls evaluated at height z above ground.

q = q_h  for Leeward walls, side walls, and roof evaluated at mean roof height h above ground.

q_i is internal pressure evaluated as follows:

• Enclosed building: q_i = q_h evaluated at mean roof height for windward, leeward, and side walls, and roof.

• Partial enclosed building: q_i = q_h for negative internal pressure, q_i = q_z for positive internal pressure at height z at the level of highest opening.

GC_p is external pressure coefficient from Figure 6.11 to 6.17 of ASCE 7-02.

GC_pi is internal pressure coefficient from Figure 6.5 of ASCE 7-02.

Note: The internal pressure shall be applied simultaneously on windward and leeward walls and both positive and negative pressures need to be considered.  Therefore, it cancels each other for enclosed building except for roof.  For partially enclosed building, internal pressure shall be added to leeward wall at the height of opening.

Wind pressure on parapets

The design wind pressure shall be calculated as

P = q_p (GC_p) – q_i (GC_pi)

Where

q_p = velocity pressure at top of parapets.

GC_p is external pressure coefficient from Figure 6.11 to 6.17 of ASCE 7-02.

GC_pi is internal pressure coefficient from Figure 6.5 based on porosity of the parapet envelope.

Wind load on open building and other structures

The design wind load shall be calculated as

P = q_z G C_f A_f

Where

q_z = velocity pressure at height z at the centroid of A_f.

G is gust effect factor.

C_f is net pressure coefficients from Figure 6-18 to 6-22 of ASCE 7-02.

A_f is project area normal to the wind.

Example 3:  Wind load on a billboard along highway.

Design Data:

Design code: ACE 7-02

Dimension of sign: 20 ft by 15 ft

Height from ground to center of sign: 60 ft

Basic wind speed: 90 mph

Exposure category: B

Topographic feature: flat land

Requirement: Determine design wind load on billboard to be used with load combination

Solution

1.Determine basic wind speed from Figure 6.1 and directionality factor Kd from Table 6-6, V = 90 mph, K_d = 0.85

2. Determine Important factor from Table 6-1:  I = 1

3. Determine Exposure category from section 6.5.6 and velocity exposure coefficient K_z and K_h from Table 6.5.

Exposure B, exposure coefficient, a = 9.5, z_g = 900 ft

Height, z = 60 ft, K_z = 2.01*(z/z_g)^2/a= 1.1

4. Determine topographic factor from Figure 6.2, K₁ = 0, K₂ = 1, K₃ = 1

K_zt = (1+K₁K₂K₃) = 1

5. Determine gust effect factor from section 6.5.8, G = 0.85

6. Determine external pressure coefficient C_f from Table 6-11

M = 20 ft, N = 15 ft, M/N = 1.3, C_f = 1.2

7. Determine velocity pressure qz = 0.00256 K_zK_ztK_dV²I = 20.03 psf

8. Determine wind pressure: p = q_zGC_f = 20.43 psf

9. Determine wind load on billboard, F = pMN = 6130 lbs