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ASCE 7-05 Wind load calculation - Method II

Method of wind calculation:

ASCE 7-05 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

            qz = 0.00256 Kz Kzt Kd V2 I (lb/ft2)

where V is basic wind speed, I is important factor, Kd is wind directionality factor, Kzt is topographic factor, and Kz is velocity pressure exposure coefficient.

 

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

Kz = 2.01 (z/zg)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 zg are taken as follows:

Exposure

a

zg  (ft)

B

7.0

1200

C

9.5

900

D

11.5

700

Topographic Factor,

Kzt = (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 Cp – qi (GCpi)

Where

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

q = qh  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.

Cp 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

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

qi is internal pressure evaluated as follows:

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

  • Partial enclosed building: qi = qh for negative internal pressure, qi = qz 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):

Surface

L/B

Cp

Use with

Windward Wall

All values

0.8

qz

 

Leeward Wall

0-1

-0.5

 

qh

2

-0.3

³ 4

-0.2

Side Wall

All values

-0.7

qh

 

 

Low-rise building.

The design wind pressure shall be calculated as

P = qh[ (GCpf )– qi (GCpi)]

Where

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

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

GCpi 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 GCpf (from Figure 6-10 of ASCE 7-02)

Roof Angle

Building Surface

1

2

3

4

5

6

1E

2E

3E

4E

0-5

0.4

-0.69

-0.37

-0.29

-0.45

-0.45

0.61

-1.07

-0.53

-0.43

20

0.53

-0.69

-0.48

-0.43

-0.45

-0.45

0.8

-1.07

-0.69

-0.64

30-45

0.56

0.21

-0.43

-0.37

-0.45

-0.45

0.69

0.27

-0.53

-0.48

90

0.56

0.56

-0.37

-0.37

-0.45

-0.45

0.69

0.69

-0.48

-0.48

 

Parapets

The design wind pressure shall be calculated as

Pp = qp GCpn

Where

qp  is velocity pressure at top of parapet.

GCpn 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 = qh[ (GCp )– qi (GCpi)]

Where

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

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

GCpi 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 (GCp) – qi (GCpi)

Where

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

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

qi is internal pressure evaluated as follows:

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

  • Partial enclosed building: qi = qh for negative internal pressure, qi = qz for positive internal pressure at height z at the level of highest opening.

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

GCpi 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 = qp (GCp) – qi (GCpi)

Where

qp = velocity pressure at top of parapets.

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

GCpi 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 = qz G Cf Af

Where

qz = velocity pressure at height z at the centroid of Af.

G is gust effect factor.

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

Af is project area normal to the wind.

 

Example 3:  Wind load on a billboard along highway.

Design Data:

Design code: ACE 7-05

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, Kd = 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 Kz and Kh from Table 6.5.

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

Height, z = 60 ft, Kz = 2.01*(z/zg)2/a= 1.1

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

Kzt = (1+K1K2K3) = 1

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

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

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

7. Determine velocity pressure qz = 0.00256 KzKztKdV2I = 20.03 psf

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

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