CEREF.COM
ASCE 705
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 nonbuilding
structures. Each
procedure has two categories: wind for main wind forceresisting
system and wind for component and claddings.
Apply to all
buildings and other structures.
Velocity
pressure:
Velocity pressure
is calculated as
q_{z} =
0.00256 K_{z} K_{zt} K_{d} V^{2} I
(lb/ft^{2})
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 63 of ASCE 702 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:
Exposure 
a 
z_{g }(ft) 
B 
7.0 
1200 
C 
9.5 
900 
D 
11.5 
700 
Topographic
Factor,
K_{zt} =
(1+K_{1}+K_{2}+K_{3})^{2}
where K_{1},
K_{2}, K_{3} are
determined from Figure 64 of ASCE 702 based on hill, ridge or
escarpment.
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. 64.
C_{p} is
external pressure coefficient from Figure 6.6 to 6.8 of ASCE 702.
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 702.
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 702):
Surface 
L/B 
Cp 
Use with 
Windward
Wall 
All
values 
0.8 
q_{z} 
Leeward
Wall 
01 
0.5 
q_{h} 
2 
0.3 

³ 4 
0.2 

Side Wall 
All
values 
0.7 
q_{h} 
Lowrise
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 702.
GC_{pi} is
internal pressure coefficient from Figure 6.5 of ASCE 702.
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 610 of ASCE 702)
Roof
Angle 
Building
Surface 

1 
2 
3 
4 
5 
6 
1E 
2E 
3E 
4E 

05 
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 
3045 
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 
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.
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.
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 702.
GC_{pi} is
internal pressure coefficient from Figure 6.5 of ASCE 702.
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 702.
GC_{pi} is
internal pressure coefficient from Figure 6.5 of ASCE 702.
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 702.
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 618 to 622 of ASCE 702.
A_{f} is project area normal to the wind.
Example 3: Wind
load on a billboard along highway.
Design Data:
Design code: ACE 705
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 66, V = 90 mph, K_{d} =
0.85
2. Determine
Important factor from Table 61: 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, K_{1} =
0, K_{2} = 1, K_{3} =
1
K_{zt} =
(1+K_{1}K_{2}K_{3}) = 1
5. Determine gust
effect factor from section 6.5.8, G = 0.85
6. Determine
external pressure coefficient C_{f} from
Table 611
M = 20 ft, N = 15
ft, M/N = 1.3, C_{f} =
1.2
7. Determine
velocity pressure qz = 0.00256 K_{z}K_{zt}K_{d}V^{2}I
= 20.03 psf
8. Determine wind
pressure: p = q_{z}GC_{f} =
20.43 psf
9. Determine wind
load on billboard, F = pMN = 6130 lbs