An example of ASCE 716 wind load calculations (directional procedure) for an Lshaped building
In this article, an example wind load pressure calculation for an Lshaped building in Cordova, Tennessee will be shown. This calculation will be in accordance with ASCE 716 wind load calculations (directional procedure).
For this case study, the structure data are as follows:
Location  Cordova, Memphis, Tennessee Elevation +110.0m 
Occupancy  Miscellaneous – Plant Structure 
Terrain  Flat farmland 
Dimensions  28m (12m width) x 24m (8m width) in plan Eave height of 5 m Apex height at elev. 8 m Roof slope: 1:2 for main frame (26.57°) 3:4 for extension (36.87°) With opening 
A similar calculation for a gable roof construction using ASCE 710 (imperial units) is referenced in this example and can be accessed using this link. The formula in determining the design wind pressure are:
For enclosed and partially enclosed buildings:
\(p = qG{C}_{p} {q}_{i}({GC}_{pi})\) (1)
For open buildings:
\(p = q{G}_{f}{C}_{p} {q}({GC}_{pi})\) (2)
Where:
\(G\) = gust effect factor
\({C}_{p}\) = external pressure coefficient
\(({GC}_{pi})\)= internal pressure coefficient
\(q\) = velocity pressure, in Pa, given by the formula:
\(q = 0.613{K}_{z}{K}_{zt}{K}_{d}V^2\) (3)
\(q\) = \({q}_{h}\) for leeward walls, side walls, and roofs,evaluated at roof mean height, \(h\)
\(q\) = \({q}_{z}\) for windward walls, evaluated at height, \(z\)
\({q}_{i}\) = \({q}_{h}\) for negative internal pressure, \(({GC}_{pi})\) evaluation and \({q}_{z}\) for positive internal pressure evaluation \((+{GC}_{pi})\) of partially enclosed buildings but can be taken as \({q}_{h}\) for conservative value.
\({K}_{z}\) = velocity pressure coefficient
\({K}_{zt}\)= topographic factor
\({K}_{d}\)= wind directionality factor
\(V\) = basic wind speed in m/s
Risk Category
The first thing in determining the design wind pressures is to classify the risk category of the structure, which is based on the use or occupancy of the structure. Since this example is a plant structure, the structure is classified as Risk Category IV. See Table 1.51 of ASCE 716 for more information about risk categories classification.
Basic Wind Speed, \(V\)
In ASCE 716, the wind speed data can be obtained from Figures 26.51 to 26.52. From Figure 26.51A, Cordova, Memphis, Tennessee is near the red dot shown in Figure 3 below, and subsequently, the basic wind speed, \(V\), is 52 m/s. Take note that the values should be interpolated between known wind contours.
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Exposure Category
See Section 26.7 of ASCE 716 details the procedure in determining the exposure category.
Depending on the wind direction selected, the exposure of the structure shall be determined from the upwind 45° sector. The exposure to be adopted should be the one that will yield the highest wind load from the said direction. The description of each exposure classification is detailed in Section 26.7.2 and 26.7.3 of ASCE 716.
For our example, since the location of the structure is in a farmland in Cordova, Memphis, Tennessee, without any buildings taller than 30 ft, therefore the area is classified as Exposure C. A helpful tool in determining the exposure category is to view your potential site through a satellite image (Google Maps for example).
Wind Directionality Factor, \({K}_{d}\)
The wind directionality factors, \({K}_{d}\), for our structure are both equal to 0.85 since the building is the main wind force resisting system and also has components and cladding attached to the structure. This is shown in Table 26.61 of ASCE 716.
Topographic Factor, \({K}_{zt}\)
Since the location of the structure is in a flat farmland, we can assume that the topographic factor, \({K}_{zt}\), is 1.0. Otherwise, the factor can be solved using Figure 26.81 of ASCE 716. To determine if further calculations of the topographic factor are required, see Section 26.8.1, if your site does not meet all of the conditions listed, then the topographic factor can be taken as 1.0.
Note: Topography factors can automatically be calculated using SkyCiv Wind Design Software. For more information on calculation of topography factor, check this article.
Ground Elevation Factor, \({K}_{e}\)
The ground elevation factor, \({K}_{e}\), is introduced in ASCE 716 to consider the variation in the air density based on ground elevation above mean sea level. This factor can be calculated using:
\( {K}_{e} = {e}^{0.000119{z}_{g}}\) (4)
Where:
\({z}_{g}\) is the ground elevation above mean sea level in meters
Hence, for this case study, since the ground elevation is +110.0m, \({K}_{e}\) is equal to 0.987.
Velocity Pressure Coefficient, \({K}_{z}\)
The velocity pressure coefficient, \({K}_{z}\), can be calculated using Table 26.101 of ASCE 716. This parameter depends on the height above ground level of the point where the wind pressure is considered, and the exposure category. Moreover, the values shown in the table is based on the following formula:
For 4.6 m < \({z}\) < \({z}_{g}\): \({K}_{z} = 2.01(z/{z}_{g})^{2/α}\) (5)
For \({z}\) < 4.6 m: \({K}_{z} = 2.01(4.6/{z}_{g})^{2/α}\) (6)
Where:
Exposure  α  \({z}_{g}\)(m) 
Exposure B  7.0  365.76 
Exposure C  9.5  274.32 
Exposure D  11.5  213.36 
Usually, velocity pressure coefficients at the mean roof height, \({K}_{h}\), and at each floor level, \({K}_{zi}\), are the values we would need in order to solve for the design wind pressures. For this example, since the wind pressure on the windward side is parabolic in nature, we can simplify this load by assuming that uniform pressure is applied on walls between floor levels. We can simplify the windward pressure and divide it into 2 levels, at the eave height (+5.0m), and at the mean roof height (+6.5m). Moreover, α = 9.5 and \({z}_{g}\) is equal to 274.32 m since the location of the structure is classified as Exposure C.
Elevation (m)  \( {K}_{z} \) 
5 (eave height)  0.865 
6.5 (mean roof height)  0.914 
Velocity Pressure, \( q \)
From Equation (3), we can solve for the velocity pressure, \( q \) in Pa, at each elevation being considered.
Elevation, m  \( {K}_{z} \)  \( {K}_{zt} \)  \( {K}_{d} \)  \( {K}_{e} \)  \( V \), m/s  \( q \), Pa 
5 (eave height)  0.865  1.0  0.85  0.987  52  1202.87 
6.5 (mean roof height)  0.914  1.0  0.85  0.987  52  \( {q}_{h} \) = 1271.01 
Gust Effect Factor, \( G \)
The gust effect factor, \( G \), is set to 0.85 as the structure is assumed rigid (Section 26.11 of ASCE 716).
Enclosure Classification and Internal Pressure Coefficient, \( ({GC}_{pi}) \)
The plant structure is assumed to have openings that satisfy the definition of a partially enclosed building in Section 26.2 of ASCE 716. Thus, the internal pressure coefficient, \( ({GC}_{pi}) \), shall be +0.55 and 0.55 based on Table 26.131 of ASCE 716. Therefore:
\(+{p}_{i} = {q}_{i}(+G{C}_{pi}) \) = (1271.01)(+0.55) = 699.06 Pa
\({p}_{i} = {q}_{i}(G{C}_{pi}) \) = (1271.01)(0.55) = 699.06 Pa
External Pressure Coefficient, \({C}_{p}\)
For enclosed and partially enclosed buildings, the External Pressure Coefficient, \({C}_{p}\), is calculated using the information provided in Figure 27.41 through Figure 27.43. For a partially enclosed building with a gable roof, use Figure 27.41. External Pressure Coefficients for the walls and roof are calculated separately using the building parameters L, B and h, which are defined in Note 7 of Figure 27.41.
For this example, since the structure is asymmetric, four wind directions will be considered: two (2) for wind direction parallel to 24m side, and two (2) for wind direction parallel to 28m side.
For Wind Direction parallel to 24m side
Thus, we need to calculate the L/B and h/L:
Roof mean height, h = 6.5 m
Building length, L = 24 m
Building width, B = 28 m
L/B = 0.857
h/L = 0.271
h/B = 0.232
Wall Pressure Coefficients, \({C}_{p}\), and External Pressure, \({p}_{e}\)
.For walls, the external pressure coefficients are calculated from Figure 27.31 of ASCE 716 where \({q}_{h}\) = 1271.011 Pa and \( G \) = 0.85.
Surface  h, m  Wall Pressure Coefficients, \({C}_{p}\)  \({p}_{e}\), Pa 
Windward wall  5.0  0.8  817.953 
6.5  0.8  864.288  
Leeward wall  6.5  0.5  540.180 
Sidewalls  6.5  0.7  756.252 
Roof Pressure Coefficients, \({C}_{p}\), and External Pressure, \({p}_{e}\)
For roof, the external pressure coefficients are calculated from Figure 27.31 of ASCE 716 where \({q}_{h}\) = 1271.011 Pa. Note that for this wind direction, windward and leeward roof pressures (roof surfaces 1 and 2) are calculated using θ = 36.87° and θ = 0° for roof surfaces 3 and 4.
Surface  Location  Roof Pressure Coefficients, \({C}_{p}\)  \({p}_{e}\), Pa 
Windward roof  –  0.4  432.144 
Leeward roof  –  0.6  648.216 
Parallel to wind (along the ridge)  0 to h from edge  0.9 0.18 
972.324 194.465 
h to 2h from edge  0.5 0.18 
540.180 194.465 

> 2h from edge  0.3 0.18 
324.108 194.465 
Therefore, combining \({p}_{e}\) and \({p}_{i}\), the corresponding design pressures can be obtained:
Type  Surface  Elevation/Location, m  \({p}_{e}\), Pa  \({p}_{e}\) – +\({p}_{i}\), Pa  \({p}_{e}\) – \({p}_{i}\), Pa 
Walls  Windward wall  5.0  817.953  118.897  1517.009 
6.5  864.288  165.231  1563.344  
Leeward wall  –  540.180  1239.236  158.876  
Sidewalls  –  756.252  1455.308  57.196  
Roof  Windward  –  432.144  266.912  1131.200 
Leeward  –  648.216  1347.272  50.840  
Flat (along ridge)  0 to h  972.324 194.465 
1671.380 893.521 
273.267 504.592 

h to 2h  540.180 194.465 
1239.236 893.521 
158.876 504.592 

> 2h  324.108 194.465 
1023.164 893.521 
374.948 504.592 
For Wind Direction parallel to 28m side
Thus, we need to calculate the L/B and h/L:
Roof mean height, h = 6.5 m
Building length, L = 28 m
Building width, B = 24 m
L/B = 0.857
h/L = 0.232
h/B = 0.271
Wall Pressure Coefficients, \({C}_{p}\), and External Pressure, \({p}_{e}\)
.For design wall pressure, the external pressure coefficients are calculated from Figure 27.31 of ASCE 716 where \({q}_{h}\) = 1271.011 Pa and \( G \) = 0.85.
Surface  h, m  Wall Pressure Coefficients, \({C}_{p}\)  \({p}_{e}\), Pa 
Windward wall  5.0  0.8  817.953 
6.5  0.8  864.288  
Leeward wall  6.5  0.467  504.528 
Sidewalls  6.5  0.7  756.252 
Roof Pressure Coefficients, \({C}_{p}\), and External Pressure, \({p}_{e}\)
For roof, the external pressure coefficients are calculated from Figure 27.31 of ASCE 716 where \({q}_{h}\) = 1271.011 Pa. Note that for this wind direction, windward and leeward roof pressures (roof surfaces 3 and 4) are calculated using θ =26.57° and θ = 0° for roof surfaces 1 and 2.
Surface  Location  Roof Pressure Coefficients, \({C}_{p}\)  \({p}_{e}\), Pa 
Windward roof  –  0.2 0.3 
216.072 324.108 
Leeward roof  –  0.6  648.216 
Parallel to wind (along the ridge)  0 to h from edge  0.9 0.18 
972.324 194.465 
h to 2h from edge  0.5 0.18 
540.180 194.465 

> 2h from edge  0.3 0.18 
324.108 194.465 
Therefore, combining \({p}_{e}\) and \({p}_{i}\), the corresponding design pressures can be obtained:
Type  Surface  Elevation/Location, m  \({p}_{e}\), Pa  \({p}_{e}\) – +\({p}_{i}\), Pa  \({p}_{e}\) – \({p}_{i}\), Pa 
Walls  Windward wall  5.0  817.953  118.897  1517.009 
6.5  864.288  165.231  1563.344  
Leeward wall  –  504.528  1203.584  194.528  
Sidewalls  –  756.252  1455.308  57.196  
Roof  Windward  –  216.072 324.108 
915.128 374.948 
482.984 1023.164 
Leeward  –  648.216  1347.272  50.840  
Flat (along ridge)  0 to h  972.324 194.465 
1671.380 893.521 
273.267 504.592 

h to 2h  540.180 194.465 
1239.236 893.521 
158.876 504.592 

> 2h  324.108 194.465 
1023.164 893.521 
374.948 504.592 
Structural Engineer, Product Development
MS Civil Engineering
References:
 Coulbourne, W. L., & Stafford, T. E. (2020, April). Wind Loads: Guide to the Wind Load Provisions of ASCE 716. American Society of Civil Engineers.
 American Society of Civil Engineers. (2017, June). Minimum design loads and associated criteria for buildings and other structures. American Society of Civil Engineers.