在这篇文章中, we will walk you through the how to determine the terrain or exposure categories of the upwind side the site location, which are essential for calculating wind loads. We will cover the specific procedures outlined in ASCE 7, NBCC 2015, 和AS / NZS 1170.2 for determining the terrain categories and discuss how these apply to each reference code available in the SkyCiv Load Generator.

ASCE 7-16/ASCE 7-22

我们将通过计算地形因子来讨论地形对结构风荷载的影响 7, the procedure to determine the Exposure Category of the upwind exposure of a site location is discussed in Section 26.7, depending on the terrain. 在这篇文章中, to simplify the reference, we shall be using ASCE 7-16. For each wind source direction, it should be analyzed from two upwind sectors extending ±45°.

数字 1. Terrain sectors for each wind source direction.

For each sector, the Surface Roughness category should be checked based on the following definition based on Section 26.7.2 ASCE的 7-16:

桌子 1. Surface Roughness definition based on Section 26.7.2 ASCE的 7-16.

From the definition of Surface Roughness, we can determine the Exposure Category of the terrain bounded by the upwind sector. The definition for each Exposure Category is stated in Section 26.7.3 ASCE的 7-16 如下:

桌子 2. Exposure Category definition based on Section 26.7.3 ASCE的 7-16.

The Table 2 can be visualized thru the following figures based on Figure C26.7-2:

数字 2. Upwind Surface Roughness conditions required For Exposure B.

数字 3. Upwind Surface Roughness condition required For Exposure D – 案子 1.

数字 4. Upwind Surface Roughness condition required For Exposure D – 案子 2.

The Exposure Category shall be determined for each wind source direction. Using an example site location – “1200 S DuSable Lake Shore Dr, 芝加哥, 的 60605, 美国”, lets analyze this for each direction.

数字 4. Sample location for Exposure Category analysis.

Assuming the mean roof height of the structure is 25 英尺 ( \( 20h = 500 英尺 \)), we will use the following procedure to check the Exposure Category for each sector:

Condition 1. Determine if Exposure D using Figure 3:

Using Figure 3 – where the distance \( d_{1} \) 是 5000 英尺 (1524 米), we need to check for Exposure D, where Surface Roughness D is dominant for the whole 5000 ft stretch:

数字 5. Offset distance of 5000 ft from site location for Exposure D check using Figure 3.

从图 5, we can already conclude that wind source directions ñ, 出生, 和E have Surface Roughness D for the whole 5000 ft stretch. 因此, these wind source directions are 曝光D.

Condition 2. Determine if Exposure D using Figure 2

Using Figure 4 – where the distance \( d_{1} \) 是 5000 英尺 (1524 米) and distance \( d_{2} \) 等于 600 英尺 (183 米), we need to check for Exposure D. 从图 5, this can only be applied for wind source direction from SE:

数字 6. Offset distance of 600ft and additional 5000 ft from site location for Exposure D check using Figure 4.

For wind source direction SE, 使用 \( d_{2} = 600 英尺 \), we can consider that this section is Surface Roughness B. 然而, for distance \( d_{1} = 5000 英尺 \), the section is not 100% Surface Roughness D. 因此, SE should not be considered as Exposure D.

Condition 3. Determine if Exposure B using Figure 1

Using Figure 3 – where the distance \( d_{1} \) 是 1500 英尺 (457 米) 以来 \( H < 30 英尺 \), we need to check for Exposure B.

数字 7. Offset distance of 1500 ft from site location for Exposure B check using Figure 3.

从图 7, we can determine that for wind source directions NW, w ^, 西南, and S are classified as Exposure B as the surface roughness for each direction sector is Surface Roughness B.

Condition 4. If conditions 1 至 3 are not true, 因此, the terrain is Exposure C.

因此, for wind source direction SE, it is classified as Exposure Category C. 综上所述, the exposure categories for each wind source direction is shown in Figure 8 下面.

数字 8. The exposure categories for each wind source direction.

These data can be used to determine what will be the worst wind source direction as the Velocity Pressure Coefficients \( K_{与} \), 地形因素 \( K_{Ť} \), and Gust-effect Factor \( G \) using detailed calculation are affected by the Exposure Category.

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NBCC 2015/2020

For NBCC 2015, the procedure to determine the Exposure Category of the upwind exposure of a site location is discussed in Section 4.1.7.3(5), depending on the terrain. For each wind source direction, it should be analyzed from two upwind sectors extending ±45°.

 

数字 9. Terrain sectors for each wind source direction.

For each sector, the terrain category should be checked based on the following definition based on Section 4.1.7.3(5) 全国广播公司 2015:

桌子 3. Definition of terrain categories as defined in Section 4.1.7.3(5) 全国广播公司 2015.

Visualizing the options in Table 3:

数字 10. Definition of Rough Terrain as defined in Section 4.1.7.3(5) 全国广播公司 2015.

数字 10. Definition of Open Terrain as defined in Section 4.1.7.3(5) 全国广播公司 2015.

Based on Section 4.1.7.3(5) 全国广播公司 2015, it is permitted to interpolate the 曝光系数 \( C_{Ë} \) in intermediate terrain. If the rough terrain distance from the structure location is greater than or equal to 1km or 20 times the structure height, 以较大者为准, 地形可以认为是 崎岖地形, 如果距离小于 50 米, 它被认为是 开放地形. 除此以外, the Exposure Factor \( C_{Ë} \) 按节 4.1.7.3(5) will be calculated from the boundary values. This can be visualized in Figure 11 下面.

数字 11. Definition of Intermediate Terrain as defined in Section 4.1.7.3(5) 全国广播公司 2015.

To further illustrate this, let’s use an example site location – “657 Masters Rd SE, 卡尔加里, AB T3M 2B6, 加拿大,” assuming the structure height \( H \) 是 25 米 ( \( 20高= 500 米 \)).

数字 12. Sample location for Terrain Category analysis.

First step is to classify the obvious rough and open terrain categories for each wind source direction. We can draw 50m and max of 1 km or \( 20 H \) radius from the site location.

数字 13. Offset distance of 50m and 1km for determining terrain category based on Table 1 definitions.

从图 13, we can say that the wind source directions 出生, Ë, 例如林区中的孤立塔 are classified as Open terrain as the rough terrain length for each direction is less than 50m from the site location. 此外, for wind source directions W and NW can be classified as Rough Terrain as the rough terrain length for these directions is greater than 1 公里. For wind source direction ñ, we can conservatively assume that the Open Terrain is dominant in this direction. For the rest, S and SW, we can conclude that these are Intermediate Terrain and we will need to measure the distance of the rough terrain from the site location.

数字 14. Approximate rough terrain length measured from site location for SW wind source direction equal to 574 米.

数字 15. Approximate rough terrain length measured from site location for S wind source direction equal to 249 米.

From the analysis above, definitely the wind source directions with Open Terrain will definitely yield the conservative values. 然而, if all wind source directions are classified to Intermediate Terrain, the procedure above is how you can determine the appropriate Terrain Category for each direction.

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AS / NZS 1170.2 (2021)

对于AS / NZS 1170.2, the same procedure with the above references applies in determining the Terrain Category of the upwind exposure of a site location. This is discussed in Section 4.2 AS / NZS的 1170.2 (2021). For each wind source direction, it should be analyzed from two upwind sectors extending ±45°. The definition of each terrain category are shown below based on Section 4.2.1 AS / NZS的 1170.2 (2021):

桌子 4. Definition of terrain categories as defined in Section 4.2.1 AS / NZS的 1170.2 (2021).

In determining the terrain category for a direction, a lag distance equal to \( 20 与 \) from the structure location shall be neglected. 从此, an offset distance (averaging distance) 的 500 或 \( 40 z), whichever is larger, shall be used as shown in Figure 16 下面. 的 \( 与 \) value is equal to the average roof height, \( H \), when it is less than or equal to 25 米. It is possible that within this averaging distance to have multiple terrain categories, and as such, linear interpolation of shall be used in determining the \( M_{与,猫} \) 价值观, depending on the length of each terrain category, 如图所示 4.1 AS / NZS的 1170.2 (2021). 在这篇文章中, we shall only consider a homogeneous terrain category within the averaging distance.

数字 16. Illustration of the distances used in determing Terrain Category based on AS/NZS 1170.2 (2021).

To further illustrate this, let’s use an example site location – Lat: 32°43’46″S Lng: 151°31’47″Ë – assuming the mean roof height \( H \) 是 10 米 ( 哪里 \( 20z = 20h = 200 米 \) 和 \( 40z = 400 米 \)).

数字 17. The site location with lag distance equal to 200 m and averaging distance equal to 500 m for each wind source direction.

Since we are only to consider the terrain category as homogeneous throughout the entire 500m or \( 40与 \) 距离, we can already classify each wind source direction. Assuming the buildings on N, NE and E, are buildings that are 5 至 10 米高, we can classify these to Terrain Category 3 (TC3) 如表所示 4. For wind source directions SE, 小号, 西南, and W, since these are grass plains without obstructions, we can classify these as Terrain Category 1 (TC1). 最后, for wind source direction NW, we can deduce that there are more than two but less than 10 buildings per hectare, with scattered houses. 因此, we can classify this as Terrain Category 2.5 (TC2.5).

数字 18. Summary of terrain category classification for each wind source direction for our sample location.

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Using SkyCiv Load Generator

In SkyCiv Load Generator version v4.7.0, new map tools are introduced – 测量距离 和 距离半径 工具.

数字 19. SkyCiv Load Generator 引入距离测量工具.

的 测量距离 工具用于从地图中单击的点生成圆并以米为单位显示其半径. 这条路, 您可以测量距正在分析的位置（某些位置）的距离. This can be used in measuring in NBCC 2015 为了 崎岖地形的逆风范围 used in calculating 曝光系数 \( C_{Ë} \). Clicking the circle generated will clear it from the map.

数字 20. Measure distance tool which creates an offset from the location and showing the radius/offset distance from the center introduced to SkyCiv Load Generator.

另一方面, 的 距离半径 的引入，以便用户可以为每个风源类别绘制距位置指定距离的圆圈. It is a toggle button to show or hide the distance radii on the map, with the site location as the center of the circles.

数字 21. Distance Radii tool which specified offset distances from the site location introduced to SkyCiv Load Generator.

The radii values can be edited upon opening the Settings.

数字 22. 设置中的选项可编辑 SkyCiv 负载生成器中距离半径工具的距离.

Take note that users must edit the distance values as these are not automatically calculated by the software. Using this for ASCE 7 和NBCC, the worst exposure or terrain category for each wind source direction shall be adopted. With regard to using it in AS/NZS 1170.2 (2021), the software doesn’t use the radii values to calculate for the average \( M_{与,猫} \) 价值观. 代替, the averaging distance is used as the applicable range where we can assign a homogeneous Terrain Category, adopting the worst category for each wind source direction.

From the sections discussed above, you can use these new tools to determine the exposure or terrain categories for each wind source directions. The procedures above can give you a quick terrain classification of each wind source direction. Using GIS and AI tools, you can further check the criteria that we used above for each wind source direction and can get a better and efficient result.

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参考资料:

• 建筑物和其他结构的最低设计负荷. (2017). 轴/六 7-16. 美国土木工程师学会.
• 加拿大国家研究委员会. (2015). 加拿大国家建筑法规, 2015. 加拿大国家研究委员会.
• Standards Australia (2021), Structural Design Actions. 部分 2 风动作, Australian/New Zealand Standard AS/NZS1170.2:2021, Standards Australia, 悉尼, NSW, 澳大利亚.
• 你可以通过这个查看我们的 API 文档