Σε αυτό το άρθρο, 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, και 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

Για την ASCE 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. Σε αυτό το άρθρο, 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°.

Φιγούρα 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 του ASCE 7-16:

Τραπέζι 1. Surface Roughness definition based on Section 26.7.2 του ASCE 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 του ASCE 7-16 ως εξής:

Τραπέζι 2. Exposure Category definition based on Section 26.7.3 του ASCE 7-16.

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

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

Φιγούρα 3. Upwind Surface Roughness condition required For Exposure D – συνδυάζοντας την επίδραση της εξωτερικής και εσωτερικής δράσης πίεσης 1.

Φιγούρα 4. Upwind Surface Roughness condition required For Exposure D – συνδυάζοντας την επίδραση της εξωτερικής και εσωτερικής δράσης πίεσης 2.

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

Φιγούρα 4. Sample location for Exposure Category analysis.

Assuming the mean roof height of the structure is 25 πόδια ( \( 20h = 500 πόδια \)), 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 \( ρε_{1} \) είναι 5000 πόδια (1524 Μ), we need to check for Exposure D, where Surface Roughness D is dominant for the whole 5000 ft stretch:

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

Από το σχήμα 5, we can already conclude that wind source directions Ν, ΓΕΝΝΗΜΕΝΟΣ, και Ε have Surface Roughness D for the whole 5000 ft stretch. Επομένως, these wind source directions are Έκθεση Δ.

Condition 2. Determine if Exposure D using Figure 2

Using Figure 4 – where the distance \( ρε_{1} \) είναι 5000 πόδια (1524 Μ) and distance \( ρε_{2} \) είναι ίσο με 600 πόδια (183 Μ), we need to check for Exposure D. Από το σχήμα 5, this can only be applied for wind source direction from SE:

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

For wind source direction SE, χρησιμοποιώντας \( ρε_{2} = 600 πόδια \), we can consider that this section is Surface Roughness B. Ωστόσο, for distance \( ρε_{1} = 5000 πόδια \), the section is not 100% Surface Roughness D. Ως εκ τούτου, SE should not be considered as Exposure D.

Condition 3. Determine if Exposure B using Figure 1

Using Figure 3 – where the distance \( ρε_{1} \) είναι 1500 πόδια (457 Μ) Από \( η < 30 πόδια \), we need to check for Exposure B.

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

Από το σχήμα 7, we can determine that for wind source directions NW, Δ, ΝΔ, and S are classified as Exposure B as the surface roughness for each direction sector is Surface Roughness B.

Condition 4. If conditions 1 προς το 3 are not true, επομένως, the terrain is Exposure C.

Επομένως, for wind source direction SE, it is classified as Exposure Category C. Συνοψίζοντας, the exposure categories for each wind source direction is shown in Figure 8 παρακάτω.

Φιγούρα 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 \( Κ_{με} \), Τοπογραφικός παράγοντας \( Κ_{τ} \), and Gust-effect Factor \( σολ \) 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°.

 

Φιγούρα 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) του NBCC 2015:

Τραπέζι 3. Definition of terrain categories as defined in Section 4.1.7.3(5) του NBCC 2015.

Visualizing the options in Table 3:

Φιγούρα 10. Definition of Rough Terrain as defined in Section 4.1.7.3(5) του NBCC 2015.

Φιγούρα 10. Definition of Open Terrain as defined in Section 4.1.7.3(5) του NBCC 2015.

Based on Section 4.1.7.3(5) του NBCC 2015, it is permitted to interpolate the Συντελεστής έκθεσης \( ΝΤΟ_{μι} \) in intermediate terrain. If the rough terrain distance from the structure location is greater than or equal to 1km or 20 φορές το ύψος της δομής, όποιο είναι μεγαλύτερο, το έδαφος μπορεί να θεωρηθεί ως Ανώμαλο Έδαφος, και αν η απόσταση είναι μικρότερη από 50 Μ, θεωρείται ως Ανοιχτό Έδαφος. Σε διαφορετική περίπτωση, the Exposure Factor \( ΝΤΟ_{μι} \) σύμφωνα με την Ενότητα 4.1.7.3(5) will be calculated from the boundary values. This can be visualized in Figure 11 παρακάτω.

Φιγούρα 11. Definition of Intermediate Terrain as defined in Section 4.1.7.3(5) του NBCC 2015.

To further illustrate this, let’s use an example site location – “657 Masters Rd SE, Κάλγκαρι, AB T3M 2B6, Καναδάς,” assuming the structure height \( Η \) είναι 25 Μ ( \( 20H = 500 Μ \)).

Φιγούρα 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 \( 20 Η \) radius from the site location.

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

Από το σχήμα 13, we can say that the wind source directions ΓΕΝΝΗΜΕΝΟΣ, μι, και SE are classified as Open terrain as the rough terrain length for each direction is less than 50m from the site location. Εξάλλου, 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 χιλιόμετρα. For wind source direction Ν, 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.

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

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

From the analysis above, definitely the wind source directions with Open Terrain will definitely yield the conservative values. Ωστόσο, 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)

Για 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 του AS / NZS 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 του AS / NZS 1170.2 (2021):

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

In determining the terrain category for a direction, a lag distance equal to \( 20 με \) from the structure location shall be neglected. Από αυτό το σημείο, an offset distance (averaging distance) του 500 m ή \( 40 z), whichever is larger, shall be used as shown in Figure 16 παρακάτω. ο \( με \) value is equal to the average roof height, \( η \), when it is less than or equal to 25 Μ. 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 \( Μ_{με,Γάτα} \) αξίες, depending on the length of each terrain category, όπως απεικονίζεται στο Σχήμα 4.1 του AS / NZS 1170.2 (2021). Σε αυτό το άρθρο, we shall only consider a homogeneous terrain category within the averaging distance.

Φιγούρα 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″μι – assuming the mean roof height \( η \) είναι 10 Μ ( όπου \( 20z = 20h = 200 Μ \) και \( 40z = 400 Μ \)).

Φιγούρα 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 \( 40με \) απόσταση, we can already classify each wind source direction. Assuming the buildings on N, NE and E, are buildings that are 5 προς το 10 ύψος, we can classify these to Terrain Category 3 (TC3) Το αποτέλεσμα της εν λόγω διαδικασίας είναι η αξιολόγηση ατομικού κινδύνου 4. For wind source directions SE, μικρό, ΝΔ, and W, since these are grass plains without obstructions, we can classify these as Terrain Category 1 (TC1). Τελικά, for wind source direction NW, we can deduce that there are more than two but less than 10 buildings per hectare, with scattered houses. Επομένως, we can classify this as Terrain Category 2.5 (TC2.5).

Φιγούρα 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 – Μετρήστε την απόσταση και Ακτίνες απόστασης εργαλεία.

Φιγούρα 19. Εισάχθηκαν εργαλεία μέτρησης απόστασης στο SkyCiv Load Generator.

ο Μετρήστε την απόσταση Το εργαλείο χρησιμοποιείται για τη δημιουργία ενός κύκλου από ένα σημείο του χάρτη στο οποίο γίνεται κλικ και δείχνει την ακτίνα του σε μέτρα. Με αυτόν τον τρόπο, μπορείτε να μετρήσετε τις αποστάσεις από την τοποθεσία που αναλύεται τις συγκεκριμένες τοποθεσίες. This can be used in measuring in NBCC 2015 για το Ανεμοδαρμένη έκταση ανώμαλου εδάφους used in calculating Συντελεστής έκθεσης \( ΝΤΟ_{μι} \). Clicking the circle generated will clear it from the map.

Φιγούρα 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.

Αφ 'ετέρου, ο Ακτίνες απόστασης εισάγεται έτσι ώστε οι χρήστες να μπορούν να σχεδιάζουν κύκλους με καθορισμένες αποστάσεις από την τοποθεσία για κάθε κατηγορία πηγών ανέμου. 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.

Φιγούρα 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.

Φιγούρα 22. Επιλογή στις ρυθμίσεις για να επεξεργαστείτε τις αποστάσεις για το εργαλείο Distance Radii στο SkyCiv Load Generator.

Take note that users must edit the distance values as these are not automatically calculated by the software. Using this for ASCE 7 και 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 \( Μ_{με,Γάτα} \) αξίες. αντι αυτου, 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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βιβλιογραφικές αναφορές:

• Ελάχιστα φορτία σχεδιασμού για κτίρια και άλλες κατασκευές. (2017). ΑΞΟΝΕΣ / ΕΞΙ 7-16. Αμερικανική Εταιρεία Πολιτικών Μηχανικών.
• Εθνικό Συμβούλιο Έρευνας του Καναδά. (2015). Εθνικός Κώδικας του Καναδά, 2015. Εθνικό Συμβούλιο Έρευνας του Καναδά.
• Standards Australia (2021), Structural Design Actions. Μέρος 2 Δράσεις ανέμου, Australian/New Zealand Standard AS/NZS1170.2:2021, Standards Australia, Σίδνεϊ, NSW, Αυστραλία.
• = Απόσταση αντίθετα από τον άνεμο της κορυφής μέχρι όπου η διαφορά στο υψόμετρο του εδάφους είναι το μισό του ύψους του λόφου ή του λόφου