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AS/NZS 1170.2 Wind Load Calculations

Using the SkyCiv Load Generator in AS/NZS 1170.2 Wind Load Calculations

To calculate the wind load pressures for a structure using SkyCiv Load Generator, the process is to define first the code reference. From there, the workflow is to define the parameters in Project Tab, Site Tab, and Building Tab, respectively. However, free users can only use the calculation for a gable roof for a maximum of 3 solves per day. With a Professional Account or by purchasing the standalone Load Generator module, you can use all the features of this calculation as long as you want. You can purchase the standalone module thru this link.

Calculating the wind speeds can be a complex process in AS/NZS 1170 for site locations in Australia and New Zealand. That’s why SkyCiv has developed an online wind load tool to help calculate the design wind speed and pressures via our interactive Google Map. Users can also click and drag the marker to move the site location:

Figure 1. Site parameters.

Site Data

Basic Wind Speed

The software will calculate the basic wind speed, VR, based on AS/NZS 1170.0 and AS/NZS 1170.2.

  • Average Recurrence Interval – how often this wind speed is expected to occur. It can also be calculated as the inverse of the annual probability of exceedance. For instance, 1 year is a very high expectancy of recurrence so this wind load will occur a lot. A higher ARI will result in a higher basic wind speed, VR. This is because you are designing for more extreme events.

Serviceability and Ultimate Limit State Wind Speeds

Users can also pull the Serviceability Limit State (SLS) and Ultimate Limit State (ULS) wind speeds for both Australia and New Zealand.  It still uses the ARI, however these can be calculated via the following input. Simply click the accordion and enter in the following input:

  • Country – Australia or New Zealand
  • Design Working Life – how long the structure is intended to be used. For instance, is the structure used for construction purposes (e.g. scaffolding) or is the design working life longer term, for say buildings and bridges. The longer the design working life, the higher the basic wind speed (to account for significance). Here, the SLS only increases up to DWL of less than 25 years.
  • Importance Level – The importance level is governed the type of structure and it’s potential impact. Click the (i) for more information about which importance level is correct for your structure.
  • Site Location – the address where the site is located
  • Here is an example of the software calculating the SLS and ULS wind speeds for 11 York Street Sydney (by default the basic wind speed will choose the largest of the two):
Figure 2. SLS/ULS input parameters.

Note that the user should double check if the wind region detected for the location is accurate based on Figures 3.1(A) and 3.1(B) of AS/NZS 1170.2 in order to obtain the appropriate wind speed for the structure. The Site Data tab should look like this:

Figure 3. Wind speed results from database.

Terrain Data

The next step is to define both the Wind Direction and Terrain Category parameters. The Wind Direction parameter is used in obtaining the upwind (left side) and downwind (right side) ground elevations to calculate for topographic multiplier, Mt, and the wind directional multiplier, Md, for the 8 cardinal directions. On the other hand, Terrain Category is used in calculating terrain/height multiplier, Mz,cat.

Figure 4. Elevation chart and other terrain parameters.

Structure Tab

The structure data and the wind and snow parameters are separated into different accordions. In order to calculate the design wind pressures, the wind load checkbox should be checked. For AS/NZS 1170, Building is the only option for Structure dropdown.

  • Building – supports the following roof profile:
    • Gable, Hip, Monoslope
    • Troughed, Pitched, Open Monoslope
Figure 5. Building parameters.

Note that for free users, only Gable roof is available for Building. Once you have completed all the structure data inputs, you can visualize the structure by clicking the 3D Render at the right side.

Figure 6. 3D render of structure.

Next step is to define the Wind Parameters depending on the structure selected based on AS/NZS 1170.2. You can either select calculations for Buildings or Components and Cladding. For Wall Condition and Action Combination Case parameters, you need to click the title of each parameter and an alert will be shown where the options are visually displayed. You can click any of the figures to select the appropriate definition for each parameter.

Figure 7. Alert option for wall condition.

The Ratio of Dominant Opening to Total Open Area parameter is used in calculation of internal pressure coefficients, Cp,i. Default value should be equal to 0. Moreover, the Is building elevated? option is calculated to obtain the external pressure coefficient, Cp,e, for windward walls of enclosed buildings.

Results

Once all the parameters are defined, clicking the Generate Loads button will give a result as shown below:

Figure 8. Pressure results.
Figure 9. Results figures.

Detailed Calculation

The detailed wind load calculations can be accessed only by Professional account users and those who purchased the standalone load generator module. All the parameters and assumptions used in the calculation are displayed on the report to make it transparent to the user. You can download a sample detailed calculation thru this link.

Figure 9. Detailed report.

For additional resources, you can use these links for reference:

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