Wind Load Classification for Manufactured Structural Products


A collection of workbooks for calculating wind loads using AS4055 based on the values from AS1170.2 assigned to the various symbols. Note that whilst AS4055 is referred to as being for residential construction, the intent for the wind classification system was for repetitively manufactured structural products for which it makes no sense, and for which it is uneconomical to design directly to AS1170.2 for each and every installation. It is preferable to have a strength or load grade for such products, though a 25% increase in load would have better matched needs compared to the 50% increase in loading adopted {There are various historical reasons why there is not a constant ratio of 1.5 between the classes.}.

Wind speed maps are not based on a detailed view of an individual site. Most sites identified as wind class N1, when looked at in detail, do not have the full shielding in all directions to justify classification as wind class N1. On the other hand the site wind speed is at the lower end of wind class N2. Similarly most sites identified as wind class N3, do not have the terrain and topographical characteristics which would justify N3, and they are typically found to be at the upper end of the range for wind class N2. In these situations it would thus appear that design to AS1170.2 is preferable, but doing so does not help with the supply of product. Certain products are not determined by wind loading, but rather from handling requirements and imposed loading. Therefore some products would not benefit from a more refined wind classification system, other products however would benefit. Manufacturers are not however going to be aware of such things if they simply have their product designed for the AS4055 wind class. It is therefore preferable to be able to see AS4055 and AS1170.2 side by side.

It should be noted that these spreadsheets do not assist in providing an assessment to AS1170.2. The results to AS1170.2 are based on the values in AS4055:1992 mapped to the symbols. For example TC2 is symbolic notation for M[z,cat] = 0.92 for regions A & B, or M[z,cat]=0.96 for regions C & D, where z=8.5m. Away from AS4055 TC2 means terrain category 2, and M[z,cat] is determined for a given height ‘z’. AS4055 is not really a code, it is a prescriptive solution, or design aid which simplifies the application of AS1170.2. The only really useful part of AS4055 is the classification of wind speed, which I personally believe should be placed in AS1170.2, and sever its relationship to housing and emphasise its importance to manufactured structural products.

The spreadsheets:

  1. AS4055V1.xls {An example of a simple MS Excel application, with spreadsheet features removed}
  2. AS4055.xlsx {Main lookup table used by other spreadsheets. But also stand alone calculator for design wind speed Vz and design wind pressure qz}
  3. mspWindLoaderMkI.xls {similar to above, but with addition of pressure coefficient, it can calculate surface design pressure and also uniformly distributed load (udl) kN/m.}
  4. mspWindLoaderMkII.xlsx {Further extension, uses annual probabilities, surface roughness length rather than terrain category, and greater detail for topography.}
  5. schWindAssessment_r02.xlsx {Assessment of wind speed in all 8 cardinal directions, relative to orthogonal axes of the building}
  6. schTechLibrary.xlam {Library of add-in functions}

Spreadsheets are in Ms Excel 2003 format (,xls), others have been revised to MS Excel 2016 format (.xlsx, .xlsam), all are bundled in a single zip file.