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RE: Topographic Effects

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If ASCE7-05 clause has similar intent to the Australian wind loading code AS1170.2. Then I believe the idea is to consider 8 compass directions ( N, NE, E, etc..) , each of these directions could have a different regional wind speed except in hurricane prone regions, where such directional variation is ignored for strength limit state at least. In addition the environment surrounding the building may differ: generating different wind speeds at the site, when account has been taken of exposure, topography and other factors such as shielding (ASCE7-5 not permitting).


Two orthogonal axes are determined for the building, though not relevant for a circular building. Then 45 degrees to either side of these axes (90 degree quadrant), the maximum wind speed from the cardinal directions (compass) occurring in the sector is the wind speed for that orthogonal direction. This may produce 4 different design wind speeds for the building. If wish to ignore the variations, then the maximum of all directions is the design wind speed for the building. Winds approaching from directions other than along the orthogonal axes may produce the worst effects and theses directions may also have to be considered. (For example 45 degrees to the free edge of a free standing wall, or is that the free edge to the wall of an open building?)


To AS1170.2 if consider directional wind speeds, the directionality factor turns the dominant regional wind speed into lower wind speeds for each compass direction. If adopt the maximum regional wind speed then there is a single directionality factor which accounts for the conservatism and allows some reduction like ASCE7.


If have different wind speeds flowing up the hill towards the buildings, and flowing down the hill towards the building, and choose to use the maximum of these wind speeds as the design wind speed, then the directionality factor kd in effect is doing the averaging for you: weighted average.


Since the research on topographical effects isn’t all that great, erring on the high side is preferable. I’ve had minor involvement with a project where:


1)       Light weight water tank, comprising simple steel frame and polymer liner was picked up and thrown over the crest of the hill.

2)       The swing doors opening out onto decking failing to stay closed, with driving rain getting in.

3)       I think there was also mention of rain driven up through suspended timber floor.


Whilst some of this occurred during a storm (80km/h), it wasn’t any where near the regional design wind speed (162km/h) and the weather proofing problem occurring at even lower wind speeds. As a consequence of this project the tank manufacturer is now getting his tanks engineered though of a size not normally requiring approval. Whilst the builder is now paying more attention to the wind class requirements for windows and making sure supplier can provide to suit, and no longer believes wind is not an issue. (For residential we have a simplified wind classification system AS4055. Having calculated wind speed to AS1170.2, the site can be assigned a wind class, which assigns a design wind speed which is usually much greater than that from AS1170.2: most sites at lower end of the class. {N1 to N6 non-cyclonic, and C1 to C4 cyclonic. Knowing the site is wind class N3 the builder can order windows to suit: assuming the builder and window supplier understand. Timber framing and trusses also ordered on this basis. Timber estimators can determine the wind class symbolically: Region A/TC3/T2/NS = N3, they just look the combination up in a table.)




Conrad Harrison

B.Tech (mfg & mech), MIIE, gradTIEAust



South Australia