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# Re: Wind Load Topographic Effects

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Wind Load Topographic Effects
• Date: Fri, 2 Jan 2009 15:34:47 +1030

Jeff

Use a topographic map, to get spot heights and contours, draw a profile of the hill, and find the local level plains. Lh and H seem reasonably clear from Fig6-4.

Avoid method 1, it is derived from method 2, and its limitations are really dependent on understanding method 2. Therefore better to simplify method 2 yourself.

Trees and forests are not really obstructions they may be removed. Though may consider their influence on determining surface roughness which is considered by exposure category. The commentary provides a means of exposure averaging, and such is dependent on averaging distance. Beneficial if want something between exposure C and B, rather than opting for C.

As for 1 and 2 of section 6.5.7.1:

I read 1 as defining isolation of the hill. If in a mountain range then there may be higher mountains behind or infront, if more than 2 miles away then their influence to obstruct wind flow is not significant, if near by wind flow will be changed.

Item 2 I read has a reference to undulating surface. A hill may not simply rise, but have a series of rises and dips until reach the very peak. These small upwind peaks may obstruct wind flow. If the highest peak of the rising hill is more than 2 miles away then its influence on increasing wind speed is not as great.

These two items basically answer the question as to when the hill relative to the location of the building is not a hill. Basically wind speed increases with increasing height away from surface roughness. More obvious using AS1170.2 since we apply equivalent of kz, kzt to the wind speed Vb to get a design wind speed Vz, then calculate qz.

For a very large and slowly rising hill, the wind is effectively flowing over flat land. But for a steep rising hill there is a rapid change in wind speed. For small surrounding undulating hills, then have more affect on selection of exposure category, influencing surface roughness. It is largely a judgement call influencing the magnitude of the reference pressure qz. Steep hills and wind can cause a lot of problems, like double balcony doors which won't stay put and leak rain. So if have a real hill then do want qz high.

regards