20090420

Engineering Misconceptions, Part 2 - Design Wind Speeds

Wrong: A wind speed of 60 m/s has been recorded recently in this area.  Therefore the 55 m/s design wind speed (50-year return period) shown in the Building Code is now wrong.

Right: A wind speed of 60 m/s has been recorded recently in this area.  The basic wind speed for this area as shown in the Building Code is only 55 m/s. I should convert 55 m/s to the actual design wind speed for this area based on the height, topography, and surface roughness to see if the recorded 60 m/s is over and above the actual design wind speed for this specific area. I should also read books to find out what exactly are return periods to better understand them.

Facts:
The "design" wind speeds shown in building codes are sometimes called "basic wind speeds" - i.e. they follow a certain, basic standard (as agreed to by different meteorological institutions around the world, represented by the World Meteorological Organization). The basic wind speed is usually a 50-year return period wind speed measured at 10 meters height in flat, open "country" terrain. If the conditions are otherwise, the actual wind speed is different.

For example, if the area being referred to in the Wrong statement is actually a few hundred meters from the ocean, the recorded wind speeds there are generally expected to exceed the basic wind speeds for the
same area.

If the area being referred to in the Wrong statement is actually on top of a 100-meter tall skyscraper, the recorded wind speeds there are generally expected to exceed the basic wind speeds for the same area.
If the area being referred to in the Wrong statement is actually on top of a cliff, hill, or escarpment, the recorded wind speeds there are generally expected to exceed the basic wind speeds for the same area.
Refer to the earlier Engineering Misconceptions, Part 1 for a discussion about return periods. The point is, it is based on statistical analysis.

Lastly, the 60 m/s recorded wind speed may be a gust wind speed, a sustained wind speed, a 1-minute average wind speed, an hourly average wind speed, etc. The 55 m/s design wind speed in the code may be a
different animal.

If the engineer wishes to use a basic wind speed over and above the code-prescribed basic wind speed, he may do so because codes usually prescribe only minimum values. Ideally he should have scientifically
viable justification for use of other values. That is, he should do his own statistical analysis.

3 comments:

daicee said...

Good day sir! I'm a civil eng'g student. I just want to ask about the list of provinces in each wind zone here in the Philippines. Can you help me where to find it? Thank you.

ronjie.com said...

CE student from where? You can find the Philippine wind zone map in NSCP 2001. Then just buy a regular map of the Philippines to see which provinces (or portions thereof) are in which zone by comparison with the Philippine wind zone map.

ronjie.com said...

The latest NSCP 2001 will have a list of wind zoning for each province in the Philippines. The new NSCP 2001 map will also have wind zone boundaries that are generally concurrent with political (provincial) boundaries.