How do we get "Site-Specific Wind Loads" in the Philippines?

I was asked this question, and here is how I would answer it.

In the structural engineering community especially in my home country, the interest and knowledge in earthquake engineering and earthquake-resistant design has grown rapidly over the past two decades especially after the 1990 Luzon Earthquake. Some of the recent "buzz words," if I may use that term, in the community are "site-specific seismic hazard assessment," and "performance-based seismic design."

Don't get me wrong here; it's a good thing that our engineers are aware of these relatively recent developments in earthquake engineering.

Naturally, structural design codes which provide for design earthquake loads, especially previous versions of the local code, the National Structural Code of the Philippines or NSCP, rely on minimum loads that are specified in general terms. For example, earthquake loads are designed according to seismic zone maps, which essentially show the Philippines to be divided into two seismic zones. This means that depending on which zone a specific site or building falls into, the earthquake loads could be either in one range of earthquake loads or another. Incidentally, the two zones would generally result in at least a 2-to-1 ratio of loads. The code, especially in the higher seismic zone, actually provides for some "site-specific" earthquake loads depending on the distance from a known active fault, and depending on the soil conditions at the site. But the soil conditions themselves are simply generalized into 6 different soil types, and the fault types into 2.

However, engineers now know that while this is site-specific, it is not very specific enough, and that this is very different from what is referred to as "site-specific seismic hazard assessment" or SSSHA, which is usually a prerequisite of performance-based seismic designs. In a SSSHA, the properties of the soil layers themselves are utilized, similar actual ground motion records that could be generated by the considered fault (or faults) are selected, and the site distances to the actual fault locations are more precisely determined to arrive at a truly more site-specific seismic hazard assessment. Once a SSSHA has been completed, the benefits can be maximized when the results are used by a structural engineer with a background in nonlinear structural dynamics and performance-based seismic design. The latter is not a prerequisite though; the end result could simply be in a form of a base shear coefficient which any structural engineer would know how to use.

In my experience, the results could result either in larger or smaller earthquake loads, translating to either safer structures or cost savings. But the end benefit is that the confidence level in the estimated loads become higher; the estimated loads have a more rational basis than the rather simpler code-prescribed earthquake loads. This is not to say that the code-based loads are "wrong." These are generally all based on projections -- it's either both are wrong or both are right. But indeed, one is more rational than the other.

What differentiates the SSSHA as a result of using the code and a true SSSHA is that the latter is carried out by numerous experts in seismology, geotechnical earthquake engineering, and as mentioned, perhaps with a structural engineer with a nonlinear structural dynamics and performance-based design background.

Now, perhaps since Typhoon Milenyo of 2006 and Typhoon Ondoy of 2009 which heavily affected Metro Manila, the political and economic capital of the country, and perhaps the location with the heaviest concentration of civil/structural engineers in the country, among other events around the world (e.g. Hurricane Katrina, and so on), there has likewise been a growing interest in wind engineering in the country.

Naturally, our engineers, armed with such a good background in earthquake engineering, would also clamor to learn "site-specific wind hazard assessment" to come up with "site-specific wind loads" and so forth.

Firstly, I should mention that "site-specific wind loads" is not a term that you might encounter in the wind engineering community. But it is quite intuitive; wind engineers would readily know what is meant by "site-specific wind loads." Instead, the term "site wind speed" is used. And wind loads are actually building-specific; not site-specific.

But, similar to earthquake design, the code actually provides for a site-specific design already. First, you need to locate in which wind zone the site belongs to. If the site is on a topographic feature, there are multipliers that should be used to consider the effects of such. Finally, there is a need to establish the terrain surrounding the building from all wind directions to be considered.

Similar to the code-based earthquake design procedure, some simplifying generalities are used. For example, the wind zone map only divides the country into 3 zones. There are only 3 very generic types of topographic features considered. And there are only 4 types of terrains to choose from. This would provide for a "site wind speed," and it is an all-direction one, meaning it is to be applied in all horizontal directions, with only a reduction factor to account for the probability that the estimated site wind speed will be reached in any given direction.

In reality, the site wind speed could vary every 5 degrees (or much less). Even though the terrain category is the same, depending on the arrangement and orientation of surrounding buildings or topographic features, the design wind pressures could change. And most actual topographic features cannot be described by simple shapes; they are by nature very complicated formations. Finally, the use of a wind zone map is a matter of convenience.

The use of wind zones dates back to a study carried out by a US-based group of scientists (wind engineers / meteorologists) back in the early 1970s, which analyzed data from our meteorological stations and for convenience, grouped them into 3 zones. But in more recent years, the US has recognized that hurricane-prone (or in our case, typhoon-prone) regions should be using wind contour maps, not zone maps. They now only use zone maps for non-hurricane-prone regions.

Our current wind zone map came to be only for continuity and a smoother transition from previous versions of the NSCP. But the truth is, a contour map has been generated based on the same data that our wind zone map was generated from. Garciano et al (2005) has also proposed a 6-zone wind map, which is more closer to a contour map. However, there are still issues that need to be resolved in the data used to generate these maps. First is the application of data correction for height and for terrain, among other things. Secondly, and this is especially applicable to the southernmost parts of the country, non-typhoon wind speeds should be excluded from the data set. Once all of these have been addressed, we can have a better estimate of the basic wind speed for a specific site -- but not yet the site wind speed. For that, again, the terrain and topography and wind direction need to be considered.

There is also the issue of return period to be used for the basic wind speeds. We are using 50 years. Consultants are suggesting 100 years for typhoon-prone regions.

The truth is, to really arrive at more realistic site-specific wind speeds and building-specific wind loads, finally, a wind tunnel test need to be carried out with the surrounding obstructions and terrain, and the final architectural envelope of the building properly modelled. Some engineers are trying to use commercial CFD software, but except for environmental purposes and perhaps for determining basic wind speeds over complex topographic features, CFD results are generally not acceptable for structural design purposes. Not even by people who have been studying CFD for almost their whole lives. The thing about CFD is it requires physical validation, so why not just go directly the physical route via wind tunnel testing? Finally, people think CFD is just like structural analysis in that it is fast and cheaper, and that it will give you results faster (and less costlier) than wind tunnel testing. But the truth is, it takes just as long and just as costly to do CFD -- and that still excludes the validation aspect.

In short, to determine "site-specific" wind speeds, someone with a background in engineering meteorology (i.e. not just regular meteorologists) or "micro-meteorology" -- a wind engineer -- needs to be able to look at the data from PAGASA and use only the appropriate ones, and apply corrections where necessary. He will be able to determine the basic wind speed for each wind direction (every 5 degrees azimuth), if need be. Then, the wind engineer will be able to carry out wind tunnel testing to ascertain the building-specific wind pressures. Finally, together with these results, a wind engineer or a structural engineer with a good knowledge of recent developments in dynamic wind response, will be able to calculate building-specific wind loads that are, in the same way as for SSSHA, more rational than the code-prescribed site-specific wind speed and building-specific wind loads. But again, this is not to say that the code-based design is "wrong."

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