The Power of Wind
While driving through central Illinois recently, I was struck by the beauty of a wind turbine farm. The vanes of the turbines responded to the movement of the wind over the open plains with grace and dignity, providing a visual indication of the wind’s movement that cannot otherwise be seen. The wind is like electricity and God–the primary evidence of its existence is in its effects.
The turbines brought to mind the wind-powered pump on my grandparent’s farm. The pump brought cool water from an underground cistern into the milk house, where the products obtained from my grandparent’s herd of cows were stored until delivery to their customers. This type of self-reliant, and self-sustaining system was needed to survive in northern Minnesota when the farm was built in the early 1900s.
The concept of being self-sustaining is one we are seeing brought up more and more in the development of the International Energy Conservation Code, as well as the International Green Construction Code. We can design buildings to require less electricity. Recent changes to the prescriptive provisions of the IECC have focused on doing exactly that, with significant impact on our industries’ products.
But to truly achieve the U.S. Department of Energy’s goal of net zero energy buildings, we must also determine how to provide electricity in the most energy efficient manner possible. Relying upon electricity that is conveyed from huge power plants across electrical lines that span for miles and miles is extremely inefficient due to the huge transmission losses that occur. One can be aware of these losses by standing under a group of high voltage overhead lines and listening to the sizzling noise they create.
In consideration of that, there are numerous requirements in the IECC that do not have to be put in place when site sourced energy is used. Proposals to the IgCC would go even further, establishing an energy budget for commercial buildings based upon the type of use of the building, with multipliers based upon the electrical source that will be serving the building.
In terms of environmental friendliness, the generation of power onsite via nondepletable resources is the best choice. Examples include the incorporation of photovoltaic cells in the building design or the use of an onsite wind turbine. Now in the scenarios above, the wind is a positive element from which both we and Mother Earth can benefit. But the opposite can also be true.
The wind is also a destructive force that can destroy trees, signs and buildings. For this reason we design buildings, including components and cladding such as windows, to withstand the stronger forces of the wind. The art of designing building components to resist the wind tends to be somewhat transitory. As time goes on we learn more about the wind, and are better able to predict its effects.
Recently the wind load provisions of ASCE7 Minimum Design Loads for Buildings and other Structures were revised. ASCE7 is the standard referenced in the International Building Code for the determination of design loads for buildings. The revision to ASCE7 reflects a change in the model used to predict wind loads.
Previous editions relied upon what was known as “the 50-year wind” as the design wind speed to which buildings were to be designed to remain serviceable. The phrase “50-year wind” meant there was considered to be a 2 percent probability of that wind speed being experienced in any given year. If a long enough period of time were considered, the frequency of occurrence of that design wind speed would be comparable to it occurring at 50-year intervals. For example, over a period of 1,000 years, we would expect that design wind speed to occur at a specific location approximately 20 times.
The new models seek to have buildings designed to remain standing when subjected to wind speeds considered to have a much lower probability of occurring. The exact model to be used depends upon the threat to life safety that a collapse of the building would pose. Those considered to be a low threat to life safety, such as agricultural buildings, must be designed to resist a 300-year wind. Buildings where a collapse is considered a significant threat to life safety–such as those occupied by large groups of people, or hospitals, police and fire stations, etc.–must now be designed to resist a 1,700-year wind. All other buildings are to be designed to resist a 700-year wind.
I was beginning to think I had my mind wrapped around this concept, when an EF-4 tornado hit the St. Louis airport on Good Friday. The designation of EF-4 means winds in excess of 168 mph. The recently revised ASCE7-10 established design wind speeds of 120 mph as a 1,700-year wind for that location, however, which is considerably less than the 168+ mph actually experienced.
Alright, I thought. So a wind of extremely low probability hit Lambert Field. Such things can happen. But then I learned that an EF-4 tornado also hit St. Louis County, including Lambert Field, in 1967. In fact, since the late 1800s, a total of four EF-4 tornados have hit St. Louis County. Furthermore, there was the severe damage that occurred across the South due to tornados the end of April. On average, over 200 tornados hit North America every April. That hardly seems like a 1700-year return period to me.
I was wondering what I was missing with regards to understanding the new model, so I asked a couple of experts in the field. What I was told is that even though four EF-4 tornados have hit St. Louis County in the past 100+ years, the area affected by each one was so small that the specific locations they each hit actually experienced only one tornado. I suppose the same might be true of the 200+ tornados that touch down in North America every April. North America is a pretty large area. And the path of a tornado is often less than a half mile in width. Plus, obviously, not all of those tornados reach EF-4 intensity.
But that doesn’t change the fact that two EF-4 twisters have hit Lambert Field in St. Louis within the past 50 years–one in 1967 and the more recent one on Good Friday of this year. Perhaps they didn’t destroy the same buildings, but they damaged the same airport. I must confess, I am still questioning the modeling being used.