As a structural engineer, I have had to serve many different types of clients over the years. With so much wind recently, I was reminded of one of the most unusual clients that I had ever had as young engineer was three little pigs.
Of course, I asked them, why did you come to me? I don’t build houses. Well, they said, the Big Bad Wolf threatened to “huff and puff and blow their houses down, and eat them.” We also heard that if we wanted a good, strong house, we should hire a structural engineer.
Of course, I asked them what they had in mind. Immediately, Tom piped up and said he had a friend who built his house out of straw.
“How did it go?” I asked.
“He’s no longer with us,” Tom answered.
Dick said he also had a friend who built his house out of sticks and he also is no longer with us.
Finally, Harry chimed in and said his friend used bricks, and he is still with us.
After much discussion, the three little pigs decided that they felt a timber or brick house would work for them and they would like to build it in the forest next to a lake. Being inquisitive little rascals, they asked what the next step was to design their little house to resist the wolf and other wind storms that frequented the area. First of all, they had to agree to pay me for my services, which they agreed to do.
“Now that you have selected a basic location for your little house somewhere in Archuleta County, we have to break out the building code to select the parameters that we will need to determine the appropriate factors that we will need to calculate the wind load on your house,” I told them.
“Building code, what is that?” asked Tom. “My friends up in Aspen Springs never mentioned a building code when I talked to them.”
Well, I told them, a building code is basically a set of rules and regulations adopted by local governing entities from various agencies to ensure that your house is safe to live in. In this case, we have to use the 1997 Uniform Building Code.
“Really,” said Tom. “And I suppose they are going to charge us money.”
“Of course,” I told them, “and that money goes to pay the building department’s expenses and salaries for its staff. Once I finish the design and drawings of your proposed structure, your building contractor will submit my documents to a building department that will review everything to ensure that proposed design complies with the current building code in use at the time.”
“So, if that’s the case, who comes up and says what the code is?” asked Dick.
Basically, building codes come from various research agencies and universities. As an example, the American Institute of Timber Construction (AITC), the American Forest and Paper Association, and the American Wood Council all do research in timber construction.
Of course, the main one is the American Society for Testing Materials (ASTM). It researches all kinds of materials. And, universities and colleges receive government grants to do research by professors well known in their field of study. For professors, it is well known that it is “publish or perish.” (Note, the Uniform Building Code is no longer used; it has been replaced with the International Building Code.)
So, based on all of this research, building codes can be established such that designers and engineers have at their disposal the information needed to ensure that their designs are based on factual, known data.
As far as wind loads are concerned, there are several factors that must be considered. These are exposure, shape, height, wind speed, and I sometimes include elevation.
Exposure deals primarily with where the structure is to be located — on the sea shore, in a forest, or in a city or town. Because you guys are locating your house in a forest, the factor for Exposure B is to be used in calculating the wind stagnation pressure. Because the overall height is less than 30 feet, we will use what is called the Normal Force Method. The shape of your house is basically a box with flat surfaces versus a round structure, such as water tank. From a map in the code book, we find that the basic design wind speed for the Pagosa Springs area is 90 miles per hour. And, lastly, we must assign an importance factor, which in your case, is one.
“Hey, wait a minute” said Tom. “Can’t we make our house more important?”
“You can if you want to pay more money for it,” I told him.
If we were designing a hospital, we would increase the stagnation pressure by 15 percent, in essence, increasing the factor of safety, which increases the cost. Although not in the code, I sometimes decrease the stagnation pressure based on elevation because the air density decreases with elevation.
Now it’s time to get down to brass tacks or, in this case, the mathematics.
Starting with the wind speed, a formula has been developed for the stagnation pressure at sea level. This formula is Ps=0.00256V2, where V is wind speed in miles per hour. So at sea level with a 100 mph wind, the stagnation pressure is 0.00256 x 100 x 100 = 25.6 pounds per square foot (psf). For Archuleta County, a wind speed of 90 mph is required, hence, Ps = 0.00256 x 90 x 90 = 20.7 psf. But, let’s modify the stagnation pressure based on an elevation of 7,500 feet. Then Ps=(0.00256-0.000067 x 7,500/1000)V2 = 16.7 psf. If we go to another table in the code book, we find that the height, exposure and gust factors have been tabulated in to one factor, namely, 0.62. Thus, the stagnation pressure is now 0.62 x 16.7, or 10.4 psf.
We are not finished yet. Now we must account for the shape of your house and the configuration of the roof.
On the windward side of the house, the design wind pressure now becomes 0.8 x 10.4 = 8.3 psf and on the leeward (suction) side of the house, the wind pressure is now 0.5 x 10.4 = 5.2 psf. For the roof with a 6:12 pitch, the wind ward pressure is 0.3 x 10.4 = 3.1 psf and the leeward pressure (suction) is 0.7 x 10.4 = 7.3 psf.
Now, the question is: What is the total force trying to blow the house off of its foundation?
For your house that is 8 feet tall at the eave with a 6:12 roof pitch, 20 feet wide and 40 feet long, the total wind force is as follows.
For the wall up to the eave on the windward side, the force is 8.3 psf x 8.0 feet high x 40.0 wide, which equals 2,656 pounds. We then look at the roof on the windward side and find that the force is 3.1 psf x 5.0 feet high x 44.0 feet wide (assuming a 2-foot overhang on each end) equals 682 pounds. For the wall on the leeward side, we find the force is 5.2 psf x 8.0 feet high x 40.0 feet wide equals 1,664 pounds. And for the roof on the leeward side the force is 7.3 psf x 5.0 feet x 44.0 feet wide equals 1,606 pounds.
Thus, the total wind force trying to move the house off of the foundation is 6,608 pounds. But, it should be pointed out that even though this appears to be an exact number, good old Mother Nature can throw us a curve ball in the form of a microburst, as discussed below. In that case, it never hurts to bump up the calculated wind force slightly, possibly rounding it off to, say, 7,000 pounds.
So, if you build your house to withstand a 7,000 pound wind load, you should be OK.
“Wow,” said Dick, “I bet the big bad wolf can’t blow that hard.”
As you can see, we have only calculated the wind load on your new house. Of course, to arrive at the final design of your house, the wind load must be combined with the dead load of the structure itself, along with the snow load. Once the entire load analyses are completed and combined to find the worst-case loading condition, the individual members, such as the bracing, columns and beams, can be designed.
Now that the little pigs’ curiosity has been raised to a higher level, they began to wonder about a 200-foot-tall structure.
The same basic principles are involved, but to a “higher” degree of complication. Obviously, the higher above the earth, the greater the wind speeds. In lower structures overturning is not so much a factor, but in taller structures the wind force not only tries to slide the structure off of its foundation, but just tip it over.
When designing tall structures in cities, quite often the engineer will submit his design to a firm that models the size and shape of the structure in question and places the model in a wind tunnel. These firms quite often have complete models of major cities so that the wind affect of the surrounding buildings can be observed on the proposed building. All of the models are mounted on a turntable so the effect of the wind from all directions can be observed as the table turns.
Hopefully, what we have talked about here provides you with an idea of wind forces and how they affect buildings. Of course, analyzing wind loads on structures of different shapes, such as water towers, drive-in movie screens, traffic signs, you name it, involves a more complicated analysis.
Lastly, the Rocky Mountains are known for what are called microbursts. These are winds of a short duration and extremely high velocities, sometimes as high as 250 mph.
As you can imagine, at a velocity of 250 mph, the static wind pressure at 7,000 feet would be about 131 psf. It would take an extremely strong, and expensive, structure to resist such a wind load. Thus, it becomes a question of risk versus cost — a difficult decision to make in some cases.
Author’s note: The discussion above is accurate based on an old, simple wind code that was in use for many years. However, the newer building codes for wind load calculations are much too complicated to discuss in a simple article such as this. It is interesting to note that under a newer code, the calculated wind force for this structure is 7,558 pounds versus 6,608 pounds as calculated by the old code, which is a 14 percent increase. By rounding up the old code to 7,000 pounds, the increase is approximately 8 percent. Thus, the roundup appears reasonable.
This column may include both fiction and nonfiction, and views expressed do not necessarily represent those of The SUN. Submissions can be sent to editor@pagosasun.com.
>