The short answer is no. There is a section covering tornadoes, and this section consist of the following sentence:

ASCE 7-16 Para 26.14 Tornado Lmitation

“Tornadoes have not been considered in the wind load provisions. “

Well that was simple, so I guess this is where the article ends? Not exactly, in the commentary of ASCE 7-16, Para C26.14 there is some more guidance on the topic. There is quite a bit of additional guidance in this section, and I will pull out the highlights in this article.

After a tornado, the meteorologists usually determine the magnitude of the tornado by specifying an EF number. The smallest tornado’s are EF0 and the largest are EF5. The EF value is the “Enhanced Fujita” scale, and it determines the range of wind speeds.

In the central United States the probability of a specific location experiencing an EF0 or EF1 rated tornado, is on the order of 4,000 years. ASCE 7 commentary recommends that if you are designing a Category IV (Essential) structure, that it would be prudent to use the EF1 wind speeds for design, since it would normally be a minimal increase in wind speed.

The probability of a site experiencing an EF4 or EF5 tornado are on the order of 10,000,000 years. According to the National Weather Service (NWS) between 1950 and 2013 there were 56,221 recorded tornadoes. Of these, only 4% were rated as EF3 and 1% were EF4-EF5; therefore the chances of experiencing an EF3 or higher is very remote.

Studies have found that tornadoes are more likely to generate wind borne debris compared to non-tornadic winds of the same speed. The momentum of wind-borne debris generated by EF3 speeds may exceed the impact test criteria adopted from hurricane opening protection.

ASCE 7 has provided a comparison to compare various wind speeds due to tornadoes in Table 26.14-3. You can see that even for lower tornadic wind velocity, larger roof and wall pressures are generated rather than a straight line wind of the same or greater speed.

ASCE 7 doesn’t include provisions in the main standard for calculating wind pressures for tornadoes; however, there are some provisions provided in the ASCE 7-16 Commentary C26.14. If you refer to that section you will find some guidance on how to estimate wind pressures for tornadoes.

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]]>We calculate critical wind speed (Vc) for a stack, and this is the wind speed at which the stack can experience wind induced vibration. The stack design codes provide an upper limit and when the critical wind speed is above this limit, then we no longer have to consider vortex shedding. The basic logic is that if the Vc is higher than any wind speed we expect to ever see on the stack, then it’s only logical that we can assume that vortex shedding will never occur.

For example, I’m located in Broken Arrow, Oklahoma, and here the design wind speed per ASCE 7-16 for Category II structures is 110 mph. I decide to design a steel stack for my back yard just to irritate my neighbors, and this stack happens to have a critical wind speed of 180 mph. That means at 180 mph the stack could experience vortex shedding; however, this is much higher than 110 mph for my region, and so it’s highly unlikely that my stack will ever see 180 mph in it’s life. Therefore, it’s not necessary that I even consider vortex shedding. The actual numbers we compare for with the upper limit aren’t quite this simplistic, but that is the basic concept.

We can use this concept to our advantage in certain situations. Now if your critical wind speed is 25 mph and your design wind speed is 100 mph, then this concept isn’t going to help you. The additional steel necessary to increase your critical wind speed from 25 mph to well above 100 mph is enormous. Where this concept comes into play is when your critical wind speed is maybe 5% to 10% less than the upper limit.

Another time we could use more speed is if we have problems with higher modes. Your mode 1 critical wind speed may only be 20 mph; however, your mode 2 critical wind speed might be just below your upper limit. In this case you could increase the stiffness of the stack and raise mode 2 above the upper limit, and then you only have to address mode 1.

From a dynamic standpoint, we can reduce our complicated stack into the simplified system shown to the right, with a beam of stiffness “K” and a mass “M” at the top. Visualize hitting the mass with a big hammer, the mass will start oscillating from side to side. We can also calculate the frequency of oscillation with the “K” and “M” values. The critical wind speed (Vc) is directly proportional to the frequency. The math is simple:

f = (K / M)^0.5

Vc = f * D / S

D = Average Outer Diameter of Top 1/3 of stack

S = Strouhal number which is usually 0.2

We can see from this simple equation that to increase Vc, we simply need to either increase D or increase f. We can increase f by either increasing stiffness (K) or decreasing mass (M). This gives us several options to increase Vc.

1) Increase the outer diameter of the top 1/3 of the stack.

2) Increase the frequency of the stack, which can be done multiple ways:

a) Decrease the mass, and most importantly the mass near the top. This isn’t always a feasible option, but if it’s possible to reduce the mass near the top it can help increase the frequency.

b) Increase the diameter of the stack, which will increase the stiffness (K). For the biggest impact, increases in diameter at the base of the stack will have a far bigger impact on the frequency than changes in diameter at the top.

c) Increase the thickness of the stack, which will increase the stiffness (K). For the biggest impact, increases in the thickness at the base of the stack will have far more impact on the frequency than changes in the thickness at the top (which also increases mass, and lowers the frequency).

To illustrate this trick lets use a real example. If we have a stack that is 100 ft [30.48 m] tall, 10 ft [3.048 m] OD and 0.25 in [6.35 mm] thick, then MecaStack will give a natural frequency of 3.3 Hz. This corresponds to a critical wind speed of 111.7 mph. The upper limit for vortex shedding is 115.7 mph. This means that if we can increase the Vc from 111.7 mph to something greater than 115.7 mph then we will no longer need to consider vortex shedding.

The easiest way to increase Vc is to increase the stiffness. This can be done by making the stack lighter, or making it more rigid. We can have the biggest impact by increasing the diameter, but increasing thickness can also have an impact. Generally changes at the bottom of the stack have a bigger impact than changes at the top of the stack.

Let’s just make a simple change of increasing the bottom 10 ft [3.048 m] from 0.25 in [6.35 mm] to 0.375 in [9.5 mm]. These results are shown below, and we are able to increase Vc > Vupr, eliminating the need to check vortex shedding. This change would generally be far less expensive than adding a damping solution or adding helical strakes, and it’s far simpler.

Increasing speed isn’t going to work in all instances, but there are many instances where this approach will work and save you time and money. The size and stiffness of stacks vary widely, and so the amount of additional steel needed to raise Vc above Vupr will vary widely. Sometimes just adding a small amount of steel can get you out of the danger zone, and other times it can take a tremendous amount of steel just to raise the Vc by 0.5 mph [0.2 m/s]. You just have to try and see if it will work on your stack.

If we can be of assistance, please don’t hesitate to contact us at support@mecaenterprises.com

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]]>There was a lot a panic a couple of years ago that FBC 2017 referenced ASCE 7-16. This isn’t true, the FBC 2017 still references ASCE 7-10. We have had a few Florida customers that have indicated that some local jurisdictions have required that they use ASCE 7-16. We haven’t been able to determine if this is a jurisdiction imposing their own requirements, or if it’s a misunderstanding of the FBC 17 requirements. Either way, we have both situations now covered in the MecaWind software since ASCE 7-10 or ASCE 7-16 can be used.

FBC 17 actually allows you to use ASCE 7-10. It goes further and gives the designer the option to use some criteria within FBC 17 to determine wind loads if the building meets certain requirements:

1609.6.1

41. Building or other structure <= 75 ft (22.86 m)

height-to-least width ratio of 4 or less

fundamental frequency >= 1 Hz

42. Not sensitive to dynamic effects

43. No located on a site for which channeling effects or buffeting are a concern

44. Building meet requirements of Simple Diaphragm Building

If the above criteria are met, then the method per Para 1609.6 can be followed. From our perspective, the method in Para 1609.6 appears to be a simplified method that is intended to be used by a Designer looking up values in Table 1609.6.2. This table has values for Cnet, and then the Cnet value is used to calculate the wind pressure using Equation 16-35.

How does FBC 17 compare to ASCE 7-10? To answer this we did a comparison of a simple gabled building using FBC 17 vs ASCE 7-10. For the ASCE 7-10 option we used Ch 27 Pt 1 for the Main Wind Force Resisting Design (MWFRS) and Ch 30 Pt 1 for the Components and Cladding (C&C).

FBC 17 does address other nonbuilding structures, such as chimneys, tanks and similar structures. On the surface the table for these structures appear to be different from that in ASCE 7; however, after some investigation we see that it yields identical pressures to ASCE 7-10. Thereforce, we recommend just using ASCE 7-10 since FBC 17 offers no advantage.

There are some other special considerations when following FBC 17.

1) Rigid Tile – There are special guidelines for calculating the wind loads acting on Rigid Tile in Para 1609.5.3.

2) Garage or Rolling Doors – There are special requirements for Garage or Rolling doors that are handled in Para 1609.7.

Please note that both of these special provisions are included in the FBC 17 option of MecaWind.

In ASCE 7-10, we use ultimate wind loads and then the load combinations factors are applied if Allowable Stress Design (ASD) is being performed. FBC 17 handles the case of ASD design a little different, it modifies the wind speed used in the calculations to an ASD velocity using Equation 16-33.

As mentioned earlier, the FBC 17 has been added to MecaWind. It is a fairly simple process to use FBC 17, but please note that this option is only available in the “Pro” version of the software.

To use FBC 17, select that standard in the dropdown. Then in the MWFRS selection menu, select the FBC Sec 1609.6 Alternative method.

Similiarly, in the Components and Cladding method selection menu, select the FBC Sec 1609.6 Alternative method.

If when selecting the MWFRS or C&C methods you are unable to select FBC and the circle to the right is RED, that means that you don’t meet all of the criteria to select the FBC Alternative method. For example, in the image below the circle is red because the building is NOT a simple diaphragm building and that is a requirement of the FBC alternative method. In that case, you would designate the building is a Simple Diaphragm Building (if that is in fact true) and then the circle will change to green and you will be able to select that option since all criteria have been met.

In conclusion, it doesn’t appear based upon the examples we have checked that there is any clear advantage to using FBC 17 rather than ASCE 7. Now with a MecaWind Pro license, a user can check both codes and determine which they wish to use in their design. The FBC wind provisions appear to be set up with the intent that the values can be looked up easily and calculated using the tables; however, with computer software no one method is any easier than the other to calculate, it usually is about the same number of keystrokes either way. So use the method or code that you are required to use, or that you feel is the best fit for your structure.

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]]>To answer this question we first need to cover some basics. If a structure vibrates and has no damping, then we would have the blue Undamped curve to the right. The amplitude is the same for each cycle and continues indefinitely. If our structure is Critically Damped, then we have the green curve where the vibration almost immediately attenuates and goes to zero.

The structure damping represents the structures inherent ability to absorb energy. There are many factors that contribute to the damping:

- Foundation and Soil (A soft soil/foundation system absorbs energy and a very stiff soil/foundation system doesn’t).
- Construction of the stack (Bolted connections increase damping, welded decrease damping)
- Lining inside the Stack (If stack has a substantial insulation/refractory/liner then it will increase damping).
- Internal Liner or Dual Wall Stack (Higher damping).
- Guy Wires
- Attachments to the stack (Piping, Ladders, etc..).

This is the difficult part, how do you put a number to all of these factors. The stack codes provide some guidance, but much of this is subjective. If you are too liberal, you run the risk of your stack having a major problem. If you are too conservative, you increase your cost and hurt your companies profitability. I can’t give you the answer, you will have to do a risk-benefit analysis to determine the right answer; however, I can give you some resources below to help you arrive at the right answer for your situation.

No! The software can not and will not estimate damping. It will assign a value based upon your selections (Elastic vs Rigid foundation, Lined vs unlined, etc..), but at the end of the day it is the stack designers responsibility to make sure the value is appropriate. This parameter is very important and shouldn’t be taken lightly. Just increasing the damping by 0.001 could make a huge impact to the results, so the designer needs to understand when it is justified to use higher values.

This whole article is focused on structural damping. There is another type of damping called “Aerodynamic Damping”; however, the aerodynamic damping is only beneficial for Along Wind loads, it can not be considered for Across Wind Loads (vortex shedding); therefore, we will not cover it in this article.

If you have a damper, such as a Damping Pad or Tuned Mass Damper (TMD), then estimating the damping becomes easy. The damper supplier will give you the estimated damping for your structure. You can then enter this value into MecaStack and perform your analysis to confirm that everything is working. If you use Meca for your damping needs, we ask for the MecaStack input file (*.stk) to be sent to us, and then we will return the file with the appropriate damping parameters as well as damper info (weight, dim’s, etc.) entered into your file.

The estimation of structural damping is an in-exact science. To really know the damping you would have to build the stack, erect it, and then do measurements on the stack to measure the damping. This is obviously not feasible, and so we make conservative assumptions based upon the resources we have available (see recommended damping values above). When in doubt, my recommendation is to guess a lower value to be conservative, because the last thing any stack designer wants is to find out their stack is vibrating.

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]]>I am an amateur cyclist, and I typically ride my bike on trails in my home of Broken Arrow, Oklahoma at an elevation of 700 ft [213 m]. One year I took my bike on a vacation to Breckenridge, Colorado at an elevation of 9,000 ft [2743 m]. The first thing I noticed when riding in Breckenridge was that there was a significant decrease in oxygen. The second thing I noticed was that the wind resistance was significantly less when I was pedaling into a headwind than it would have been in Oklahoma. This was due to the density of air being less, and therefore the wind resistance was reduced.

When calculating wind pressures per ASCE 7-16 there is a new consideration, that allows you to consider lower wind pressures due to the thinner air at high altitudes. This is not to be mistaken with the elevation above grade. Since this has been the source of some confusion, I wanted to explain the difference.

The height above ground level has always been a consideration in wind pressure calculations. This is referring to the elevation above ground level. At ground level, there are many obstructions (buildings, trees, terrain, etc.) that slow down the wind, and at very high elevations above grade there is nothing interferring with the wind. It makes sense that as the elevation above ground increases, so does the wind velocity. This is accounted for in the ASCE 7 standard by use of the “Kz” and “Kh” factors. Referring to Table 26.10-1, the “Kz” factor is dependent upon the exposure, and gets larger as the elevation increases. The exposure is a consideration because at exposure B there are more ground level obstructions than there would be at exposures C and D.

In the 2016 version of ASCE 7, a new parameter was introduced, Ground Elevation Factor “Ke”. This parameter is dependent upon the ground elevation above sea level. As seen in Table 26.9-1, the “Ke” decreases as the altitude increases. The theory here is that the wind pressure is based on the density of air, and as the density decreases, the pressure also decreases. If we have the same wind velocity, then we can see from Table 26.9-1 that the wind pressure at a site that is 6,000 ft [1829 m] above sea level will be 80% of the pressure calculated for the same wind speed at sea level. This is a significant decrease. As seen in the illustration below, the ground level then becomes the local ground level for the structure. Then the “Kz”/”Kh” factors are calculated just as they were before, but z is measured from the local “ground level” up to the area of interest.

Did you know that the versions of ASCE 7 prior to 2016 actually had provisions to account for altitude above sea level, but they were back in the commentary. In Paragraph C27.3.2 of ASCE 7-10, the constant of 0.00256 (or 0.613 in SI) in the pressure calculation, was based upon the mass density of air at sea level. There was then a table for the density of air at various altitudes, and a method to calculate a new constant to be used. Here is a sample calculation for the constant at an elevation of 0 ft (sea level) and 6,000 ft [1829 m] above sea level.

**0 ft [0 m] Average Air Density = 0.0765 lb/ft^3**

Constant = 0.5*[(0.0765 lb/ft^3)/(32.2 ft/s^2)]*[(5280 ft/mi)*(1 hr/3600 sec)]^2 = 0.00256

**6,000 ft [1829 m] Average Air Density = 0.0639 lb/ft^3 **

Constant = 0.5*[(0.0639 lb/ft^3)/(32.2 ft/s^2)]*[(5280 ft/mi)*(1 hr/3600 sec)]^2 = 0.00214

If you ratio 0.00214 / 0.00256 you get 0.835. If you will note that the Ke factor at 6000 ft from Table 26.9-1 is 0.8, so the two are in the same range.

The wind pressure equation is as follows:

qz = 0.00256 * Kz * Kzt * Kd * V^2 {ASCE 7-10, Eqn 27.3-1}

Considering the altitude, it is then modified to be:

qz = (Constant) * Kz * Kzt * Kd * V^2

The ASCE 7-16 (and previous releases) do give provisions to reduce the wind pressure to account for the air being less dense at higher altitudes. I can’t say that I have personally taken this reduction in my designs of equipment, because I prefer to stay conservative and use Ke = 1; however, it is a legitimite reduction that could play a major benefit for very tall structures.

In our MecaWind software, the entry of each value is shown below. Once entered correctly, the software will calculate Kz and Ke automatically, and the designer doesn’t need to do anything else.

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]]>On the form, there are circles and circles. Think of these like a traffic light, is bad and is good. MWFRS Methods that are green mean that you meet the criteria for that section, and can use it. indicates that you do not meet the criteria. For example, in the image of the MWFRS Method Selection you can see for Ch 28 Pt 2 we meet all criteria () except the Simple Diaphragm (). If we change the building to be a Simple Diaphragm, then we can select Ch 28 Pt 2.

Select a MWFRS method that is applicable, and then you can run the analysis and look at the results. If you want to compare results from different methods then simply go back to the MWFRS method form and change to another valid ( ) option and perform analysis.

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]]>**“Save Current File as Default”**.

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