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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]]>Both versions of MecaLug allow the user to design a single lifting lug from a variety of configurations that are most commonly used in the industry. Understandably, there are countless other lifting lug configurations that can be used by companies and it’s not feasible to include every possibility. Once the configuration is chosen, the user will see a graphical representation of the lug configuration that will dynamically update when lug parameters are updated.

What do you do if you don’t know the loads the lug will see during the lifting operation? That’s where the Pro version of MecaLug comes in.

A lifting system is comprised of 2 or more lifting points, and finding the loads acting on each lifting point can be challenging. MecaLug Pro allows the user to pick one of several standard lifting system designs, and then simply enter the dimensions that match your design. The user can select which lifting lug is to be used for each lifting point and has the ability to rotate the lug about all 3 axes. The software also has an option to select “None” for the lifting lugs, if the user only wants to calculate the loads in each cable and the reaction(s) at the crane point(s).

For lifting systems where the lifting cables come up to a single point (as shown below), the geometry of the system can be a little complicated. MecaLug takes the geometry into account and calculates the loads acting on each cable. In addition, the lug is experiencing loads in all three directions (X/Y/Z) and it is analyzed for all of those load components to check the overall integrity of the system.

- +1 918 258 2913
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]]>**What is Vortex Shedding?**

The calculations can be quite involved, depending upon the code, but I can give some of the basics. The first thing you need is a natural frequency, and unfortunately there aren’t many easy ways of estimating the frequency unless you are dealing with a constant diameter and constant thickness stack. Software is usually used to calculate frequency of stacks, such as our MecaStack software. Once you have the frequency then you can calculate the critical wind speed (Vc).

* Vc = f * D / S *

* f = Natural Frequency (Hz)*

* D = Outer diameter of Stack over top 1/3*

* S = Strouhal Number, usually taken as 0.2 for round shapes*

Let’s take a real world example, lets say we have a stack with a f = 1 Hz, D = 2 m [6.56 ft] and S = 0.2.

*Vc = (1 Hz ) * (2 m) / 0.2 = 10 m / s [22.4 mph]*

The critical wind speed Vc is the wind speed at which vorticies are shed, and the stack is most likely to vibrate. In this example, 10 m/s is quite low and so it is highly unlikely that the stack could experience vortex shedding. The specific criteira is more complicated, but as a rule of thumb, if your design wind speed is 40 m/s then you could potentially have a problem if the Vc <= 40 m/s.

Not necessarily, there are calculations involved. Even though Vc may be low enough that it can occur on a regular basis, the question becomes whether the stack has enough inherent damping. If you look at the graph to the left, an undamped system will vibrate indefinitely. A low damped system will vibrate for some time before it eventually reduces to a negligible amplitude. Most stack design codes (ASME STS-1, CICIND, Euro, etc..) have equations that estimates the amplitude of vibration that is to be expected on this stack. These equations take into account frequency, mass distribution, stiffness and most importantly structural damping. The structural damping plays a **HUGE** role in the predicted vibration amplitude. A typical value for structural damping is 0.002, which is a very small number. Just increasing this to 0.003 can make a huge impact, since it’s a 50% increase in value.

This is the tough part, there isn’t any calculation you can do to estimate damping. It is a measure of how much energy is dissipated by the system. A very stiff stack mounted on a very stiff foundation has very low damping. A stack with a lot of heavy refractory and a more flexible foundation will absorb energy, and thus will have a higher damping. It’s a very inexact science and one that requires a lot of tough decisions.

My recommendation is to be conservative. If you aren’t sure, then estimate low. You don’t want to win the job and then have a stack show up in our Vortex Shedding Video of examples of what not to do. When vortex shedding does occur, the resolutions can become very expensive.

When you have a problem there are a few options. We will go into these options in more detail in subsequent posts, but we will highlight the most common solutions.

1) Add Helical Strakes

2) Add a Tuned Mass Damper (TMD)

3) Add a Damping Pad

4) Add a Tuned Liquid Damper (TLD)

5) Increase stack stiffness to increase Critical Wind Speed above upper limit

6) Decrease stack frequency to decrease Critical Wind Speed

7) Add Guy wires

We will discuss some of these options in future posts, and we are always available to provide a free consultation on evaluating a specific situation to find the best solution. Simply email us at support@mecaenterprises.com

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]]>The new ASCE 7-16 standard offered many changes to the ASCE 7-10. MecaWind includes ASCE 7-05, ASCE 7-10 and ASCE 7-16, and includes all of the changes in the new standard. I wanted to take a minute to explain some of the major changes that occurred in ASCE 7-16.

- Addition of a new Classification: “Partially Open Building”
- Wind Maps for Hawaiian Islands added
- New Wind map for Risk Category IV added
- Wind Speeds in non-hurricane regions decreased, and contours in Northeast USA revised
- Now a new parameter “Ke” added to account for reduced wind pressures at higher altitudes.
- Edge zone side dimension ‘a’ has been reduced for very large buildings with low sloped roofs.
- Transverse Frame criteria has been added
- Criteria for Rooftop Equipment has been revised and expanded
- Significant changes for Bins, Silos and Tanks
- Solar Panel criteria for Rooftop mounted panels added
- Flat and Sloped Roofs <= 60 ft saw an increase in component and cladding pressures; however, for h < 30 ft in Exposure B also saw some reductions.
- Gabled & Hipped Roofs <= 60 ft have significantly revised figures for Components and Cladding pressures that cover a wider range of slopes.
- Criteria on loads on Canopies has been added
- Components and Cladding Ch 30 Pt 2 completely revised the tables.
- Components and Cladding Ch 30 Pt 4 now refers to Ch 30 Pt 2 when h <= 60 ft.

Although most of these changes are not relevant to stacks, we have incorporated all of these changes into our MecaWind software.

MecaStack includes the comprehensive stack design code ASME STS-1 ‘Steel Stack Design’. In this standard, the ASCE 7-05 wind load calculations are used, which were based upon an Allowable Stress Wind Speed. Recently the ASME STS-1 standard was revised to allow Ultimate Wind Speeds to be entered per ASCE 7-10. The revision was made by simply converting the ASCE 7-10 Wind Speed (Ultimate) to an equivalent ASCE 7-05 Wind Speed (Allowable). From that point on the calculations are the same as they would have been using ASCE 7-05. The STS committee has not yet determined what they will do with ASCE 7-16, but it seems that eventually the committee will have to revise the STS standard to use Ultimate wind speeds, since that is going to the basis for ASCE 7 from now on it appears.

There aren’t many changes in ASCE 7-16 that impact stacks. There are some revisions to the wind maps, and there is the possibility to consider a reduction in wind pressures at higher altitudes due to the air being less dense. It is unclear if STS will allow for this reduction within the Stack code.

MecaStack has a Wind Conversion Utility which few users know about It’s found by going to the Design Code menu, and then select Wind Code and then Click the Wind Conversion Utility button. You select the original code on the left, and then select the code you are converting to on the right.

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]]>The design of a Tuned Mass Damper (TMD) is specialized, and not a simple process. Meca sells TMD’s, offers quotations for a TMD, and provides preliminary estimates of the sizing. Typically the designer sends Meca the Input file from MecaStack and then we size the TMD and provide a quote. Then we return the MecaStack file with the relevant TMD parameters included in the file. MecaStack will then use the values entered to perform the analysis. MecaStack does NOT design the TMD, it only considers the effects of the TMD, after the appropriate parameters are entered. Meca provides these parameters (structural damping, dimensions of TMD, etc.) and then you use those parameter to evaluate the impact on the stack using MecaStack.

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