Helical strakes can be added in MECASTACK.  Go to the “Design” “Damping” menu, and then there is a helical strake option.  The button at the right “Damping Device Menu” lets you enter the parameters.   Although MECASTACK does let you simulate helical strakes, their use still requires some engineering judgment.  The software will consider the additional along wind loads, by considering the higher shape factor that is entered on this "Damping Device Menu" (Default Cf = 1.4).  as far as Vortex Shedding is concerned, the software assumes that you know what you are doing by adding the  helical strakes, the software can not determine how well they will work at addressing vortex shedding.  This is completely a judgment by the user, because there are no equations or methods to predict how effective they will be in suppressing vibration.  There are specific instances where you CAN NOT use helical strakes, such as when you have closely spaced stacks (or other tall structures).  Each code varies in the limit, but generally this is considered unacceptable when the center to center spacing is 10 to 15 diameters or less.  In these instances, a damper or another solution is required.   

For a stack with helical strakes, some codes reference a shape factor of 1.2 and others recommend 1.4.  The reason for the difference is the way in which the codes assume that the shape factor is being applied, if you are consistent in the applying of the factor then the values are actually almost identical.  MecaStack applies the shape factor to the bare diameter of the stack, and that's why MecaStack recommends 1.4.  Codes that use 1.2 are applying the shape factor to the "straked" width of the stack.  

MecaStack (Apply shape factor to "Bare" OD of stack:

Effective Width = 1.4*D

Codes that recommend 1.2 shape factor (Apply to Straked Width of stack).

Strake Width = 0.1*D

Effective Width = (D + 2*0.1*D) * 1.2 = (1.2 D) * 1.2 = 1.44 D

If you really intend to use a 1.2 shape factor, then in MecaStack you must enter 1.44*D since the software applies the shape factor to the "Bare OD" of the stack.  

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