I have not use AS1170 in my calculations; but I designed on steel stack for a petrochemical company by ASME STS-1. If you have AS1170, please send it to me to study and if I have a specific opinion about it, I will share with you –if Allah wants-
I believe that ASME STS-1 does not allow exceeding the first natural frequency. Vortex shedding is a phenomenon that occurs in all the wind speeds and cause stack to vibrate. Frequency of vibration of stack has a linear relation with wind speed. As wind speed increases, frequency of vibration will increase. It continues until frequency of vibration coincides with first natural frequency. At this wind speed, resonance will occur. If the stack does not have enough strength, failure is an expected result; but if it has enough strength -as wind speed increases- signs of resonance will be dissipated and amplitude of vibration will decrease considerably.
ASME STS-1 includes above idea. Since there is not a specific natural frequency in practical situation, ASME defines three zones:
1. In the first zone that wind speed is small enough that makes us sure that resonance will not occur.
2. In the second zone, we are not sure that resonance will occur or not. So ASME submits more accurate calculation for this subject. If this calculation leads to resonance then it is time to calculate huge loads caused by resonance.
3. In the third zone, we are absolutely sure that resonance will occur and we should calculate subsequent loads caused by resonance.
You should consider that in design, there are two criteria that shall be satisfied.
1. Maximum stress (that shall not be greater than allowable stress based on your design code)
2. Maximum deflection (that shall not be greater than 0.5% of total length of stack)
I said all above, because most of engineers think that vortex shedding occurs at a specific wind speed, while vortex shedding –except in low wind speeds- absolutely will occur in all wind speeds. What you should take enough care about it, is resonance not vortex shedding. (For more information please refer to Bednar handbook)
Although ASME is not applicable for second natural frequency; but you can even exceed the second natural frequency base on a correct engineering justification and calculation.
(((The Vendor's opinion is that the stack does not have a likelihood of large amplitude vibration due to vortex shedding at the second mode of frequency, and calculated vortex shedding equivalent loads are very small, and fatigue analysis is not required. Do you agree with this opinion, what do you think? Is it sufficient statement for overturning bending moment and shear forces at the base?)))
About above phrase, I should say that vendor's opinion is quit incorrect. Because maximum wind speed probably will occur once in 50 years; but at lower speed your structure may fall within the first mode of frequency. I think that first mode of frequency may be more severe than the second mode. So at first please ask your vendor to perform calculation for all possible modes of frequency. It is also a good idea to force your vendor to check his own design with a well-known software. Also ask your vendor to give design guarantee.
Also it is not important that mass in stack is distributed uniformly or not. You can obtain natural frequency by manual calculation or by a 3D modeling software.
However vortex shedding (according to experience) occurs at top 1/3 of the structure. For this reason, ASME insists to use vortex breakers at this zone. Using vortex breaker below this zone has no affect for dissipating vortex shedding.
Based on my studies and also experiences of my colleagues, it is strongly recommended to use guy wire; if there is not enough space around your stack, make a steel structure around it. Finally if none of two choices is not practical, you can use vortex breaker. In this recent case, vortex shedding and consequently resonance will not occur. Instead please consider that vortex breaker causes a shape factor of 1.4 that multiplies in static wind load. This high shape factor leads to a higher static wind load. As a result maybe thickness of stack as well as load at base is increased.
If you need more discussion, please don't hesitate to contact me at firstname.lastname@example.org
Also if you have an engineering judgment about my opinion or if you think something is wrong with me, I would be glad to hear about it.