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# AS 1170.2 and ASME STS-1 Wind sensitive structures like chimney

## AS 1170.2 and ASME STS-1 Wind sensitive structures like chimney

(OP)

Greetings,

I am going through a Vendor calculation on the wind sensitive structure like chimney. I can see that the vendor has selected large aspect ratio H/D around 31, and single self standing stack can experience cross wind (vortex shedding) vibration at the second mode of frequency as well.

"AS1170 Section 6.1 Evaluation of Dynamic Response factor" clearly says that "The dynamic response factor shall be determined for structures or elements of structures with natural first mode fundamental frequencies ......If the first mode of vibration greater than 1 Hz Cdyn = 1.0.

"AS1170.2  Clause 6.3.3 Crosswind response of chimneys,..." is looking for only first mode of frequency of vibration in the calculation.

Can we say the following from above statements?

1. AS1170.2 is not interested in higher modes of vibration by these.
2. Or AS1170.2 does NOT accept the structures with high H/D ratio.
3. Or the procedure provided by the standard is for the first mode of vibration only, and can not be used for higher modes of vibration.

AS/NZS1170.2 Supp 1- C6.3.3.1 Crosswind tip deflection says the following:

"The calculated value of crosswind tip deflection will have an accuracy of no better than 30 % to 50%. However, if the calculated value is a large fraction of the diameter than methods of mitigating the vibrations should be considered, such as the following:
(a) Increasing damping.
(b) Increasing mass.
( c ) Installing aerodynamic devices such as strakes or shrouds."

C6.3.3.2 Equivalent Static wind force
This represents an inertial load that varies with height, according to the mode shape on the first mode of vibration. A more accurate first mode shape may be determined for a particular shape."

These seem to support the statement 3 above.

However ASME STS-1-2011 Section 5.2.2 (a) (1) Wind responses – Vortex Shedding clearly says that "Vortex shedding loads shall be calculated for all modes of vibration where Vc < Vzcr. The procedure in Nonmandatory Appendix E may be used. Fatigue analysis must be considered. The vortex shedding loads need not be considered with (a)long wind loads.".
Appendix E sample calculation for Vortex shedding under section E04 does give calculation for the first mode of vibration only, unfortunately.

I see that all the formulations in AS1170.2 and ASME STS-1 are for linear mass distribution along the chimney with some tolerance, and this tolerance is less than 10% over the top third of height. In our case, there is a mass at the top of the chimney which does not comply with the tolerance given by the code either to be able to use the code formulation as they are. I guess the mass at the top is considered in the calculation of first and second mode of frequencies.

Additionally, the largest displacement is not expected to be at the top of the chimney under the second mode of the vortex shedding vibration, the chimney may be experiencing large displacement at lower level, and  corresponding overturning bending moment and shear force at the base might be a lot larger because of the large critical wind speed. I have not provided any calculation to support this.

I am looking for a comment from the engineers experienced on the calculation methods given by AS1170.2 and ASME STS-1.

The vendor has provided very limited information on this side which only calculates the top deflection for the second mode of frequency vortex shedding. I am questioning this part of the calculation because it does not provide a proper calculation for the overturning bending moment at the base.
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?

Ibrahim Demir

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