Interpretation of ACI 307-98 - Design of Concrete Chimneys

[structuralecstasy]

I am need of some advice on ACI 307-98 in regards to the "grouped chimney" effect (vortex shedding) for concrete chimneys. We are adding a new chimney to an existing plant for a project I am working on, and we have a code issue we are trying to resolve.

We had stack modelers do a wind analysis of the existing chimney based on the proposed location of the new chimney, and their results showed that the existing chimney had a 21% overstress in one section. For the overstressed chimney, they proposed keeping the new chimney a distance of 530' from the existing chimney.

ACI 307-98, Section 4.2.3.4 sets a minimum chimney spacing so that grouped chimney effects (magnified vortex shedding forces) do not have to be considered. It says if the chimneys are approximately the same height & diameter (they are not in our case), then the minimum spacing is 12.75*(chimney diameter at 5/6 of height). Using the larger of the two chimney diameters we have for our case, this minimum distance is 380'. This distance is much less than the 530' recommended by the modelers, but the code says this criteria is only for identical chimneys.

I believe that since we used the larger of the two diameters to calculate the critical distance of 380' that we are still within the guidelines of the code despite not having identical chimneys, but at the same time I realize that this is an engineering interpretation of the code and may not be valid for reasons not outlined in the ACI code. Does anyone have enough knowledge of ACI 307-98 to know if this is a valid assumption? Or is this a black and white issue: follow the code, get a chimney model done per the code, follow the recommendations of the modelers.

Thanks for your thoughts.

 

[VoyageofDiscovery]

With regards to vortex shedding, it is very much a function of the actual situation and since your situation does not meet all the requirements for the simple approach you must utilize modelers with wind tunnel testing.

VOD

 

[JStephen]

Have you discussed this with the stack modeler people? That sounds like the first choice.

My first thought would be that both ACI and your modeler people are checking if the effect from the new chimney is below some threshhold- but that perhaps they are using different threshholds?

 

[dbuzz]

How was the 530' separation distance determined, from a code, a quantitative result of the modelling or inferred from the modelling?

Can the specialists model the two stacks at a separation distance of 380' and determine the interaction effects?