Issue with stack structural analysis 1

[atrizzy]

I'm analyzing an existing steel stack for power plant effluent to determine adequacy for wind loading and am running into a bit of an issue. Please see the attached sketch.

The stack was designed with 4 vertical stiffeners at 90 degrees to each other running vertically for the top 40' of the stack. There are intermediate horizontal angle stiffeners, but none exists at the lower termination of the vertical stiffeners. The stiffeners just stop, abruptly, about 40' below the top of the stack.

I'm trying to determine if there is a risk of premature local buckling in the primary shell due to bending in the stack at the point where the stiffener terminates.

I'm having a tough time wrapping my mind around this. On one hand it seems if there is stress in the stiffener, it has no path but to transition to the main body but through a localized point. On the other hand, how could the entire stiffener be under stress if it terminates and there's nowhere for it to go?

I'm thinking of providing a beefy horizontal ring stiffener at this location to mitigate any potential for this issue, so that I can be sure that the flexural capacity of the entire tube section can be utilized.

Any thoughts and insight welcome. Thanks in advance.

 

[SAIL3]

by providing a horiz stiffener @ the termination point then any flexural stress still present in the stiffener which could cause a local moment in the shell can be transferred to the horiz stiffener resulting in a radial point load in the horiz stiff @ that location....we are not addressing classical shell buckling here but rather local stress @ the point of termination of the longitudinal stiff....

 

[KootK]

by providing a horiz stiffener @ the termination point then any flexural stress still present in the stiffener which could cause a local moment in the shell can be transferred to the horiz stiffener resulting in a radial point load in the horiz stiff @ that location

I get it, it just doesn't ring at all true to me.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SAIL3]

intuitive engineering judgement would lead one to assume that there is a gradual migration of flexural stress into the longitudinal siffeners...in cases like this where that assumption sounds reasonable, the negative consequences of that assumption not being entirely correct would cause me to address the problem and also in light of the relative difference in the size of the stiffeners(1x12") verse the shell thickness (0.15")...

 

[Sawbux]

I agree that these "stiffeners" are actually "strakes", used to inhibit vortex shedding which can cause horizontal flexing of the stack and fatigue failure. The spiral layout is usually used to make them more Omni-directional as far as the wind direction.

I would be surprised if a stack of this size and height did not have insulation and a inner liner. Any info on that? The insulation keeps the stack gasses warm enough to prevent condensation on the interior (sulfuric acid).

I would be hesitant to add horizontal stiffeners, unless welded carefully to avoid a circumferential stress raiser/ fatigue problem. The upper one, probably not a problem since bending stresses are probably very low at the upper portion of the stack.

The failures I've seen were at the base of the stack, at the horizontal ring at the top of the anchor bolt chairs. A combination of corrosion and fatigue loading.