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Using wind as an external pressure?

Using wind as an external pressure?

(OP)
Hi all,

When considering buckling as a failure mode due to wind, I was wondering if you could approximate windward wind(pressure) as an external pressure? I know wind should be seen a bending force but I am curious if setting them equal makes sense for the primary membrane stress or would just be a conservative estimate.

Cheer,
Alex

RE: Using wind as an external pressure?

Wind and external pressure are not the same and wind cannot be considered as an external pressure. Wind pressure can cause a local buckle whereas external pressuure would cause a global buckle.

RE: Using wind as an external pressure?

Are you talking about an API tank or an ASME pressure vessel?

I've never worked with pressure vessels where buckling due to wind was a controlling mechanism, but I guess anything is possible. I have however worked with API tanks, and buckling due to wind is a very real concern.

EDIT:
I forgot to mention that regardless of the tank/pressure vessel, I agree that you need to look at wind loading separate, not as a uniform external pressure, for the same reason noted above.

Jim Breunig P.E.
XCEED Engineering
Mechanical Engineering
FEA Consultants

RE: Using wind as an external pressure?

JBreunig,

Just as an aside, wind can become a controlling mechanism in pressure vessels when you're talking about tall and slender distillation columns with a low design pressure. In these cases you are forced to thicken lower shell sections.

RE: Using wind as an external pressure?

As the primary stress induced by either internal or external presses, the wind load, similar to the seismic load, is classified as a secondary stress.
It could be over-conservative if the design to a primary stress which includes the wind load with the the external pressure.

RE: Using wind as an external pressure?

Thanks Marty007. Most tanks I've worked with have been indoors, and rather stout. I've never worked with tall, slender distillation columns.

With regards to treatment of the wind load, I didn't see anything in section VIII that gave detailed analysis methods. API-650 however does have some guidance for evaluating this if you're looking for a method.

Jim Breunig P.E.
XCEED Engineering
Mechanical Engineering
FEA Consultants

RE: Using wind as an external pressure?

mk3223 please explain how wind or seismic are secondary. As defined in Sec. VIII Div. 2 paragraph 5.14:

Quote (Sec. VIII Div. 2 5.12)

Secondary Stress: A normal stress or a shear stress developed by the constraint of adjacent parts or by self-constraint
of a structure. The basic characteristic of a secondary stress is that it is self-limiting. Local yielding and minor distortions
can satisfy the conditions that cause the stress to occur and failure from one application of the stress is not to be expected.
Examples of secondary stress are a general thermal stress and the bending stress at a gross structural
discontinuity.

Wind or seismic are directly applied loads and are not self limiting. Consider a column under wind loading; if it displaces due to the applied wind load it is still under the same amount of load. However, for a true secondary stress, such as would be seen at a head to shell junction, once yielding occurs, the load does not increase. Wind and seismic should be considered to be primary loads.

RE: Using wind as an external pressure?

Pdiculous963:
Thanks for the correction. I mixed up it with the loads in piping system.

RE: Using wind as an external pressure?

While checking for secondary wind girders/ rings, the EEMUA 159 and the European code EN 14015 ( both are for storage tanks) combine in one formula the Wind (thru wind speed Vw) and the external pressure (Pv) to check buckling lengths and the need of wind girders .

take care and thanks

RE: Using wind as an external pressure?

See API 650
4) Calculate the minimum shell thickness required for the combined loading from design external pressure
and wind - PAG 281 / 302 in pdf.

Regards
r6155

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