wind pressure on silo

[ronster]

I am checking an existing barn silo which is being added to a building that I am designing. The silo will be empty (used a stair tower). Does anyone know how to check an empty silo for wind pressure. In particular I am concerned about localized bucking or cruching of a side, much like when you step on the side of a beer can.

 

[steve111]

What type of silo is it? Staves with tension rods, cast in place concrete, or perhaps even wood with tension rods (theres a few of them around), also some masonry ones as well.

 

[ronster]

Steel plate rings about 1/4" to 3/8" thick that are bolted together in sections (about 6 feet on center vertically). Similar to a large beer can (about 16 foot diameter)hence the analogy. The entire section works easily for wind loads however I'm concerened about localized buckling due to the thin steel plate.

 

[dlew]

ronster,

The book "Formulas for Stress and Strain" by Roark, has formulas for circular rings that would give you the maximum concentrated or uniform load that would produce certain level of stress and/or deflection in the shell.

The formulas are based on the shell being perfectly circular, it is good practice to provide an additional safety factor for any imperfections.

If needed, the best way to reinforce the shell is using reinforcing rings spaced along the length of the cylinder.

Regards

AEF

 

[Ron]

There are several ways you can approach the analysis. Bethlehem Steel published a steel tank guide about 25 years ago that has some excellent info along these lines. Check around for a copy. Also, you can approach it as an empty water tank, thus AWWA has some guidelines.

The simple approach it to treat the whole silo as a pipe to check for localized buckling.

 

[ChipB]

The "simple approach" method stated above will unfortunately not work. As far as I know, AISC does not have any recommendations on something this size.

Assuming 36 ksi steel, you would be allowed a maximum compressive stress of about 16ksi with a tank diameter of 11'-3". That is the upper limit of allowed ratios.
The formula is Fa=(662/(D/t)) + .4Fy for (3300/Fy)<D/t<(13000/Fy)where D is the diameter and t is the shell thickness.

Because of irregularities in this type of section, it should be substantially reduced. I read somewhere (I'm sure it's in the ASD manual), that theorectical design will substantially (>200%) overestimate the buckling strength.

Unless AWWA helps you out in this, I would reduce my theoretical allowable stresses by a factor of 3.

Hope this helps.
Chip

 

[curvbridger]

I would contact Harvestore, a US manufacturer of silos similar to the one you describe. Since silos are often empty, and usually exposed to high wind velocities, I would not be too concerned unless there is evidence that the original design was faulty.