Beam- Column question

[jmen68]

(Sorry mods for posting this question in the "other topics" board too, perhaps this board is more suitable.)

Ill preface this by saying this is very rough number and my schooling is failing me...
I am trying to find the equivalent moment on the columns of a simple structure. See my attached sketch and here is what I am thinking for simple numbers.

1) convert wind speed to pressure = (140^2)/383.6 = 51.1 psf
2) pressure X ft^2 = 51.1*188 = 9605 lbs
3) (Theoretically since dealing with wind max load is at the top of structure.) 9605 lbs X 20' = 192,100 lb-ft
4) divide by # of columns = (192100 lb-ft/1000 lbs/kip) / 12 columns = 16 k-ft.

How wrong is this without breaking out ASCE 7-10?

Thanks.

 

[JAE]

The wind load is not applied at the max. height but at the center of the area.

You also have an open structure below the floor and this will generate some upward pressures - perhaps on the order of 40 psf up on 1/2 the area (near wind side) and, under a separate load condition generate downward roof pressures of about the same amount.

These will add axial tension or compression to the columns.

The fixity of the columns to the structure will affect how much bending is in the columns.
The fixity of the columns at the base will also affect bending. If piling in sand then there will be some level of rotation/lateral translation at the soil surface.





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[jmen68]

Thanks for the response, currently the plan is to have a rigid connection (welded all the way around) to a base plate and top plate. The foundation is a slab on grade and the top plate bolted to the framework supporting the structure.

The drawing shows a box as if the structure is a trailer or a building of some sort, but in actuality it is three cylindrical tanks setting on a grillage of beams and an expanded metal type grating, would you still apply an uplift?

 

[JAE]

There is still wind pressure developed on grating - it is not linear with the percent of open space. Bars cause drag forces with wind flowing through.

A slab-on-grade is not a foundation unless it is at least 6 to 8 inches thick and properly reinforced to take the column applied bending. In addition, if the columns are intended to be fixed at the base the slab may be way too flexible to achieve a level of fixity and affect the structure's behavior. Also a concern would be punching shear forces under axial and moment at the base.



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