Concrete Column Lateral Deflection Check for Industrial Sheds 4

[MSUK90]

Dear All, I was going through a design of industrial shed with concrete columns till roof(around 7m eaves height). The roof sheeting is supported by rafters which rests on the concrete columns connected by a base plate. My question is, do I need to check lateral deflection of concrete columns due to wind loads with a code prescribed limit of range H/400 to H/600? Do I need to consider just the wind load reaction from roof rafters or also due to wind loads acting on all four sides of shed, distributed to columns? Does someone has experience designing such kind of structure?

 

[Lomarandil]

You would consider the wind load acting on the entire shed, and the effect on each element according to how it distributes through the structure to the ground.

However, for an industrial shed, there may not be as many concerns about lateral deflection due to wind. Occupant comfort may not be a factor, and the cladding materials (you mention sheeting for the roof) are often more flexible. H/400 and H/600 are limits more common for structures incorporating masonry, glass, etc.

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just call me Lo.

 

[MSUK90]

Occupant comfort may not be a factor, and the cladding materials (you mention sheeting for the roof) are often more flexible

Yes. But there will be block walls all over the edges supported by columns. Aren't those dictate the limits for column deflection?

 

[Lomarandil]

Yes, if the shed includes masonry walls, then those will necessitate a tighter limit on deflection.

(At least you appear to have avoided the pinnacle of deflection concerns -- masonry walls actually hanging on steel PEMB columns)

----
just call me Lo.

 

[MSUK90]

Yes, if the shed includes masonry walls, then those will necessitate a tighter limit on deflection.

Yes, now that's my problem. When I use the limits I specified in my initial post, the concrete column depths goes to more than 1m which seems too high. People have previously designed similar sheds here with a column size of merely 200mmX400mm or so without thinking much of the concrete column deflections.
I wonder if I am going wrong somewhere.

 

[rowingengineer]

Is a cantilever or portal frame? Deflections are different for both, reinforced black walls can have lighter deflection limitations to masonry unreinforced.

 

[MSUK90]

Is a cantilever or portal frame?

They are cantilever concrete columns supporting steel rafters.

 

[rowingengineer]

Deflections limits seem high for this case, I would think more like h/200 under service or there abouts, I would try to get some help from the walls as well.

 

[MSUK90]

Deflections limits seem high for this case, I would think more like h/200 under service or there abouts, I would try to get some help from the walls as well.

I understand the thing you are saying. But I need some codal reference to prove if something may happen in future? Also, even if the limit is reduced to what you have suggested, still the column sizes for deflection will be too much due to their cantilever nature. Totally stuck.

 

[MSUK90]

Still wondering if someone can guide me to some reference for deciding deflection limits for such cantilever columns.

 

[SlideRuleEra]

...guide me to some reference for deciding deflection limits for such cantilever columns.

"Structure Magazine" December 2019

http://www.SlideRuleEra.net

 

[MSUK90]

SlideRuleEra, I am aware of this clause. Am I doing the check correctly considering H/500 as displacement limit in your view then?
The only issue I face is very deep columns for such limits and no client likes it.

 

[SlideRuleEra]

MSUK90 - What is the calculated displacement with the column size you want to use?

http://www.SlideRuleEra.net

 

[MSUK90]

What is the calculated displacement with the column size you want to use?

Its a general question, varies from project to project.
Lets say for example, I get deflections of around 70mm for 400mmx400mm columns. Total height of column being around 7m.
If I check allowable limit by H/500, it will be around 14mm.

 

[SlideRuleEra]

Ok, 70mm deflection for a column 7000mm high, H/100.

What happens to the industrial shed with a deflection of 70mm?

Is some part to the shed damaged by the 70mm defection?
Is some item that is being protected by, or adjacent to the shed affected by the 70mm deflection?
Will the users of the shed notice the 70mm deflection and complain?

Don't expect some "code" to address these questions, but what to do depends largely on your answers.

http://www.SlideRuleEra.net

 

[MSUK90]

Is some part to the shed damaged by the 70mm defection?
Is some item that is being protected by, or adjacent to the shed affected by the 70mm deflection?
Will the users of the shed notice the 70mm deflection and complain?

1) The columns, beside roofing, will support walls all along the edges of shed and I don't think a lateral deflection of 70mm is anyhow accepted for concrete block walls.
2) I don't think so.
3) May be, May be not, I am not sure as no one has ever complained about that in past.

Don't expect some "code" to address these questions, but what to do depends largely on your answers.

I have to depend on codal preferences as authority will ask me to prove myself through some codal reference. I know basic sense should be applied but no one accepts your own beliefs when you are submitting for approvals.

 

[SlideRuleEra]

MSUK90 - So, you consider 70mm lateral deflection to be to much for concrete block walls. That rules out H/100.

From your previous post you consider H/500 acceptable.

Code reference is required for approval.

Then you have narrowed down possible answers to two choices by the process of elimination:

Based on reference cited in the Structure Magazine article, H/400 or H/500

Use H/500, since that is the value you considered acceptable.

http://www.SlideRuleEra.net

 

[human909]

I have to depend on codal preferences as authority will ask me to prove myself through some codal reference. I know basic sense should be applied but no one accepts your own beliefs when you are submitting for approvals.

The get around this by declaring the design criteria. AKA if you declare that the design criteria is H/250 and show that the building achieves H/250 then that could be a good way of getting you over the line. Generally the 'authorities' whoever they might be know a lot less about the codes than you do. If they knew as much as you do then they'd likely be more aware about what is an engineers judgment vs a code requirement.

As structural engineers many of us expect a recipe for everything. Partly because we generally do find that there is a recipe for almost everything and that recipe is conservative. So we become used to it. Deflection control sometimes put ourselves in unfamiliar territory as it is often a CHOICE we have to make based on our JUDGMENT.

I'd suggest the H/500 is quite a high value for a portal frame at ultimate wind loads let alone serviceability wind loads.

Regarding the masonry walls it depends on your detailing. I wouldn't be concerned about a properly restrained wall deflecting a decent amount in its minor axis. If the building was trying to deflect it along the length of the wall then sure expect cracking or other concerns due to the stiffness contrast between your building and the wall.

 

[MSUK90]

Thanks a lot for your inputs SlideRuleEra and human909.

I wouldn't be concerned about a properly restrained wall deflecting a decent amount in its minor axis

Why is it so?