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Cantilever carport footing size 2

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Gus14

Gus14

Civil/Environmental
Mar 21, 2020
186
I designed a steel cantilever carport out of curiosity as shown on the attached plans. When designing for negative wind load pressures, the footing size is 8'x5.5'x18".
Do you have any tips or ideas to reduce the footing size ?

The plan view shows the pressures I found from the ASCE acting on the roof.


The attached elevation plan shows a summary of the calculation I made.

Note, the wind pressures were obtained at 10 degree angle roof inclination, I just did not show it on the elevation plan as the angle is relatively small.
 
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Some questions... Do you have snow load? The columns might be rotated 90deg for stiffness in the other direction or are they part of a frame structure? Framing seems heavy for the spans and the deck span of 7.5' feet is large. How is the lateral taken care of? Is the carport secured to the house at the side opposite the columns? If regular 1-1/2" 22 GA deck this would be flimsy to work on. Do you have any roofing over it? If not 3psf, may be a little light and maybe 5psf maybe a little more appropriate. [ponder]

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik
 
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There is no need to center the footing under the column. You can reduce footing size as shown below.

Capture_ikkbdl.gif


BA
 
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Thank you dik for replying, I don't have a snow load. Regarding the columns, I did a poor elevations job, they should be as you mentioned and as shown on the plan view.

The carport is ideally in an open area and it will be an open building so no significant lateral load will affect the structure and the columns will be fixed with footing.

dik said:
If regular 1-1/2" 22 GA deck this would be flimsy to work on. Do you have any roofing over it? If not 3psf, may be a little light and maybe 5psf maybe a little more appropriate
Click to expand...
You are right a (5-7) psf for the steel deck and the secondary beams would be more realistic, which would reduce the overturning moment significantly. I was just trying to be conservative with the deck dead load value.

note, I did not show the secondary beams on the plan and the dotted line refers to the area with different wind pressures and not the secondary beams.
 
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Thank you BAretired for replying, very interesting suggestion you mean something similar to the silver man scheme.

But I disagree with the direction since the uplift case is controlling I should move to the left.
Then, I could check for the case when there is positive wind pressure and move it to the right and slowly find the effective balance to size the footing.

TRIAL_onyh5r.png
 
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Thank you rowingengineer for replying. I did not know about this type of footings before. This really is ideal for cantilever carports.
 
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Gus14 said:
But I disagree with the direction since the uplift case is controlling I should move to the left.
Click to expand...

Yes. I should have noticed you were in Kuwait. I was thinking of snow load controlling.

BA
 
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Are your soils cohesive or non-cohesive? Might have an influence on friction piles. As far as the deck is concerned, 1-1/2" deck should not be used over 7' and preferably less.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik
 
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BART... everyone's not as fortunate as we are. [pipe]

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik
 
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BAretired said:
Yes. I should have noticed you were in Kuwait. I was thinking of snow load controlling.
Click to expand...

Thank god. I can't stand cold weather. [smile]

dik said:
Are your soils cohesive or non-cohesive? Might have an influence on friction piles.
Click to expand...
The soil is non-cohesive. How much influence does that have ? Which is better ?

 
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dik said:
BART... everyone's not as fortunate as we are.
Click to expand...

Yeah, pretty hard to ski on sand. But I imagine that windblown sand could create a fairly significant live load on a roof if sheltered by a higher building.



BA
 
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For a carport such as this, I would typically use Caissons. Caissons are great at providing overturning resistance (for cantilever columns) as well as the depth allows for the use of skin friction to help resist uplift.
 
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If non-cohesive, installing friction piles may not be possible... even sleeving them can be problematic.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik
 
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I've not designed for sand, but I've designed for accumulated 'stuff' on ducts, cable trays, etc. for mining 'dust' debris accumulation... sometimes in excess of 100 psf.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik
 
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Aesur said:
Caissons are great at providing overturning resistance (for cantilever columns) as well as the depth allows for the use of skin friction to help resist uplift.
Click to expand...
Thank you Aesur for replying, yes I found them very efficient.

dik said:
If non-cohesive, installing friction piles may not be possible... even sleeving them can be problematic.
Click to expand...

I don't think you're talking about it from a design standpoint as it's better to have granular soil.

If you're talking about construct-ability, then I see how that can problematic. But still it's very economical alternative to present for the contractor ( they sometimes surprise me with what they can do )
 
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