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Carport Footings
3

Carport Footings

Carport Footings

(OP)
I have a pretty large residential carport that is to be flagpoled off six 7x7 tube steel columns. My overturning moment at each column is approx. 12 ft-k (service).
The foundation will be isolated spread footings. To get a factor of safety of 1.5 for overturning, I need huge footings on the order of 5'-9" sq. x 16" deep. If I go deeper, to keep the excavation footprint smaller, at what point can I reasonably take into account some passive pressure on the sides of the footing to help resist the overturning?

Thanks in advance.

RE: Carport Footings

I am not sure passive pressure will help much.

Take advantage of the soil overburden, and make the footings as big as necessary. The size you give does not seem too excessive to me.

DaveAtkins

RE: Carport Footings

I'd be okay using it at any depth but I'd base it on a conservative estimate of at rest pressure of the adjacent material (perhaps backfill). The development of passive pressure implies real lateral movement and I don't see how one can guarantee that sliding would occur prior to overturning.

Frankly, I've always been hesitant to rely on any kind of earth pressure to resist overturning. I've seen enough theoretically impossible vertical soil cuts that I have a hard time convincing myself that lateral earth pressure development is reliable for situations where the failure mode is anything other than plowing laterally through the earth.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Carport Footings

(OP)
Good advice - thanks

RE: Carport Footings

Do you know what kind of soil will be under the footings?

Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com

RE: Carport Footings

And, make sure you're not trying to use the 1.5 overturning factor of safety at the same time you have your 0.6*DL load combinations. That would be redundant as discussed previously on this forum....

RE: Carport Footings

My concession to reasonableness/cowboy engineering will be to look at it using a rectangular soil stress distribution out at the toe, kinda like concrete in flexure. You know the drill: your cheats questionable but mine's legit!

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Carport Footings

XR250 gets a star for noting the load is a service load. Thank you, that is the first question that is always asked.

RE: Carport Footings

Why don't you design a network of grade beams to help with the overturning?

Mike McCann, PE, SE (WA)


RE: Carport Footings

What about combined overturning and uplift? Open structures have a pretty sizable uplift pressure... at least before you make it smaller. Although I assume you are set down for frost and the soil weight may be enough.

RE: Carport Footings

I like the suggestion of msquared48. Footings do not need to be square.

BA

RE: Carport Footings

(OP)
I want to keep the footings square as I am sort of counting on the carport to help brace the house that it is attached to (long, tall skinny carport projecting from a perpendicular long, tall skinny house) I do have to add the uplift as well.
Thanks for everyone's advice

RE: Carport Footings

It seems to me that a grade beam on a strip footing all around the rectangular area and a cross beam at the two middle columns would provide ample resistance to overturning in both directions with something left over for the skinny house. But of course, I have not seen the plan so there may be factors I am not considering.

BA

RE: Carport Footings

(OP)
That would certainly work, but I am into less concrete by doing the isolated big footings
The carport is 36' long x 24' wide w/ 12' tall columns and a 12:12 roof
It is pretty grand w/ timber trusses @ 12' O.C. supporting the ridge beam
Thanks

RE: Carport Footings

For columns with moment in one direction, I generally use rectangular footings with an approx. length to width ratio of 2 to 1.

RE: Carport Footings

Agree with Mike and BAretired....look at alternatives to conventional footings. Another consideration is relatively short piers that help you get the overturning resistance you need.

Another consideration is to put the footing lower as you noted and take into account the overburden soil (100+ pcf) and a slab on top of all that. Helps tremendously. Passive/active pressure won't help you.

RE: Carport Footings

(OP)
@Ron:

The contractor would prefer conventional spread footings. I prefer not to make the footing deeper than just below slab level or else the base plate and anchor bolts are going to buried (not a fan of that). It is only six footings and concrete is cheap compared to other, more labor intensive alternatives.

thanks

RE: Carport Footings

I imagine then a 'dead man' type footing that is fairly thick could be the way to go. you get the added weight for uplift, and it may marginally help against overturning. if it is thick enough it could be unreinforced, and if soils permit completely trench formed.

RE: Carport Footings

Your footing size doesn't sound huge to me. Like you said, concrete is cheap. Make them 6ft square if you need to and call it a day.

RE: Carport Footings

In my area freestanding carports usually just have knee bracing at each column. That way the moment at the footing is zero and the footing can be designed for uplift only. The only time I account for moment at the footing is if there are brick piers in which case there can't be knee bracing. Just my 2c

RE: Carport Footings

(OP)

Quote (nonplussed)

In my area freestanding carports usually just have knee bracing at each column. That way the moment at the footing is zero and the footing can be designed for uplift only. The only time I account for moment at the footing is if there are brick piers in which case there can't be knee bracing. Just my 2c

Anytime I have looked at kneebraces, the connection loads were so high at the ends of the brace and the beam to column connection that it was nearly impossible to make the thing work nicely. I guess if the carport is small with a low pitched roof, it may make sense. This carport is way too big for kneebraces.

RE: Carport Footings

I agree with CANPRO; your footing doesn't seem that large to me, especially since the carport is so large. Wouldn't a bunch of grade/tie beams be more expensive with extra forming and concrete/rebar? I'd just make them 6 ft square and forget about it.

RE: Carport Footings

(OP)
Sounds good

RE: Carport Footings

XR250 :
I like the idea of bracing the house with the carport. That’s a twofer, you design the lateral support system for the building and you get a carport for free. What a deal. How did the house stand there before you thought of putting up a carport? Lateral bracing is supposed to resist some lateral load, at some height, without too much lateral deflection (parallelograming of everything else). Are you sure your 12' flagpoles and 12/12 pitch roof trusses, on spread footings will do that?

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