Carport Footings 3

[XR250]

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.

 

[DaveAtkins]

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

 

[KootK]

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.

 

[XR250]

Good advice - thanks

 

[TehMightyEngineer]

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

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[JoshPlumSE]

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....

 

[KootK]

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.

 

[BUGGAR]

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

 

[msquared48]

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

Mike McCann, PE, SE (WA)

 

[structSU10]

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.

 

[BAretired]

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

BA

 

[XR250]

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

 

[BAretired]

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

 

[XR250]

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

 

[jike]

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

 

[Ron]

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.

 

[XR250]

@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

 

[structSU10]

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.

 

[CANPRO]

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.

 

[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