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.
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
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
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
RE: Carport Footings
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com
RE: Carport Footings
RE: Carport Footings
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
RE: Carport Footings
Mike McCann, PE, SE (WA)
RE: Carport Footings
RE: Carport Footings
BA
RE: Carport Footings
Thanks for everyone's advice
RE: Carport Footings
BA
RE: Carport 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
RE: Carport Footings
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
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
RE: Carport Footings
RE: Carport Footings
RE: Carport Footings
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
RE: Carport Footings
RE: Carport Footings
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?
RE: Carport Footings
The house is being built at the same time as the carport. It is a small 1 1/2 story guest house - barely bigger than the carport in footprint. The house has its own lateral stability system - just not quite enough. (see attached)