Footing embedment depth

[whitley85]

Most current building codes state that footing should be embedded a minimum of 12 inches below the lowest adjacent grade, or below frost depth. We don't have any issues with frost depth where I'm at, but we always recommend that footings be embedded at least 18 inches, not just 12 inches. And we are not the only ones, all the local geotechs say the same thing. So, my question is, why? If code says 12 inches is ok, why go to 18 inches? Any thoughts? Thanks!

 

[MA_PE]

Most current building codes state that footing should be embedded a minimum of 12 inches below the lowest adjacent grade, or below frost depth. We don't have any issues with frost depth where I'm at, but we always recommend that footings be embedded at least 18 inches, not just 12 inches. And we are not the only ones, all the local geotechs say the same thing. So, my question is, why? If code says 12 inches is ok, why go to 18 inches? Any thoughts? Thanks!

Probably to allow for field construction variances and to provide a more conservative estimate of lateral resistance. Not to mention that a thicker concrete member allows for better cover of the reinforcement. Designing to meet code minimum allowables is not necessarily the best design practice, IMHO.

 

[Guest]

It's just a thought ...

Doesn't ACI also require that you have 3 inches of concrete cover that must be included from ground contact, which takes into consideration irregularities in the excavation and corrosion factors? If this is so, and you include other 'extras' like size of rebar and factor of safety, I can see why 18" minimum would be considered.

FWIW - I have always seen 18" as minimum embeddment depth as well.

JR

 

[whitley85]

I agree, codes minimums are not always the best idea. We are going to continue to recommend 18 inches, but we have a client who read the code and wants to know why we want him to do more than is required. It's one of those things that I believe that I'm right, I'm just having a hard time verbalizing why.