Cantilevered retaining wall - footing design

[pwht1]

Hey, sorry if this is been posted before I did a quick search and couldn't find anything.

Where does the idea come from that a footing is considered structurally safe when the resultant of the moments is within the central third of the footing? Can anyone recommend a decent text/paper that covers the issue?

Thanks,
Paul

 

[msquared48]

That "requirement" although it is technically not one, just makes sure that the entire footing area is seeing a soil pressure at the bottom. It is zero at one extreme (toe or heel), and some maximum at the other (Heel or toe).

It is not structurally unsafe to design the wall with the kern outside the middle third, it's just that the soil is likely to be overstressed due to the leveraging action that ensues, and the heel is likely to see tension at the top, but none at the bottom, so the reinforcing pattern is a little different. The wall is just less efficient. Sometoimes, due to site restraints, this is the only solution. Nevertheless, it can work.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[weab]

pwht1,

I fully agree with Mike. Like you, I tried to find this after I graduated and was designing my first retaining wall.

I would only add that when your resultant is outside of the kern and if actual loads ever exceed service design loads, maximum bearing pressures can increase dramatically. In the end, try to keep resultant in middle third if possible.

 

[pwht1]

Toooo easy, thanks.