AASHTO LRFD Long or Short Footing Heel 1

Figure C3.11.5.3-1a is the top figure shown on page 3-65 (LRFD 2nd Ed.).

Hope this helps.

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I don't have that manual. Only looking for a basic explanation between the failure surface criteria and when each occurs.

There doesn't seem to be a written definition, at least I can't find one. Looking at the figures, it appears that a long heel is such that the failure shear zone is not interupted by the stem. That is, if you begin at the end of the heel and draw a line at an angle to the vertical equal to 0.5*(90+beta-phi-epsilon), that line doesn't touch the stem. beta is the angle of the backfill to the horizontal; phi is the angle of internal friction of the backfill; epsilon is the angle whose sin is sin(beta)/sin(phi).