Footing Stability Ratio
Footing Stability Ratio
(OP)
If a soil supported mat supports 4 piers with 2 of the piers subjected to compressive loads and the other 2 piers subjected to uplift loads for a certain load combination, what is the correct way to calculate stability ratio about an edge of the mat?
1. Would the uplift loads contribute to the overturning moment about an edge of the footing or is it just the applied shears and moments on the piers that contribute to overturning?
2. In continuation of 1 above, in calculating resisting moment, is it appropriate to algebraically add all the vertical loads on the piers (both compressive and uplift), add the weight of the piers, footing and the soil and use this resultant load as contributing towards resisting?
3. In general, SR is defined as Mr/Mo where Mr is the resisting moment and Mo is the overturning moment. Would SR = L/2e be correct under all circumstances and equate to SR calculated using Mr/Mo?
Thank you
1. Would the uplift loads contribute to the overturning moment about an edge of the footing or is it just the applied shears and moments on the piers that contribute to overturning?
2. In continuation of 1 above, in calculating resisting moment, is it appropriate to algebraically add all the vertical loads on the piers (both compressive and uplift), add the weight of the piers, footing and the soil and use this resultant load as contributing towards resisting?
3. In general, SR is defined as Mr/Mo where Mr is the resisting moment and Mo is the overturning moment. Would SR = L/2e be correct under all circumstances and equate to SR calculated using Mr/Mo?
Thank you






RE: Footing Stability Ratio
DaveAtkins
RE: Footing Stability Ratio
Is that true? ACI 336.2R states that overturning must be considered for combined footings and mats.
RE: Footing Stability Ratio
RE: Footing Stability Ratio
RE: Footing Stability Ratio
RE: Footing Stability Ratio
RE: Footing Stability Ratio
RE: Footing Stability Ratio
Thank you all for your responses.
RE: Footing Stability Ratio
RE: Footing Stability Ratio
RE: Footing Stability Ratio
Definitely the uplift would contribute.
I'll include any resisting load that I'm confident will be present during whatever environment will create the overturning tendency. I'll take foundation weight, piers, columns, structural walls, soil, slab on grade... I won't include live load, snow load, equipment, or architectural finishes with the rare exception of where those things are actually the cause of the overturning.
The e<L/2 business should still apply in principle if you do it like this:
1) e must include all load effects.
2) apply your load factors as discussed above.
3) instead of e<L/2 just determine whether or not your aggregate eccentric load still lands on the footing.
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: Footing Stability Ratio
The attached pdf shows 2 different methods of calculating overturning moments and resisting moments for the same foundation loads and geometry. It seems that both of these methods are being used. Which method is the correct way to calculate overturning moments?
RE: Footing Stability Ratio
I would not use either method. I would check the footing for the various applicable load combinations per code (you don't show what load combination has yielded the P loads). As I said before, if the footing is not overturning assuming 0.6DL, then stability is satisfied. But the load case with 0.6DL may or may not give you the highest bearing pressure.
DaveAtkins
RE: Footing Stability Ratio
RE: Footing Stability Ratio
DaveAtkins
RE: Footing Stability Ratio
My previous post explained how the safety factor is included in the combination.