basic foundation overturning question 1

[delagina]

been using too much software. i forgot the basic lol.

if i dont have horizontal force and only unbalanced vertical force, is there overturning moment for stability?

i'm confused because if i do summation of moment at the edge of footing, all forces are righting moment and no overturning.

but there is overturning due to unbalanced vertical force at the center.

 

[dcarr82775]

I don't think there is any overturning moment. If you are doing some kind of combined footing to you would need to check the bearing pressure for the eccentricity, but your sketch doesn't imply that. Even then there wouldn't be any overturning moment

 

[ToadJones]

take the unbalanced force and convert it to a moment.
Then go form there.

 

[hokie66]

Without a horizontal force, you don't have an "overturning moment", but you still have unbalanced forces which have to be resisted. Actually, it just depends on the point about which the moments are taken. If you take the moments, e.g., about the other end of the footing, you could say the downward forces cause "overturning".

 

[BAretired]

Am overturning moment can be generated by gravity loads only if the loads are outside the edge of the footing. Gravity forces inside the width of the footing can produce high bearing pressure but not overturning.

BA

 

[faisal45]

if footing depth is sufficient and foundation is not eccentric then there is no possibility of overturning. but if you are thinking that there is a chance of overturning then go for combined footing. just check the eccentricity in that case.

Thank you.

Regards
Engr. Faisal

 

[paddingtongreen]

If the resultant of the total of the loads is inside the middle third of the foundation, there will be pressure under the whole thing, if not, part will not be under pressure.

The center of resistance, will be exactly under the resultant load and will be higher at one end than the other.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Teguci]

The summation of the forces about a point must equal 0 otherwise you have a problem. In this case you have a moment due to the applied loads, but the soil bearing will couteract that moment with a triangular pressure 7.5 ft wide and 2.67 k/ft at the right side plus a footing weight / width pressure.

 

[msquared48]

To me, an "Overturning Moment" is any moment causing a potential for instability in the structure that has to be countered by a corresponding "Resisting Moment".

That would include moments due to lateral forces, but also, any moments causing anything but uniform soil pressures on the foundation pad.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[DST148]

@delagina: Overturning of the footing could be due to horizontal load, eccentricity of the gravity loads or a combination of both.
For simplicity let us ignore the weight of the footing.
R = 10 Kips. eccentricity w.r.t. c.g. of footing = 2'-6".
Overturning moment about the base = 25 kip.ft.
Restoring moment about the toe = 10 * 5 = 50 kip.ft.
F.O.S. against overturning = R.M. / O.M. = 50 / 25 = 2
Eccentricity lies outside the kern of the section. 75% of the footing is effective. The c.g. of soil pressure coincides with the 10 kips load.

 

[faisal45]

Dear DST148,
can you suggest me any book reference or link of your calculation please?

Thank you.

 

[JoshPlumSE]

This doesn't seem like that complex of a question to me. I would convert this to an equivalent load condition (axial force + moment at centroid of the footing).

I believe this is what DST148 has done in his calculation.

 

[delagina]

@dst148, joshplum,

thats the confusion. some engineers dont consider unbalanced vertical force to be included in FS against OT.

 

[DST148]

@delagina - If the subject footing was only 7 feet wide, R.M. would be 35 kip.ft and F.O.S. against overturning would be 1.4 and only 43% of the footing effective. You can look at it in different ways but still the effect of unbalanced loading has to be taken care of.
On few occasions (in low seismic zones) we had truly isolated footing not connected to the main structure. In such rare cases we designed the footing for axial load + moment due to horizontal load of about 2.5% of axial load applied at the top of the pier, even though the axial load on the pier was concentric.

 

[JennyNakamura]

There is no overturning. The only way that thing is going to overturn around the point of consideration is if the left side of the footing magically lifts itself off the ground.

This is a bearing pressure issue: P/A + M/S.

 

[delagina]

i have staad foundation software. it considers unbalanced vertical forces in F.S OverTurning calculation, same as poster dst148.

not saying staad foundation is the authority for this though. but staad is a respectable software.