Stability of free standing cantilever wall with eccentric footing(safety against overturning) 1

[precast123]

Hi

In calculating sum of overturning moment, apart from wind load or soil back fill load, should I have to consider Moment due to eccentricity of foundation? Please see attachment.

I think Mw + Nxe I should consider when calculating stresses under foundation.

Mw = Moment due to wind load
N = Weight of Wall
e = eccentricity of wall (distance from center of footing to center of wall)

Thanks

 

[SlideRuleEra]

In calculating sum of overturning moment, apart from wind load or soil back fill load, should I have to consider Moment due to eccentricity of foundation?

Moment from weight of the wall, acting though it's center of gravity plus moment from the weigh of the foundation, acting through it's center of gravity, should be considered.

I don't understand the attached sketch... no soil loading is shown. I would expect wind loading only on the wall projecting above the soil, not full height of the wall, see marked up sketch below:

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[PEinc]

The eccentricity is already taken into account when you sum the moments about the toe of the footing and calculate the maximum (toe) and minimum (heel) bearing pressures.

http://www.PeirceEngineering.com

 

[precast123]

@SlideRuleEra , I was trying to create a general question.
Please see attachment for actual problem.
In calculating stability against overturing, i doubt I have to use 40.6kN moment due to eccentricity.

For bearing stress I am sure I have to use this 40.6kN because eccentricity will cause extra compressive stress which will be concentrated on one edge. Tensile stress i have to distribute and convert it to compressive stress .

 

[PEinc]

I have to admit that I am confused by your sketch. What holds the fence/wall onto the footing? Is there a moment connection between the fence/wall and the footing?

http://www.PeirceEngineering.com

 

[JoshPlumSE]

There are different way to approach this problem.
a) Calculate the total overturning moment (due to all loads and eccentricities). And, compare it to the total vertical load times 1/2 the footing width.
b) Calculate a stabilizing moment due to gravity loads for overturning about the toe. Compare that to the overturning wind moment in the other direction about that same toe.

For simplicity, most textbooks show method a). And, I think that's what most of us learned in school. Personally, I prefer method b) because it gives me a true overturning safety factor for wind moments. Though either way gives you the same soil bearing values.

 

[PEinc]

Method A might give a "close" answer but Method B is more correct.

http://www.PeirceEngineering.com

 

[IDS]

Josh's method b) seems to me to be both the simplest and the least likely to give the wrong answer (through ignoring some component of the overturning moment).

For the specific example, clearly the wall will blow over with a lower wind speed with wind from one direction than the other, so yes, the eccentricity of the wall should certainly be taken into account.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[SlideRuleEra]

Agree with PEinc about the apparent lack of a moment connection between the wall and footing. Without a suitable moment connection there overturning cals are worthless.

I am sure I have to use this 40.6kN because eccentricity will cause extra compressive stress which will be concentrated on one edge.

Take another look at that, with an open mind. Forces W1, W2, and W4 were accounted for twice in the overturning calcs (you can not do that...see my comments below)

Your concern about "stresses under the foundation" is another problem. Traditional overturning calcs assume the subgrade (what I call "Good Soil" in my sketch below) will provide ALL needed support to the foundation. If the soil is "Bad", traditional overturning calcs are not valid.

For "Bad" soil, the assumed point of rotation for overturning is shifted away from the toe. Exactly how much it is shifted will take detailed knowledge of soil properties and/or some really insightful (1930's style) engineering judgement:

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net