Footing Moments

[EngTipper.]

I have some questions for moments on spread footings:

1- Lets say overturning moment is present, but the dead load (including footing and soil self weights) is large enough such that there is always compression on the soil below the footing for the entire footing area. Putting the net pressure diagram together would clearly show the bottom of the footing always in tension, and therefore top steel is not required. But would top steel be required at all to transfer the moment into the rest of the footing? Or is the assumption to just put the net bearing pressure on the footing all at once sufficient?

2- In the other case, M/S>P/A and one end the footing does not induce downward pressure at one end of the footing (ill call this negative pressure). In this case id only use the actual bearing area of the footing when finding if the bearing capacity is exceeded. In this case, there is a chance that one side of the footing may go into negative flexure (i.e. require top steel) if the negative pressure is large enough relative to the positive pressure on that side of the column.


Is it correct to say that the only time you'd need top steel due to footing moments (not uplift!) is in option 2? And that option 1 will never need top steel if there is never any heel uplift? I know some people in the office who put top steel right when there is any moment, even if M/S<P/A, which I'm thinking is unnecessary.

Cheers!

 

[CJLCivilStruct]

Is this a design for a retaining wall? Taking a section cut through your 'critical point' and taking moments looking left or right will get your answer

Happy to discuss further if there's a sketch to look at, I may (probably) have got the wrong end of the stick!

 

[DaveAtkins]

The only time you need top steel in a spread footing is when the top of the footing is in tension (as you already stated). This can only occur when there is load pushing down on the top of the footing, or when the selfweight of the footing causes tension in the top of the footing. A bending moment transferred to the footing in and of itself will not cause tension in the top of the footing. This is true even when there is a triangular bearing pressure below the footing (although in that case the selfweight of the footing may cause enough tension in the top of the footing to be a concern).

DaveAtkins

 

[milkshakelake]

Is it correct to say that the only time you'd need top steel due to footing moments (not uplift!) is in option 2?

There are other ways to induce negative moment on a spread footing, like having two axially loaded columns in a combined footing, even with zero moment at the column base. So strictly speaking, what you're saying is not correct. But I think it's correct based on what I think you're talking about, which is like a wall footing with fixity at the bottom. I have some other thoughts on this, but it's just shooting from the hip and imagining what you're talking about. If you provide a sketch, I might give a better answer.

 

[BAretired]

The simple rule to follow is the one DaveAtkins stated, namely provide top reinforcement when the top of the footing is in tension.

 

[canwesteng]

If there is a triangular distribution of stress in the soil below the footing, and there is soil above the footing itself, it's more likely that there is tension in the top of the footing compared to the case of footing under selfweight alone. Since this a more involved calc, an experienced engineer might just put the top steel in anyway rather than do the calcs. I see you are in Ontario, so this is probably what is going on. Also consider for a pier on a spread footing, a portion of the unbalanced moment can be transferred to the footing as moment instead of shear (shall be in A23, not sure if you can transfer it all as shear in ACI 318).

 

[hokie66]

If the soil UDL is greater than the bearing pressure, you are in a world of trouble.

 

[DaveAtkins]

If the soil UDL is greater than the bearing pressure, you are in a world of trouble.

I do not agree. When the bearing pressure below the footing is trapezoidal or triangular, the weight of soil above plus selfweight of footing will almost always be greater than the bearing presssure, thus causing tension in the top of the footing.

DaveAtkins