Wood Shear Walls with large Overturning Moments on Continuous Footings

[swooneyWOODSTRUCT]

Hello,

Whenever I have a large overturning moment on a continuous footing due to a wood-framed shear wall above, my boss has me check the shear and moment under the footing to determine if it has the necessary capacity. If it doesn't, he says to design the footing as a "grade beam". I have never really understood the rationale behind the way he designs these footings despite him explaining it multiple times. In my opinion, it isn't a grade beam because it is continuously supported; it is a beam on an elastic support, correct? But I know he doesn't analyze it as such so he must use a simplified method. Does anyone know how to design for this condition without designing it as a beam on an elastic foundation?

Thank you

 

[Harbringer]

...it is continuously supported; it is a beam on an elastic support, correct?

Yes

But I know he doesn't analyze it as such so he must use a simplified method.

You should ask him for an example from one of his previous projects.

Generally I understand a grade beam to be a continuous footing that is spanning between piers without utilizing the soil below for support. I would guess he is checking the footing spanning between perpendicular continuous footings. Best way to know what he wants is to ask.

 

[KootK]

I'd ask your boss for a free body diagram of the grade beam solution. My best guess is that it's meant to use a stem wall as a non-soil supported beam spanning between pad footings at the ends of the wall or beyond. That arrangement can be used to increase the section modulus of the foundation.

 

[Celt83]

Beam on elastic foundation is probably the most appropriate approach.

I've seen a rigid continuous foundation approach as well, where the assumption is that the stem wall/shear wall act like a rigid element over the foundation so you can look at design sections at the faces of the stem wall only. I have a spreadsheet for the rigid footing approach that is only partially developed if you want to take a look you can find it here: Link

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[DaveAtkins]

I think the boss is using the term grade beam somewhat incorrectly. I bet he is looking at it as a long rectangular footing with a wall on top.

DaveAtkins