Foundations for shear walls

[Okiryu]

Just looking for input from the SEs here: if I have a building which the structural system is composed of exterior shear walls with columns and interior columns, do the exterior shear walls require their own foundations (i.e wall footings) or can they be supported on grade beams which will transfer the loads to the column’s foundations?

I expect the building to have a SDC of E.

Thanks!

 

[CURVEB]

I'm not sure it matters, but what is the material for the shear walls? In general, I am not aware of any limitations on using grade beams to transfer the loads to pad footings at the columns, assuming you've accounted for a complete load path, connections between elements, adequate development of the reinforcement, etc.

What is the mechanism for transferring the shear into the soil? Are you relying on friction or passive earth pressure?

 

[Okiryu]

CURVEB, thanks for the reply. The wall will be RC and the plan is to use both, the contribution of friction and PEP for lateral resistance.

 

[CURVEB]

If the ends of the walls aren't over the pad footings, I think you might be putting a lot of shear into the grade beam. Often we will try to put drilled piers (or footings in your case) directly under the ends of the walls because of this. In SDC E you might need to consider this a discontinuity in the vertical load path, and that may require you to design the grade beam for overstrength. Definitely something to check on.

 

[ggcdn]

Tangent question - Would anything really change if it was a wall footing tied into pad footings, vs grade beam tied into pad footings? Isn't it just semantics?

 

[BAretired]

What is an SDC of E? Is it too much trouble to type it out the first time?

BA

 

[CURVEB]

Maybe I'm assuming too much - but Seismic Design Category, correct OP?

 

[Okiryu]

Sorry BA, yes, it is seismic Design Category

 

[CURVEB]

Sonof: The difference in my mind is that the soil pressure due to overturning would be more of an axial force at the end of the wall that transfers into the footing. I would consider the grade beam to be a clear-spanning element that doesn't rely on the soil for support. That said, depending on the force distribution you could certainly still have a shear issue in a wall footing/pad footing setup. I think you'd have to consider the specific scenario as it really depends on the geometry and forces.

 

[BAretired]

Thanks for the clarification.

BA

 

[Okiryu]

Thanks for your responses. CURVEB, yes, large shear forces are expected at the end of the walls. I will need to talk with the structural engineer and coordinate this. Thanks again !