Restrained Concrete Retaining Wall Diaphragm Force Transfer

[bbjafa]

I'm designing a 10' restrained basement retaining wall. The wall will be restrained by a 16" concrete slab at grade level. The building shear walls extend down to the basement level. I'm a little confused on the transfer of the soil pressures into the diaphragm as I haven't dealt with this kind of problem before. What diaphragm checks are required? For the static soil pressure, would you ensure the slab dowels into the wall can transfer the force into the diaphragm and that's it? Are there any additional checks required (for example the axial capacity of the diaphragm)? Then for the seismic load from the soil, again check the dowels and ensure the diaphragm has enough capacity to transfer the load into the shear walls? If someone can provide me with typical design steps for this scenario I would greatly appreciate it.

 

[KootK]

1) The basement wall will dump load into the slab that may increase the shear demand on your slab, depending on your situation. Often this load is opposed at the other side of the building.

2) With a 16" floor slab, the load from the basement wall shouldn't create enough axial stress in the slab that it warrants explicit attention.

3) The big issue is usually the connection between the slab and basement wall for our of plane wall shear (assuming that the slab is poured over top of the walls). Some options:

a) My preferred method is to use field bent dowel bars from the wall into the slab and then to call that a shear friction joint. This favors the use of fairly small bars, usually #5 for me.

b) Run the inside face wall bars up into the slab for a dowel connection.

c) Case the walls a little higher than the slab soffit so that you have something of a bearing connection. I don't use this method as a stand alone connection.

 

[bbjafa]

Thanks, but what happens to the load after it's transferred into the diaphragm? Would I need to check the shear walls in the basement level for the additional load (Static soil and seismic soil).

 

[jayrod12]

Potentially not if you have opposing soil on the opposite side, which is what KootK was alluding to on his point 1).

You'd have cancelling forces applied on opposite ends so the shear walls in the basement wouldn't see the additional load per se. However, if there are large openings adjacent the basement walls, or something else that would cause the diaphragm to not be able to transfer the loads clear across to the opposite side, then you may need to dump it into the basement shear walls.

 

[bbjafa]

There are soils on all sides of the building. I can see how static soil pressures would cancel out from each side but what about seismic, since seismic loads wouldn't act at opposite directions at the same time.

 

[jayrod12]

Good question, I'll leave that to the seismic guys since I don't do any seismic design.

However, in my limited understanding of seismic design, if the building is moving due to the ground shaking, wouldn't the soil be moving the same as the building and therefore not imparting any additional load to the system than it was prior to the shaking beginning?

 

[jittles]

jayrod12 brings up a question that many a structural engineer has asked - what's the deal with seismic soil loads? How real are they...

In any case, seismic need not be resisted by seismic. Seismic soil pressure + static soil pressure can be resisted by passive soil pressure on the other side.

Given that passive pressure is typically 5x larger or more than static and seismic pressures, you typically have ample resisting capacity available on the other side.