Drilled Pier Supported Shear Wall

[tbone73]

There has been much debate in our office regarding the following scenario (see attachment). A shear wall supported on a grade beam supported by three drilled piers one at each end and one in the middle. The only loads resisting overturning are the uniform weight of the wall and concentrated beam loads at every level at mid length of the wall. The question is to what degree do the concentrated loads help resisting uplift in the outer drilled piers if the loads are centered over the center pier? Any thoughts are appreciated. Thanks

 

[ishvaaag]

As long as the vertical walls are concreted and reinforced to integrity, I would say that as efficiently as its value and position allow. If no continuity can be assumed, depending upon the values the horizontal joint (or joints) might open, what could be one indesirable condition.

 

[ron9876]

I would say P/A +/- M/S as long as the wall can be considered a rigid body. Then the wall has to be designed for the applied loads.

 

[dcarr82775]

The loads help resist uplift at the outer piers. The wall can't rotate over or lift without reacting against those load.

 

[Teguci]

I have wondered what the need is to place a drilled pier centered under the shear wall. To me it has always been redundant seeing as the overturning and lateral loads control.

 

[tbone73]

dcarr82775, I agree with you, but the question was, to what to degree do the loads help?



Teguci, in this particular instance it is required to resist the large lateral loads. Two piers are not enough.

 

[Teguci]

tbone - make sure you have a disucssion with the geotech regarding adjacent piers shadowing each other under lateral loads. Closely spaced piers in line with the lateral load do not activate as much soil as individual piers (ie they push against the same soil). In the past I have opted to increase the diameters of the end piers prior to adding the middle pier.

 

[ron9876]

Teguci I agree with the point about the center pier. If not needed to reduce compression (which proably means the loads are heavy) or horizontal loads I would try to span the wall between the outer piers.

tbone what I meant by my previous response is that the vertical loads will be distributed between all three piers and the moment will be resisted by the outer piers.

 

[tbone73]

Teguci, yes you are right and we have had a lot of discussions with geotechs regarding lateral resistance of piles in groups.

ron9876, I do agree with both your posts. But as far as the vertical load distribution to the three piers, what % each would get is the main question.

Thank you all for your continued input.

 

[ron9876]

If the wall is rigid and centered on the pile group the load will distribute equally to all three piers. That is what I meant with my first response. A=number of piles. M is moment about the centroid of the pile group. S is for the pile group.

 

[Teguci]

I seem to recall a limitation placed on beam loads placed on precast shear walls in high seismic zones but I can't put my finger on the reference. Short of that, the sum of the moments around an exterior drilled pier should include the dead loads from the IT beams as well as the uplift resistance of the other drilled piers. If there are still problems resisting uplift, spread the drilled piers further out or use a pump handle to an adjacent column (8'-0"+ deep x 6'-0" grade beams are not unheard of).

The contribution of the IT beams to the shear walls is why I like to place the shearwalls on the interior bays. For turning bays, openings in the shearwalls can help alleviate site line issues.

 

[tbone73]

ron9876, thank you, that's what I was looking for.

Teguci, I can tell we design similar structures. I agree and I also prefer to have the shear walls on the interior bays to get the extra dead load contribution. However we are always fighting the architects to get this.

Thanks again, I appreciate your valuable input.