Ignoring Non-Orthogonal Concrete Shear Walls

[bumbler]

Hello all,
We're having a discussion in our office about the use of non-orthogonal concrete shear walls. The structure is a 6-on-1 wood on PT podium slab. Concrete shear walls support the PT podium slab. We have some disagreement about how to treat the concrete walls in the corner of the slab shown in the attached sketch. We are modeling this as a rigid diaphragm in RISA Floor/RISA 3D. The structure is in a high seismic area (Design Category D).

The three arguments are as follows:
a) Ignore the non-orthogonal concrete walls for lateral analysis because they are difficult to analyze and we don't need them for lateral strength. Design the walls only for gravity. Detail with the same non-slip connections used for the concrete shear walls. Provide minimum vertical and horizontal reinforcement in the non-orthogonal walls.
b) Design the non-orthogonal concrete walls as shear walls. We are detailing the non-orthogonal walls with the same non-slip connections as the concrete shear walls so they will attract load no matter what we put in the program. The building doesn't know we told RISA this is actually just a gravity wall if we don't detail for that.
c) Design the non-orthogonal walls as gravity walls only. Detail a slip connection at the slab to allow for seismic deflection.

I am on the side of arguments (b) and (c). The other two (senior) engineers are arguing for (a). I was told by one of the older engineers that a slip connection for the top of concrete walls is prohibited by ASCE 7 because concrete walls are required to be braced, but I can't find the reference and the engineer couldn't produce it so I'm not sure if that is a real requirement or not. Anybody have any thoughts here? I'm decades behind the other engineers in experience but just turning it off in the program doesn't feel good to me.

 

[EZBuilding]

If you were trying to do this by hand - I could see the argument for foregoing the contributions of those walls. But with current analytical tools - I don't see too much difference between analysis a orthogonal or a non-orthogonal wall. Additionally, the non orthogonal wall will likely be acting composite with your orthogonal walls - greatly increasing it's stiffness and capacity...

 

[Trenno]

Ignoring stiff concrete shear walls may underestimate your natural period and thus your seismic loads. The earthquake cares not for your simplistic assumptions, those walls will see lateral load, especially being on the perimeter of the diaphragm.

Wouldn't you rather know what that load is? Surely it's not that much more effort if you're using an analysis program.

 

[driftLimiter]

I'm also not sure about this slip connection being prohibited. I have used slip connections that have dowels extending into the slab but embeded in foam, this combined with heavy building felt on the top of the wall to uncouple walls from the diaphragm.

I agree with the others' reply that if your not going to slip the wall then you aught to consider its effect on the building because A) It does effect the seismic response and B) it isn't that hard to consider the effects of a rotated wall, even by hand this is not that hard. The wall has a stiffness contribution in both orthogonal directions that can be computed and input into a rigid diaphragm analysis.

 

[ggcdn]

Including the walls will trigger cl. 12.5.3.1 which requires designing for E = 100Ex +/- 30Ey and E = 100Ey +/- 30Ex for non-orthogonal systems.

Personally, I would try either replacing the non-orthogonal walls with a column or two, or including them and analyzing and detailing them accordingly as part of the LFRS.





-JA
try [link calcs.app]Calcs.app[/url] and let me know what you think