Special Seismic Walls - Midzones
Special Seismic Walls - Midzones
(OP)
When using a stress based approach of requiring boundary zones (stress > 0.2 f'c) , what is the requirement for providing confinement steel in the zone between the two boundary elements, if stress exceeds 0.2 f'c in the middle of the wall?
The boundary element has been sized such that the entire axial force due to gravity and overturning moment couple is carried by length of wall at each end, which is confined with transverse reinforcement. The web region between Boundary elements only has minimum steel 0.25%. I have a peer reviewer who's saying that this minimum vertical reinforcement should be confined as well.
I am using the Indian code, which is based on the older versions of ACI, prior to the displacement based approach for boundary elements.
I will post a sketch later if my description isn't quite clear.
The boundary element has been sized such that the entire axial force due to gravity and overturning moment couple is carried by length of wall at each end, which is confined with transverse reinforcement. The web region between Boundary elements only has minimum steel 0.25%. I have a peer reviewer who's saying that this minimum vertical reinforcement should be confined as well.
I am using the Indian code, which is based on the older versions of ACI, prior to the displacement based approach for boundary elements.
I will post a sketch later if my description isn't quite clear.






RE: Special Seismic Walls - Midzones
RE: Special Seismic Walls - Midzones
I'm curious about one of the details of your approach. If I understand correctly, you've built a column at the end of your wall capable of carrying all of the gravity load on the shear wall in addition to the compression force from overturning. That sounds safe and logical to me. Is it necessary by code though? I've never been clear on that. It seems somewhat analogous to tying the compression zone of a beam which is generally not required.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Special Seismic Walls - Midzones
In that case, if the zone of stress>0.2 f'c extends beyond the edges of the confined boundary element, should those zones be confined as well?
The old UBC had limits on how far the boundary element needed to extend depending on the axial forces in the walls. In some cases, if the wall had axial load beyond a threshold, that wall could not be counted for lateral stiffness.
RE: Special Seismic Walls - Midzones
The check is performed in M/Sx fashion assuming that the section is elastic and uncracked. Once your wall cracks and starts developing a compression block, the stress distribution won't resemble the M/Sx business much at all. The maximum stress will probably be much greater than 0.2 f'c and it will be concentrated over a relatively small length of the wall.
I view the 0.2 f'c limit as simply a rough index. At that level of stress, we're expecting the compression zone to have taken a serious excursion into inelastic territory, justifying the confinement reinforcing. I believe that the check was derived with a particular R value in mind although I can't remember the details.
In short, since 0.2 f'c doesn't represent an actual state of stress in your wall, I don't think that it is sensible to interpret the provision as requiring confinement everywhere that M/Sx exceeds that value.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Special Seismic Walls - Midzones
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Special Seismic Walls - Midzones
Outside of this horizontal region I do not believe you need boundary confinement.
RE: Special Seismic Walls - Midzones
RE: Special Seismic Walls - Midzones
I think your plan checker is misinterpreting the provisions.