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Special Seismic Walls - Midzones

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.

RE: Special Seismic Walls - Midzones

(OP)
Anyone? Is the question not very clear?

RE: Special Seismic Walls - Midzones

ACI provides some guidance on how far the boundary zones should extend, as I'm sure you know. I'm not sure that it's necessary to do anything beyond that. The 0.2f'c thing is pretty rough work anyhow since it's based on the gross section. Is this mid-wall vertical reinforcing surrounding an opening in the wall or something of that sort?

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

(OP)
Thanks KootK, I think the confusion may be stemming from the fact that the code says if stress > 0.2 f'c at the ends of the wall, then confinement must be provided.

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

It's important to recognize that, at ULS levels, 0.2 f'c likely does not represent the actual stress in your wall at any location.

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

I'll also add that the method that you described in your original post seems perfectly sound to me. Should the wall somehow become overloaded and buckle at an interior location, I expect that the load would simply make its way out to the capable, confined compression member that you've provided at the end of your wall. Sleep easy.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Special Seismic Walls - Midzones

It's been a while since I have done a concrete shearwall, and I'm not sure which version of the ACI you are using, but section 21.9.6.4 of aci 318-08 says the boundary element needs to extend a distance of c - o.1Lw or c/2.

Outside of this horizontal region I do not believe you need boundary confinement.

RE: Special Seismic Walls - Midzones

(OP)
I am actually using the Indian Code IS:456, whose equations are based prior to the days of the displacement based approach for boundary elements. ACI 318-99. I don't believe there was an equation that calculated the length of the boundary element back then. The BZ's were sized to carry the entire axial load and overturning moment couple.

RE: Special Seismic Walls - Midzones

In reading ACI 318-95 (closest I could find) I agree that there are no defined limits for the extent of the boundary element (aside from force requirements) Personally, if it was me doing the design I would make sure that I meet the requirements of both the code in effect and the newer ACI code (since 99 is so old). Therefore, I would extend the boundary element horizontally at least as far as required in the current aci. If that calcs out for the full compression forces I'd be done. If the method of aci 318-99 results in a larger area I would extend the boundary. However, I think just checking the 318-99 procedure would be totally acceptable and reasonable as well (I'm just conservative).

I think your plan checker is misinterpreting the provisions.

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