ACI 21.6.4.2 (08) - Transverse Reinforcement

[MrClean3d]

The issue is regarding the orientation of the intermediate cross ties within a boundary element. Per ACI 21.6.4.2, the crossties shall be alternated end for end for proper confinement (see section 21.9.6.4c for required transverse reinforcement in a boundary element). This week we found out that the cross ties were installed incorrectly for approximately 8 boundary elements; where, the 90 degree hooks were not alternated within the shear wall boundary elements. We have been looking for a solution to provide confinement that satisfies ACI 21.6.4.2; this has included, FRP wrap/anchors (FYFE, BASF), reinforced concrete jacket, but each solution has not been successful. Has anyone ever run into a situation where this has occurred? Is there anyway to justify/provide confinement after a wall has been cast. I appreciate your feedback.

 

[KootK]

My first stop would be to see if there's any way that you could do nothing. I like this option because:

1) The tie alternating business is certainly good practice. However, I'm sure that you still get pretty good confinement even the ties aren't alternated. Of course, now that I've committed to this, somebody's sure to bust out a research paper indicating that non-alternated ties are utterly worthless.

2) I feel that this is one of those situations where your efforts to "fix" the code problem will actually result in a weaker, less ductile structure.

If you choose to execute a fix on this, I think that your goal should be simply to confine the 90 degree tie legs. One option could be to bolt a strip of steel plate over top of the concrete surface that covers the 90 hooks. You may even be able to avoid fireproofing it on the basis that, by the time the fires arrive, the earthquake will likely be over. Although I suppose there are aftershocks to worry about...

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

 

[MrClean3d]

Thanks KootK. I appreciate the feedback.

 

[KootK]

Too bad we didn't get a little more feedback. So what did you decide to do?

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

 

[LD81]

IMHO you lose the confinement from the cross-ties if the 90-degree hook sides are not alternated.
If you can put your hands on "Reinforced Concrete Design of Tall Buildings" by Taranath there is a pretty good illustration of concrete confinement from ties in Chapter 2. He also says something along the lines of: you don't see much lateral strain until you reach about 85% of the axial capacity of the unconfined column. This may be your way out.
If you exceed that I would go for fiber-wrapping these elements that are deficient.

 

[TXStructural]

If the discontinuous end of the hook is confined by anything other than normal (thin) cover, such as an adjacent element (wall, slab), you should be fine.
If these ties are situated such that the 90 is confined solely by cover subject to spalling during a seismic event they will need additional confinement.