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Concrete Out-of-Plane Design Not Part of SRFS

Concrete Out-of-Plane Design Not Part of SRFS

Concrete Out-of-Plane Design Not Part of SRFS

For a concrete bearing wall that is not part of the lateral force resisting system I am designing/detailing the wall reinforcing per ACI 318 Sect. I have seismic crossties in the transverse direction (perpendicular to longitudinal wall axis) with the ends "anchored" around the vertical reinforcing. Per the spacing of the crossties is d/2. For a 8" concrete wall with reinforcing each face the d = 6.5". This would put the vertical spacing of the ties at 3.25". This seems excessive to me in terms of the vertical spacing. I am curious how others interpret this code requirement.


RE: Concrete Out-of-Plane Design Not Part of SRFS

Yes, you should using the other direction for your d. Be careful when not counting walls as part of the lateral system, walls are stiffer than frame members. As such, they will still see sizable seismic loads even if you are not counting on it.

RE: Concrete Out-of-Plane Design Not Part of SRFS

For any wall - seismic loads and displacements can be applied in either in-plane or out-of-plane directions.

For in-plane seismic, section 21.13 applies and as sandman21 stated, d is the long direction d.

For out-of-plane seismic, you apply the delta(u) displacement and derive from it shears and moments in the wall. These forces would be relatively small as the wall stiffness is small in that direction.
However, in this case "d" is the short direction. However, I'm not convinced that ACI 21.13 applies here. It doesn't make sense to have 3" bar spacing in a wall for such small forces and in a non-critical direction.

Nothing in 318 suggests that out-of-plane conditions don't apply however.

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RE: Concrete Out-of-Plane Design Not Part of SRFS

Regarding out of plane considerations:

Logically, I think it reasonable to assume that the d/2 requirement is intended to ensure that you get at least a stirrup or two crossing any hypothetical shear crack.

21.13.4 Kicks you over to 21.5.4. Has you capacity design your shear strength for the probable moment strength of your wall (assuming fixed at ends). Has you assume Vc = 0 as there would be shear reversal in the wall.

This would definitely require the use of ties spaced ridiculously tight and would, generally, be terrible.

I'd say that your best approach would be to estimate du and then use the out given in 21.13.3. That basically says that you're good to go if your member capacity exceeds the specified load combo in conjunction with the forces induced by du. This should be fairly easy to satisfy so even a very rough, conservative estimate of du ought to get the job done.

Like JAE and Sandman, I'm pretty skeptical that the intent of the code was to encourage such tightly spaced ties in ride along walls.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

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