horizontal reinforcing development into boundary elements for wind governed walls

[said the noob]

i suspect this maybe a quick answer but just wanted to confirm a general consensus,

if core walls are wind governed we would not need to provide fig. R18.10.6.4.1 - development of wall horizontal reinforcment in confined boundary element, where it states that reinforcing can be a straight bar but must be extended to the end of the wall within a boundary element of 6inches or less. I believe we would only need to require to follow Ld (straight horizontal development) into boundary element core as directed by chapter 11 - Walls to refer to chapter 25.4 for development lengths.

our boundary element is quite long, and to extend 15dia horizontal reinforcing to within 6in end of these walls will be quite excess in reinforcing,

 

[JAE]

"wind governed" is not always an appropriate way to look at things.

A shear wall in a building must resist both wind and seismic loads in their respective load combinations.
The requirements for wind load, strength and detailing must be met as required by the level and character of those wind loads.

Likewise, the shear wall must also resist the seismic loads (however small) and still meet all the relevant and applicable provisions of the code for strength and detailing as well.

Now with very small seismic conditions the special seismic detailing may not apply, not because its loads are lower than the wind, but because it is lower than a code-prescribed limit where that detailing is mandated.

For example, an X-brace must meet wind loads for strength - and the seismic loads for strength, but the seismic design category might be A, where special seismic detailing for the brace connections (using Omega, etc.) is not required.

So if you are in a seismic category that requires a specified development of wall reinforcement into a boundary element, you have to do that even if the wind load is greater.

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[Daydreamer]

If taking horiz rebars to 6in of face is too much, I would, within plastic hinge zone of shear wall, use 1.5 ld. Remember to also increase for top bar concept.

 

[said the noob]

thanks for the replies, I had talked to a senior engineer in our firm about this and agrees that it is not required for wind governed walls to carry to the ends within 6inches although it is good practise regardless, 1.5ld (x1.3 for horizontal) seems like a reasonable inbetween,

 

[JoshPlumSE]

I agree with what JAE said completely. Though I would phrase it slightly differently.

1) Design forces may be higher for wind than seismic.
2) Just because wind forces are higher doesn't mean that you can ignore seismic DETAILING requirements.
3) This is because the seismic forces you calculated were based on the R value (response modification factor) of your structural system. The R value is based on the ductility of the system. So, if you don't have the proper detailing, you won't get the proper ductility. If you don't have the proper ductility then you should use a much lower R resulting in a much higher seismic force.

This is a common mistake among younger engineers, or among engineers without a lot of seismic design experience.

 

[KootK]

I think that the answer to this lies in recognizing that the horizontal bars are performing the function of beam stirrups. And, like beam stirrups, the purpose of the bars is to shear connect the flexural compression and flexural tensions zones. In this respect, this comes down to why your boundary members are so long to begin with. If they're long to satisfy compression requirements, then it may well be okay to not extend the horizontal bars all the way. However, if they're long to satisfy tension requirements, then your compression block may not actually be nearly as long as your boundary element. In that case, you'd want to extend your bars into the boundary element at least far enough that you're confident that you're anchoring them within the compression block. A compromise strategy that could be used could be to call your boundary ties lapped extensions of your horizontal bars which gets you back to anchoring the "stirrups" back to the compression block even if you don't take the horizontal bars all the way to the back. Obviously, this requires a few extra checks but, for most configurations, this is entirely doable without any strange detailing.

In my work, I take the horizontals all the way back in all scenarios although I often do that via the tie extension method that I just mentioned. It's easy/cheap to do and saves all the accounting of trying to figure out what should be done.