Special Reinforced Concrete shearwall with coupling beam?

[GreekSE]

Hi, first time poster, long time reader.

I have a question related to special reinforced concrete shear walls (SDC D) and coupling beams. Here are my questions:

1. If I have a single story RC wall, with a single opening in it, is the portion of wall above the opening considered a coupling beam? Lets assume that the ln/h ratio for this piece of wall is 1.0, and lets assume I have a rigid diaphragm at the top of the wall.

2. If this piece of wall above the opening is considered a coupling beam, it must satisfy 21.9.7.1, 21.9.7.2, or 21.9.7.3 (ACI 318-08). In my case, the ratio is less than 4, therefore 21.9.7.1 is out. Also, Vu is less than 4sqrtf'cAcw, therefore, 21.9.7.2 is out, and I am left with 21.9.7.3.

3. Assuming my wall is not thick enough to place diagonal bars in it, I am left to design this piece of wall as a special reinforced moment frame beam....again, assuming it is considered a coupling beam.

4. I can handle the requirements for special reinforced moment frame beam longitudinal reinforcing, but it seems ridiculous to provide the hoop reinforcing spacing that the code requires, especially when my shear demand is not very high.

5. I have been using a NEHRP seismic design technical brief #6 to wade through the special structural wall requriements, and it has been very helpful. I am confused however by Figure 5-12, which illustrates "Details for conventionally reinforced coupling beams." Since it calls this "conventionally" reinforced, I take it this is not for seismic applications, or is this intended to be the same requriements as a special reinforced moment frame beam? It seems like some components are slightly different (hoop spacing).

6. Building in question is a load bearing cast in place concrete bearing wall building, single story, all exterior walls are concrete walls, with some interior concrete shear walls.

Thanks for any help/insight!

 

[dik]

I would likely ignore it and design it as a lintel with fixed ends. Any lateral load should be taken by the adjacent concrete walls. If you do include it, determine the shear from any lateral loads and treat the middle of the lintel as a point of contraflexure and use the moment V*s/2 for the added moment. Does that answer your question?

Dik

 

[GreekSE]

Thanks, your response does help. I have been reading more, and it seems like if I ignore it, I will need to design it as a member not designed as part of the seismic force resisting system.

Do you have any specific reference on why you would ignore it? I am looking for a definitive case when I need a piece of shear wall to be considered a coupling beam.

 

[dik]

If your wall lengths are proportionally larger than the opening, the effect of the opening on the overall behaviour of the complete wall will be negligible, in particular for a single storey building. If the building height was 60' and you had a 7' opening, then it would make to reinforce for the opening treating the entire wall. If your building height is 12' and you have a 7' high opening.. you can reinforce it as I noted, just to minimise cracking... but the overall effect of the opening on the behaviour of two single walls will be much similar.

Dik