Column supporting shear wall

EDub24 · Feb 21, 2020

I've got a special reinforced concrete shear wall building with a discontinuous shear wall supported by columns at either end (vertical irregularity type 4). I'm designing the columns using the overstrength factor per ASCE 7-16 §12.3.3. but I'm not sure if I should be following the requirements for moment frames per chapter 18 of ACI 318-14. Anybody have any thoughts on this? My gut tells me that since the overstrength factor is meant to make sure the column stays in the elastic range then I don't need to do any special detailing. At most I would follow the requirements of the ordinary moment frame (which is basically nothing). Anybody have any thoughts on this? I don't think I can use the section 18.14 in ACI 318 for elements not part of the seismic force resisting system as they clearly are.

KootK · Feb 21, 2020

Are the columns in question actually part of a moment frame? Or are they just axial columns dealing with the shear wall boundary axial forces and the wall shear is getting transferred elsewhere by the diaphragm? The former is pretty common; I've never seen the latter.

EDub24 · Feb 21, 2020

Well that’s the part that’s a bit of a grey area for me. There are gravity beams framing into it but they are not part of the lateral system. The floor is a concrete diaphragm that spans to the shear walls.

KootK · Feb 21, 2020

You don't need to do the moment frame column special stuff but you do need to do some special stuff. Decent summary below.

radiocontrolhead · Feb 21, 2020

KootK,

I've done the latter a handful of times in response to your 1st reply

KootK · Feb 21, 2020

I did get my pretentious former and latter reversed. Does that change your answer? I've seen plenty of shear walls dump on columns. I've not yet done or seen special shear wall that turned into a moment frame. I'm not even sure that would make sense as the beam would be too much stiffened by the wall to form any beam side plastic hinges.

radiocontrolhead · Feb 21, 2020

Thanks for the clarification Koot. Your reply made me really think about the approach i was taking on those jobs. Nonetheless i'm never a fan when dealing with those situations but what choice are we left with.

I agree although could it then become say a column in pure bending by a infinitely "idealistic" rigid support above?

EDub24 · Feb 22, 2020

KootK said:
I did get my pretentious former and latter reversed. Does that change your answer? I've seen plenty of shear walls dump on columns. I've not yet done or seen special shear wall that turned into a moment frame. I'm not even sure that would make sense as the beam would be too much stiffened by the wall to form any beam side plastic hinges.

I understood what you were trying to say [thumbsup]

Thanks for the screenshot. You're right that's very helpful. I was digging through all of my references and totally forgot about the NEHRP Tech Briefs. The ACI 318-11 §21.6.4.6 referenced in that document is §18.7.5.6 in ACI 318-14. See below. You can also see in that section that the compressive stress Ag*f'c/10 is increased to Ag*f'c/4 if you are including the overstrength factor which would be the case in this instance. Don't ask me why that section is under the 'Columns of Special Moment Frames" section. The organization of ACI 318-14 is still confusing to me.

I also agree it doesn't make sense to have a shear wall supported by a moment frame. The beam would be so stiff you would never be able to form hinges.

2020-02-21_21_06_03-ACI_318-14_Building_Code_Requirements_for_Structural_Concrete_and_Commentary_-_B_msj4s9.png

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