To couple or not to couple?

[P205]

This is a question I've been wondering for a long time and am finally going to ask to get to the bottom of this (hopefully). Now, I've mainly done steel buildings with steel framed stair and elevator shafts, so this is the first time I'm having to design concrete stair and elevator shafts.

My questions is, say you have a stair shaft with a 3ft opening on each floor with a concrete beam above to opening, must I design this beam as a coupled beam? I'm choosing to design the shear walls as individual components as opposed to as composite sections (C-shape). In my ETABS model, I was planning on using a pin-pin connected concrete beam above the opening in order to ensure that the model doesn't "couple" the walls.

I have a number of textbooks (5 in total, including ones covering seismic design in concrete) on concrete design and I haven't found any information discussing this issue, other than the design and detailing of coupling beam. But nothing on "when to" or "must I" couple the walls.

I should mention that this building is in a low seismic region in Canada and I'm using "conventional construction" (Rd=1.5, Ro=1.3).

 

[KootK]

I'm assuming that we're talking about a mid-rise building here rather than a super tall.

1) One negative consequence of ignoring the coupling is that the coupling beams may become damaged at higher levels of lateral load. That said, I've never actually heard of any problems developing except in high seismic ares. And one can design the beams for ductile behavior if desired to mitigate this. In a modest, low-rise structure, nobody will put up with the diagonal reinforcing so you'd have to get this cone with a convention reinforcing scheme which is easier to make ductile if it has a higher aspect ratio.

2) A second negative consequence of ignoring the coupling will be that the stiffness of your wall systems will be inaccurate which will make your lateral load distributions inaccurate up until the beams fail and allow load to redistribute.

3) In my experience, for low rise aseismic work, it is very common for practicing structural engineers to disregard coupling. I've done it and continue to do it.

4) For shorter structures, accurately predicting the behavior of rather complex core wall systems is very difficult. Shear deformations and foundation flexibility will enter into the picture in a big way. So I don't feel that getting super fancy with things is justified.

5) Accidental coupling beams can draw a lot of load in an elastic model. Without getting into redistribution and pushover analysis etc, they can become quite challenging to design for those demands.

 

[P205]

Yes, this is a 5 storey concrete building. Every wall between units is concrete so that building is very stiff.
1) Given how many shear wall this building has, the storey drifts will be very small and so there shouldn't be much damage to these beams.
2) Yep, understood.
3) Excellent, this is the sort of experience/judgement that I was looking for.

As an aside, It amazes me (if that's the right word) how much engineering "best practices" and heuristics are missing from formal education and most readily available textbooks.

 

[KootK]

As an aside, It amazes me (if that's the right word) how much engineering "best practices" and heuristics are missing from formal education and most readily available textbooks.

It's my impression that, for the stuff that requires true engineering judgement (most of what matters), pretty much nobody has the balls to put anything in print for fear of being wrong. And that's a rational fear. I intend to finish out my tenure as a structural engineer by authoring the definitive work on shear wall analysis and design. Unfortunately, by the time that I get it ready, you'll be bouncing grandchildren on your knee and drooling involuntarily.

 

[dhengr]

P205:
I’d still do a few std. details showing some crack protection at the reentrant corners of those shallow beams and the walls. As long as they are poured together, they will try to act together and you could end up with some unsightly diag. cracking. You just won’t realize the full potential of the coupled system, and as Koot says, that’s often o.k.

RE: your last para., “It amazes me…,” that’s because the Prof’s. don’t know these types of things, they’re too busy not teaching engineering fundamentals; the students wouldn’t be smart enough to absorb then anyway, without a software; and you couldn’t write a thick enough text book to cover all these special conditions in the real world. You don’t have a high enough ladder to reach the first chapter, when that book is placed on a desk.

 

[Enrique2015]

You will only have to design the beam as a "seismic" coupled beam if you want to use this beam as part of your lateral system, otherwise you will size and design the beam as gravity only. Of course if you end up with a 50" deep beam (exaggerating) you will need to include it since even with minimum reinforcement it will affect your lateral stiffness. But if this is a 12" deep beam, no need. For a 3ft opening I usually use the concrete slab no beam unless is need it for architectural purposes. In high semi-mic risk areas or structures with high SDC, you may need to check compatibility of deformations.

http://www.structuralapps.com

http://www.Tilmanllc.com

 

[jdgengineer]

This doesn't apply to this particular question as the wall doesn't seem to be a special frame, but I don't really design special concrete shearwalls often, but what do you think is the intent of the commentary to 18.10.7 "For coupling beams not used as part of the lateral-force resisting system, the requirements for diagonal reinforcement may be waived". This to me indicates that aside from diagonal reinforcement all of the other detailing requirements for coupling beams, in addition to compatibility requirements of 18.14 would be required even if the coupling beam is not utilized as an element for the lateral force resisting system for the concrete shearwall. This seems to apply for any special concrete shearwall. Thoughts?

 

[KootK]

in addition to compatibility requirements of 18.14 would be required even if the coupling beam is not utilized as an element for the lateral force resisting system for the concrete shearwall.

While I think that there is considerable room for interpretation on that, my personal opinion is that the compatibility requirements only apply if the damage resulting from incompatibility would impair the damaged member from performing an aspect of its structural function. Slab column joints, as ubiquitous example, can't fail in drift induced punching shear because they would then be ineffective for carrying gravity loads. With inadvertent coupling beams, I would say that many are of such proportions that drift related damage would not impair gravity function. If your coupling beam is 2' deep x 6' wide with a CIP slab on top carrying hallway loads, you're probably okay. If you're coupling beam is 2' deep x 12' wide and carrying a massive steel girder at mid-span... not so much.