Concrete Coupling Beams 3

[msquared48]

This is a new animal for me and I am trying to understand the intended design phylosophy of the ACI regarding these elements. Did a search and really couldn't find much on the topic here.

I have a situation where the local jurisdiction is requiring the use of these between concrete shear walls. I have a real problem with this in that the structure was designed using R=5 under Table 12.2-1 for a Ordinary Plain Concrete Shearwall System. Section 21.7.7 of ACI refers to using these connectors between the shear elements of Special concrete Moment Frames (R=8) and Special Reinforced Concrete Shear Walls (R=6), which this is neither.

My question is whether their interpretation of the code is correct. Is it the intention of the code to require these elements even in Ordinary Concrete Shear Wall Systems?

I noticed, too, that these elements are only effective if the slope on the steel is great enough - i.e. for use with short, deep beams between the concrete shear elements. In that the comments were concerning the shear walls around elevator and stairwell walls that had little if any rigidity with respect to the rest of the system due to the openings, I don't see the code logicically applied. I feel that these walls with the openings are really too short to be effective shear walls so no coupling beam should be required anyway. Plus, having to add the coupling beam means having to widen the walls from 8" to 10".

Any comments?

Mike McCann
MMC Engineering

 

[hokie66]

Mike,

I am not familiar with the code provisions you quote, but coupling beams are used extensively in high rise buildings. The typical application is when the elevator walls are used as a core, and are the primary lateral load resisting element. When you have elevators on each side of a lobby, the walls behind each bank of elevators form a C shaped element which is strong in one direction, but not in the other. In the other direction, coupling beams force the two sides to act as a T-C couple. The coupling beams act like the webs in a cantilevered Vierendeel truss.

The couplers are typically heavily reinforced, and the junction with the end walls requires special detailing. These beams, and thus the walls, are typically 10" to 20" wide, depending on the building height, but sometimes much thicker.

If the walls you are referring to are not the load resisting elements, I think the requirement of coupling beams is illogical.

 

[prsconsultant]

If you are using an Ordinary Reinforced Concrete Shearwall System, seismic detailing of coupling beam is not required. You may still provide additional reinforcement around the openings (top, bot and sides). It is a good practice to add some diagonal bars but is not mandatory.

What version of ACI-318 & which is the governing building code? I believe you are not using Ordinary Plain Concrete Shearwall System with an R=5, which is incorrect.

 

[msquared48]

Hokie:

That's exactly what I thought. Thanks.

prsconsultant:

I am looking at ASCE7, Table 12.2-1, Seusmic Force Resisting Syetem B6, Ordinary Reinforced Concrete Shear Walls, R=5. The codes in use are IBC 2006, and ACI 05.

Mike McCann
MMC Engineering

 

[prsconsultant]

Just finished designing a building with Ordinary Reinforced Concrete Shear Walls, R=5 with IBC 2006, and ACI 05. Definitely you don't have to detail the link beams for seismic.

 

[csd72]

Check out AISC Civil engineering magazine May 2006 for an article on coupling beams in lift cores.

 

[csd72]

Sorry that was ASCE not AISC