Diagonal reinforcement in coupling beams

[winburg22]

Hi,

Can someone please help me understand why the longitudinal reinforcement (reinforcement running left-to-right) for coupling beams with diagonal reinforcement isn't embedded/developed into the walls much at all?

Please see image below:

https://www.structuremag.org/wp-content/uploads/2018/12/0119-sd-2.jpg

This detailing style appears very common - the diagonal bars are properly developed into the wall, but the longitudinal bars are not embedded much at all.

Does it have to do with trying to ensure they are only being used for the gravity load and to direct the lateral forces to the diagonal bars? (As I wrote this sentence I realised it doesn't make sense and I really have no idea.)

 

[bobbyboucher]

page 24

https://www.nehrp.gov/pdf/nistgcr11-917-11.pdf

 

[winburg22]

Thanks bobby

So page 23 says:
The longitudinal
reinforcement, typically No. 4 or No. 5 bars, should extend
only a short distance into the wall boundary so that it will
not develop significant tensile stress due to beam flexure.

So what's the purpose of the longitudinal reinforcement?

 

[ggcdn]

It confines the concrete around the diagonal and prevents strength loss. It’s basically a cage to keep the beam intact.

There are quite a few tests where the longitudinal bars are developed, and the beam generally performs okay, but there is much higher over strength compared to code equation.

-JA (working on [link calcs.app]Calcs.app[/url])

 

[KootK]

Can someone please help me understand why the longitudinal reinforcement (reinforcement running left-to-right) for coupling beams with diagonal reinforcement isn't embedded/developed into the walls much at all?

Because the longitudinal bars are not active as primary reinforcement during coupling action. I feel that the best way to visualize the action of these coupling beams is as a pair of three bar, pin connected trusses. Those trusses don't need the longitudinal bars in order to do their work in any primary sense.

Also, as ggcdn mentioned, once you develop the bars they will be more inclined to increase the load at which targeted system plasticity occurs. This is pretty similar to this comment of yours.

Does it have to do with trying to ensure they are only being used for the gravity load and to direct the lateral forces to the diagonal bars?

The diagonals still resist gravity load but the longitudinal are, indeed, left undeveloped so as to direct the lateral forces to the diagonal bars.

 

[Jinal Doshi]

Some thoughts here on the typical design, detailing and construction practices:
The longitudinal bars are utilized only for the confinement for most part, but it will also provide some minimum tension capacity at mid span. Also, 6" embedment is not insignificant for #5 bar say, but it is not enough to add to plastic capacity of beam specifically during a seismic event. The coupling beam is designed for the shear developed based on the plastic moment capacity, and in design, we consider the capacity being contributed from diagonal group only.

So if we develop these longitudinal bars, we will increase the plastic moment capacity of the coupling beam, which in turn will increase shear demands (2Mp/L). The beam may not be designed for this additional shear, leading to less than expected ideal performance that it was tested as. Thus, it just make it simpler not to develop the longitudinal bars in general for diagonally reinforced coupling beam.

Also from constructability standpoint, we have vertical shear reinforcement (at times boundary element) crossing the diagonal bars already, and if you develop these bars further into the wall, it will definitely pose a constructability issue as well and may be concrete consolidation issue in one of the most critical zone of a beam to wall connection. So better to simplify as much as we can from detailing standpoint.