Coupling beams are a very important member of a lateral force-resisting system. It couples or you can say combines two independent systems.
Let us say there are two buckets, one completely filled with water and the other is half-filled now you have to level up the amount of water in both the buckets without lifting up the bucket and I will give you a small flexible tube to connect the bucket. What will you do?
You will connect both the buckets and considering the siphon action you can transfer water from a filled bucket to the half-filled bucket.
Similar to this effect when you have two independent shear walls or concentrically braced frames or anything that is helping you to resist lateral loads and you want to connect them to reduce the overturning effect or increase the overall stiffness of the system then you will use a coupling beam to connect both the systems.
As shown in the image, it is connecting two individual piers of the shear core. Imagine the deformation of the shear wall when a lateral load is applied from left to right and they were not coupled (connected).
Both piers will move independently without any harmony and this will further complicate the behaviour so to make them a solid core we use the coupling beam to transfer force from one pier to another.
As seen in the image, while deforming the extreme ends of wall piers experience tension and compression. If we couple the system, these tension and compression forces transfer through the coupling beam as a shear force for which the beam is designed.
The stiffness of the coupling beam plays an important role in adding the stiffness of the system. But can we make a coupling beam very stiff?
No, because there is one more important role that a coupling plays during an earthquake. It acts as a fuse or a source of energy dissipation. It undergoes cyclic deformations as the wall rock back and forth and resists the seismic force.
When hysteresis loops of the moment experienced by beam versus rotation also called the cyclic curve, it looks something like as shown in the image.
There are two different types of coupling beams:
1. Conventionally reinforced coupling beam
2. Diagonally reinforced coupling beam
## 1. Conventionally reinforced beam
This is just a regular type of beam with horizontal top and bottom reinforcement and shear ties. This beam is used when the aspect ratio of the beam is large or the beam is pretty long compared to its depth.
A long beam is a flexible one which means it can form moment hinges at the ends and the failure is ductile.
Thus the shear reinforcement is generally under control and you can provide a feasible and constructible cross-section of the beam.
## 2. Diagonally reinforced beam
When the beam's aspect ratio is small, say the depth of the beam is very similar to length, then the failure of the beam is shear governed and the beam will see a brittle failure.
To resist brittle failure, we provide diagonal bars in the coupling beam which will help in resisting shear and will also reduce the amount of shear reinforcement required.
As shown in the image, we would like the core wall to experience a large number of moment forces at the base of the system, but sometimes the core is so big that it does not yield and thus the system becomes inefficient and at that time we can only rely on coupling beams to dissipate all this earthquake energy.
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