Coupling beam concept and detailed analysis
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 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 concentric 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 image above, it is connecting two individual piers of 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 behavior so to make them as a solid core we use the coupling beam to transfer force from one pier to another.
So as I was saying 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. A stiffness of the coupling beam plays an important role in the 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 earthquake. It acts as a fuse or a source of energy dissipation. It undergoes cyclic deformations as the wall rock back and forth and resist the seismic force. When you plot the loops of moment experienced by beam versus rotation also called the cyclic curve, it looks something like this
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 ration 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 failure. 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 depth of beam is very similar to length, then the failure of beam is shear governed and the beam will see a brittle failure. To resist brittle failure we provide diagonal bars in coupling beam which will help in resisting shear and it will also reduce the amount of shear reinforcement required.
As shown in first image, we would like the core wall to experience large amount of moment forces at the base of the system, but sometimes the core is so big that it does not yield and thus system becomes inefficient and at that time we can only rely on coupling beams to dissipate all this earthquake energy. In a way they act as the guardian angel of the building. To read further more I would suggest you to go through Professor Wallace’s experiments.