Coupling beam is an important member of the lateral force resisting system in a structure. Consequently It couples or combines the two independent systems like shear walls.
WHY WE PROVIDE COUPLING BEAM ?
For example, Let’s assume there are two buckets, one is completely filled with water whereas other is half filled. Now we have to level up the amount of water in both of the buckets. oh..! Seems quit easy but not.. For doing this there is a condition, it should be done without lifting up the bucket and a small flexible tube is given to us to connect the bucket. What will we do? We will just connect both of the buckets and due to the siphon action we would be able to transfer water from a filled bucket to the half filled bucket.
Similarly to above effect when we have two independent shear walls or two concentric braced frames or anything which is helping us to resist the lateral loads and we want to connect them to reduce the overturning effect or improve the overall stiffness of the system then we will use a coupling beam to connect both the systems as a result distributing forces in between.
Effect of coupling BEAM
As shown in above image,in first figure there is no shear beam is provided. Whereas in second figure shear beam is connecting two individual piers of shear core. Now Imagine the deformation of shear wall, when the lateral load is applied from left to right . In first case Both piers will move independently without any harmony resulting in further complicated behavior. Therefore to make them as a solid core the coupling beam is used to transfer force from one pier to another.
Hence, while deforming the extreme ends of wall the piers experiences tension and compression. If we coupling/combining the system results in transfer of tension and compression forces through the coupling beam as a shear force for which beam is designed. The stiffness of coupling beam plays an important role in adding the stiffness to the system.
But can we make the coupling beam very stiff ? Well no , because coupling plays one more important role during earthquakes. It acts as a fuse or a way of energy dissipation. By undergoing cyclic deformations as the wall rock back and forth and resisting the seismic force. When we plot the loops of moment experienced by beam versus rotation called the cyclic curve, it looks something like this :
There are two types of coupling beam :
1. Conventionally reinforced beam
This is just an regular type of beam. It consists a horizontal top and the bottom reinforcement and shear ties. This is used when the aspect ration of the beam is large i.e. the beam is pretty long compared to its depth. The long beam is a flexible, which means it will form moment hinges at ends and the failure will be ductile failure. Thus the shear reinforcement is basically under control. Hence, we are able to provide a feasible and constructible cross section of beam.
2. Diagonally reinforced beam
When the beam’s aspect ratio is small, i.e. the depth of beam is very similar to length. Then the failure of beam is governed by shear and the beam will see a brittle failure. To resist the brittle failure diagonal bars are provided in coupling beam. This bars help in resisting shear and will also reduce the amount of shear reinforcement required.
As indicated in first image, we assume that core wall will experience large amount of the moment forces at the base of system, but sometimes the core is so big that it was unable yield and thus system becomes inefficient. At that time we can only rely on the coupling beams to dissipate all of this earthquake energy. Hence in other way they act as the safety factor to the building.