What is Shear? Can Concrete Resist Shear?

What is Shear? Can Concrete Resist Shear?

Well, we have to consider a reinforced concrete beam.

How it might fail, if there was bending, and we have a nice bending failure.

But unfortunately, there is another form of failure which can occur in a reinforced concrete beam which is called shear, and what that is? Usually near the ends of a concrete beam, where the supports are.

We have very high vertical forces at the ends of a concrete beam and that can cause the end of the concrete beam to suddenly just fail in shear.

So let's have a look now at this concrete beam that is being loaded to ensure that we have a shear failure. Now look carefully the beam doesn't look as though it's under much stress that this beam is being loaded to cause shear failure.

So under load, it goes along and suddenly it fails in a very explosive manner. So this is a form of failure in a reinforced concrete beam which we have to ensure never happened.

Can concrete resist shear?

Well, yes to some extent, but remember that the shear failure we've just seen is very brittle. So what we need to do is ensure that never happens and that means the concrete needs some help to resist shear.

So what is that help?

Well, remember that the shear failure occurs along an inclined plane, so the two pieces of concrete move apart like that which means we need vertical reinforcement to prevent that from happening.

This is what reinforcement looks like on a site. We have the vertical reinforcement in place which resists, and in fact, it looks a bit like this it looks like a shear stirrup and we have the longitudinal reinforcement in place which resists bending together. This thing is called a Reinforcement Cage and you might have sealed it in any construction site.

The beauty of this sort of Reinforcement Cage is that it resists shear failure with the vertical stirrups, the longitudinal bars resist bending. So we were not fair in bending, but if it were ever to be overloaded and it did fail in bending then that's okay because it would fail in a gradual manner because we've chosen the amount of longitudinal reinforcement carefully.