Electrical Fuse vs RBS Connection

Let us talk about electrical circuits and fuses.

I am pretty sure everyone must have seen fuse at some point in their life. They are used in electric circuits just to protect important electrical components. They save the component in case of power surge. They look something like this:

As soon as the power on one side of the circuit is more than the other side, then this fuse is not able to bear that additional demand and basically breaks down.

If we take the same concept and use it in structural engineering for moment frames, then we get something called Reduced beam Section a.k.a. RBS.

Let us first take a look at possible plastic hinge locations in a moment frames, then we are potentially looking for plastic hinges in beams, columns and panel zone.

But this is not necessarily the case and in Northridge earthquake specifically, engineers observed that column flanges and welds were ruptured, and there was imperfect plastic hinge developed in the beam. Ideally we want the building to behave like this:

Image courtesy of AISC

But this did not happen. People observed plastic deformations in panel zone and brittle shear failure in columns as well as connections. This proved that we cannot rely significantly on connections as they do not have any ductility capacity. So there were two options:

1. Make the connections and panel zone of column significantly stronger and more rigid. Forcing the beam to form hinge away from the connection.
2. Weakening the section of beam away from the beam column joint to get more predicted response of the structure.

Reduced beam section uses the second approach.

Image courtesy of AISC

What we do in a reduced beam section is we cut the flange of the beam away from the connection at a particular radius. The radius of cut depends on the geometrical shape of the section. This reduced capacity of the beam section should be greater than the moment demand due to seismic demand as well as the gravity loading. The seismic demand we are talking about is a reduced demand which is the reduced by a force reduction factor R. By doing this, in case of an earthquake, the plastic hinge in beam is forced away from the beam column joint. Higher ductility demand can be achieved and possible brittle failure because of uncertainties is eliminated.

Want to learn more about Response or Force reduction factor? You can read another blog post that talks about the philosophy of response reduction in depth over here:

The only downside is the weight of the beam it is tested for. AISC gives a section weight limit and ductility checks for members that can be used as a reduced beam section. Thus a Reduced Beam Section is useful in order to achieve higher ductility of the seismic system and improve the performance of the structure under earthquake.

When analyzing the structure, stiffness of beam should be reduced to an RBS equivalent stiffness so that a software program will be able to capture more drifts and deformations in the structure. Drifts generally control a moment frame structure, thus this becomes an important issue especially when beams contribute more to the overall stiffness of the moment frame. More about equations to solve and design a Reduced Beam Section can be found in AISC.

I hope you enjoyed reading this short post.

Thank you