CURTAILMENT OF REINFORCEMENT BARS

Curtailment of reinforcement bars is a practice used in structural works.

Have you ever noticed the steel bars sticking out from the edges of a concrete floor or ceiling? Those steel bars are part of what makes the building strong and safe. Let’s talk about how they work. We all know every RC structure has reinforcement bars that help the concrete carry heavy loads (dead, live, etc.)

Now, curtailment of reinforcement bars means ensuring these steel bars are properly placed and ended in the concrete. Imagine building a puzzle, each steel bar has a specific place and length where it’s needed to make the concrete strong. Curtailment is about making sure those bars are long enough and in the right spots so they can do the jobs properly.

It’s important because if these bars are not in the right place or long enough, it’s like having a puzzle piece missing or not fitting well. This might make the building not as strong as it should be.

Tension reinforcement is only important in regions where the bending force applied to a structural element surpasses the moment capacity of the concrete without reinforcement. Therefore, it might seem unnecessary to reinforce the entire concrete area (waste of steel) if the moment capacity of the unreinforced area is not exceeded or only surpassed in smaller areas. The reinforcement can be cut off in certain areas where it is not needed and this is what curtailing means.

PRACTICAL CURTAILMENT OF REINFORCEMENT BARS IN BEAMS

Curtailment of reinforcement bars is often done in areas with hogging moments to enhance the negative bending capacity locally. In practice, it is generally assumed to be safe to disregard the curtailed reinforcement when calculating the shear capacity. However, some tests from the literature have shown that the termination of curtailed reinforcement leads to a reduced shear capacity.

The diagram below gives a clear picture of how hogging and sagging occur in beams and the areas where both tend to be high.

Sagging happens when the beam deflects downwards in the middle under a load. In sagging, the bottom part of the beam is in tension (stretched), while the top part is in compression.

Hogging occurs when the beam curves upward in the middle and downwards at the ends. In hogging, the top part of the beam is in tension, and the bottom part is in compression

It can be observed from the diagram that the beam experiences high hogging at the support and high sagging at the midspan. To deal with this, instead of entirely placing the top and bottom bars in the beam, the bottom bars can be placed at the midspan where sagging occurs and tension is high. Similarly, since the upper part of hogging is under tension at the supports, reinforcement bars can be placed only in that area instead of the entire top area. This arrangement is shown below.

The main purpose of the primary bars is to counteract the bending of the concrete beam due to bending moments (BM), which vary in magnitude and direction along the length of the beam. Consequently, varying amounts of main steel are required. Given that the beam’s BM is positive near its center (bottom), more steel is necessary there, and in principle, none is needed at the upper center. Similarly, BM is negative near the supports (top), signifying that more steel is required there and none at the bottom. Consequently, only hanger bars (corner bars) are utilized in these BM zones to minimize steel wastage. However, in practice, this process is quite labor-intensive on-site as cutting rebar demands a significant amount of time and labor.

Benefits of reinforcing bars curtailment

• It makes construction more economical
• It adds strength to RC structures
• It helps to reduce waste
• It provides flexibility in optimizing the structural layout

Guides to help in curtailment

There are standards and codes that guide us on how to curtail our reinforcement bars. These standards vary from location to location as each jurisdiction has its own unique way of operating, influenced by their environmental characteristics. Some of these standards are;

• British Standard code (BS)
• American Concrete Institute code (ACI)
• International Building code (IBC)
• Canadian Standards Association (CSA)
• Australian Standards (AS)
• Indian Standards (IS)
• Eurocode

Each of these individual codes also have sub-categories pertaining to any structural work be it concrete, reinforced concrete, steel, etc.

Reference

• Shedbale, Nikhil & Kame, Ganesh. (2017). Parametric Analysis of Continuous Beams for Curtailment of Reinforcement Bars. International Journal of Engineering Technology Science and Research. 4. 154-159.