A beam is a flexural member which resists loading mainly by bending. The aim of the design is to decide the size or dimensions of the beam and provide suitable reinforcement. This should be performed safely during its lifetime. Let us discuss the types of beams in detail.
What is a Singly Reinforced Beam?
A singly reinforced beam is a reinforced concrete beam that only contains reinforcement in the tension zone. In simpler terms, it has reinforcement bars (also known as rebars) placed along the bottom surface of the beam to resist tensile forces. This reinforcement effectively counteracts the tendency of the beam to bend or crack under load.
Since it is impossible to tie the stirrups without providing compression reinforcement in the section, we place two bars at the ends in the compression zone of the beam. The purpose of those bars is to tie the stirrups only.
What is a Doubly Reinforced Beam?
A doubly reinforced beam is a reinforced concrete beam that contains reinforcement in both the tension as well as compression zones. In addition to the bottom reinforcement, it also includes reinforcement bars along the top surface of the beam to enhance its load-carrying capacity and overall structural strength.
The size of the beam may occasionally be limited due to headroom restrictions or architectural factors, and the same beam must resist a moment greater than Rbd2.
This can be designed in two ways:
- By using an over-reinforced section.
- By using a doubly reinforced section.
The use of over-reinforced sections is not considered in practice because they do not give an exact margin of safety before failure. Hence it gives sudden failure without any warning.
In a doubly reinforced section extra steel is provided in both tension and compression zones which constitutes an additional moment of resistance.
Difference between Singly and Doubly Reinforced Beam
| Singly Reinforced Section | Doubly Reinforced Section |
| The reinforcement is placed only on the bottom (tensile side) of the beam. | The reinforcement is placed on both the top (compressive side) and the bottom (tensile side) of the beam. |
| It is suitable for situations where bending occurs predominantly in one direction. | It is applied in situations when there is a lot of bending in both the directions. |
| The compression load is taken by the concrete only. | Here the compressive load is taken by compression steel as well as concrete. |
| It has typically a lower load-bearing capacity compared to doubly reinforced beams. | It offers a higher load-bearing capacity, making it suitable for heavy-duty applications. |
| They are simple in design and construction. | They are more complex due to the additional reinforcement. |
| Generally, they are more economical. | It may require a higher investment due to the extra steel reinforcement. |
Why do we use Singly Reinforced Beams?
Singly reinforced beams are commonly used in situations where the load primarily creates bending stresses in one direction, such as simply supported beams and cantilevers.
When the balanced reinforced concrete beam of dimension (b X d) has a moment of resistance Rbd2, the singly reinforced beam is appropriate.
Why do we use Doubly Reinforced Beams?
Doubly reinforced beams are utilized when the applied loads result in significant bending stresses in both directions. These beams are commonly used in continuous beams or slabs and structures where higher loads and strength are required.
By reinforcing both the tension and compression zones, doubly reinforced beams can withstand greater bending moments and shear forces.
When the external moment is greater than the moment of resistance of a singly reinforced section and the dimensions of the beam are limited by any restrictions, such as the availability of headroom, architectural, or space considerations.
When eccentric loads are present.
When unexpected and rapid lateral loads are applied to the beam.
Basic Rules for the Design of Beam
The following rules are considered while designing beams:
Effective Span
Simply Supported beam or Slab
The effective span of a simply supported beam or slab is taken as a minimum of the following:
- Clear Span + effective depth of beam or slab.
- Centre-to-center distance between supports.
Continuous Beam or Slab
In the case of a continuous beam or slab if the width of the supports is less than 1/12 of the clear span. If the width of the support is more than 1/12 of the clear span or 600mm whichever is less, the effective span is taken as:
- For the end span with one end fixed and the other continuous or for intermediate spans, the effective span shall be the clear span between the supports.
- For the end span with one end free and the other continuous, the effective span shall be equal to the clear span + 1/2 the effective depth of the beam or slab or clear span plus 1/2 the width of discontinuous support whichever is smaller.
Cantilever Beam or Slab
For cantilever beam or slab the effectiveness is taken as:
- Length of overhang plus half the effective depth.
- Except where it forms the end of a continuous beam where the length up to the center of support is taken.
Effective Depth
The effective depth of a beam is the distance between the centroid of the area of the tension reinforcement and the topmost compression fiber. It is equal to the total depth of the beam minus the effective cover.
Control of Deflection
The vertical deflection limits may be assumed to be satisfied for beams and slabs If the span to depth ratios are less than the following:
1. Span up to 10m
| For Simply supported beam | Span/ Effective depth = 20 |
| For Cantilever beam | Span/ Effective depth = 7 |
| For Continuous beam | Span/ Effective depth = 2 |
2. For a span more than 10m, the above values should be multiplied by 10/span, except for the cantilever for which the is to be calculated the exact deflection.
Advantages of Doubly Reinforced Beam
- The long-term deflection of the beam is decreased with the aid of compression steel.
- Compared to single-reinforced beams, the doubly reinforced beam is more cost-effective.
- Steel is used in both the tension and compression zones of the doubly reinforced beam to minimize section deflection.
- It enhances the rotating ability of the section.
- The flexibility of the section is increased by the addition of compression and tensile steel, which is why doubly RCC sections are always utilized in earthquake-prone regions.
- Doubly reinforced beams offer greater resistance to cracking, deflection, and overall structural failure, ensuring the long-term stability of the structure.
FAQs
Can doubly reinforced beams be used in residential construction?
Doubly reinforced beams are commonly used in commercial and large-scale construction projects. They can also be used in residential construction, especially for structures with unique design requirements or higher load demands.
What are the disadvantages of using singly reinforced beams?
Singly reinforced beams have certain limitations in terms of load-carrying capacity and span lengths. They may not be suitable for structures that require high load resistance or longer spans.
Where is a doubly reinforced beam used?
When there is a significant bending moment the strength of the singly reinforced beam is insufficient and the depth of the section is constrained.
What is the difference between singly and doubly reinforced beam?
A singly reinforced beam is a type of beam that contains reinforcement in the tension zone only, whereas the doubly reinforced section contains reinforcement in both tension and compression zones.
Why stirrups are provided in the beam?
The function of stirrups is to hold the longitudinal (main) reinforcement of the beam and also to prevent the beam from diagonal shear cracks.
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