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Singly and Doubly Reinforced Beam
A beam is a structural component that mainly carries vertical loads, applied vertically to the beam’s axis and prevents deflection.
Its deflection model is mainly due to bending when the load is applied to the beam resulting in reaction forces at the support point of the beam.
Beams are identified by their support method, length, material profile (cross-section shape) and equilibrium conditions.
Different types of material are used for the construction of beam-like wood, steel etc. But the RCC beam is most prevalent.
Beams carry the vertical loads and transfer them to the supporting column or masonry that move them to the foundation.
Mainly simply-supported beam has two types which are as follows.
What is Singly Reinforced Beam?
The beam that contains steel reinforcement only in the tension zone refers to a singly reinforced beam.
In this beam, reinforcement carries the tension force, while concrete has the compressive load caused by the deflection of the beam.
Since we have to tie the stirrups, it isn’t easy to reinforce the tension zone. Therefore we also provide two bars in the compression to tie the stirrups that act as a dummy bar only for holding the stirrups.
Where is the singly reinforced beam used?
The singly reinforced beam is suitable when the moment of resistance of the balanced reinforced concrete beam of dimension b × d is Rbd2.
Concrete has significant compressive strength and minor tensile strength. So In a singly reinforced beam, steel is given in the tensile zone, which is good in tension and compression.
Singly reinforced beam has a limited moment of resistance with specified width and depth grade of concrete and steel.
What is Doubly Reinforced Beam?
The reinforced concrete beams in which steel bars are provided in the tension and compression zone refers to doubly reinforced beams.
The balanced reinforced concrete beam of dimension b x d has a moment of resistance Rbd2.
In some situations, due to architectural consideration or headroom limitation, the beam size is limited, and the same beam(b × d) is expected to resist the moment higher than Rbd2.
Only two methods are there to do this.
- Use an over-reinforced section.
- Use a doubly reinforced section.
Since the over-reinforced sections, failure is unexpected without warning, and also, they are uneconomical. Hence the first method is not a valid option.
Consequently, it is suitable to use a doubly reinforced beam section in such conditions. The additional reinforcement is placed in the tension, and the compression zone develops the extra required moment of resistance (greater than Rbd2).
When We UseDoubly Reinforced Beam?
Doubly reinforced beam sections are essential in the following circumstances.
- When the beam’s dimension (b x d) is limited due to many limitations like the headroom restriction, architectural or space consideration, and the singly reinforced section has less resistance to the external moment.
- It is required when either face of the member experiences an external load that means the load is reversing or changing that cause tension on both faces.
- It is practised when the load is eccentric.
- It is idle when accidental or sudden lateral loads govern the beam.
- The continuous slab or beam support sections are generally designed as a doubly reinforced section.
- If a continuous beam’s, some section changes the bending moment sign due to moving load, which creates the compression zone to tension zone or vice versa.
- It is used if the bending moment changes sign.
Advantages of Doubly Reinforced Beam
- The doubly reinforced beam has steel in tension and compression zone to reduce the section’s deflection. It increases the section’s rotation capacity.
- Compression steel and tensile steel increase the section’s flexibility; hence, the doubly RCC section is always used in earthquake-prone areas.
- Compression steel helps in reducing the beam’s long-term deflection.
- The doubly reinforced beam is economical as contrasted to singly reinforced beams.
Difference Between Singly and Doubly Reinforced Beam
The purpose of the design to determine the beam’s size or dimension and provide suitable steel reinforcement. The beam is a flexural member that resists load mainly by bending. The difference between singly and doubly reinforced beam are as follows.
- A Singly reinforced beam holds a steel bar in the tension zone, but in doubly reinforced beams, steel bars are given in both zones, tension, and compression.
- In singly reinforced beam compression, stress resists by the concrete, while in doubly reinforced beam compression steel, counters the compression stress.
- In singly reinforced beam compression, steel acts as a false member provided to tie the stirrups with bars. While in the doubly reinforced beam, compression steel resists compressive stresses and constitutes the addition moment of resistance.
- It is used when the section has to resist moment Rbd2. While it is used when the section has to resist a moment greater than the Rbd2.
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