In this article, we will discuss
What is Beam? | Singly Reinforced Beam and Doubly Reinforced Beam | Where is the singly reinforced beam used? | Where is the singly reinforced beam used? | The Procedure for Doubly Reinforced Beam Design
What is Beam?
Difference between Singly Reinforced Beam and Doubly Reinforced Beam: A beam is a structural component that supports all vertical loads and prevents bending. Beams can be made out of a variety of materials, including steel, wood, and fibers. But reinforced cement concrete is the most widely used material.
Singly Reinforced Beam and Doubly Reinforced Beam
A beam is a structural element that prevents deflection and primarily carries vertical loads that are applied vertically to the beam's axis.
When a load is applied to the beam, the main cause of its deflection is bending, which produces reaction forces at the beam's support point.
When all of these forces act on a beam, the resultant shear force and bending moment cause internal stresses, strains, and beam deflection.
Beams are recognized using the support method, length, material profile (cross-sectional shape), and equilibrium conditions.
A variety of materials are used to build beams made of steel, wood, and other materials. But the RCC beam takes center stage.
Beams transport the vertical loads to the supporting column or masonry, which then transports them to the foundation.
The two main types of simply supported beams are as follows.
Read More
What is a Singly Reinforced Beam?
A singly reinforced beam is one that only has steel reinforcement in the tension zone.
The tension load in this beam is carried by the reinforcement, and the compressive load brought on by the beam's deflection is carried by the concrete.
It is difficult to reinforce the tension zone because we have to tie the stirrups. As a result, we also include two dummy bars in the compression to tie the stirrups, which serve only to hold the stirrups.
Where is the singly reinforced beam used?
When the balanced reinforced concrete beam of dimension b x d moment of resistance is Rbd2, the singly reinforced beam is appropriate.
Significant tensile and compressive strength are present in concrete. Steel is therefore provided in the tensile zone of a single-reinforced beam, which is good in tension and compression.
With a concrete and steel grade that is specified for its width and depth, a single reinforced beam has a limited moment of resistance.
What is the Doubly Reinforced Beam?
Double-reinforced beams are those made of reinforced concrete that has steel bars in both the tension and compression zones.
The balanced reinforced concrete beam has a resistance moment of Rbd2 and a dimension of b x d.
In some circumstances, the beam size is constrained due to architectural considerations or headroom restrictions, and the same beam (b x d) is expected to resist a moment greater than Rbd2.
There are only two ways to do this.
- Use an over-reinforced section.
- Use a doubly reinforced section.
Failure comes suddenly and without warning due to the over-reinforced sections, and they are also unprofitable. Therefore, the first approach is not a viable choice.
Therefore, in such circumstances, a doubly reinforced beam section is appropriate. The compression zone develops the extra necessary moment of resistance while the additional reinforcement is placed in the compression zone (greater than Rbd2).
Why We Use Doubly Reinforced Beam?
The following situations necessitate the use of doubly reinforced beam sections.- The singly reinforced section is less resistant to the external moment when the beam's dimension (b x d) is constrained due to various restrictions, such as headroom restriction, architectural, or space considerations.
- It is necessary whenever one of the member's faces is subjected to an external load, meaning the load is shifting or reversing and putting tension on both faces.
- When the load is eccentric, it is used.
- When unexpected or sudden lateral loads control the beam, it is idle.
- Typically, the continuous slab or beam support sections are constructed with two layers of reinforcement.
- If the beam is continuous, a section may experience a change in the bending moment sign due to a moving load, which could result in a compression or tension zone.
- If the bending moment's sign changes, it is used.
Advantages of Doubly Reinforced Beam
- Steel is used in the tension and compression zones of the doubly reinforced beam to lessen section deflection. It boosts the section's ability to rotate.
- The section's flexibility is increased by the addition of compression and tensile steel, which is why doubly RCC sections are always used in earthquake-prone regions.
- 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
Difference Between Singly Reinforced Beam and Doubly Reinforced Beam
The design's goal was to establish the size or dimension of the beam and provide the appropriate amount of steel reinforcement. The flexural member known as the beam primarily resists loads by bending. The following is the distinction between single and double reinforced beams.
- A steel bar is held in the tension zone of a single-reinforced beam, but in doubly reinforced beams, steel bars are provided in both the tension and compression zones.
- In a single-reinforced beam, concrete acts as a stress resister, whereas in a double-reinforced beam, steel acts as a stress counter.
- Steel serves as a false member in the compression of a singly reinforced beam and is used to connect the stirrups to the bars. Compression steel, on the other hand, resists compressive stresses and provides an additional moment of resistance in the doubly reinforced beam.
- When the section must resist moment Rbd2, it is employed. When the section must withstand a moment greater than the Rbd2, it is employed.
The Process of Designing a Single Reinforced Beam
1. Calculate the value of N using the formula provided:[Were N = AxixConstant neutral critical.]
Where J is known as Lever arm constant
4. Select the appropriate width (b) and compare the bending moment and the resistance moment with sufficient depth for that section.
5. Calculate the value of At using the formula provided
Where At = Area of elastic steel.
It is necessary to apply reinforcement (steel bars) in the compression and tension zones for a particular section of the beam.
Single Reinforcement Beam refers to reinforcement that is only in the tension zone, while Doubly Reinforced Beam refers to reinforcement that is both in the tension and compression zones.
There will be a load in the tension and compression zones in both scenarios. This is because a beam structure cannot be found without stirrups.
Two reinforcements must be placed in the compression zone of the reinforced beam singly to keep the stirrups in their upright position. This is simply untrue because these two never carry loads or carry them on your body.
The upper portion of a beam is referred to as the compression zone and the lower portion is the stress zone.
The Procedure for Doubly Reinforced Beam Design
1. Determine the limit moment of the resistance.
Mu Lim = 87.fy.Astl.d [1 – 0.42 Xu max]
Mu – Mu lim = fsc.Asc (d-d’)
Ast2 = Asc.fsc / 0.87fy
Ast, Ast = Ast1 + Ast2
Doubly reinforced beam analysis
The following is a summary of the analysis steps for doubly reinforced rectangular beams:
Step(1) Using clause 38.1 of code IS 456:2000, determine the upper limit of neutral axis depth (Xu, max).
Step(2) Find Xu while taking into account force equilibrium and assuming fsc = fst = 0.87fy;
Where fcc is the level of compressive steel in concrete's compressive stress.
So, we can take
Fcc = 0.446fck
Step(3) Compare Xu with Xu, max to determine whether the section is under or over-reinforced. Therefore, the assumption that st = 0.87fy was made in step 1 is valid.
If Xu>Xu, max, the section is over-reinforced, and the fit can be determined using the strain compatibility method.
Step(4) Find the strain in compressive steel
Fcc = 0.446fck
Step(3) Compare Xu with Xu, max to determine whether the section is under or over-reinforced. Therefore, the assumption that st = 0.87fy was made in step 1 is valid.
If Xu>Xu, max, the section is over-reinforced, and the fit can be determined using the strain compatibility method.
Step(4) Find the strain in compressive steel
Step(5) Using the equation, determine the moment of resistance (Mu)
