To start the design of doubly reinforced beam you should have an idea about this type of section. As the name implies doubly reinforced beams, we are providing the steel in both zones. In the case of a beam, the zone is basically compression and tension zone.
They are used in construction projects that require high load – bearing capacity, such as bridges, high – rise buildings. These beams allows for more efficient use of materials, as the additional steel provides added strength without the need for a larger c/s area. This can save on materials and construction costs. Furthermore, these beams are a practical solution for old structures, as they can be added to beams to increase their load bearing.
Check this post to know more about a doubly reinforced beam section.
As in this type of beams, the beam size is a constrain. Our main aim is only to calculate the area of reinforcement required in each zone of the beam section.
Procedure to design a doubly reinforced beam
The step by step procedure to design a beam section as per IS:456:2000 are as follows:
- Step 1: Calculate the depth of neutral axis.
Apply the equilibrium condition for the beam section. $$ C_{uc} +C_{us} = T_{u}$$
where Cuc and Cus are the compressive force in concrete and steel resp. and Tu is the tensile force in the steel.
The depth of neutral axis is $$ x_u = \frac{f_{st} A_{st} – (f_{sc} – 0.447 f_{ck}) A_{sc}}{0.362 f_{ck} b} $$
- Step 2 : Calculate the factored moment
In this case, we split the total bending moment into two parts.
$$M_u = M_{u lim} + \Delta M_{u} $$
where Mu lim is the limiting moment capacity which is equal to the moment of resistance of a balanced RCC beam section and the second component is for the moment capacity which should be taken care by the compression reinforcement.
- Step 3 : Design of compression reinforcement
Calculate the value of the area of steel in the compression zone using :
$$ A_{sc} =\frac{\Delta M_{u}}{f_{sc}(d -d ‘)}$$
Area of steel in tension zone is the sum of area of steel to resist the compressive and tensile forces in the tension zone.
$$ A_{st } = A_{st1} + A_{st2} $$
$$A_{st1} = \frac{0.362 f_{ck} b x_{u}}{0.87 f_y}$$
$$A_{st2} = \frac{A_{sc}f_{sc}}{0.87f_y}$$
This is not the end of the design procedure.
Then you have to check for shear reinforcement and deflection control of the RCC beam section. This part I will discuss in the up coming posts.
Conclusions
In conclusion, designing a doubly reinforced beam section is a simple process that requires a thorough understanding of the Indian standard codes.
Additionally, it is important to consider the potential for creep, and temperature effects when designing a doubly RCC beam. By following these steps, engineers can design a doubly reinforced beam section that is both efficient and effective.
Check this post to know more about min. and max. steel of a beam section
Beam design
- RCC Beam Design is a free app for designing reinforced concrete beams as per Indian Standards.
- RCC Design and detailing could be performed by Limit State Method specified in IS456:2000
- Option to save the design projects in local storage.
- Detailed calculation steps presented for verification and validation.
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