Doubly Reinforced Beam: ESE & GATE CE Crash Course

A doubly reinforced beam is a reinforced concrete beam that contains steel reinforcement on both the tension side and the compression side. It is used when the depth of the beam is restricted or when the bending moment is too high for a singly reinforced beam. The compression steel helps increase the load-carrying capacity and improves ductility.

 Doubly reinforced beams are commonly used in buildings, bridges, and structures subjected to heavy loads. Understanding their behavior, design principles, and applications is essential for civil engineering students and is especially important for competitive exams, as it helps in designing safe, economical, and efficient concrete structures.

PW Online GATE Coaching Courses

Doubly Reinforced Beam

A reinforced concrete beam often requires strengthening beyond just tension reinforcement. This leads to the use of doubly reinforced sections.

Concept of Doubly Reinforced Beam

A Doubly Reinforced Beam incorporates steel reinforcement in both the tension and compression zones of the concrete section. This design is necessary when the concrete's compressive strength alone is insufficient to resist the compressive forces resulting from the applied bending moment. Adding compression steel allows the beam to carry a greater bending moment without increasing its cross-sectional dimensions.

When to Use Doubly Reinforced Beams

Doubly reinforced beams are primarily used in specific scenarios.

• Restricted Dimensions: When the cross-sectional dimensions of the beam are limited due to architectural or other constraints.

• Excessive Bending Moment: If the bending moment from loads exceeds the capacity of a singly reinforced section of the same dimensions.

• Reversal of Moments: In cases where the beam may experience moment reversal, such as in continuous beams or earthquake-resistant structures.

• Reduced Long-Term Deflection: Compression steel helps reduce creep and shrinkage effects, leading to less long-term deflection.

Check: GATE Civil Engineering Notes

Advantages of Doubly Reinforced Beams

There are several benefits to using this design.

• Increases the moment-carrying capacity of the beam.

• Reduces the long-term deflection due to creep of concrete.

• Enhances ductility, providing more warning before failure.

• Helps in placing and holding stirrups in position during construction.

Disadvantages of Doubly Reinforced Beams

This design also has some drawbacks.

• Increases the cost due to additional steel and labor for placement.

• Adds complexity to the design and analysis process.

Design Philosophy

The design of Doubly Reinforced Beams often involves treating the section as two parts. One part is a singly reinforced beam with the maximum possible tensile steel. The second part consists of the remaining moment carried by the compression steel and the additional tension steel.

Neutral Axis (NA) Position

The position of the Neutral Axis is crucial in determining the stress distribution within the beam. For a doubly reinforced section, the NA shifts, impacting the forces in concrete and steel.

Moment of Resistance Calculation

The total moment of resistance for a Doubly Reinforced Beam is typically considered as the sum of two moments:

• Moment of resistance provided by the concrete and tension steel (forming a singly reinforced section).

• Moment of resistance provided by the compression steel and the additional tension steel.

Key Mechanisms of Doubly Reinforced Beam

Understanding the internal forces and stress distribution is essential for analyzing these beams.

Stress-Strain Diagram for Concrete and Steel

The behavior of concrete and steel under load is defined by their respective stress-strain curves. For concrete, a parabolic-rectangular stress block is often used in Limit State Design. For steel, an elastic-perfectly plastic or bilinear curve is typically assumed. These diagrams help calculate internal forces.

Assumptions in Limit State Design

Key assumptions for Doubly Reinforced Beam design under Limit State Method include:

• Plane sections remain plane after bending, implying linear strain distribution.

• Maximum strain in concrete at the outermost compression fiber is 0.0035.

• Tensile strength of concrete is ignored.

• Stress-strain curves for concrete and steel are as per code (e.g., IS 456).

• Compression steel yields if its strain is above the yield strain.

Formulae for Moment of Resistance (Result)

The total ultimate moment of resistance () of a doubly reinforced section is the sum of moments from the concrete-tension steel couple () and the compression steel-tension steel couple ().

Where is the moment of resistance of the balanced singly reinforced section, and is the moment of resistance provided by the additional compression and tension steel?

Elevate your GATE Exam readiness with Physics Wallah’s GATE Online Courses . The PW GATE Online Coaching offers comprehensive live sessions tailored to the syllabus, invaluable study materials, practice tests, and much more. Explore Now!