Cracking in concrete is common on any civil site. However, cracks do not always mean failure. In fact, once a crack forms, the structure does something very important. It adjusts itself. This process is known as Redistribution of stress after cracking.

In simple words, when concrete cracks, the internal forces move to other parts of the section. Because of this movement of forces, the member can still carry load safely. Therefore, understanding Redistribution of stress after cracking is essential for every civil engineer and site supervisor.

What Happens When Concrete Cracks?

Concrete is strong in compression but weak in tension. When a beam bends, the bottom fibres go into tension. Once the tensile stress becomes higher than the tensile strength of concrete, cracks appear.

However, reinforced concrete is not plain concrete. It contains steel bars. After cracking:

• Concrete stops carrying tension.
• Steel reinforcement starts carrying most of the tensile force.
• Compression remains in the upper concrete zone.
• Internal forces adjust within the section.

This internal adjustment is called Redistribution of stress after cracking.

Why Redistribution of Stress After Cracking Is Important

Redistribution of stress after cracking helps in:

1. Preventing sudden failure
2. Improving ductility
3. Allowing warning before collapse
4. Making structures economical

If stress could not redistribute, structures would fail suddenly and dangerously. Fortunately, reinforced concrete behaves in a ductile way when designed correctly.

Simple Example

Let us consider a simply supported reinforced concrete beam.

Before cracking:

• Concrete carries both compression and small tension.
• Steel shares a small part of tension.

After cracking:

• Tension zone concrete cracks.
• Steel bars carry most tensile force.
• Neutral axis shifts upward.
• Compression block in concrete increases.

This change in internal force distribution is Redistribution of stress after cracking.

Behaviour in Continuous Beams

Redistribution of stress after cracking becomes more visible in continuous beams.

In a continuous beam:

• Negative moments form at supports.
• Positive moments form at mid-span.

When cracking occurs at supports:

• Moment capacity reduces locally.
• Some bending moment shifts to adjacent spans.

This shifting of bending moment is also a form of Redistribution of stress after cracking.

Role of Steel Reinforcement

Steel plays the most important role in Redistribution of stress after cracking.

Because steel is ductile:

• It stretches before failure.
• It gives warning signs like large deflection and cracks.
• It allows stresses to move gradually.

If steel ratio is too low, cracks become wide.
If steel ratio is too high, failure becomes brittle.

Therefore, proper reinforcement design is necessary for effective Redistribution of stress after cracking.

Neutral Axis Shift

After cracking:

• The neutral axis moves upward.
• The compression zone becomes deeper.
• Tensile stresses concentrate in steel.

This shift helps maintain equilibrium of internal forces.

Case Study: Two-Span RC Beam

A two-span reinforced concrete beam in a residential building developed cracks at the internal support due to higher negative moment.

Observations:

• Cracks appeared at top surface near support.
• No sudden collapse occurred.
• Load was redistributed to mid-span regions.
• Deflection increased slightly but remained within limits.

Reason:
Proper reinforcement detailing allowed Redistribution of stress after cracking.

Lesson:
Good detailing and ductile design prevent sudden structural failure.

Design Standards

1. IS 456

This Indian standard allows moment redistribution up to 30% under certain conditions. It ensures:

• Adequate rotation capacity
• Proper ductility
• Minimum reinforcement ratios

2. Eurocode 2

This code also permits controlled moment redistribution, provided strain limits are satisfied.

Both standards recognise Redistribution of stress after cracking as a normal structural behaviour.

Factors Affecting Redistribution of Stress After Cracking

Several factors influence how stresses redistribute:

• Reinforcement ratio
• Concrete strength
• Steel grade
• Member depth
• Span length
• Support conditions
• Loading pattern

For example, higher ductility in steel improves Redistribution of stress after cracking.

Cracking and Serviceability

Cracks are not always dangerous. However, crack width must be controlled.

Excessive cracking may lead to:

• Corrosion of reinforcement
• Durability issues
• Water leakage
• Reduced aesthetics

Therefore, Redistribution of stress after cracking must be controlled within serviceability limits.

Plastic Hinge Formation

In advanced structural behaviour, Redistribution of stress after cracking leads to plastic hinge formation.

A plastic hinge:

• Forms when steel yields.
• Allows rotation without sudden collapse.
• Helps redistribute bending moments.

This concept is widely used in seismic design.

Real-World Example: Earthquake Performance

During earthquakes, buildings experience large bending moments.

Structures designed with ductile detailing:

• Develop cracks.
• Form plastic hinges.
• Redistribute forces.
• Avoid sudden collapse.

Thus, Redistribution of stress after cracking improves safety in seismic zones.

Practical Site Considerations

On site, engineers should:

• Check crack patterns.
• Ensure proper curing.
• Verify reinforcement placement.
• Avoid under-reinforced or over-reinforced sections.
• Follow detailing drawings carefully.

Proper supervision ensures safe Redistribution of stress after cracking.

Common Mistakes to Avoid

• Ignoring crack patterns
• Using insufficient reinforcement
• Poor anchorage length
• Improper lap splices
• Not following design codes

These mistakes reduce the capacity for Redistribution of stress after cracking.

Conclusion

Redistribution of stress after cracking is a natural and beneficial behaviour of reinforced concrete structures. Instead of causing failure, cracking allows internal forces to adjust. Steel reinforcement carries tensile forces, while concrete resists compression.

When designed according to standards like IS 456 and Eurocode 2, Redistribution of stress after cracking improves ductility, safety, and economy. Therefore, engineers must understand this concept clearly.

Cracks do not always mean danger. They often show that the structure is behaving as expected.

FAQs

1. Is cracking always dangerous?

No. Small cracks are normal in reinforced concrete. Controlled cracking allows Redistribution of stress after cracking.

2. What allows stress redistribution?

Steel reinforcement and ductile behaviour allow internal forces to shift safely.

3. How much moment redistribution is allowed in IS 456?

IS 456 permits up to 30% redistribution under specified conditions.

4. Does redistribution reduce strength?

No. It adjusts internal forces while maintaining equilibrium and safety.

5. Why is ductility important?

Ductility allows deformation before failure, which is essential for safe Redistribution of stress after cracking.