Understanding Stirrup Spacing in RCC Beams

Vertical stirrups in a Reinforced Cement Concrete (RCC) beam serve a crucial role in resisting shear forces. Shear forces are internal forces that occur within a beam due to external loads, tending to cause one section of the beam to slide past an adjacent section.

In a typical simply supported beam subjected to downward loads, the shear force distribution is not uniform along the beam's length. Shear forces are generally:

• Maximum near the supports.
• Minimum (often zero) near the center of the beam.

The amount of shear reinforcement required in a beam is directly proportional to the magnitude of the shear force it needs to resist. Where the shear force is high, more reinforcement is needed to safely resist the forces. Where the shear force is low, less reinforcement is needed.

Relating Shear Force to Stirrup Spacing

To provide more shear resistance in areas of high shear force, the stirrups need to be placed closer together. Conversely, in areas where the shear force is low, the stirrups can be spaced farther apart.

• Near the supports, where shear force is maximum, the spacing between vertical stirrups should be minimum. This provides a higher density of shear reinforcement to combat the large shear forces.
• Near the center of the beam, where shear force is minimum, the spacing between vertical stirrups can be maximum (up to permissible limits specified by design codes).

Therefore, the spacing between two vertical stirrups in a rectangular RCC beam is minimum near the supports, gradually increasing towards the center of the beam.

Analyzing the Options

• Option 1: Minimum near the supports. This aligns with the principle that maximum shear force occurs near supports, requiring closer stirrup spacing.
• Option 2: Decreased towards the centre of the beam. This is incorrect. Spacing increases towards the center where shear force is lower.
• Option 3: Maximum near the supports. This is incorrect. Maximum shear force near supports requires minimum spacing.
• Option 4: Minimum near the centre. This is incorrect. Minimum shear force near the center allows for maximum spacing.

Based on the distribution of shear forces and the function of stirrups as shear reinforcement, the spacing of vertical stirrups is minimum near the supports where the shear force is highest.