Ductile Detailing of
RCC Structures

Scope & Applicability

IS 13920 prescribes provisions for the detailing of Reinforced Concrete (RCC) members subjected to seismic forces. It ensures that the structure possesses adequate Ductility (ability to deform plastically) and energy dissipation capacity. Cl. 1

The code covers:
• Flexural Members (Beams)
• Compression Members (Columns)
• Special moment resisting frames
• Structural Walls (Shear Walls)
• Footings supporting these members

Material Requirements

The 2016 revision introduced stricter material grades to ensure higher strength and ductility. Cl. 4

Elongation Requirement: Steel should have a minimum elongation of 14% (measured on a gauge length of 200mm).

Ductile Beam Detailing

Beams must be designed as "Stronger than Columns" to prevent soft-story mechanisms. Plastic hinges should form in beams, not columns. Cl. 6 & 7

Longitudinal Reinforcement

• Min Bars: At least 2 bars at top and 2 at bottom throughout the length.
• Max Area: Total longitudinal area shall not exceed 2.5% of the gross cross-sectional area ($bh$).
• Top Reinforcement: Positive moment steel at supports shall be at least 50% of the negative moment steel provided at that support.

Shear Reinforcement (Stirrups)

Close-spaced closed ties (stirrups) are required in the "Plastic Hinge Zones" near joints.

• Hook: Must be $135^\circ$ bend with a straight extension of $\ge 10d_b$ (where $d_b$ is bar diameter).
• Spacing ($S_v$):
  Within $2d$ (where $d$ = effective depth) from face of joint:
  $S_v \le \min(d/4, 8 \times d_{b(min)}, 100 \text{ mm})$
• Outer Zone: Spacing can increase up to $d/2$.

Ductile Column Detailing

Columns are critical vertical elements. Their failure leads to total collapse. Hence, strict confinement is required. Cl. 7

Longitudinal Bars

• Min Bars: At least 4 bars (one at each corner).
• Min Ratio: Longitudinal steel area $\ge 0.8\%$ and $\le 6\%$ of gross area.
• Splices: Splicing is allowed only in the central half of the column height (avoid plastic hinge zone). Lap length shall be calculated based on development length in tension.

Ties (Transverse Reinforcement)

• Shape: Closed rectangular or circular hoops. Every alternate tie must have a $135^\circ$ hook and $10d_b$ extension into the core.
• Diameter: Min 6 mm (preferably 8 mm or 10 mm). If diameter of main bar $> 14$ mm, use min 8 mm ties.
• Spacing:
  Within plastic hinge zone (length $L_p$):
  $S \le \min(D/4, 8 \times d_{b}, 100 \text{ mm})$ where $D$ is the smallest dimension of the member.
  Outside hinge zone: Spacing $\le \min(3 \times S_{hinge}, D)$.

Confinement Length ($L_0$)

Special confining reinforcement (closer spacing) is required over a length $L_0$ from the joint face:

• $L_0 \ge$ Largest of:
  1. Largest lateral dimension of the member
  2. $1/6$ of clear span
  3. 450 mm
• If discontinuity exists (soft storey), extend this length into the adjoining member.

Beam-Column Joints

Joints transfer moments and shears between beams and columns. They must be designed to remain elastic while beams yield. Cl. 8

• Shear Force: Calculate horizontal shear force in the joint panel. Provide horizontal ties if the calculated shear exceeds the capacity of the concrete alone.
• Detailing: The special confining reinforcement required in the column (ties) must extend through the beam-column joint region fully.
• Bar Arrangement: Ensure beams frame into the column on opposite sides. If beams frame in perpendicular directions, provide extra ties in the column to resist the unbalanced load.

Shear Wall Detailing

For tall buildings, shear walls act as deep cantilevers. They require boundary elements for confinement. Cl. 9

Boundary Elements

Special confined boundary elements are required at the ends of walls in zones of high compression (plastic hinge zone).

• Dimensions: Width $\ge$ max(Lateral dim./4, 450mm).
• Reinforcement: Similar to columns (min 0.8% steel, closer ties).

Wall Web Reinforcement

• Vertical & Horizontal: Min 0.0025 of gross area in each direction.
• Spacing: Max 300 mm.
• Diameter: Min 8 mm.

Critical Changes in 2016 Revision

The 2016 edition brought significant updates compared to 1993:

• Mandatory Zones: Explicitly stated mandatory nature for Zones III, IV, V.
• Steel Grade: Allowed Fe500 (previously limited mostly to Fe415 for better ductility, though Fe500D is now common).
• Hook Angle: Re-emphasized $135^\circ$ hooks with $10d_b$ extension (crucial for anchorage).
• Slab Interaction: Added provisions for slab reinforcement acting as part of the flange in T-beams during seismic cycles.
• Pile Foundations: New clauses for detailing pile caps and pile heads to transmit forces from superstructure.
• Soft Storey: Stricter requirements for columns in soft-storeys (ground floor with parking/commercial). Ties must extend full height.

Related Codes

IS 13920 cannot be read in isolation. It is the detailing arm of IS 1893.