Clause 6.2 governs the arrangement and splicing of longitudinal reinforcement in seismic beams. At least two bars must be continuous on both top and bottom faces throughout the beam. Lap splices are permitted only in the central half of the span and are prohibited within a distance of 2d from the face of the column (plastic hinge zone). The positive moment capacity at the face of the joint must be at least half the negative moment capacity.
Key Requirements
•At least 2 bars shall be provided continuously on both top and bottom faces of the beam
•Positive steel at a joint face shall be at least half the negative steel at that face
•Neither the positive nor negative moment capacity at any section along the beam shall be less than one-quarter of the maximum moment capacity at either face of the joint
•Lap splices shall be provided only in the central half of the member span
•Lap splices shall not be provided within a distance of 2d from the face of the column/joint, where d is the effective depth
•Lap splice length shall not be less than the development length (Ld) calculated for 1.25 fy
•Not more than 50% of the bars shall be spliced at any one section
•Welded splices and mechanical connections may be used at any location, subject to IS 456 requirements
Formulas
Ld = 1.25 × (fy × φ) / (4 × τ_bd)
Development length for lap splices in seismic beams (25% increase over standard Ld)
Ld = Development length in mmfy = Yield strength of steel in MPaφ = Bar diameter in mmτ_bd = Design bond stress per IS 456 Table 26
Practical Notes
✓The 'central half' rule means for a 6 m span beam, laps are allowed only between 1.5 m and 4.5 m from either support — plan bar lengths accordingly.
✓In practice, run 2–16 mm or 2–20 mm bars full length top and bottom, and add extra bars at supports/midspan as needed.
✓The positive-to-negative steel ratio of 0.5 at joints often governs bottom steel — check this before finalizing the bar schedule.
Common Mistakes
⚠Lapping bars at or near column faces (within 2d) — this is the plastic hinge zone where ductility demand is highest and laps will fail.
⚠Not maintaining continuous bars on both faces — sometimes bottom bars are curtailed near supports, violating the 2-bar continuity requirement.
⚠Splicing more than 50% of bars at one section — stagger laps to avoid creating a weak plane.