Clause 23.2 defines the effective span used for analysis and design of beams and slabs. The effective span depends on the support condition — simply supported, continuous, or cantilever — and accounts for bearing width. Using the correct effective span is critical as all bending moments and shear forces are computed based on it.
Key Requirements
•Simply supported beam/slab: Leff = clear span + d, or c/c of supports, whichever is less
•Continuous beam/slab: if support width < 1/12 of clear span, Leff = same as simply supported. If support width ≥ 1/12, Leff = clear span
•Cantilever: Leff = clear span + d/2 (for fixed end at face of support)
•Cantilever at end of continuous beam: Leff = clear span + half the bearing width or clear span + d/2, whichever is less
•For frames: centre to centre of supporting members
Formulas
Leff = min(Lc + d, Lcc)
Effective span for simply supported beams/slabs
Leff = Effective spanLc = Clear span (face to face of supports)d = Effective depth of memberLcc = Centre to centre distance of supports
Leff = Lc + d/2
Effective span for cantilever members
Leff = Effective spanLc = Clear span from face of support to free endd = Effective depth at support
Practical Notes
✓For most practical cases (beams on 230 mm walls), Leff ≈ clear span + 115 mm on each side = clear span + 230 mm, which is also close to c/c of supports.
✓In frame analysis (columns as supports), always use c/c span — this is standard in STAAD/ETABS models.
✓For slabs spanning one way, the effective span is measured along the shorter span direction.
Common Mistakes
⚠Using clear span instead of effective span for bending moment calculations — this underestimates moments.
⚠For continuous beams, not checking whether support width > 1/12 clear span — this changes the effective span formula.
⚠Confusing effective span with clear span in software input — STAAD uses c/c by default.