One-Way & Continuous Slabs Notes

Last reviewed 16 Sept 2026 · 6 min read

One-way action

A slab supported on two opposite edges, or on four edges but much longer than it is wide, bends mainly in one direction — the short span.

DefinitionWhen a slab is one-way

Supported on two opposite sides only, or
Supported on all four sides with $\frac{l _{y}}{l _{x}} > 2$ (long span / short span).

Main steel runs along the short span; distribution steel runs along the long span.

Typical examples: verandah and corridor slabs, chajjas, slabs spanning between parallel beams, landings.

Effective span

For a simply supported slab: clear span + effective depth, or centre-to-centre of supports, whichever is less. For a cantilever: length to the face of support + $d /2$ . Continuous slabs: centre-to-centre of supports (with IS 456 variations for wide supports).

Loads

Dead load: self-weight (25 kN/m³ × thickness for RCC) + floor finish (typically 1.0–1.5 kN/m²) + partitions where applicable.
Imposed load (IS 875 Part 2): e.g. residential floors 2.0 kN/m², office floors 2.5–4.0 kN/m², roofs with access 1.5 kN/m².
Design load $w_{u} = 1.5 (D L + I L)$ per metre width.

Design a 1 m wide strip as a rectangular beam with $b$ = 1000 mm.

Detailing rules for slabs

Code ProvisionIS 456 — slab reinforcement

Minimum steel (each direction): 0.12% of gross area $b D$ for HYSD bars; 0.15% for mild steel bars. In slabs this rule replaces the beam minimum $0.85 b d / f_{y}$ .
Maximum bar diameter: $D /8$ (one-eighth of total slab thickness).
Maximum spacing of main bars: $3 d$ or 300 mm, whichever is less.
Maximum spacing of distribution bars: $5 d$ or 450 mm, whichever is less.
Nominal cover as per exposure (20 mm for mild exposure; 15 mm allowed for bars up to 12 mm in mild exposure).
At least 50% of the positive steel extends into the support; bars at discontinuous ends may be bent up or provided as top steel to control cracking from partial fixity (commonly 50% of mid-span steel extended over the support for 0.1 $l$ ).

Deflection control

Basic span/effective depth ratios: 20 (simply supported), 26 (continuous), 7 (cantilever) — for spans up to 10 m. Multiply by the modification factor for tension steel (IS 456 Fig. 4), which depends on the steel percentage and the service stress $f_{s} = 0.58 f_{y} \frac{A _{s t, r e q u i r e d}}{A _{s t, p r o v i d e d}}$ . Lightly reinforced slabs get a factor well above 1 (often 1.4–2.0), allowing thinner slabs.

Shear in slabs

Shear stress is usually low. Check $τ_{v} = V_{u} / (1000 d)$ against $k τ_{c}$ where $k$ = 1.30 for slabs 150 mm or thinner, falling to 1.00 at 300 mm (see Shear & Torsion). Shear reinforcement is almost never provided in ordinary slabs; the depth is increased instead.

Continuous one-way slabs — coefficient method

For slabs continuous over three or more approximately equal spans (variation within 15% of the longest), carrying uniform loads, IS 456 allows bending moments and shears from coefficients (with $l$ = effective span; for moments at supports, $l$ is the average of the two adjacent spans).

Code ProvisionIS 456 Table 12 — bending moment coefficients (multiply by

w l^{2}

)

Type of load	Near middle of end span	At middle of interior span	At support next to end support	At other interior supports
Dead load (fixed)	$+ \frac{1}{12}$	$+ \frac{1}{16}$	$- \frac{1}{10}$	$- \frac{1}{12}$
Imposed load (not fixed)	$+ \frac{1}{10}$	$+ \frac{1}{12}$	$- \frac{1}{9}$	$- \frac{1}{9}$

Code ProvisionIS 456 Table 13 — shear force coefficients (multiply by

w l

)

Type of load	At end support	At support next to end support: outer side	Inner side	At all other interior supports
Dead load	0.40	0.60	0.55	0.50
Imposed load	0.45	0.60	0.60	0.60

Imposed-load coefficients are larger because live load may act on some spans and not others (pattern loading).

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One-Way & Continuous Slabs