Reinforced concrete slab serviceability to SIA 262

Checks a strip of reinforced concrete slab under uniform, partial and line loads in service, to SIA 262:2025: the moments of the load combinations, concrete compression and creep, bar stresses and crack control, crack width, long-term deflection with cracking, creep and shrinkage, finishes, comfort and natural frequency. Every step shows its formula and the clause it comes from.

What it checks

• Load combinations: the self-weight g₁ = γ_RC · h is added to the loads entered; for each combination, ΣF, R_A, R_B, M_A = −η_A · M_0,max, M_max at x_M and M_B = −η_B · M_0,max, per metre of width, then M_qp, M_fr and M_ra at the critical sections.
• Section per metre of width: a_s and a′_s, d and d′_sup, with f_cm = f_ck + 8, clause 3.1.2.2.3, equation (6).
• Cracking moments M_cr and M′_cr (4.2.1.14), with f_ct,eff = k_t · f_ctm, clause 4.4.1.3, equations (99) and (100).
• Concrete compression, check 1: σ_c,qp ≤ 0.6 · f_cm (3.1.2.6.3).
• Bar stresses in the span and over the support against σ_s,adm = √(9 · E_s · f_ctm · w_nom / Ø_s) ≤ f_yk, clause 4.4.2.3.4, equation (102), with w_nom from Table 17: at first cracking (minimum reinforcement, 4.4.2.3.3) and under the quasi-permanent load.
• No yielding: σ_s,ra ≤ f_yd − 80 N/mm² under the characteristic combination (Table 17).
• Crack width (optional) to EN 1992-1-1 §7.3.4, equations (7.8) to (7.11), against w_max.
• Deflection: w_c (4.4.3.2.4), the estimates w_cφ = w_c · (1 + φ), equation (103), and w₁₀₄ = (1 − 20 · ρ′)/(10 · ρ^0.7) · (0.75 + 0.1 · φ) · (h/d)³ · w_c, clause 4.4.3.2.5, equation (104), and a detailed calculation of the curvatures along the span to EN 1992-1-1 §7.4.3, equations (7.18) to (7.21).
• Deflection limits (4.4.3.2.1): w_∞ for appearance, Δw = w_∞ − w_g1,0 for finishes and partitions, and w_q,fr under ψ₁ · q_k for comfort.
• Natural frequency f₁ ≥ f_min (4.4.4.2), from the Rayleigh quotient of the static deflection curve.

What you enter and what you get

You enter the static system (simply supported, fixed at one or both ends, or a span of a continuous member with the support moments η_A and η_B as fractions of M₀, the largest moment of the same loads on a simple span), the span l and, for a panel spanning both ways, the share α of the load carried by this strip. Then h, the bottom bars Ø at s with cover c, the top bars Ø′ at s′ with cover c′, whether the top bars run through the span, the materials, the creep and shrinkage data and the faces exposed to drying. You enter up to five uniform loads, each on the whole span or on a part of it, and up to five line loads across the strip at x₁, each with a permanent and a variable part (g_2,1 and q₁, G₁ and Q₁ for the first), then ψ₁ and ψ₂, the requirement level, the crack width option and the deflection limits as fractions of the span.

You get φ and ε_cs, the section stiffnesses, the support forces and moments of each load combination, the moments at the critical sections, every check with its utilisation and clause, and both deflection estimates. With the Basic plan, it prints as an A4 calculation note.

Assumptions and limits

• Moments come from an elastic, uncracked analysis; the redistribution caused by cracking is not followed.
• η_A and η_B apply to every load combination: take them from an elastic analysis of the continuous member under a similar layout of the loads.
• A line load is entered in kN per metre of width; a load on a small area is entered divided by the width over which it is spread.
• A two-way panel is checked as a beam strip under α · p, with α from your plate analysis.
• The bottom bars are assumed to run to the supports.
• φ follows EN 1992-1-1 Annex B and ε_cs EN 1992-1-1 §3.1.4, or your own values. The notional size h₀ = 2 · A_c/u counts only the faces exposed to drying.
• Equation (104) is an upper bound (4.4.3.2.6). The detailed calculation is the detailed study of 4.4.3.2.3.
• For finishes, the creep under g₁ before they are placed is counted as occurring after, on the safe side.

Worked example

A 240 mm slab, end span of a continuous slab with l = 6 m, Ø12 at 150 mm at the bottom and Ø14 at 150 mm over the support, f_ck = 30 N/mm², g_2,1 = 2 kN/m² and q₁ = 2 kN/m² on the whole span, a further q₂ = 2 kN/m² from 3 m to 6 m, a permanent line load G₁ = 4 kN/m at 2 m, increased requirements, w_max = 0.3 mm and f_min = 5 Hz. These are the module's own results for its worked example, computed when this page was built.

How this module is verified

Verified clause by clause against SIA 262:2025 and against independent calculations used on real projects, before release.

Questions

Why is equation (103) valid in the worked example?

The quasi-permanent moment in the span, 25.9 kNm, stays below M_cr = 27.8 kNm. The distribution coefficient ζ at the point of largest deflection is then 0, and equation (104) gives an upper bound only.

Which deflection is used in the checks?

You choose. The detailed calculation follows the curvatures along the span. The SIA estimate takes equation (103) when the span stays uncracked, otherwise equation (104). Both are always shown.

Which plan includes this module?

The Basic plan, with its calculation note.

Related

• Two-way slab on edge supports
• Slab section resistance table
• Concrete slabs: all modules
• SIA 262: modules and clauses

Norm: SIA 262:2025 · Page updated: 5 November 2026