Slab Design
RC slab flexural design and capacity check. ACI 318-25, Eurocode 2, IS 456:2000.
Input Parameters
ACI 318-25Section Geometry
Input Parameters
EN 1992-1-1Section Geometry
Input Parameters
IS 456:2000Section Geometry
Input Parameters
ACI 318-25Input Parameters
EN 1992-1-1Input Parameters
IS 456:2000Input Parameters
ACI 318-25 · kN-mInput Parameters
EN 1992-1-1 · kN-mInput Parameters
IS 456:2000 · kN-m📚 Design Background & Code References
One-Way Slab — Structural Behaviour
A one-way slab transfers load predominantly in one direction — the short span — to its supporting beams or walls. It is classified as one-way when the aspect ratio of the panel exceeds 2:1 (long side / short side), meaning the short-span stiffness dominates and the contribution from the long direction is negligible.
Structurally, a one-way slab behaves like a wide shallow beam. The same bending theory applies: a compressive zone in the concrete above the neutral axis, a tension zone below carried by the steel reinforcement, and a moment arm between the two resultants.
Unit Strip Method
One-way slabs are designed by isolating a 1 m wide strip running in the span direction. This strip is treated as a simply-supported or continuous rectangular beam with b = 1 m. All loads (kN/m²), moments (kN·m/m), and reinforcement areas (mm²/m) are expressed per unit width. The design then follows the standard beam flexure procedure.
Key ULS Assumptions
- Plane sections remain plane after bending (Bernoulli-Euler hypothesis).
- Concrete reaches its limiting compressive strain: εcu = 0.003 (ACI) or 0.0035 (EC2 / IS 456).
- Tensile strength of concrete is neglected — all tension is carried by the steel.
- Steel has yielded at ULS: σs = fy / fyd.
- Concrete stress distribution idealised as a rectangular (Whitney) stress block.
Shear in One-Way Slabs
Punching and beam shear are generally not critical in solid one-way slabs designed with adequate span/depth ratios. All three codes allow the concrete alone to resist shear in typical slabs (no shear reinforcement required) provided the nominal shear stress does not exceed the concrete shear capacity. If the applied shear exceeds this limit, the slab thickness should be increased rather than adding stirrups.
Temperature & Shrinkage Steel
The primary (span direction) reinforcement resists bending. In the transverse direction, temperature and shrinkage reinforcement is required to control cracking from restrained volume change. Each code specifies a minimum transverse steel ratio — typically 0.0018 to 0.0020 times the gross cross-section — applied across the full slab thickness h.
Effective Depth
No stirrups are present in typical slabs, so there is no additional offset for shear links. The cover ccover is measured to the face of the main bar.
Unit Strip Method
One-way slabs are designed per unit width (b = 1 m). The slab is treated as a series of rectangular beams, 1 m wide, spanning in the short direction. Loads, moments, and reinforcement are all expressed per meter width.
Effective Depth
(No stirrups in slabs — no additional offset for shear links)
Flexural Design (ACI 318-25)
ρ = (0.85f'c / fy) · [1 − √(1 − 2Rn / 0.85f'c)]
As,req = ρ · b · d
Minimum Reinforcement — Table 7.6.1.1
As,min = ρmin · b · h (uses gross depth h, not d)
Maximum Bar Spacing — §7.7.2.3
ACI 318-25 Table 7.3.1.1 Minimum Slab Thickness (Deflection Control)
ACI 318-25 Table 7.3.1.1 gives minimum one-way slab thicknesses to avoid deflection calculations for slabs not supporting or attached to partitions or other construction likely to be damaged by deflection:
| Support Condition | Minimum h (fy = 420 MPa) |
|---|---|
| Simply supported | l/20 |
| One end continuous | l/24 |
| Both ends continuous | l/28 |
| Cantilever | l/10 |
For fy ≠ 420 MPa, multiply tabulated values by (0.4 + fy/700). Minimum h = 90 mm for non-prestressed slabs per §7.3.1.1.
ACI 318-25 §7.6.4 Temperature and Shrinkage Reinforcement
In the transverse direction (perpendicular to the span), temperature and shrinkage reinforcement is required to control cracking from restrained volume change. This steel is not designed for flexure — it carries only the shrinkage and thermal strains.
For Grade 60 (fy = 420 MPa): As,T&S = 0.0018 · b · h
Maximum spacing: s ≤ min(5h, 450 mm) (§7.7.2.3)
ACI 318-25 §22.5 Shear Capacity Without Stirrups
In typical one-way slabs, shear reinforcement is not provided. The slab relies entirely on the concrete shear capacity Vc. If the applied shear Vu exceeds φVc, the slab thickness must be increased — adding stirrups to slabs is impractical.
φVc ≥ Vu required (φ = 0.75 for shear)
Effective Depth
Flexural Design (EN 1992-1-1 §6.1)
z = d · [0.5 + √(0.25 − K / 1.134)] ≤ 0.95d
As,req = MEd / (fyd · z)
where fcd = fck/γc, fyd = fyk/γs
Minimum Reinforcement — §9.3.1.1
fctm = 0.30 · fck2/3 for fck ≤ 50 MPa
Maximum Bar Spacing — §9.3.1.1(3)
EN 1992-1-1 §7.4 Span-to-Depth Ratio (Deflection)
EC2 permits deflection to be verified implicitly by checking the span-to-effective-depth ratio l/d against limiting values from Table 7.4N. These limits assume the slab is not supporting partitions or other elements sensitive to deflection.
| Support Condition | Highly stressed (ρ=1.5%) | Lightly stressed (ρ=0.5%) |
|---|---|---|
| Simply supported | 14 | 20 |
| End span, continuous | 18 | 26 |
| Interior span, continuous | 20 | 30 |
| Cantilever | 6 | 8 |
Values for fyk = 500 MPa. Multiply by 310/σs for other service stress levels, where σs = fyk/γs · As,req/As,prov.
EN 1992-1-1 §9.3.1.2 Secondary (Distribution) Reinforcement
Secondary bars spaced ≤ min(3.5h, 450 mm)
EN 1992-1-1 §3.1.6 Material Strengths Summary
fyd = fyk / γs = fyk / 1.15
Common EC2 concrete classes: C20/25 (fck=20), C25/30 (25), C30/37 (30), C35/45 (35), C40/50 (40)
Effective Depth
Flexural Design (IS 456:2000 Annex G)
Ast = 0.36 · fck · b · xu / (0.87 · fy)
Limit: xu ≤ xu,max (singly reinforced limit)
Neutral Axis Limit — Table E
Minimum Reinforcement — Cl 26.5.2.1
Ast,min = 0.0015 · b · D for mild steel (fy = 250 MPa)
Maximum Bar Spacing — Cl 26.3.3(b)
IS 456:2000 Cl 23.2 & Table 15 Span-to-Effective-Depth Ratios
IS 456 Cl 23.2 controls deflection by limiting the span-to-effective-depth ratio. Basic values from Table 15 are modified by factors depending on the reinforcement percentage and the tension steel stress at service load:
| Support Condition | Basic l/d Ratio |
|---|---|
| Cantilever | 7 |
| Simply supported | 20 |
| Continuous | 26 |
Multiply by modification factors Mt (tension steel, Cl 23.2.1) and Mc (compression steel, if any). For flanged beams, multiply by the flanged-section factor from Cl 23.2.2.
IS 456:2000 Cl 26.5.2.2 Distribution (Temperature & Shrinkage) Steel
In the direction perpendicular to the main span reinforcement, distribution steel controls cracking from temperature change and concrete shrinkage.
As,dist ≥ 0.15% · b · D (mild steel, fy = 250 MPa)
s ≤ min(5d, 450 mm) (Cl 26.3.3b)
IS 456:2000 Table 1 & 2 Material Design Values
Design steel strength: fs,design = fy / γm = 0.87 fy
Common grades: M20 (fck=20), M25 (25), M30 (30), M35 (35), M40 (40) MPa
Steel: Fe 415 (fy=415), Fe 500 (500), Fe 550 (550) MPa
References
- [1]ACI 318-25 — Building Code Requirements for Structural Concrete. American Concrete Institute, 2025. Chapter 7 (One-Way Slabs): §7.6.1 (minimum reinforcement), §7.7.2 (bar spacing), §22.2 (flexural design).
- [2]EN 1992-1-1:2004 — Eurocode 2: Design of Concrete Structures. CEN, Brussels. §9.3 (Solid slabs), §9.3.1.1 (minimum and maximum reinforcement), Table 7.4N (span/depth limits for deflection control).
- [3]IS 456:2000 — Plain and Reinforced Concrete — Code of Practice, 4th Rev. Bureau of Indian Standards, New Delhi. Cl 23 (Slabs), Cl 26.3.3 (bar spacing), Cl 26.5.2.1 (minimum tension reinforcement).
- [4]Wight, J.K. & MacGregor, J.G. — Reinforced Concrete: Mechanics and Design, 7th Ed. Pearson, 2016. Chapter 13 (One-Way Slabs).
- [5]Mosley, W.H., Bungey, J.H. & Hulse, R. — Reinforced Concrete Design to Eurocode 2, 7th Ed. Palgrave Macmillan, 2012. Chapter 8 (Slabs).
- [6]Pillai, S.U. & Menon, D. — Reinforced Concrete Design, 3rd Ed. Tata McGraw-Hill, 2009. Chapter 11 (Design of One-Way Slabs).