Isolated Footing Design as per IS 456: Step-by-Step Solved Example + Free Checker

An isolated footing is the first RCC member most civil engineers design on their own. The arithmetic is short, but four separate checks govern it: bearing on soil, bending, one-way shear and punching shear. In practice the depth is almost always set by shear, not by bending. This guide works through a complete design to IS 456:2000, with every clause reference. It shows a trial depth that fails, explains why, and ends with a reinforcement detail, a bar bending schedule and a free checker you can use on your own footing.

Design data

Step 1: Size the footing for soil bearing

Size the plan area with service loads, not factored loads, because SBC is itself a service-level value. Allow about 10% for the footing’s self-weight and backfill.

• Area required = 1.10 × 1200 / 200 = 6.6 m²
• Side = √6.6 = 2.57 m → adopt B = 2.6 m, area = 6.76 m²
• Check once D is known: (1200 + 6.76 × 0.6 × 25) / 6.76 = 192.5 kN/m² ≤ 200 ✓

Step 2: Net factored upward pressure

For bending and shear, use the factored column load only. The footing’s own weight acts straight down on the soil it sits on, so it does not bend the slab.

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• P_u = 1.5 × 1200 = 1800 kN
• q_u = 1800 / 6.76 = 266.3 kN/m²

Step 3: Bending moment at the column face

IS 456 cl. 34.2.3.2 places the critical section for bending at the face of the column. Each side of the footing behaves as a cantilever projecting from that face.

• Cantilever a = (2.6 − 0.4) / 2 = 1.1 m
• M_u = q_u × B × a² / 2 = 266.3 × 2.6 × 1.1² / 2 = 418.8 kN·m
• Depth needed for bending alone (Fe500, M_u,lim = 0.133 f_ck b d²): d = √(418.8 × 10⁶ / (0.133 × 20 × 2600)) = 246 mm

246 mm looks small, and that is the trap. Shear will need roughly twice this depth, so never stop at the bending check.

Step 4: Trial depth 550 mm, and why it fails

Take D = 550 mm. With 50 mm cover and 12 mm bars in two layers, the average effective depth is d = 550 − 50 − 1.5 × 12 = 482 mm.

• Punching shear (cl. 31.6, perimeter at d/2 from the face): side = 400 + 482 = 882 mm, b₀ = 3528 mm. V = 266.3 × (6.76 − 0.882²) = 1593 kN. τ_v = 1593 × 10³ / (3528 × 482) = 0.94 N/mm² ≤ k_s × 0.25√20 = 1.12 ✓
• Steel: A_st required = 2085 mm²; 12 mm @ 130 mm gives 20 bars = 2262 mm², p_t = 0.18%.
• One-way shear (cl. 34.2.4.1(a), section at d from the face): distance to edge = 1.1 − 0.482 = 0.618 m, V = 266.3 × 2.6 × 0.618 = 428 kN. τ_v = 428 × 10³ / (2600 × 482) = 0.341 N/mm². From Table 19 at p_t = 0.18%, τ_c = 0.304 N/mm². ✗ Fails.

Punching passes comfortably but one-way shear fails. Footings carry no shear stirrups, so there are only two fixes: make the slab deeper, or add steel to raise p_t and with it τ_c. More 12 mm bars at 100 mm c/c would just pass (τ_c = 0.340). But that is 25 bars each way to save 50 mm of depth, and the margin is razor-thin. A deeper slab is the better design.

Step 5: Final depth 600 mm, all checks

D = 600 mm, d = 600 − 50 − 18 = 532 mm.

Flexural steel (Annex G)

• A_st = 0.5 (f_ck/f_y) [1 − √(1 − 4.6 M_u / (f_ck b d²))] b d = 1874 mm²
• Minimum (cl. 34.5.1 → 26.5.2.1): 0.12% × 2600 × 600 = 1872 mm²
• Provide 12 mm @ 125 mm c/c: (2600 − 100 − 12) / 125 + 1 = 20 bars → 2262 mm² ✓. Spacing 125 mm ≤ min(3d, 300) ✓
• For a square footing the steel is spread uniformly across the full width in both directions (cl. 34.3.1(a)).

One-way shear (cl. 34.2.4.1(a))

• V = 266.3 × 2.6 × (1.1 − 0.532) = 393.2 kN; τ_v = 393.2 × 10³ / (2600 × 532) = 0.284 N/mm²
• p_t = 100 × 2262 / (2600 × 532) = 0.164% → τ_c = 0.291 N/mm² (Table 19, interpolated) ✓

Punching shear (cl. 31.6.1 and 31.6.3)

• Critical perimeter side = 400 + 532 = 932 mm, b₀ = 3728 mm
• V = 266.3 × (6.76 − 0.932²) = 1568.7 kN; τ_v = 1568.7 × 10³ / (3728 × 532) = 0.791 N/mm²
• k_s = 0.5 + β_c = 1.5 → limited to 1.0; τ_c = 1.0 × 0.25√20 = 1.118 N/mm² ✓

Development length (cl. 26.2.1 and 34.2.4.3)

• τ_bd = 1.2 × 1.6 = 1.92 N/mm² (M20, deformed bars)
• L_d = 0.87 × 500 × 12 / (4 × 1.92) = 680 mm ≤ available 1100 − 50 = 1050 mm ✓ (straight bars are enough)

Bearing at the column base (cl. 34.4)

• Actual = 1800 × 10³ / (400 × 400) = 11.25 N/mm²
• Permissible = 0.45 f_ck √(A₁/A₂), with √(A₁/A₂) capped at 2 → 0.45 × 20 × 2 = 18 N/mm² ✓ (no dowel design needed for bearing; extend column bars into the footing anyway)

Check your own footing

The checker below runs the same steps and clauses as the worked example for a square column on a square footing with a concentric load. Change the depth or bar spacing until every row turns green.

Common mistakes in isolated footing design and on site

• Sizing the area with factored load. SBC is a service value. Using 1.5P gives an oversized footing; using P without self-weight gives an undersized one.
• Stopping at the bending check. As the example shows, bending needed 246 mm but shear needed over 530 mm.
• Checking one-way shear with the wrong p_t. τ_c depends on the steel actually provided at that section. A designer who checks shear before choosing bars has to come back and recheck.
• Cover blocks missing on site. Bars resting on the PCC have zero cover. That is exactly where the footing is wet for its whole life. Use 50 mm cover blocks at about 1 m centres.
• Column starter bars not tied to the mesh. Starters must be L-bent, sit on the bottom mesh and be held by at least two ties in the footing depth so they do not move during the pour.
• Casting on loose or wet soil. The SBC in the design assumes undisturbed ground. Remove loose soil and slush, and cast the PCC bed the same day the excavation is approved.

When this simple method is not enough

This procedure assumes an axial load with uniform soil pressure. Use a full eccentric-footing design when the column carries a significant moment (frames, wind or seismic load cases, or edge columns), when two footings overlap and should be combined, or when the soil report gives settlement limits rather than just an SBC. Sloped (trapezoidal) footings save concrete but need the shear and bending checks at several sections. Footings for structures above 4 storeys or in seismic zones IV and V should always be designed and checked by a qualified structural engineer.

FAQ

What is the minimum depth of an isolated footing as per IS 456?

IS 456 cl. 34.1.2 requires a thickness at the edge of at least 150 mm for footings on soil. In practice, one-way and punching shear set a much greater depth, typically 400 to 700 mm for building columns.

Where is the critical section for bending in a footing?

At the face of the column for a concrete column (IS 456 cl. 34.2.3.2). Each projection is treated as a cantilever loaded by the net upward soil pressure.

Where are one-way and punching shear checked?

One-way shear is checked at a distance d from the column face (cl. 34.2.4.1(a)). Punching shear is checked on a perimeter at d/2 from the column face (cl. 31.6.1), with permissible stress k_s × 0.25√f_ck.

What is the minimum steel in a footing?

Footings follow the slab rule (cl. 34.5.1 and 26.5.2.1): 0.12% of the gross cross-section for HYSD bars (0.15% for mild steel), in each direction.

What clear cover is used for footings?

A minimum of 50 mm for footings (IS 456 cl. 26.4.2.2), measured from the bottom bars to the concrete surface, not to the PCC.

How this was checked: every number in the worked example was recomputed independently in code from IS 456:2000 (Annex G, Table 19, cl. 26, 31.6, 34) and matches the checker on this page. Clause numbers refer to IS 456:2000 with amendments. This article teaches the method; site designs must use project loads, the soil investigation report and a qualified engineer’s review.