Design of Column and Footing

Column and footing design ties the superstructure to the ground. A short column carrying mainly axial load is designed with the IS 456 formula P_u = 0.4 f_ck A_c + 0.67 f_y A_sc, where the concrete and longitudinal steel share the load.

Every column must be checked for minimum eccentricity; if the actual eccentricity exceeds e_min the column is designed as a member subject to combined axial load and bending using interaction diagrams. Slender columns (l_eff/D > 12) attract additional moments that must be added.

For the footing, the plan area comes from dividing the service load by the safe bearing capacity, but the thickness is governed by shear — both one-way (beam) shear at d from the face and two-way punching shear on the perimeter at d/2. Flexure then sizes the reinforcement.

(short axially loaded column, IS 456)

(minimum eccentricity)

(combined footing)

Relation 1 —

(short axially loaded column, IS 456)

Write this relation with symbols exactly as in Reinforced Concrete Design — Pillai & Menon before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 2 —

(minimum eccentricity)

Write this relation with symbols exactly as in Reinforced Concrete Design — Pillai & Menon before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 3 —

(combined footing)

Write this relation with symbols exactly as in Reinforced Concrete Design — Pillai & Menon before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

In a short axially loaded column concrete and steel act together; the coefficients 0.4 f_ck and 0.67 f_y are reduced from the pure material strengths to account for the minimum eccentricity implicitly built into the formula. A_c is the net concrete area (gross minus steel) and A_sc the longitudinal steel.

Minimum eccentricity recognises that perfect axial load never occurs in practice; construction tolerances and load misalignment always introduce some bending, so the code enforces e_min = l/500 + D/30 (≥ 20 mm). When eccentricity is larger, the column is designed from an axial-force/moment interaction diagram.

Footing behaviour is dominated by shear. Two-way punching shear acts on a critical perimeter at d/2 from the column face and often decides the footing depth, while one-way shear is checked on a section at d from the face. Only after the depth satisfies both is the flexural steel designed for the cantilever moment of the soil pressure about the column face.

Development length and lap length ensure bars can transfer their force to the concrete; inadequate anchorage at the column-footing junction is a common design and detailing failure.

State assumptions explicitly before using any relation for design of column and footing — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.

Wrong assumptions invalidate the entire solution even when the formula is correct. In RCC Design viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

1. Read the question and list given data with SI units (common in RCC Design papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to design of column and footing.
4. Use equation 1: .
5. Use equation 2: .
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Design of Column and Footing appears in buildings, bridges, and water tanks. In Indian civil curricula this topic is tested because it connects theory to reinforced concrete per IS 456.

GATE and semester exams often combine design of column and footing with earlier units — revise prerequisites before attempting mixed problems.

Industry interview panels sometimes ask: "Where did you use design of column and footing?" — answer with a lab, mini-project, or plant visit example if possible.

• Using gross area instead of net concrete area A_c in the column formula.
• Ignoring the minimum eccentricity check and treating every column as purely axial.
• Checking only one-way shear and missing the usually critical two-way punching shear in footings.
• Forgetting to verify development length of dowels at the column base.

Before attempting design of column and footing problems, confirm you can:

1. Slenderness: l_eff/D > 12 → consider biaxial bending and additional moments
2. Isolated footing: check one-way and two-way shear (punching)
3. Lap length: 49φ (HYSD) or as per IS 456 development length

Revise the solved examples in Reinforced Concrete Design — Pillai & Menon and one previous-year GATE or university paper for this unit.