Footing detailing under columns — bottom reinforcement bars passing beneath a column should be turned and extended into the footing slab interior by a distance not less than:

Introduction / Context:
In isolated and combined footings, column bars and bottom footing bars must develop full anchorage to transfer forces safely. Bars running beneath the column are extended (hooked or bent as required) into the footing slab interior to achieve adequate development length beyond the critical section at the column face.

Given Data / Assumptions:

• Bottom bars under the column require development beyond the column face.
• Diameter of the bar is the reference (d).
• Customary detailing rule uses a multiple of bar diameter.

Concept / Approach:

The critical location for anchorage is the column face (inner edge), not the column centre. A widely used detailing guide is to extend bottom bars at least 42d beyond the column face into the footing slab to ensure bond and proper distribution of stresses, especially for punching shear and bending transfer regions.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Compute actual development length Ld using Ld = (ϕ * σ_s)/(4 * τ_bd) and confirm that provided extension (≥ 42d) meets or exceeds Ld as required.

Why Other Options Are Wrong:

Referencing the column centre or outer edge mislocates the critical anchorage point; 24d is typically inadequate for full development in footing regions.

Common Pitfalls (misconceptions, mistakes):

Measuring from the wrong reference; ignoring hooks/bends when space is limited; insufficient cover leading to durability issues.

Final Answer:

42 diameters from the inner edge (face) of the column