In a combined footing for two columns spaced L apart, the top (negative) reinforcement must extend into the projecting cantilever portions beyond each column face. What practical detailing rule is commonly adopted for this extension?

Introduction / Context:
Combined footings support two columns on a single base when individual isolated footings would overlap or when soil pressure distribution must be controlled. Top bars resist negative bending in the cantilevered projections beyond the column faces and must be adequately developed for moment transfer.

Given Data / Assumptions:

• Two columns separated by distance L on a common footing.
• Top reinforcement over columns must be extended into the projecting regions.
• Need a practical rule for minimum extension to ensure anchorage and moment capacity.

Concept / Approach:
Under column loads, the footing slab behaves as a beam-slab with negative bending over columns and positive bending in mid-panel. To prevent premature cracking at the column face and to ensure full development of negative moment capacity, top bars are commonly extended a fraction of the column spacing from the column’s centre line; a widely used detailing guide adopts 0.1L beyond the column centre line into the projection.

Step-by-Step Solution:
Identify column spacing L and projection lengths beyond faces.Provide top bars continuous across both columns.Extend the bars into each projection by about 0.1L measured from the centre line of the column.

Verification / Alternative check:
Checking against development length requirements shows that 0.1L typically exceeds minimum bar anchorage in many practical layouts, providing redundancy against local stress concentrations near column faces.

Why Other Options Are Wrong:

• 0.1L from the outer edge: not the conventional reference point used in detailing rules.
• Half or one-fourth projection: may be insufficient or inconsistent across varying geometries.
• No extension: unsafe and contrary to sound practice.

Common Pitfalls:
Stopping bars close to the column face without adequate anchorage; neglecting bar hooks or bends where cover is limited; ignoring development length checks in thinner footings.

Final Answer:
Extend 0.1 L from the centre line of each column.