Construction Joints – Preferred locations within structural members Where should a construction joint ideally be provided in a reinforced concrete member to minimize adverse effects on structural behavior?

Introduction / Context:
Construction joints are unavoidable stoppages in concrete placement. Their locations must be chosen to minimize negative impacts on strength and serviceability. Understanding the internal force distribution helps decide the best placement zone for a joint in beams, slabs, and walls.

Given Data / Assumptions:

• Monolithic behavior is preferred; reinforcement provides continuity across the joint.
• Joints will be keyed, roughened, and properly cleaned before continuation of concreting.
• Load effects include bending and shear.

Concept / Approach:

Shear transfer across a joint is more sensitive to discontinuity than bending because joints can reduce aggregate interlock and interface shear capacity. Therefore, joints are usually placed at zones of minimum shear. In simply supported beams and one-way slabs, minimum shear occurs near midspan; consequently, joints are commonly located at or near the middle third. For walls and columns, joints are often placed away from high-shear regions and close to level transitions where forces are lower.

Step-by-Step Solution:

Verification / Alternative check:

Standard recommendations put beam/slab construction joints at the middle third (low shear zone) and column joints at about one-third height from the floor or just below a beam soffit, avoiding high shear transfer planes.

Why Other Options Are Wrong:

“Where bending moment is small” may lead to regions of high shear near supports. “At member supports” typically coincides with peak shear and is undesirable. “All of the above” cannot be correct because the best single criterion is low shear.

Common Pitfalls:

Placing joints near supports without shear keys; poor surface preparation; inadequate curing leading to weak interfaces.

Final Answer:

Where shear force is small