In reinforced concrete detailing, the lap length for tension reinforcement shall not be less than bar_dia * (actual_tension / (4 * permissible_average_bond_stress)) if this calculated value is greater than which minimum benchmark (expressed in bar diameters)?

Introduction / Context:
Lap splices ensure continuity of reinforcing bars where a bar is terminated and another begins. In tension zones, adequate lap length is critical to transfer stress safely through bond with concrete and avoid premature slip or cracking.

Given Data / Assumptions:

• The expression for lap length in tension: L_calc = bar_dia * (actual_tension / (4 * permissible_average_bond_stress)).
• We must compare L_calc with a minimum benchmark expressed in bar diameters.

Concept / Approach:
Codes and standard practice stipulate a minimum lap length in tension to prevent short splices. Even when the calculated value is small, a floor value in terms of bar diameters is enforced. A widely adopted benchmark is 30 bar diameters for tension laps (subject to additional detailing requirements, cover, confinement, and bar type).

Step-by-Step Solution:
Compute L_calc from the given formula.Compare L_calc to the minimum benchmark.If L_calc exceeds the benchmark, use L_calc; otherwise, not less than the benchmark is permitted.

Verification / Alternative check:
Typical detailing guides and many code interpretations adopt 30φ as the minimum for tension splices in ordinary conditions, with adjustments for bar type (plain vs deformed), confinement, and concrete class where applicable.

Why Other Options Are Wrong:

• 18φ and 24φ: too short for tension splices under most design scenarios.
• 36φ: conservative but not the standard threshold referred to in this formulation.

Common Pitfalls:
Confusing development length with lap length, and neglecting reductions or increases due to hooks, confinement, or poor bond conditions. Always verify with current code provisions for the project jurisdiction.

Final Answer:
30 bar diameters.