Bundled reinforcement — development length increase When three reinforcing bars are bundled together, the required development length of each bar shall be increased by:

Difficulty: Easy

Correct Answer: 20%

Explanation:


Introduction / Context:
Bundling bars reduces the perimeter available for bond for each individual bar, which lowers bond efficiency. To compensate, codes require an increase in development length (Ld) for bars in bundles, with the increment depending on the number of bars bundled.


Given Data / Assumptions:

  • Three bars are bundled together in the same plane.
  • Standard bond stress provisions for deformed bars in tension/compression apply.
  • Question asks the percentage increase in Ld for each bar.


Concept / Approach:
Typical tabulated increments are: +10% for two bars, +20% for three bars, and +33% for four bars bundled together. These multipliers reflect the reduction in effective surface area per bar and the tendency for slip within bundles.


Step-by-Step Solution:
Step 1: Identify number of bars n = 3 in the bundle.Step 2: Apply codal increment: Ld_increase = +20% for n = 3.Step 3: Hence, required Ld per bar = 1.20 × Ld (for a single bar).


Verification / Alternative check:
Detailing handbooks and solved examples reproduce the same 10/20/33% sequence for 2/3/4 bars respectively, confirming the 20% increase for triplets.


Why Other Options Are Wrong:
10%: Applies to two-bar bundles, not three.
33%: For four-bar bundles.
50%: Excessively conservative; not specified for three-bar bundles.
5%: Too small; not a recognized increment.


Common Pitfalls:

  • Applying the increment to lap length only; it applies to development length too.
  • Ignoring minimum spacing/stirrups needed to confine bundled bars.


Final Answer:
20%

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