For plain (mild) reinforcing bars placed in compression, by what percentage may the permissible design bond stress (based on tension) be increased to account for improved anchorage performance?

Introduction / Context:
Bond stress represents the interface shear transfer between steel and surrounding concrete. Bars in compression develop improved confinement and bearing effects, allowing a modest increase in permissible bond stress compared with the tension case. This adjustment is routinely used in anchorage and development length calculations for compression bars.

Given Data / Assumptions:

• Bars considered are plain (mild) bars in compression.
• Baseline permissible design bond stress is the one specified for tension.
• We seek the percentage increase allowed in compression.

Concept / Approach:

Because compressive action enhances bar-concrete interaction and reduces splitting risk, codes allow a percentage increase in bond stress for compression bars versus tension bars. This provides shorter required development lengths while maintaining safety.

Step-by-Step Solution:

Verification / Alternative check:

Design examples for column longitudinal bars in compression typically adopt 1.25 times the tension bond stress, confirming the rule-of-thumb increase.

Why Other Options Are Wrong:

• 10%, 15%, 20%, 30%: Do not match the commonly accepted 25% increase for plain bars in compression.

Common Pitfalls:

• Applying compression increase to deformed bars without checking the specific code clause.
• Forgetting to verify adequate confinement and cover, which still govern splitting capacity.

Final Answer:

25%