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?
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A10%
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B15%
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C20%
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D25%
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E30%
Answer
Correct Answer: 25%
Explanation
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:
Start with permissible bond stress for tension as the base value.Apply the allowed increase for compression in plain bars: +25% of the tension value.Use this increased value when checking development length and anchorage at compression ends.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%