Loss of prestress due to 'elastic shortening' in a pre-tensioned beam occurs:
Choose the correct answer
Gradually over months
Immediately when the tendon force is transferred to the concrete (at release)
Only in post-tensioned systems
During service under live load
Correct Answer
B. Immediately when the tendon force is transferred to the concrete (at release)
Why is this the answer?
In pre-tensioning: when tendons are cut (released), concrete shortens elastically due to prestress → prestress in tendon reduces by ΔP_e = Es × εc = Es × fc/Ec = (Es/Ec) × fc = m × fc. This is immediate and certain. In post-tensioning: elastic shortening of sequentially stressed tendons (earlier tendons lose prestress when later ones are jacked).
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