NASCLA Accredited Commercial General ContractorConcreteHard
A contractor is installing post-tensioned concrete slabs for a parking garage. During the stressing operation, the force applied to the tendons is monitored. If a tendon is stressed to 80% of its ultimate tensile strength, and its cross-sectional area is 0.15 square inches, with an ultimate tensile strength of 270,000 psi, what is the total jacking force applied to this single tendon?
- A32,400 lbs
- B40,500 lbs
- C48,600 lbs
- D24,300 lbs
Show answer & explanationAnswer & explanation
Correct answer: A. 32,400 lbs
First, calculate the ultimate tensile force: Ultimate Tensile Strength (psi) x Cross-sectional Area (in²) = 270,000 psi * 0.15 in² = 40,500 lbs. Then, apply the stressing percentage: 40,500 lbs * 80% = 32,400 lbs. This is the total jacking force.
Why the other options are wrong
- B. This is the ultimate tensile force, not the applied jacking force (which is 80% of ultimate).
- C. Incorrect calculation, possibly 120% of ultimate tensile strength.
- D. Incorrect calculation, perhaps 60% of ultimate force.
Post-Tensioning Stressing Force
The controlled tensile force applied to post-tensioning tendons after concrete has gained sufficient strength, inducing compressive stresses in the concrete.
- Applied using hydraulic jacks.
- Monitored by gauges and elongation measurements.
- Typically stressed to a percentage (e.g., 80%) of ultimate tensile strength.
Memory trick: Stress is strength x area, then take a percentage share.