Reinforced Concrete Design / module 13
Prestressed concrete
How prestress changes concrete stresses at transfer and in service, and how losses, strength and serviceability affect the design.
What you will understand
- Explain prestressing as pre-compression, and say why eccentricity matters
- Write the three stress terms and say what each does
- Check a section at transfer and in service, and say why they are opposites
- Estimate the losses, and say which dominate
- Size a section from the two inequalities, before choosing any prestress
- Use load balancing as a design tool
Lessons
The idea, and the two stages
One sentence of principle, three terms of algebra, and two load cases that pull in opposite directions.
Losses, and load balancing
Why the force falls by a fifth, and the mental model that makes prestressed design intuitive.
Module summary
- Prestress applies compression before the load, so the tension never appears
- Three terms: −P/A uniform, ∓Pe/Z from eccentricity, ±M/Z from the load
- Eccentricity is what makes prestress efficient — concentric prestress is wasteful
- TRANSFER and SERVICE are opposite cases and both must be satisfied
- Losses total 20 to 25%, dominated by creep and shrinkage
- SERVICEABILITY governs and the ultimate check is usually comfortable — the reverse of reinforced concrete
- Load balancing: a parabolic tendon of drape d applies an upward load of 8Pd/L²
Teaching basis and review status
These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.
Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.
Read the lesson register and outstanding review notesCheck your understanding
Check yourself
Why is a prestressing tendon placed eccentrically rather than at the centroid?
Practice
A 400 × 900 mm rectangular section. What is its section modulus, in units of 10⁶ mm³?
Practice
A prestress of 1800 kN acts on a section of area 250,000 mm². What is the uniform compressive stress, in N/mm²?
Check yourself
Which loss is normally the largest?
Practice
A tendon carries 2000 kN with a drape of 400 mm over a 20 m span. What load does it balance, in kN/m?
Check yourself
In prestressed concrete, which limit state normally governs the design?