Module 13
Prestressed concrete
Compress the concrete before the load arrives, so the tension never appears — and design the whole thing on serviceability, which is the reverse of everything so far.
What this module covers
- 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
One sentence of principle, three terms of algebra, and two load cases that pull in opposite directions.
Start lesson →Why the force falls by a fifth, and the mental model that makes prestressed design intuitive.
Start lesson →
Module checkpoint
Check what you have taken in
6 questions
Question 1
Why is a prestressing tendon placed eccentrically rather than at the centroid?
Question 2
A 400 × 900 mm rectangular section. What is its section modulus, in units of 10⁶ mm³?
Question 3
A prestress of 1800 kN acts on a section of area 250,000 mm². What is the uniform compressive stress, in N/mm²?
Question 4
Which loss is normally the largest?
Question 5
A tendon carries 2000 kN with a drape of 400 mm over a 20 m span. What load does it balance, in kN/m?
Question 6
In prestressed concrete, which limit state normally governs the design?