What is prestressing?
Prestressing is putting and keeping steel under tension in a structure, so that a favourable stress is created in the building element in advance. The term is mainly used for prestressed concrete: steel strands or bars are put in tension, which places the surrounding concrete in compression.
How does it work?
A beam on two supports deflects under load. The underside is then in tension, and concrete is poor at resisting tension: it cracks. With prestressing, the underside is put in compression beforehand. Under the service load, this compression is cancelled out first, before any tension develops. The result:
- fewer or no cracks;
- less deflection;
- longer spans with a smaller structural depth.
The prestress is usually applied eccentrically, that is below the centre of the cross-section, because that is where tension develops under load.
Pre-tensioning and post-tensioning
- Pre-tensioning: the steel is tensioned before the concrete is cast. Typical of factory-made elements.
- Post-tensioning: the steel is only tensioned after the concrete has hardened, in ducts cast into the concrete.
Prestress losses
The prestress decreases over time due to shrinkage and creep of the concrete, relaxation of the steel and, with post-tensioning, also friction and anchorage slip. The structural engineer allows for this in the calculation.
Applications and safety
- You find prestressing in hollow-core slab floors, precast beams, piles, bridges and car park floors.
- A tensioned strand contains a lot of energy. If it is cut through or hit, this can weaken the element and the work itself carries risks. Demolition or drilling work on prestressed elements is therefore only carried out after consulting a structural engineer.
Related terms
- Prestressed concrete
- Reinforced concrete
- Reinforcement
- Reinforcing steel
- Creep
- Stress
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