The main principle of prestressed concrete structure is to introduce internal stresses that counteract the stresses caused by external loads.
The fundamental principle behind prestressed concrete is brilliantly simple yet highly effective in structural engineering. Based on the Basic Concept reference provided:
"Prestressed concrete is basically concrete in which internal stresses of a suitable magnitude and distribution are introduced so that the stresses resulting from the external loads are counteracted to a desired degree."
This means that unlike traditional reinforced concrete, which is designed to carry loads once cracks form (relying on rebar), prestressed concrete proactively applies a compressive force to the concrete before external loads are even applied.
Here's a breakdown of this core principle:
- Introduction of Internal Stress: Before the structure is subjected to its service loads (like traffic on a bridge deck or occupants in a building), high-strength steel tendons (cables or bars) are tensioned. This tension is then transferred to the concrete, typically resulting in a compressive stress within the concrete element.
- Counteracting External Loads: When external loads (such as the weight of the structure itself, live loads, wind, etc.) are applied, they cause tensile stresses in the concrete. The pre-existing internal compressive stresses introduced by the tensioned tendons effectively cancel out or significantly reduce these tensile stresses.
Why is this principle important?
Concrete is very strong in compression but relatively weak in tension. By ensuring that the concrete remains largely in compression or experiences greatly reduced tension under service loads, prestressed concrete structures can:
- Achieve longer spans without excessive deflection.
- Be more durable by controlling or eliminating cracking.
- Utilize materials more efficiently, potentially leading to lighter structures.
In essence, the principle shifts the stress state of the concrete element from one where tension might dominate under load to one where beneficial compression is maintained, ensuring the concrete's strength is utilized more effectively.
