How to design a pre-stressed concrete beam

It is known that concrete has a good strength against compression and feeble strength in tension. To overcome this issue, reinforcement, in the form of steel bars, are provided to withstand tension/tensile force that operates on beams/columns/slabs etc.

RC structures under service load experiences deflection that leads to elongation of the bottom of the beam (tensile zone) to produce cracks. Normally, steel bars are arranged to restrain the crack widths as well as withstand the tensile force that is absent in concrete.

Here, the rebar performs as ?passive reinforcement?. Rebars (steel reinforcement) are arranged at the bottom of the bar and does not bear any forces as far as the concrete has already been deflected as much as necessary to crack.

The percept behind pre-stressed concrete is that compressive stresses stimulated by high-strength steel tendons in a concrete member prior to loads are employed will balance the tensile stresses enforced in the member throughout service.

Basically, permanent pre-compression is formed in the areas based on tension with high tensile strength steel wires or alloys. Now, a part of tensile stress is neutralized, as a result the cross-sectional area of steel reinforcement is reduced.

Due to this, no cracking occurs in the concrete as the pre-stressing has lowered the tensile stress in the section under cracking stress. So, concrete is treated as a elastic material.

Given below, the process to design a pre-stressed concrete beam according to BS 5400 standard :-

The process is briefly explained with a solution to the following problem :-

Design a simply supported pre-stressed concrete Y beam that contains a 150mm dense concrete slab and 100mm of surfacing, along with a nominal live load udl of 10.0 kN/m2 and kel of 33kN/m.

The distance of the beam is 24.0m centre to centre of bearings and the beams are placed at 1.0m gappings.

To learn the complete process, go through the following link bridgedesign.org.uk