A concrete beam prestressed with a parabolic tendon is shown in the sketch. The eccentricity of the tendon is measured from the centroid of the cross-section. The applied prestressing force at service is $1620$ kN. The uniformly distributed load of $45$ kN/m includes the self-weight
The stress (in $N/mm^2$) in the bottom fibre at mid-span is
- tensile $2.90$
- compressive $2.90$
- tensile $4.32$
- compressive $4.32$