EXAMPLE $8\mathrm{.}1$ A prestressed concrete beam $($span$=10$m $)$ of rectangular section, $120$ mm wide and $300$ mm deep, is axially prestressed by a cable carrying an effective force of $180$ kN$.$ The beam supports a total uniformly distributed load of $5$kN $/$ m which includes the self$-$weight of the member. Compare the magnitude of the princi$-$ pal tension developed in the beam with and without the axial prestress.