Figure $2\mathrm{.}1$

$\frac{{}_{w}P}{A}-\frac{{}_{w}Pe}{Z_b}-\frac{M_{\text{m}\text{a}\text{x}}}{Z_b}-f_{tw}$

Bottom tension workingA $12$m span prestressed simply supported I beam, with the section

shown in Figure $2\mathrm{.}1,$ carries a maximum working uniformly distributed

load of $30$kN$/$m $($this includes self$-$weight$).$ Assuming that the

prestressing tendons act at a constant level of $75$mm above the lower

face of the beam, compute the prestressing force $($P$)$ required for the

maximum moment location. Assume fck$=40$N$/$mm$2$

$,$w$=0\mathrm{.}70,$

ftw$=3\mathrm{.}0$N$/$mm$2$

and fcw$=24$N$/$mm$2$

$.$ Use only the bottom tension working

condition shown below.

Note: working load means the serviceability load.