(Solved): (a) In analysing prestressed concrete sections under working loads, briefly itemized any tive assum ...

(a) In analysing prestressed concrete sections under working loads, briefly itemized any tive assumptions you will make? [5 nuarks] (b) A prestressed concrete beam supports an imposed load of $5\mathrm{kN}/\mathrm{m}$ over a simply supported span of $10\mathrm{m}$. The beam has an I-section with an overall depth of $450\mathrm{mm}$. The thickness of flange and web are $75\mathrm{mm}$ and $100\mathrm{mm}$ respectively. The width of the flange is $230\mathrm{mm}$ (Fig. Q2). The beam is to be prestressed by an effective prestressing force of $350\mathrm{kN}$ at a suitable eccentricity such that the resultant stress at the soffit of the beam at the centre of the span is zero. Find the escentricity required for the force. $A$ lso, find the magnitude of prestressing force for the resultant stress to be zero at mid-span if the tendons are concentric. (Unit weight of concrete $=24\mathrm{kN}/{\mathrm{m}}^3$ ) $[15\mathrm{marks}]$ ection modulis. $\begin{array}{l}=\frac{T/y}{}\\ =\frac{P_{ne}}{2t}+\frac{M_g}{2t}+\frac{M_q}{2t}\\ +\frac{P_{uc}}{2b}?\frac{M_j}{2b}?\frac{M_q}{2b}\end{array}$ Fig. Q2