A cylindrical concrete tank, 30 m external diameter, is to be prestressed circumferentially by means of high-strength steel wire (E, = 2 x 105 N/mm2) jacked at 4 points, 900 apart. If the minimum stress in the wire immediately after tensioning is to be 450 N/mm2 and coefficient of friction is 0.5, calculate :
(a) The maximum stress to be applied to the wires at the jack, and
(b) The expected extension at the jack.