three steel bars AD, BE, & C(Est=200GPa Coeffcient of Thermal expansion=15*10^-7/C) with area of cross section 450mm^2 and suspended on rigid bar DEF. If the temperature of rod CF is increased by 10 degrees C and that of rod BC is decreassed by 40 degrees C, determine the normal stress in rod AD. The rods are two meters long. and they are spaced 2 meters apart with fixed supports at the top of each rod.