12. The rigid horizontal bar ABC of negligible mass is connected to two rods as shown in the figure. If the system is initially stress-free, calculate the temperature change that will cause a tensile stress of 90 MPa in the brass rod. Assume that both rods are subjected to the same change in temperature. B C Brass L=2m A=1200 mm² E = 100 GPa a 18.7 x 10-y°C 3 m 2 m Copper L-3m A = 1500 mm² E=120 GPa a-16.8 x 10-C