A simply supported beam AB of span, $L=24$ m is subjected to two wheel loads acting at a distance, $d=5$ m apart as shown in the figure below. Each wheel transmits a load, $P=3$ kN and may occupy any position along the beam. If the beam is an $I$-section having section modulus, $S=16.2 \:cm^3$, the maximum bending stress (in GPa) due to the wheel loads is _______.
