Maximum bending moment in simply supported beam under equally spaced in the middle uniform load.
Bending moment
$$ M_{B_{d_{max}}} = {\left({ {W_d \over 4} }\right) \cdot \left({ 2 \cdot a + b - {b \over 2} }\right)} $$
Calculate normal stress in a section, with known section modulus, due to bending caused by distributed load
Bending stress
$$ \sigma_{bend_{d_{max}}} = {10 ^ 3 \cdot {M_{B_{d_{max}}} \over W_{y_{section}}}} $$