All beams on the plan view shown have the same web width $b_w=300mm.$ All columns are $400$ x $400mm$ and the slab thickness is $150mm.$ What is the effective flange width of the beam B$3C3($spanning from point $B3$ to point $C3).$

The beam is reinforced with $4$#$32$ bars on the tension side in the positive moment region, the overall depth is $600mm,$ and slab thickness is $150mm.$ Concrete compressive strength is $35$MPa, and steel yield strength is $420$MPa. The beam is not exposed to weather and not in contact with the ground. What is the flexural design capacity of the flanged beam? Include strain diagrams and force diagrams to support your calculations.

$5$