If d is the thickness of a concrete pavement, the equivalent radius b of resisting section for an interior loading, is

• b = 0.6 + d2 - 0.675 d
• b = 1.6 + d2 + 0.675 d
• b = 1.6 + d2 - 3.675 d
• b = 1.6 + d2 - 0.675 d