This is considered to be equal to 0.06 MPa, which is the difference of the maximum and minimum principal stresses as obtained in the analysis using the 245-kN (25-t) axle load. The factor f(β) is the geometry factor that is equal to 1.3 for a circular arch (Prashant 1999). An initial crack size of 0.1 mm is assumed at the crown of the arch.
Read More: http://ascelibrary.org/doi/full/10.1061/%28ASCE%29BE.1943-5592.0000338