Fig. 6 shows the plot of maximum principal stress at the crown of Span 1 with respect to the applied axle load. From this figure, it is seen that the rise in the maximum principal stress is only about 0.1 MPa for an increase of a 980-kN (100-t) axle load, which is quite small. Furthermore, at a 245-kN (25-t) axle load and without the self-weight, the maximum principal stress near the crown is 0.0245 MPa (0.16 MPa with self-weight). Because no cracks were visually observed at this load in the field studies, it is expected that the minimum tensile strength of the masonry arch is more than 0.16 MPa.
Read More: http://ascelibrary.org/doi/full/10.1061/%28ASCE%29BE.1943-5592.0000338