For the simply supported beam of Figure 1, determine the uniformly distributed load, q, at which the flexural crack forms. The beam has a length L=7m, and f’c=25Mpa.
Figure 1: Cross-section of Concrete Beam
First crack occurs when the maximum tensile stress in concrete, ft, reaches the rupture modulus of concrete, fr.
The tensile stress, calculated from elastic theory, is:
According to ACI, the modulus of rupture of concrete is:
The uniformly distributed load, q, at which the flexural crack forms is thus calculated as: