THE ROLE OF THE STRUCTURAL CHARACTERISTIC LENGTH IN FRC STRUCTURES

In the framework of the CEN Committee involved in the writing of the fiber reinforced concrete structure standards, a strong debate has been focused on the possibility to use a stress-strain rather than a stress- crack opening constitutive relationship, even if only the second one is physically meaningful after the cracking of the matrix. The use of a stress-strain model, even if it can be regarded as an effective simplification in many cases as it is in R/C structures, can be justified by the rough choice of a unique crack spacing in the range of 125 mm. In the paper, the modeling of different FRC cross sections and in particular of a thin-walled open cross-section profile longitudinally reinforced with steel bars like a FRC box-culvert (U-channel) highlights as only the use of a correct structural characteristic length when a simplified Navier-Bernoulli plane section model is adopted prevents the overestimation of the bearing capacity in bending. A comparison with F.E. model and previous experimental tests on full-scale structures are also proposed.