Structural concrete members used in bridges can be subdivided into two regions, B-and D-regions (Figure 13.1). In the B-region, Bernoulli’s hypothesis holds valid, where it is assumed that a normal cross-sectional plane remains plane and normal to the reference lines when the beam deforms. Bernoulli’s hypothesis facilitates the flexural design of reinforced concrete structures by allowing a linear strain distribution for all loading stages, including an ultimate flexural capacity. Design of the B-(Bernoulli or beam) region is well understood, and the entire flexural behavior can be predicted by simple calculations. For torsion, the sectional shape and size in its own sectional plane are assumed to be preserved during torsion, and the cross section can warp freely out of its plane.