Two-layer pre-stressed beams consisting of normal and high strength steel fibred concrete

The paper is focused on modelling of two-layers bending pre-stressed beams consisting of steel fibered (SF) high strength concrete (HSC) in compressed zone and normal strength concrete (NSC) in tensile zone. Investigation of such beams is important for RC structural design, because calculation of fibers ratio is significant like that of reinforcing steel bars for usual RC elements. In other words, such elements are made of high performance concrete. It is demonstrated that SF have little effect on the beam elastic deflections. However, the ultimate deflections of the section increase because additional potential for plastic energy dissipation (PED) in the bending element. NSC, used in the section tensile zone, contributes additionally about 20% to the section PED potential compared to one-layer HSC beams. In order to guarantee sufficient section's ductility of pre-stressed beams, required to withstand dynamic loadings, a minimum SF ratio is proposed to be considered. It allows taking the tensile stresses, caused by concrete matrix cracks, by the fibers. A design model for obtaining the SF volume ratio, as a function of the required ductility of a pre-stressed two-layer beam, is proposed. An example, illustrating the efficiency of this model for calculation of the SF volume ratio, is presented.