Reinforcement and terminology

A beam bends under bending moment resulting in a small curvature. At the outer face (tensile face) of the curvature the concrete experiences tensile stress while at the inner face (compressive face) it experiences compressive stress.
A "singly-reinforced" concrete means that the concrete element is only reinforced near the tensile face and the reinforcement, called tension steel, is designed to resist the tension.
A "doubly-reinforced" concrete means that beside the tensile reinforcement the concrete element is also reinforced near the compressive face for assisting the concrete to resist compression. The latter reinforcement is called compression steel.
An "under-reinforced" concrete means that the tension capacity of the tensile reinforcement is smaller than the combined compression capacity of the concrete and the compression steel (under-reinforced at tensile face). When the reinforced concrete element is subject to increasing bending moment, the tension steel yields while the concrete does not reach its ultimate failure condition. As the tension steel yields and stretches, an "under-reinforced" concrete also yields in a ductile manner, exhibiting a large deformation and warning before its ultimate failure.
An "over-reinforced" concrete means that the tension capacity of the tension steel is greater than the combined compression capacity of the concrete and the compression steel (over-reinforced at tensile face). An "over-reinforced" element will fail suddenly, when the concrete fails brittle and crashes before yielding of the tension steel. It is however possible to push the design of an "over-reinforced" concrete element to "under-reinforced" concrete element by adding sufficient compression steel. There is however a limit in the quantity of both tension and compression steel for practically placement of reinforcement.-Balaji.
Steel reinforced concrete elements should normally be designed to be under-reinforced so users of the structure will receive warning of impending collapse.
Characteristic strength strength of a material where less than 5% of the specimen show lower strength.
Design strength strength of a material including a material safety factor.
Ultimate limit state theoretical failure point with a certain probability. State under factored loads and factored resistances.
Nominal strength actual strength from the material properties is called the nominal strength.[2] Its relation with design strength can be represented as: