Strain curvature characteristics of R.C. sections externally reinforced in a loaded state

The study presents a numerical model for the development, in usable form, of a strain-step his-tory relating concrete strain, strains in tension and compression steels, curvature, moment, neutral axis position and crack height of reinforced concrete sections which are externally reinforced at a chosen state of preload. The determination of the balanced area of external steel poses some difficulties. This is because it is a function of the available area of internal steel (both tension and compression) and the strains induced in them by the existing service loads. The model affords an evaluation of the pseudo-balanced value of external steel for a reinforced concrete section in a state of strain under existing load, the nominal capacity that the section may develop after application of a desired ratio of the balanced value of external steel and the ductility level attained. The history developed furnishes strain and depth of neutral axis values in the service range of the loading. The results of the analysis by the model are compared with published test results. A parametric study of the effect of preload-moment on the pseudo-balanced ratio of external reinforcement and the corres-ponding moment capacity is made. An example of design for external reinforcing of an existing section is also presented