Flexural Extension Considerations in Reinforced Concrete

Abstract

When reinforced concrete members crack in flexure, the neutral axis shifts from the geometric center of the member towards the compression fibers. The amount of shift is related to the magnitude of the flexural moment, and the result is that there are tensile strains at the geometric center of the member that integrate to yield axial extensions. These extensions are not captured by the common elastic analysis programs. While non-linear analysis programs with fiber element modeling capabilities can capture these effects, they can be very complex to build and time-consuming to run. Neglecting consideration of the axial extensions associated with flexural forces in members can result in very significant underestimation of moments and shears in such members, not to mention axial forces--easily as much as 100% in specific situations. This paper presents the relevant theoretical equations for this effect, provide guidance for identifying structure types where this effect needs to be considered, and will also provide a simple method for capturing these effects in elastic analysis studies using commonly used programs.