This dissertation reports investigations on shear in cracked reinforced concrete (RC) elements including the development and implementation of a material subroutine for the commercial finite element (FE) program ABAQUS. The material subroutine UMAT is intended to substantially improve the shear behaviour of the standard concrete options of ABAQUS. At first the important shear theories are reviewed in detail and their advantages and drawbacks are summarised. The modified compression field theory (MCFT) is identified as a suitable shear theory worth being coded for its application in FE analysis. A comprehensive check on the MCFT confirms its suitability in a slightly modified form for the investigation of a variety of cracked structural RC elements. This check is conducted on a section analysis level by means of a developed program called LAYER which is coded according to the MCFT. The main part of the work is the implementation and testing of the material subroutine UMAT which is added to the source code of ABAQUS via an interface provided by the commercial FE program. Finally, the UMAT is utilised for examining the ductility of RC walls. It is concluded that shear deflections can influence the displacement and curvature ductility of squat structures in a substantial way, even though a flexural type of failure might prevail.

Doctor of Philosophy / Department of Civil Engineering / Hayder A. Rasheed / This study is conducted because of the lack of an existing theory to accurately predict the diagonal tension cracking in shallow reinforced concrete beams. A rational approach is followed to numerically derive the shear stress profile across the depth of the beam in cracked beams based on the smeared crack approach. Furthermore, the determined shear stress distribution coupled with the normal axial stress distribution are used to predict the principal stress variation across the depth and along the shear span using standard Mohr’s circle. Following a biaxial stress cracking criterion, the likely diagonal tension cracks along their orientation profile are predicted. Furthermore, this study is conducted to provide a mechanics-based understanding of the shear stress distribution in cracked reinforced concrete. This approach utilizes the transversal shear differential equation to evaluate the shear stress at any given depth by the variation of the axial stress distribution within an infinitesimal beam segment at that depth. In addition, this study presents a more accurate representation of the change in the strain profile parameters with respect to the sectional applied moment. Furthermore, the dowel action effect is derived to illustrate its significance on the shear stress distribution at various stages of loading.