Strain penetration effects on lateral strength and deformation capacity of U-shaped walls

In the present study the flexural strains in longitudinal tension reinforcement along the shear span of seismically loaded wall components are evaluated by explicitly accounting for bond-slip interaction between reinforcement and concrete. In this approach, the kinematics of flexural response are no longer controlled by the plane sections assumption, but by the relative slip between bar and concrete. The strain penetration that spreads in the tension reinforcement over extensive heights from the critical section of the member leads to increased strains in the concrete compression zones and accelerates crushing and localized lateral instability in the thin parts of wall members. This effect is studied using detailed nonlinear finite element modeling of two U-shaped walls which were tested under combined axial load and simulated seismic lateral loads (through lateral displacement reversals) at the University of Louvain; the wall tests were the subject of a blind prediction contest in 2022. In the finite element models, flexural strains in reinforcement were calculated from the assigned bond properties, and detailed mechanics of strain penetration and its effect on the deformation capacities, lateral load strength, and failure mode of the walls were illustrated. The modeling approach was first validated through correlation with previously conducted simulated seismic loading tests on a different U-shaped wall specimen and was subsequently verified through comparison with the experiments of the blind prediction contest. It is demonstrated that slip of tension reinforcement, occurring due to extensive strain penetration over the shear span of the member, controls the onset of compression zone crushing and the eventual damage observed in slender walls with open cell-type cross sections under seismic loading.