This article presents results from the experimental testing of two half-scale RC U-shaped walls under quasi-static cyclic bi-directional loading along the diagonal direction of the U-shaped section, which were recently completed at EPFL. The main objective of the article is to emphasise particularities in the behaviour of U-shaped walls under diagonal loading and to point out related design and analysis issues. Several phenomena specific to diagonal loading are discussed: (1) strain gradient across the wall width promotes out-of-plane buckling of the boundary elements in the flange ends; (2) plane section analysis does not yield reliable moment capacity estimates for the diagonal loading direction and (3) under diagonal loading the compression depth in the flange end boundary elements is larger than for the other loading directions, exposing unconfined concrete to large compressive strains. These phenomena lead to a reconsideration of the following design and analysis issues for U-shaped walls: (a) the distribution of the vertical reinforcement layout; (b) the use of plane section analysis for estimating the strength capacity of the wall; (c) the confinement length of the boundary elements of the flanges; and (d) the assessment of the out-of-plane stability of flange ends.