This paper addresses the problem of determining the stiffness and strength of steel storage rack base plate assemblies. Base plate assemblies are usually bolted to an upright and to a concrete floor, and they are used to provide resistance against the flexural buckling of the upright and/or the overall down-aisle buckling of the frame. The semi-rigid stiffness is usually determined by means of a test of two pre-loaded uprights connected to a concrete block in a dual-actuator set-up. Guidance for conducting the test is provided in the European Standard EN 15512. However, several aspects of the test need clarification, notably the test set-up and the transducer arrangement for measuring the rotations of the base plate. This paper provides recommendations for how best to conduct the test, and proposes an alternative test method to that given in the EN 15512 Specification. The paper also identifies the contributions to the deformations of base plate assemblies, including the deformations of the supporting floor, the base plate assembly itself and the upright, and proposes simple expressions for calculating the stiffness associated with each contributing deformation where applicable. Furthermore, the paper proposes a deformation criterion for determining the ultimate design moment for base plate tests which do not attain a peak moment.