This paper describes a numerical evaluation of design rules for the determination of wave loads of non-breaking waves on vertical structures. Design guidelines were proposed by Sainflou (1928) and Goda (1974). These guidelines use geometric parameters of the structure, an incident wave height and a wave period. In practice (cf. CERC, 1984), a Rayleigh distribution of individual wave heights is assumed to determine the design wave height in an irregular wave field. Their reliability and range of applicability are poorly known, especially when the incident wave condition consists of a mixed sea state, like a local wind sea and a low-frequency (swell) component. To validate the above described design methods, we applied the non-hydrostatic numerical wave model SWASH to simulate wave loading on a rigid vertical wall for single and mixed sea states. In addition, we compared the results with linear wave theory and the spectral response approach using transfer functions based on linear wave theory.