Abstract | The validity of the typical 100/30 combination rule for horizontal seismic action effects is investigated for the design of structures that are axially symmetric along the vertical direction. The 100/30 rule stipulates that one should combine 100% of the seismic action in one principal direction (as estimated by the design spectrum) with 30% of the action in the other principal direction, and vice-versa. Having been derived for azimuth-dependent structures, having e.g., a rectangular plan, it takes advantage of the fact that the two horizontal components of ground motion are only partially correlated, with peaks that in general do not happen simultaneously, to reduce the overall design loads. On the contrary, vertical liquid storage tanks, silos and chimneys are examples of azimuth-independent structures, which by virtue of their symmetry will always experience the worst-possible incidence angle of a ground motion. To quantify the effect of axisymmetry we employed a database of 150 records with three components of ground motion. The results show that an 106/106 combination rule, or more accurately a 1.12 amplification factor on the design spectrum in a single direction, rather than the 1.04 implied by the 100/30, is adequate to account for the effects of axisymmetry. Still, this value depends on the definition of the underlying design spectrum, and whether, e.g., the maximum, arbitrary or geometric mean component is employed, which should be accounted for in all calculations.