The paper discusses envelopes of combined loading corresponding to: (i) first yield (ii) plastic load, and (iii) plastic instability load. The latter two were researched in the past but for a single load, only. The past idea has been expanded in the paper to two, practically relevant, simultaneously acting loads. Conical shell serves here as an example. It is shown that the ratio of area of plastic load envelope to the area associated with the first-yield envelope is 3.2 whilst the similar ratio of plastic instability to the first-yield envelope amounts to 25.8. This indicates 'a modest' (320 %) increase of possible range of loading, and a substantial reserve of strength above the end-of-elastic behavior (approx. 26-times).
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