A fixed support will not experience any rotation at the base node, hence columns above can experience base moments. A pinned support is completely free to rotate. The decision about which to use is ultimately the job of the EOR based on the physical conditions. Since most real world conditions fall somewhere in between, most FEA programs (including STAAD, RAM Elements and Ram Structural System) also provide tools for rotational spring supports that linearly resist rotation without being rigidly fixed.
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