The IMR option allows users to generate static cases based on individual modes. One can thus study the effect of these modes acting in isolation. The results for these IMR cases have signs unlike the response spectrum cases. These can be combined with other static loads using REPEAT LOADs which is also not possible for a response spectrum case. So one can do Pdelta or tension-compression type analysis with these cases. Another application of the IMR is that for design of certain parts of the structure, one or two modes may be more important than the rest. Generating IMR cases for these modes enables user to get signed results for these specific modes which can subsequently be used for design.
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