Why does the deformed shape for linear add load combination with response spectrum load case does not look correct?
First, it is important to remember that the output of a response spectrum analysis is the likely maximum response to a given loading and the structure can experience this response in “both directions”. Deformed shape for linear add load combination that contains a response spectrum load case shows the displacements, either minimum or maximum, whose absolute value governs the response. Depending on the sign of the other loads involved in the load combination, the resulting displacement can be either in positive or negative direction, even for adjacent joints.
