The deflected configuration of a structure subjected to a load combination which includes a response-spectrum load case is meaningless since displacement sign convention is lost during modal combination.

In a response-spectrum case, for a given direction of acceleration, the maximum displacements, forces, and stresses are computed throughout the structure for each of the vibration modes. These modal values for a given response quantity are combined to produce a single, positive result for the given direction of acceleration using one of the modal combination methods. As soon as a response-spectrum case is added to a combination, it becomes a double-valued combination, once with positive values and once with negative values, for the response-spectrum results. When deflected shape is displayed for a multi-valued combination, it is based on the maximum or minimum displacement at each degree of freedom, whichever has the larger absolute value. Depending on the sign of the other loads involved in the load combination, the resulting displacement can be in either the positive or negative direction, even for adjacent joints.

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