Response Spectrum Analysis calculates and combines the pseudo
maximum responses of natural vibration modes against a given design spectrum by
assuming each mode as an independent single degree of freedom system.
During the
combination of modal result vectors, the sign of the displacements and internal
forces are lost and hence equilibrium condition is not satisfied anymore. If
you check the deflected shape of the structure under response spectrum load
cases, you may see weird displaced shapes due to this fact. Also, internal
force diagrams will always be positive after the combination. This is the
expected behavior and natural output of response spectrum analysis.
Most of the time, this is easily overcome during the design
by combining RSA load cases with dead and live load cases using both negative
and positive coefficients. However, for deflection checks and other structural
checks, you may need the signs of the internal forces and deflections.
For this purpose, ProtaStructure hosts a setting to either
use the ‘Absolute Values’ or ‘Sign of Dominant Mode’ for RSA
result signage. This option is available in Seismic Parameters > Settings tab.

If dominant mode signs are used for overall RSA results then
the dominant mode will be selected depending on its mass participation in a
certain DOF and the sign of that mode will be used for the particular RSA load
case in the relevant direction. This approach may yield unexpected results in
highly irregular structures where the dominant mode is a torsional mode or
translational mode with high coupling.
This option can be accessed from Seismic Parameters > Settings