Issue: unexplained changes of sign in seismic results

Issue

Investigating internal forces in seismic load cases, strange changes of sign appear, which cannot be explained in a meaningful way.

This typically shows as inconsistent changes of sign of vertical forces along sections in shear walls.

Possible causes and solutions

• Most common: use of automatic selection of predominant mode on a 3D model. To be avoided.
• Poorly selected predominant mode (manually)
• Jumps (discontinuities) in diagrams occur when the selected predominant mode has near-zero results (and change of sign) in a location and the modal superposition results is non-zero (values can be quite high, depending on the case) in the same location. This cannot be fully avoided. Keep in mind, that a seismic load case is an envelope of seismic results, cominb from the superposition of all considered modes. Applying a signature to those results is an approximation.
• The presence of a torsional mode in first position is usually a sign that the design of the seismic shear resisting system is not optimal. Torsional behaviour often complicates the selection of the predominant mode.

In general, pay particular attention to the selection of the predominant mode, see "Validation of seismic analysis: signature of results"