One remark here: frequencies may happen to be negative, but with a very low value, this is usually due a rigid body mode, it is the same with positive very low values. Another remark: with a model made of many beam elements, like a large frame, we may find many values corresponding to the local mode of a single element, before finding global modes, whether these local modes should be considered or not is the choice of the engineer 1 ! And one last remark: if the model is showing symmetry or cyclic sym- metry there may be several modes with almost the same Eigen frequencies and shapes due to symmetry, and this is normal, for example an hexahe- dral section tube submitted to a compressive end load shows 6 local modes with exactly the same shape repeating on each of the six faces.