• Improper material properties—Sections of the model with material properties that are not physically realistic, or adjoining sections that have a difference in material properties that is too great may cause problems. There may be typographical errors in material properties. If you estimate any physical properties, then be sure to check how the estimates compare to the properties for other materials. One possible source of errors is an element with very flexible material properties in a model containing elements with much stiffer material.

• Improper spring stiffness, beam section, or shell thickness properties—If one shell is far thinner than other shells in the model, an error can occur. Check for omissions or typographical errors in these types of properties.

• Mass density properties of 0 for a modal analysis—Mass density must not be zero for modal analyses.

• Beam releases—If there are too many beam releases in the model, they may allow the model or parts of the model to move without resistance to some arbitrary load. You must specify beam releases such that the model is structurally stable.