Effects of axial forces on flexural stiffness

The forces required to cause a unit rotation or translation in the transverse direction at one end of a member decrease when the member is subjected to axial compressive force, and conversely increase if the axial force is tensile. When the axial forces in members of frames are relatively high, the change in stiffness arising from bending by the axial force can be of importance. A set of axial forces is estimated, the stiffness or flexibility of the members is determined accordingly, and the frame is analyzed as a linear structure. The force or displacement methods of analysis for linear structures can be used for structures in which the axial forces in the members affect their stiffness, provided that a set of axial forces is assumed to be present in the members in all the steps of the calculations. The transverse deflection also produces a change in member length, which is ignored here as is generally done in linear analysis.