2-6 Bending Plus Axial Stiffness

The beam element and the truss element can be combined to form a two dimensional element that resists transverse loads as well as axial loads. The accompanying figure shows the node point variable notation for this type element.

The stiffness matrix for the two dimensional bending element with axial element added is formed by superposing the beam and truss stiffness matrices and is shown next.

{r} = [k] {d}

Note that there is no interaction between bending effects and axial effects in this element. The interaction between the two is a nonlinear process, so the element presented in this section should only be used in the modeling of situations in which no nonlinearities are expected. If there is any doubt, it may be necessary to analyze the problem using both a linear and a nonlinear model and examine the differences between the two.

This element is a standard component in most FEM codes and is suitable for the analysis of many structures including the model of the building frame shown below.