Beam Pre-stress Loads and Lack-of-fit

Pre-stress and lack of fit loads are similar, but the definition differs. A pre-stress is considered as a force applied to the element at some position in the section relative to the neutral axis. A lack of fit is assumed to affect only the axial terms relating to the element. For a pre-stress of force and offsets of y and z the resulting nodal forces and moments are

$$\begin{aligned}&f_{x} = f_{p} \\&m_{yy} = f_{p}z \\&m_{zz} = - f_{p}y\end{aligned}$$

For initial strain, the nodal force is

$$\begin{aligned}&f_{x} = EA\varepsilon\end{aligned}$$

For a lack of fit in linear or P-delta analysis, the force is

$$\begin{aligned}&f_{x} = \frac{EA}{l}\Delta l\end{aligned}$$

while for nonlinear analysis, the force is

$$\begin{aligned}&f_{x} = \frac{EA}{l + \Delta l}\Delta l\end{aligned}$$

where ${l}$ is the as-modelled length.

Note: In GSA, loads can be applied to a list of members or elements. Any loads applied onto members will be automatically expanded into the appropriate elements loads in the solver in order to analyse the model.