Question

The generated loads are the same in both Static and Collapse analysis, but there are differences in member forces between the two analyses.

Answer

PSI and Collapse Advanced modules utilize widely different formulations for member force and stress calculations. PSI uses the small deformation and strain assumption, where for a horizontal member, the axial strain is:

PSI Axial Strain = (X displacement at Joint B - X displacement at A)/ (member undeformed length)

On the other hand, the Collapse Advanced uses a large strain formulation where the axial strain becomes:

Collapse Axial Strain = (member deformed length - member undeformed length) / (member undeformed length)


The deformed length of a member is calculated based on the updated coordinates of its end joints in a deformed configuration. In other words, we update the location of each joint as:

X0 + X Displacement, Y0 + Y Displacement, and Z0 + Z Displacement

As you can see, all displacement components are involved in axial strain calculation in the collapse case, unlike in the PSI case, where only X displacement is used.

Also, the joint displacements can differ even for small loads because of different formulations. Although the difference between joint displacements is small, it can lead to significantly different axial strain in members. especially for short members using the above axial strain formulation.