I've got a perfectly straight run of GRP/FRP pipe, anchored at both ends, modeled in Caesar v5.3. I'm analyzing in accordance with BS 7159.

I understand that stress ratios are calculated in the following manner (keeping in mind that compressive stress is induced from the opposing anchors, so sect.7.4 comes into play):

long: (axial + bending) <= 1.25 Sh
circ: [hoop - vh/a*(axial+bending)] <= Sh*Eh/Ea

where vh/a is Poisson ratio; Sh is axial allowable stress; Eh and Ea are moduli of elasticity, hoop and axial, respectively.

But why does Caesar alternate allowable stress (Sa) between each node? For example, Sa at node 10 is 1.25Sh but at node 20 it's Sh*Eh/Ea, and then at node 30 it's back to 1.25Sh.