What would be the effects Warp Plates in STAAD analysis?

Question

Good day to All (especially the STAAD developers/engineers) 
 I would like to solicit your opinion and perhaps be able to clearly understand my the question that would be of the subject title above. 
 I have modeled two-intersecting tunnels in which apparently have 16-20+ warp plates. This is around 1% of the total plate elements in the model. 
 My initial take is that, it is okay for me to continue in my analysis and design with these warp plates in the model. I just need your opinion, recommendation and take on with this as I would also need your response on the effects of this warp plate in the analysis. (aside from inducing additional torsional stress on the plate) 
 A snapshot below depicts my model

Raymond Curtis · Answer

The plate element is derived as being exactly flat. Some unintended moments will be generated that will result in statics check results/differences becoming non-zero. If these differences are small then the analysis may be OK. If not, you may want to subdivide the non-flat quad plate into two tri-plates.

Use View-Options-tolerance-angular for vertex normals to get a tighter definition of warped.

Sye · Answer

The automatic stiffness correction was introduced in SS5 20.07.10.64 to improve the stiffness for the quad plates which are non-flat but some issues were observed later due to which it was taken out in SS6. So the automatic stiffness correction is not applied anymore by the analysis engine. The SET PLATE FLATNESS TOLERANCE is not active anymore due to the same reason.

It is important to note that STAAD.Pro assumes the plates as flat plates. When plates are non-flat, bending stresses and transvers shears are introduced due to in-plane forces which causes statics check, corner forces, stress calculations to be incorrect. It is best to replace such non-flat plates with four triangular plates having a node at the center.

The tolerance for warped plate detection in the GUI can be adjusted from View > Options > Tolerance > Tolerance for Warped Plate Element Detection. To detect warped plates, one can select the plates first and then go to Tools > Check for Warped Plates.

Sye · Answer

Whether it is OK to go with some warped plates or get those replaced by triangular plates, is more of a judgment call. As Raymond Curtis pointed out in his post, looking at the imbalance in the static check results could be a factor based on which you can decide which route would be appropriate for your case. I don't think there is a single value of tolerance that can be labeled as good for all models. You can definitely model curved surfaces with plates but for that you have to mesh the curve with a series of flat plates if you are using quad plates. Using triangular plates is a safer bet for the curved regions as that way you will not have this warping issue.