topic Differential deflection analysis in steel frame in Robot Structural Analysis Forum

Differential deflection analysis in steel frame

Gabriela_Bercot3JB97 — Tue, 11 Aug 2026 17:07:26 GMT

Hi all,

I'm carrying out a differential deflection analysis in Robot Structural Analysis and have been investigating the effect of semi-rigid beam-to-beam connections on the overall behaviour of the frame.

I modelled the beam-to-beam connections in IDEA StatiCa and extracted the initial rotational stiffness, Sj,ini (kNm/rad). I then applied these values as rotational releases in Robot using elastic rotational springs. This was done for almost all beam-to-beam connections within the frame.

After re-running the analysis, I found that the differential deflections were virtually unchanged from the original baseline model with pinned connections.

To verify that the modelling approach was correct, I created a simple dummy model and tested pinned, semi-rigid and fixed connections. In that model, changing the rotational stiffness produced the expected changes in deflection, which suggests the releases are working correctly.

However, when the same approach is applied to the full frame, there is little to no change in the results. Has anyone come across this before, or can suggest what I should be looking at? I'm trying to understand whether this is a modelling issue, a consequence of the frame behaviour itself, or whether the connection stiffness values are simply not significant enough to influence the global differential deflections.

Thanks!

Re: Differential deflection analysis in steel frame

Krzysztof_Wasik — Tue, 11 Aug 2026 22:48:18 GMT

Hi @Gabriela_Bercot3JB97

Share both model versions (you are comparing) at the moment only model version with elastic releases is shared.

Please also show screenshot present insignificant differentional deflection for selected member/members in both model for which you expect larger differences.

Model is defined correctly, elastic releases are applied, and DSC algorithm, necessary to consider elastic releases is activated. In my opinion there are no errors in model geometry definition

Re: Differential deflection analysis in steel frame

bhavani_sankuratri — Sat, 15 Aug 2026 07:27:04 GMT

Hi @Gabriela_Bercot3JB97,

Hope you are doing good! Just checking in to see if you've had a chance to provide the additional model version and deflection screenshots requested by Krzysztof_Wasik . This information will help further investigate the behavior you are seeing.