Discrepancy Between Results in Robot Structural Analysis and other Softwares

[mardwicaksono]

Hi everyone,

I'm doing a simple analysis comparing results from RSA and other software such as SAP2000 and STAAD.Pro. I created this simple 2D frame with no loading, just from the self weight,

and from doing analysis in each software, I got the result like in this table

Parameter	Sap2000	Robot	STAAD	Average
Moment Reaction	3,42	3,53	3,43	3,46
V Column	2,09	2,12	2,09	2,10
M Column	7,02	7,08	7,02	7,04
M Beam [-]	7,02	7,08	7,02	7,04
M Beam [+]	10,97	10,92	10,98	10,96
Vertical Reaction	44,40	44,40	44,40	44,40
P Column	44,40	44,40	44,40	44,40
V Beam	14,40	14,40	14,40	14,40

While the vertical reaction forces are matching between them, I'm seeing clear differences in internal force outputs (especially moments at fixed supports), even when the models are extremely simplified. No offsets, no releases, just straightforward frame elements. I've gone through all inputs multiple times to make sure everything is consistent.

At this point, I'm not just looking for how to "make the numbers match," but rather I’m looking to understand the why:

• What are the key differences in how each software treat frame elements and boundary conditions?

• Do they use different default solver types or stiffness formulations?

• Are there known differences in how each software calculates stiffness matrices or handles constraints?

• How do their internal numerical solvers differ (e.g., matrix storage methods, element formulations)?

Here are the models i created
Models 

Appreciate any help or resources anyone can point me to 

Thank you!

[Rafacascudo · Accepted solution]

On Geometry/additional atributes/Advanced member properties , assign this property to all bars as in the picture below , and then the results will be exactly the same

[netsonicyxf]

Is shear deformation calculation disabled by default in Robot?

[Rafacascudo]

yes