topic Re: API access to "Moment originated from force eccentricity" option for mobile/vehicle load routes? in Robot Structural Analysis Forum

API access to "Moment originated from force eccentricity" option for mobile/vehicle load routes?

noama_almou — Thu, 13 Aug 2026 09:04:53 GMT

I'm generating moving/vehicle load cases (RobotOM, Python) to model a monorail crane beam, where the wheel loads act on the bottom flange while the beam element is defined on its neutral axis. In the Robot GUI, the Route parameters dialog for a mobile case includes a checkbox: "Moment due to force eccentricity" — exactly what I need, since the route sits at an offset from the bar's own axis.

I've tried to control this option through RobotOM 2026 (robotom.tlb) without success. On IRobotMobileCaseRoute I only find:
BeginingRouteLimit, EndRouteLimit, Geometry, GetFactors, LoadDirection, SetFactors, Step, Tolerance

No property like EccentricityMoment, MomentDueToEccentricity, ConsiderEccentricity, etc.

I also checked IRobotMobileCase, IRobotVehicleData (and its Loads collection — F, X, S, DX, DY, Type), Project.Preferences, and searched the whole robotom.tlb for "eccent"/"moment"/"offset"/"mobile"/"route"/"force". The only offset-related fields I found are on static concentrated bar loads (IRobotBarForceConcentrateRecordValues.I_BFCRV_OFFSET_Y / OFFSET_Z), which don't apply to vehicle-generated loads.

To confirm the option truly isn't wired to the public API, I compared two nearly identical .rtd files (checkbox checked vs unchecked) and found the difference is a single bit in the LDATR stream (0x00100000). Patching that bit and reopening the file does toggle the checkbox correctly in the GUI — so the setting definitely exists in the file format, just not (as far as I can tell) through RobotOM.

Before resorting to something as fragile as patching the .rtd binary format directly (no guarantee of stability across versions, or of not interacting with other flags packed into the same bitfield), I wanted to ask:

Is this option genuinely not exposed through RobotOM, or is there another interface/object I'm missing?
Is there a supported way to set it programmatically (even indirectly)?

Any pointer would be much appreciated — happy to share more details on the model/setup if useful.

Re: API access to "Moment originated from force eccentricity" option for mobile/vehicle load routes?

Stephane.kapetanovic — Wed, 02 Sep 2026 03:09:07 GMT

Just out of curiosity, have you tried applying a higher tolerance value to see if the API returns the desired torsional moments ?

Re: API access to "Moment originated from force eccentricity" option for mobile/vehicle load routes?

noama_almou — Thu, 13 Aug 2026 10:56:11 GMT

Thanks for the suggestion — I tested this thoroughly.

First test, three cases with a point load F=100000N at midspan of a simply-supported beam (5m, IPE300):

Case A — route directly on the bar's own axis (no offset): FZ=70400N, MY=125000N·m, MX=0 (baseline, matches classic beam theory M=FL/4)
Case B — route offset e=0.15m below the axis, small Tolerance (0.01m, i.e. < e): FZ=0, MY=0 — the load is entirely lost (too far from the bar to be captured at all)
Case C — same offset e=0.15m, Tolerance=0.20m (i.e. > e): FZ=70400N, MY=125000N·m, MX=0 — identical to case A, no extra moment whatsoever

So a sufficient Tolerance does solve the "load is lost" problem (case B → case C), but doesn't trigger any eccentricity moment — Robot just snaps the force back onto the bar's own axis once it's within tolerance, discarding the offset entirely.

Second test — in case you meant a genuinely large tolerance (not just "slightly above e"), I swept several orders of magnitude, same offset e=0.15m throughout:

Tolerance=      0.2m -- MX=0.00
Tolerance=      1.0m -- MX=0.00
Tolerance=      5.0m -- MX=0.00
Tolerance=     50.0m -- MX=0.00
Tolerance=    500.0m -- MX=0.00
Tolerance=1000000.0m -- MX=0.00

MX stays exactly 0.00 across the entire range, from 0.2m up to 1,000,000m. No threshold effect anywhere.

Re: API access to "Moment originated from force eccentricity" option for mobile/vehicle load routes?

Stephane.kapetanovic — Fri, 14 Aug 2026 10:07:45 GMT

I do not see any setting that would resolve the issue of the missing option. Thank you for running those tests.

Re: API access to "Moment originated from force eccentricity" option for mobile/vehicle load routes?

lauri_barnhart — Mon, 17 Aug 2026 15:00:17 GMT

Welcome to the community, @noama_almou.

Hope you are well. Just checking in to see if you found a solution to your question or if you still need assistance. If you found a solution, please let us know how you resolved it. If you still need assistance, please provide an update here with any additional information about the problem. Any additional information you can provide is greatly appreciated.

Thanks,