topic Re: Cable: assembling case incorrect ? in Robot Structural Analysis Forum

Cable: assembling case incorrect ?

benoit.thiry — Mon, 24 Oct 2022 15:12:51 GMT

Hello,

I used the FO field of a calbe to define the initial tension in the cable.
The software obviously does not consider the force F0 as an initial tension in the cable but as the projection of the final
load in the cable projected on the undeformed position of the cable.
my model is pretty simple: 14m horizontal cable. section 1.33cm2, E=1886 MPa.
the cable support a vertical distributed load of 0.26kN/m.
introducing F0 = 2.5kN Robot computes
deflection = 259.8cm, F in cable= -3.09kN , reactions: Fhorizontal = 2.5kN, Fvertical=1.82kN ??????

introducing F0 =0kN Robot computes
deflection = 125.5cm, F in cable= -5.48kN , reactions: Fhorizontal = 2.5kN, Fvertical=1.82kN
now introducing the initial tension via a thermal shrink of the cable
deflection = 105.5cm, F in cable= -6.42kN , reactions: Fhorizontal = 6.16kN, Fvertical=1.82kN
which is the correct value.
I believe there is a problem with the assembling case !!!

Many thanks in advance, to check / advise !

Benoît

Re: Cable: assembling case incorrect ?

Krzysztof_Wasik — Mon, 24 Oct 2022 17:53:55 GMT

Hi @benoit.thiry

Have you used Large Displacement calculation method. It is recommended for cables

Re: Cable: assembling case incorrect ?

benoit.thiry — Tue, 25 Oct 2022 08:00:46 GMT

Hello,
I tried both options, results are the same.
The problem is not related to convergence.
My understanding (I might be wrong) of what Robot calls assembling case is:
it represent the state of the cable when it is initially installed. Either an initial
tension force is applied or an initial shrink.
after that the cable will support the loads or structure it is designed for and this will somehow
increase( or decrease..) the tension in the cable and create extra cable sagging.

the issue is that when I specify an initial force to the cable
ROBOT solves for a case where the inward pull of the said cable matches the initial tension, whatever external loads are applied to the cable... while actually one expects the tension in the cable to increase when suspended loads are applied to the cable.

introducing F0=2.5kN to the cable ROBOT computes deflection as 259.8cm !!!
introducing F0=0.0kN to the cable ROBOT computes deflection as 125.5cm!!! ???
(Robot models attached)

-> initial tension increases the deflection ?????
with initial tension F0=0.0kN

with initial tension F0=2.5kN

Now defining initial tension via a temperature case, correct behavior is observed

questions:

what is the actual meaning of the assembling case?
can we define several independent load cases in a model containing cables. It seems that the load cases are additive?
Apparently deleting one load case in a cable model deletes the cable elements ?

many thanks in advance for your help,

Benoît

Re: Cable: assembling case incorrect ?

Krzysztof_Wasik — Tue, 25 Oct 2022 08:02:27 GMT

Hi @benoit.thiry

Share your model

Re: Cable: assembling case incorrect ?

benoit.thiry — Tue, 25 Oct 2022 08:05:00 GMT

it is in the zip file attached to previous message

Re: Cable: assembling case incorrect ?

benoit.thiry — Tue, 25 Oct 2022 08:08:19 GMT

did you find it ?
here it is again.

Benoît

Re: Cable: assembling case incorrect ?

Krzysztof_Wasik — Tue, 25 Oct 2022 08:46:21 GMT

Hi @benoit.thiry

Defie assembly conditions (initial force ,or enlogation) in the first load case. It is regarded as assembly load case and determines structure stiffness (usually self weight may be included in that case). If you need to consider only prestressing forces in assembly case leave it empty- (with no loads defined in load table)

All other load cases: wind, live etc should be defined in next, separate load cases.

While analysis program applies prestressing conditions to first load cases (assembly) which determines structure stiffness. All next load cases are applied on prestresses model. When load cases are calculated assembly load case is always regarded (in behind)

Results for wind = wind load + assembly case

Results for live= live load + assembly case.

In case of your model with Fo force

When wind is defined in case 2 and case 1 has no forces defined in load table

Assembly case 1 (with no loads defined) gives expected prestress force (and no visible deformation).

Wind aplied in load case 2 gives final tension (depending on prestressing force and wind load) and corresponding deformation.

Wind load case 2 results for prestressing 2.5

Wind load case 2 for increased prestressing force - F0=5 kN (smaller displacement bigger tension as expected)

When generating combinations, all loads defined in load table are multiplied by defined coefficients

It means that when assembly case is used in combinations, all loads defined for assembly case in load table - for instance dead load if considered in assembly case) are multiplied by defined combination factor, but prestressing force defined in cable properties is not modified (regardless coefficient used in combination for case 1).

While cable structrure analysis large displacement effect should be activated for load cases and combinations.

I hope it will help

Re: Cable: assembling case incorrect ?

benoit.thiry — Tue, 25 Oct 2022 08:57:53 GMT

Thank you very much Krysztof,
this is now very clear to me. I suggest that the user manual / on line help is modified/updated to
include your useful explanation. The user can hardly imagine hat he needs to consider
one extra initial load case for the cables, as initial data are already included in the cable data..
Benoît

Re: Cable: assembling case incorrect ?

Krzysztof_Wasik — Tue, 25 Oct 2022 09:01:03 GMT

Hi @benoit.thiry

Refer to that article

https://knowledge.autodesk.com/article/ROBOT-What-is-specific-to-the-use-of-cables-in-a-model

Re: Cable: assembling case incorrect ?

benoit.thiry — Tue, 25 Oct 2022 11:08:30 GMT

Indeed, it is obvious that when using non-linear element every load case and of course
every load case combination becomes a non linear case to be run separately.

What I wanted to say is that "the assembling load case" is not described in the Robot documentation
It must be documented. Your explanation was very clear.
Had it been present in the documentation, we wouldn't have this discussion ....

Thank you again,

Benoît