Cable: assembling case incorrect ?
Have you used Large Displacement calculation method. It is recommended for cables
Krzysztof Wasik
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
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
Krzysztof Wasik
Refer to that article
https://knowledge.autodesk.com/article/ROBOT-What-is-specific-to-the-use-of-cables-in-a-model
Krzysztof Wasik
