In analysis of cables via robot, a pretension value has to be inserted and then iterations have to be made. I was wondering if there was a way to predetermine a good judgement for the pretension to be used for the cable which eliminates the need for doing many iterations. This may not be a pure robot question but I would appreciate the help nevertheless.
Structural Design Engineer
MZ & Partners Engineering Consultancy
For preliminary analysis of models containing many cables it is much more efficient to define cables not by pretension but by elongation or length:
In such case when specifying for instance zero relative elengation the unloaded cable length equal to the distance of end nodes will be used. For instance +0.01 relative elongation corresponds to unloaded cable length 1 percent longer than the distance of end nodes.
In case of defining cables by pre-tension it is sometimes very difficult to predict what unloaded cable lengths may be necessary to obtain it and moreover there is a risk that equilibrium and convergence is not possible with reasonable lengths of cables. See:
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I am having the following problem with a guyed tower (17 m) and 4 guys, when i try to run the analysis with the non-linearities on=non-linear analysis +P-d (figure below), the program gives me a error:
The cables are defined with pre-tension F0, should i change to relative elongation?
And if with a relative elongation of 1%, still does not converge, should i try to reduce the elongation to 0,1%?
I already saw the thread http://forums.autodesk.com/t5/Autodesk-Robot-Struc
And after i find the cable elongation, the pre-tension F could be obtained from F=A*dl*E ?
with A=area of section; E= modulus of elasticity and dl=elongation and F=pre-tension
The message "Matrix is not positive definite..." may result from 2 reasons:
1/ tower not stifff enough or tension forces in cables too high - resulting in buckling of the tower
2/ sometimes such message is resulting from the instability of the structure - in case of guyed towers it is very easy to forget about fixing RZ rotation at the bottom of the tower and defining only pinned support there
It is also important to remember about using full Newton_Raphson method - see message 6 of http://forums.autodesk.com/t5/Autodesk-Robot-Struc
If still problems please attach the model.
I am still having problems, i changed the cross sections of the tower to the highest angles availables in Robot and i still had the error of critical load excedeed for the case of only selfweight acting on the structure. Than i changed the cables from F0 to relative elongation to 0.0001 and just in this case i the program did not show any error.
The support of the tower is defined with RZ fixed and also all the translations degrees are fixed too.
Is there any way to send the model just to you?
Your description suggests that the reason of problems was corresponding to point 1 of my previous answer: "1/ tower not stifff enough or tension forces in cables too high - resulting in buckling of the tower"
When defining the cables by Fo=0 Robot is in reality using zero elongation so it can result in "short" cables, big tension forces in them thus big compression force in the mast - thus buckling of it.
Using 0.0001 relative elongation the length of cables was increased, tension forces in them reduced, compression force in the mast reduced too - thus no buckling.
Later today I will look closer at your model sent using PM.
Additional remarks related to the model received by PM:
F0 force in cable definition was not Fo=0 but Fo=1 kN
The model was slightly unsymmetrical and this connected with the existence of interacting cables defined by the same tension force resulted in problems.
As already suggested on this forum recommended to define cables by elongations. In such case forces can be indirectly controlled modifying the values of elongations.
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