SAP 2000: Cable-Strut structures under self-stress
SAP 2000: Cable-Strut structures under self-stress
(OP)
Hello,
I would like to know how to model 3D cable-strut structures in SAP 2000. There will be no externally applied loads and self-weight will not be taken into account. Which means, on analysis, the structure should find a new equilibrium position resulting in internal forces (prestress forces) in the members. If you have experience with this, and have examples to share, I will appreciate you taking the time to help.
I would like to know how to model 3D cable-strut structures in SAP 2000. There will be no externally applied loads and self-weight will not be taken into account. Which means, on analysis, the structure should find a new equilibrium position resulting in internal forces (prestress forces) in the members. If you have experience with this, and have examples to share, I will appreciate you taking the time to help.





RE: SAP 2000: Cable-Strut structures under self-stress
RE: SAP 2000: Cable-Strut structures under self-stress
RE: SAP 2000: Cable-Strut structures under self-stress
It is easy to make the lenticular example by drawing tendon pieces following equally the segments of the bottom chord.
It seems natural that to impart prestress one uses the tendon element suited to it. You can also use cable elements and for tensile forces following the cable use some equivalent temperature descent. I may also try to apply one of the types of cables that set some amount of tension to the cable to inspection of result; if I find something interesting that way I may post about it.
RE: SAP 2000: Cable-Strut structures under self-stress
RE: SAP 2000: Cable-Strut structures under self-stress
A third option tension at I end, or at J end, also causes the trusses where the cable is the lower chord yo take stresses in the other members, try yourself.
As indicated, the best control in the stress of what imparts the prestress is through the tendon element.
RE: SAP 2000: Cable-Strut structures under self-stress
Thanks for responding. I am unable to open your Version 15 files in Version 14 that I have. Would you be able to convert the files to this lower version?
To elaborate on my earlier message, I am thinking of cable-strut structures in which the tension members are made of cables (instead of bar elements). The members of this structure will be prestressable. This structure with cables can stand by itself only under prestress.
I want to know how I can create such a 3D cable-strut structure, with no applied loads, no self-weight, but with the 6 constraints that a 3D structure should have. Can SAP find the new equilibrium position(s) such that the structure stands on its own. Of course, the joints will have to move but this is what I am expecting that SAP could automatically do. The analysis results should give me the new joint locations and the prestress forces in the members that make the structure stand in equilibrium.
I will appreciate your help.
RE: SAP 2000: Cable-Strut structures under self-stress
I in fact tried to convert it to SAP 14 before upload but since I am learning SAP2000 these days, I couldn`t (yet) do. I may however repeat (I hope, since the cable element is changing with every version) in version 14 if I fail to produce the conversion and will do I expect reasonably soon one or another way.
RE: SAP 2000: Cable-Strut structures under self-stress
To make the model simple, it is a Warren truss of 25x24 concrete members. These members are rendered weightless trhough material properties, and, since had rebar impossible to eliminate, I also changed weight property to zero just in case. The tendon runs the bottom chord of the truss, and I have rendered weightless its material. Then I apply a 20 tonnef prestress to the tendon and the truss cambers a bit upwards.
RE: SAP 2000: Cable-Strut structures under self-stress
RE: SAP 2000: Cable-Strut structures under self-stress
Tomorrow I will add some sample/s that create stresses in rigid inclusions and against the standing restraints. It may be even more useful if you imagine one case that exemplifies your expected difficulties and we try to produce a model dealing with them.
RE: SAP 2000: Cable-Strut structures under self-stress
RE: SAP 2000: Cable-Strut structures under self-stress
In this the upper cables originally follow what is the line of lesser tension; the bottom ones follow equilibrium with somewhat higher tension at hanging points and are shorter having less sag. What said, of course, not meaning any tension, just devices provided by SAP2000 to define the shape of the cables.
Then I insterperse a rigid structure of RC 25x25cm frame element members.
All is weightless.
I subject the whole outfit, cable and frame elements to a descent of 100ºC
In this case the shortening results in the 8 members in the outside of the cross plan in compression and the rest in tension. The cables show marginal stress.
Whilst not illogical in general, displacements when looked at thigh amplification (calculated nonlinear large deformation) show some kinks improper of cable behaviour, but this may as well be a defect of my model as a device of the solving routines; I am not expert to say what.
RE: SAP 2000: Cable-Strut structures under self-stress
RE: SAP 2000: Cable-Strut structures under self-stress
For a cable with some stiffness, a initial position, and a set of excitations and environment, the actual position can only then be derived from the laws of the mechanics. In absence of such data and process, every line of equal length is a solution.
What means:
That the solution -we are not to forget it is a solution got by SAP2000 v.15- of my entry 5 Aug 12:05 is but, if something, in spite of its apparent beauty, at most, just one of many possible solutions, because whatever the predominance of the tightening produced by the cables of the top of bottom set, the other has ample deformation ability to soak the required movement without problem; hence the sets of solutions really are all those that require the cinematics (as related to the measure of the cables with some holgure) of the problem. The "solution" found by SAP2000 must then, more than anything a device of its inner workings, since nothing really impels the outfit to the final shape shown in the image other than the relaxation worked by the program, and it is patent that there are many other positions that also satisfy the necessary requirements of relative positions between the parts after the descent of temperature.
Hence, for non-taut cables, the weightless environment becomes a severe requirement from the viewpoint of structural analysis since it widens the scope of the satisfactory solutions. A such cable is a mechanism able to take every shape compatible with its length, even with kinks (like chain links of the length of the elements, as, I remark, is the case, since the deformed shape shows such unnatural kinks ... because there is not a gravity forcing to take some catenary shape, or stiffness present to force some constant angle at a node.
RE: SAP 2000: Cable-Strut structures under self-stress
I will try to see if I can open your files on a university computer. I have attached an image of what I would like to know about. If all the tensile members shown are elastic, and all the 12 members were prestressable.....I would like to model this such that the structure finds an equilibrium configuration with a certain level of prestress in all the members. To start assume that the top triangle and the bottom triangle are parallel. This is not an equilibrium configuration. Only at a particular angle of twist between the top and bottom triangle, will this structure come to equilibrium. I understand from your last message that there can be several different equilibrium configurations, which will depend on the level of prestress in the members. Hope this can be a fundamental learning model from which I can do more complicated structures. Thanks again for answering so thoroughly and taking the effort to help.
RE: SAP 2000: Cable-Strut structures under self-stress
Can you NOT use temperature, and instead use elongation and prestress to analyze the above image I sent.
RE: SAP 2000: Cable-Strut structures under self-stress
In any case, you can get equilibrium for your problem only for some final configurations and stresses and even with the help of tendon elements SAP2000 might only be able to prove equilibrium more than find it. The main difficulty is that the initial dimensions prior to relaxation of the tendons and taking loads of the strut may need be so close to the final dimension (and to the proper side of length and force) that even if in a case with the rotated symmetry your problem has, we with difficulty can get the same for one arbitrary geometry, and so will render the process unworthy for general application.
I will be addressing your problem soon, first in Mathcad for inspection of the required inner workings of the thing, then we'll try to make the use we can in SAP2000 to see what we can make.
RE: SAP 2000: Cable-Strut structures under self-stress
For a given input data you get infinite feasible solutions (within the realm of the acceptable stresses) that can satisfy equilibrium. Under such circumstances, it is quite likely that whatever SAP2000 may find be just a verification of a well set case, or just a device of its inner workings, since obviously 1 solution won't advert of the infinite others to the inadverted. Anyway we can give it a try later.
RE: SAP 2000: Cable-Strut structures under self-stress
I tried first with tendons with force in the retentors, established in several ways, then the structure didn't show any response; in the end I returned to load the retentors through a temperature descent forcing their shortening, and this is the model I am uploading.
For the particular case SAP2000 since showing what more or less one could expect might be right in this structure.
RE: SAP 2000: Cable-Strut structures under self-stress
Your MathCad solution is very instructive. Thank you again. I understand that you use temperature change for changing the tendon lengths to eventually achieve the equilibrium configuration. The analytical solution, like your MathCad results, gives alpha=30. From your message on "6 Aug 11 13:30", do you mean that SAP will not be able to do what MathCad has done for you, with regard to finding the equilibrium configuration?
I get the attached error message when I try to open your SAP files. Do you know how I can escape this? See attached error message.
So, exactly, what element type, what load cases/combinations, and other specifications did you use in SAP? Also, I think I have access to MathCad at college. Would you mind sharing your file, so I can see how it works? Do you think it is more powerful than SAP for finding equilibrium configurations?
RE: SAP 2000: Cable-Strut structures under self-stress
In the Mathcad worksheet model we assume as a simplification inextensible rods and cables that out of the geometry, and then we find that for every geometry that takes the shape of the solution in which one triangle is rotated 30 deg from the one that keeps its position we can have infinite sets of grouped tensile and compressive stresses in the members that satisfy equilibrium as long as they meet some precise proportions that depend on the dimensions of the base and length of the strut (or overall height of the outfit, if you want).
SAP2000 on the contrary is an advanced analysis program, and, partly at least, mechanical solver, and simply, as long you go within the basic understandings of the program and make a good model it is accepted by most of its users provides a reliable answer to equilibrium and other useful structural stuff. From this description and knowing that it admits property modifiers, we must expect that SAP2000 will (or at least can, with a proper model) resolve for solutions that take the final shape of the kind found through Mathcad.
But to produce such model we first than anything need make the axial rigidity of the members very big. Imagine we have rigidity enough to mimick the infinite axial rigidity of the Mathcad model to tolerance, then we can for go the obvious solution when producing the model of just using one of the geometries of the found solution, then if I set some force on, say, the retentors, wil relax it to take the values proportional that we may established through mathcad for the same geometry and that would be all. I suspect the overall high stiffness may do worrying things numerically but this is just a suspicion and we may defer it to further proof. Even so, then you may believe that the forces found are the "solution" to the structural case (within the named parameters), yet you may eventually by inspection discover what we did through the Mathcad study, that other forces, just proportional, also would be, and even if you have found "THE" one that correspond to its derivation from the unrelaxed force applied to the retentors, you might be forfeiting the other proportional answers that may also be of your interest.
This is no surprise, since we all solve everyday for just a set of hypotheses and forfeit every other understandings of the problem and secondary ways of structural response that would burden our structural design tasks.
We maybe need not to adjust ourselves just to the tight geometry of the answer, and maybe with SAP2000 we can set the case of the cables at the triangles hanging somewhat loose, whereas the retentors (to be loaded) and struts have the straight geometry. We should expect that as long the slackened cables at the triangles have the final length, and axial rigidity brings the solution of a final shape of the kind found in Mathcad, it also will find the solution, something that we also can bring to test.
Yet SAP2000 potentially at least has shown us that it needs not to abide within the hypothesis of perfect rigidity and found shapes of the Mathcad study, since it can derive solutions according to the laws of structural analysis not within these limits.
Respect the kind of error you refer to I found it when I tried to convert v15 to v14 yet since I am not SAP2000 expert, but novice, I have not found solution if it exists and maybe someone may help us in the task whilst we learn how.
Whilst you get the model working, I didn't manage to get any response till I used for all elements other than the struts the cable element; i.e., the tendon element -in whatever definition I put where the retentors are- failed to produce meaningful response, maybe because they are thought to be applied on some beam or struts or whatever other reason that for now escapes me. I inmovilized the vertices of the upper triangle in an horizontal plane (whilst allowing it to vary in position in such plane and forfeiting the lack of enough constraints that might bring the outfit in movement and so, analysis error), i.e., one node inmovilized XYZ, other YZ, and other just Z.
Everything rendered weightless one way or another.
Then I just applied a descent of temperature to the retentors, and then the solution adopted the kind of predominantly rotational displacement that we see to be consistent with the nature of the outfit. You can dump all this in just one hypotheses, even dead load case, since the members have no weight.
Respect sharing the Mathcad worksheet, I have already made in my post of 6 Aug 11 13:30. It is the wholly operative file with extension .mcd
The file is for Mathcad 2000, but, since the mathematics are simple, any of the later Mathcad 11, 12, 13, 14 or 15 should open it without any problem in the automatical conversion.
RE: SAP 2000: Cable-Strut structures under self-stress
RE: SAP 2000: Cable-Strut structures under self-stress
Can you post your .s2k files instead of your .sdb for all the above that you sent.
.s2k can be opened in older versions.
RE: SAP 2000: Cable-Strut structures under self-stress
RE: SAP 2000: Cable-Strut structures under self-stress
I am still a bit puzzled by the fact of Mathcad going definitely for the alpha=30 deg solution (I have ranged the limits for alpha by ten sexagesimal degrees, then the following, without finding other solution) and SAP2000 finding final configurations of equilibrium of other angles ... at the moment I can't see how it can be (other than the forces in the retentors and struts become both zero at the rotated angle -then obliging so as well in the triangle cables since the strut and retentor not exerting any action- and then equilibrium is feasible for other rotation angles; I have not examined the values in the 15 deg rotation case to see if such is the case to some program tolerance); in any case, when the rotation is set to 30 deg and you relax the imparted forces, the deformation occurrs quite homotetically, as is expected from the Mathcad result, whereas at the 15 deg case some rotation becomes quite apparent.
RE: SAP 2000: Cable-Strut structures under self-stress
I could open your model in v14 and I looked through the member type, loading, etc. I see that you have given a temperature decrement to all the cables, not just the vertical cables. Any particular reason for that?
Also how do you determine the various equilibrium configurations? How do you know the angle of rotation between the top and bottom triangles? I ask this because when you run the analysis, it displaces to a particular configuration, not several.
Any reason why you put the supports at the top?
I note that there are support reactions at all the constraints. Shouldn't they come out as ZERO?
I see the point you are making about MathCad giving a precise 30 deg. angle at which equilibrium is achieved, whereas SAP may not.
RE: SAP 2000: Cable-Strut structures under self-stress
Free Standing Tension Structures
From tensegrity systems to tension-strut systems
Wang Bin Bing
SPON Press
London 2004
Our already familiar 30º between triangles outfit and plan is extant at p. 24 in such book.
Respect the model, I think in the model starting at 15º I only affected the retentors to the descent of temperature, whereas in the 30º case, I subjected everything to the descent. My idea when doing this in the second case was that whatever the input desequilibrium (non relaxed temperature descents) if it has to come to forces in the members at some particular proportion it wouldn't matter what members are prestressed or not.
Respect why I retained the kind of support provided XYZ, YZ and Z atop, since not under weight, it wouldn't matter that was the bottom plane the one holding the restraints. It is just a device to provide rotational stability to the outfit, enabling for answer from the program, and a plane of reference to where look for relative deformation. Respect why I elected the 3 supports in one triangle plane, it was because from the Mathcad study I expected it remaining plane and then I was not restraining relative movement within the plane, whilst as well, for non-zero stresses in the members, the restrained joints keeping as well relative restraint between the joints given the expected tensile stresses in the triangle cables.
As I was giving insight in my previous entry, it may turn that, effectively, all non-zero stresses mean the 30º deg final status, whatever the initial unrelaxed forces, i.e., all other shapes not having the 30 deg rotation by inception would relax totally to zero if have to get to equilibrium (to, then, a different than 30º angle); this is as of now conjecture. All equilibrium situations for rotated angles different than zero would be showing zero stresses in all members. There would be a match between the initial loading and shape and final angle, yet the forces for all with non 30º rotation will have become zero, something that hints to a somewhat unstable equilibrium that must be not, since it has relaxed what have been actual and maybe significant forces to gain such zero stress state. The puzzling nature of the zero stress is patent since it seems easy to throw such system easily from such equilibrium ... to once suppressed the cause immediately returning to (maybe another, if dependant on the loading ... or maybe not, if depending on just the available geometry)... doesn't sound as unstable equilibrium if the shape is restored after disturbance.
As of now, I have not entered in detailed examination of the results nor the implications of the same. By the above suggested hypotheses, some forces should be zero, where the members may be showing some stress. I expect to give a look at that.
But when performing non-linear analyses, I have seen, even in examples in the watch and learn by CSI for cable elements, that the cables that should show zero stress, show not. It must be a device of the tolerance convergence and impossibiliy of getting a wholly accurate result numerically by the program, whatever the tolerances extant within. I don't know even where to look as of now to see where the tolerances are set (I have not even managed to read the ample documentation of SAP2000) nor what that would mean for a case like ours. But that zeros can have 0.02 or small values in SAP2000 I have seen with my eyes.
Thanks as well for your explanation on opening files of one version in other that I expect to try soon.
RE: SAP 2000: Cable-Strut structures under self-stress
All equilibrium situations for rotated angles different than 30º would be showing ...
RE: SAP 2000: Cable-Strut structures under self-stress
Everything seen points to SAP2000 being able to approximate anything that does not become a mechanism, and maybe even mechanisms should be representable as long as appropriate definition of the initial states and hinge behaviour is entered with some dynamic hypothesis; this however is an unlikely model most of us can be able to produce, particularly if not an specialist in the problem and how SAP2000 deals with things like the axial compressive stiffness of the cables and things like that. I mean, some particular inabilities of SAP2000 purportedly shown by the models I have made and -posted here and non posted- may well derive from being bad set models, particularly in not being dynamic analyses along time.
RE: SAP 2000: Cable-Strut structures under self-stress
SAP must be using Stiffness Method. Which means, the analysis will blow up because of the mechanism. Am I right?
RE: SAP 2000: Cable-Strut structures under self-stress
osquro (Structural) 15 Aug 11 9:57
Try using .s2k (or .$2k) file.
Lets say that version 14.2.4 file was created in the computer A and the computer B has SAP v14.1.0 installed.
- Make sure computer B has the same regional configuration (decimal separator) of computer A. If it is not possible to check, take a look to the file created in computed B and identify decimal separator. Then you have two options a) Change regional configuration in computer B according to computer A. b) Change decimal separator in .s2k file according to computer B (find and replace). If you select the last option, make sure that the string containing version is period-separated and the string containing units is comma-separated, regardless decimal separator, as shown bellow:
TABLE: "PROGRAM CONTROL"
ProgramName=SAP2000 Version=14.2.4 CurrUnits="KN, m, C" ...
- Change the string contatining version to the desired release. In this case:
TABLE: "PROGRAM CONTROL"
ProgramName=SAP2000 Version=14.1.0 CurrUnits="KN, m, C" ...
- Make sure desired default units for the model are selected in SAP2000 graphic user interface (bottom-right).
- Try to import the model.
- Let us know any trouble.
RE: SAP 2000: Cable-Strut structures under self-stress
http://www
Please read there.