Modeling Cables in SAP

[mtobolski]

I am looking to model traffic signal span wire structures using SAP2000 and am wondering what is the best way to model cable elements? I first attempted to model as frame elements and applied moment releases at the ends of members. This lead to errors at joints between members along the span.

 

[arigato]

Hmm Ive never tried modeling a cable before but I would try a series of 2 force members (truss memebers) pinned at their ends before I would use frame memebers. By definitions cables cant take compressive forces very well and dont resist moment all that great either.

 

[mtobolski]

The main problem I encountered when modeling as a truss member is the fact that I also need to model the sag of the cables. The problem is that when I model the sag and discretize the continuous parabolic shape of the cable into a series of straight elements and make them axial only members, there is a loss of stiffness at the joints. This is easily understood since I have two members framing into one joint at different angles and I say that they can only take load along their centerline. The forces do not line up, hence there is no stiffness there as the computer sees it. Because the stiffness of the cables are very small when compared to the supporting poles, I have modeled as a fixed connection at all points in order to model. This leads to some moments in the cable, but nothing major, especially when compared to the poles.

 

[arigato]

What stiffness is being lost at the joints? axial rotational? Did you apply nodal loads in between the truss members along the cable to represent the self weight of the cable or the applied loads?

Another idea might be to insert a ton of hinges with low Mp's into the frame memebrs you are using. That way youll get alot of rotation along the memember and therefore alot of lateral deflection without losing any axial stiffness.

Hope some of these ideas help!

AR

 

[dozer]

What version are you using? Version 9 has true cable elements. Unless I just missed something, in versions before this, what they call cable elements really aren't. It's a been a while since I played with it but I'm thinking you have to do a nonlinear analysis for it to behave properly.

Anyway, bottom line is you need version 9.

 

[ingemor]

I am working on a similar situation and the verification example 1-016 "Tension Stiffening Using P-Delta Analysis" in the SAP2000 v9 documentation has been quite helpful. Using the example model, I suggest you lower the midspan joint and hang the traffic signal from it and see what kind of axial tensions you get; I dont think you need to assign cable section, frame section will do. The lower the joint the less tension you get on the frame elements. Hope this helps.

 

[ingemor]

Forgot to suggest to remove the tension force from joint 3; add restraint "u1" to joint 3. Assign weight of traffic signal to "Tension" Load Case . Use the same Analyses Cases and run it. Example 1-016 has 7 different models, check out each one carefuly. Good luck!

 

[ingemor]

Careful, you may have to modify "Other parameters>Nonlinear Parameters>Geometric Nonlinearity Parameters>" to "P-Delta plus Large Displacements" in the Analysis Case "NLSTAT1".
More information in the Analysis Reference Manual Chapter XXI "Geometric Nonlinearity".