Catenary Cable

Question

A few questions regarding a cable spanning between two pin supports. 
 
 I watched a couple helpful videos: 
 2021-02 (February) SIG Workshop &ndash; Structural for APAC:Advanced Cable Analysis in STAAD.Pro 
 2020-17 (June) SIG Workshop: STAAD Session: Applications of Non-Linear Cable Analysis 
 In both of these videos he models the catenary cable as a segmented member. Is there any reason for this? Could he have just used a straight member spanning from support to support. Or a straight segmented member spanning from support to support? Was it necessary to model some sort of initial sag? Or is there accuracy gained in segmenting the member? 
 For a single member cable spanning between two pinned supports I get a converging result when using the advanced solver but not using the basic non-linear solver. Any idea on why that would be? Do I need to use a segmented member or provide some initial sag? 
 For a two member (2 segments) cable spanning between two pinned supports I get converging results for both, but slightly different results.

Madhan Elumalai · Accepted Answer

The reason why i modelled the catenary cable as a segmented member is because of a concentrated load that acts in-between the cable that needs to be modeled as a joint load. Further the basic nonlinear cable analysis doesn't use the catenary theory, hence it is recommended to model the cable with some intermediate nodes to capture the intermediate deflection in the cable more accurately. On the contrary the advanced cable analysis uses the true catenary theory, hence the intermediate deflections of the cables are captured more accurately even without the intermediate nodes. So, yes, accuracy is gained in segmenting the member when you use the basic nonlinear cable analysis. But this is not the case for the advanced cable analysis. Also, the reason for using a curved profile cable in my example is to demonstrate the capability of the program in handling such curved profiles in the basic nonlinear cable analysis itself. 
 For a single member cable spanning between two pinned supports the advanced solver will give you the correct results where-as for the basic solver to give you the correct result you need to segment the cable into two or more. 
 The Advanced Cable Analysis is more robust as it uses the true catenary cable element. 
 Hope this answers your queries.

Madhan Elumalai · Answer

Hi Ganesh, 
 The STABILITY parameter is an artificial stabilizing stiffness matrix value that is added to the global matrix at each translational direction for joints connected to cables and nonlinear trusses. The amount added linearly decreases with each of the Load Steps and finally reaches to zero at the last load step. Since this is a artificial stiffness that doesn't exist in the site, i would suggest to use as small value as possible as this alters the stiffness of the structure. Usually for simple problems like this i start with a value of 1 KN/m. It this doesn't work, then i increase it to 2 then 5 then 10 etc until it works (max 1000). If any value works, i reduce it half way. For example if 5 doesn't work and 10 works, i'll try 7.5. Try to keep this as small as possible as it doesn't exist in the site. 
 For your model, i would suggest a value of 1 KN/m for the stability command. 
 Regarding the decision of segments, i would suggest to keep it simple i.e just one node for the point load. TL Problemr.std 
 So, with STABILITY set to 1 KN/m and with just one node for the point load i got the max deflection of 1593 mm which is slightly more than you got in your spreadsheet for various values of STABILITY. 
 I checked this with the Advanced cable analysis in STAAD.Pro advanced and got the same results.

Catenary Cable

Top Replies