Non-Linear or Linear Cable Analysis

Question

Hi There: I was wondering if I can have some technical support on this issue. 
 I am trying to perform cable analysis for 4-cables restrained vertical vessel for seismic loading. 
 1.0 When, FRS seismic input is specified along with cable analysis: a) I don&rsquo;t see the program performing loading steps iterations. b) The sum of support reactions not matching the applied lateral loads. c) Even the cable tension is not as expected. d) Not too sure, if the cable analysis is being performed. Please note, I have "SPECTRUM SRSS " command, "Master and Slave Rigid" command. Not too sure if the cable weight and initial tension are considered. 
 2.0 When, FRS seismic input is specified along with just "perform analysis" then I am getting cable forces in only 2 cables out of 4. 
 3.0 When I apply static lateral loads and perform cable analysis, loading steps are considered but the moment developed at the base of the vessel is huge (i.e. same as unrestrained vessel).

Sye · Answer

An advanced non-linear cable analysis cannot be used with a dynamic loading like response spectrum. If you have a response spectrum loading, you may only use a linear PERFORM ANALYSIS. If your loads are applied as static loads, you may use the advanced nonlinear cable analysis. Also ensure that you are using the latest STAAD.Pro 20.07.11.82.

Sye · Answer

Advanced Nonlinear Cable Analysis is available in 20.07.11.33 but just ensure that you have the license for the Advanced Analysis Engine ( or Advanced Analysis Plus Engine). If you do, the software would use the Advanced Nonlinear Cable Analysis when you do a PERFORM CABLE ANALYSIS and you can also confirm that by looking at the messages in the Analysis/Design window. 
 
There is no linear cable analysis per say. You can define a MEMBER CABLE with an initial TENSION and do a PERFORM ANALYSIS in which case the members are treated very much like TRUSS but just that the pre tension is considered in the stiffness calculation for these members. If you have to use a Response Spectrum loading, you have to use a PERFORM ANALYSIS and not a cable analysis. 
 
If you are trying to match the applied lateral loads from the response spectrum load case with the summation of support reactions, these would not match. The reason is explained in the wiki below 
 communities.bentley.com/.../29001.base-shear-does-not-match-the-summation-of-shears 
 
To answer items 2 and 3, I need to take a look at the model. Please upload your model.

Sye · Answer

Here are a couple of changes I would suggest 
 Change the nonlinear cable analysis from BASIC to ADVANCED 
 *PERFORM CABLE ANALYSIS BASIC STEP 10 SAGMIN 1.0 
 PERFORM CABLE ANALYSIS ADVANCED STEPS 10 KGEOM 1 
 You need to have the Advanced Analysis engine license to run this analysis. When you use SAGMIN 1 as part of the basic analysis, you are not accounting for any reduction in stiffness due to sag. The Advanced Nonlinear Cable Analysis results seem to be much better. Tension is reported for all cables and the support moment at the base is much less, which is what I think you are expecting. 
 Also comment out the MEMBER TENSION command as it is not used by the analysis engine in a nonlinear cable analysis 
 *MEMBER TENSION 
 *18 TO 21

Sye · Answer

Prabir : You need to use the Advanced Nonlinear Cable Analysis and not the Basic. Also please ensure that you are using the latest STAAD.Pro version 20.07.11.82. The modified file is attached. 
 advanced_cable_analysis-trial.std

Sye · Answer

Prabir : Couple of things 
 First, please ensure that the license for Advanced Analysis Plus Engine is activated as shown below. The box should be checked and you should see a green light. 
 
 I was checking the revision history and saw that some improvements was made to the advanced nonlinear cable analysis in 20.07.11.50. So I would definitely recommend that you use the latest 20.07.11.82.