tension only command

Re: tension only command

Kris Sathia — Thu, 19 Jan 2012 22:01:11 GMT

You wrote ;

To me, this means that I don't need the change command. Please confirm.

Answer:

Correct

Re: tension only command

ryanasu — Thu, 19 Jan 2012 19:02:03 GMT

Kris -

I'm reading two different recommendations from you. In your first paragraph, you agreed that ANALYSIS AND CHANGE should occur at each load case/load combination. In your last paragraph, you say that staad automatically re-activates all members prior to the ANALYSIS command. To me, this means that I don't need the change command. Please confirm. Thanks

Re: tension only command

Kris Sathia — Thu, 19 Jan 2012 12:44:13 GMT

ryanasu,

You wrote :

I just wanted to clarify something. I'm using tension only members. So to my understanding, during a single load case/combo, staad will iterate and inactivate tension only members that are carrying any compression load. In the next load case, the members that were inactivated will remain inactive unless the change command is used. So I am not changing any structural conditions, but staad is automatically doing it. Unless I analyze the file and determine which load cases required iterations, it seems as though for tension only members I should have perform analysis under every load case. Does this seem correct to you?

Answer :

Before the start of the analysis for any given load case, the full structure is restored by the program, meaning, all members with the tension-only attribute are considered active in the model. Hence, the state of the structure at the end of the analysis for that load case (in which some of those members may be switched off) is not used as the starting condition for the next case.

In other words, this is like the program inserting the ANALYSIS and CHANGE command on its own between successive load cases. After any load case is solved, it "resets" the model to the full state automatically before it starts to solve the next case.

To summarize, you have to enter ANALYSIS and CHANGE only after the last of those cases, not for each individual case.

Re: tension only command

ryanasu — Wed, 18 Jan 2012 18:15:38 GMT

Thanks for your input. That helps a lot. I just wanted to clarify something. I'm using tension only members. So to my understanding, during a single load case/combo, staad will iterate and inactivate tension only members that are carrying any compression load. In the next load case, the members that were inactivated will remain inactive unless the change command is used. So I am not changing any structural conditions, but staad is automatically doing it. Unless I analyze the file and determine which load cases required iterations, it seems as though for tension only members I should have perform analysis under every load case. Does this seem correct to you?

Also, I found my error from earlier. I wasn't using the Set NL command. Once added, it worked fine.

Re: tension only command

SKLose — Wed, 18 Jan 2012 11:59:30 GMT

Ryanasu,

The PERFORM ANLYSIS (and CHANGE) command is required after a load case only under 2 conditions:

1) If that load case contains a ELFP based seismic analysis command, like the IBC or UBC loads. I guess this is due to a quirk of program rather than a requirement based on engineering principles.

2) If the structural condition changes, such as due to a change in member properties, material properties, support conditions, etc. A steel member selection is an example of change in member properties. So, if one wishes to solve some load cases with one set of structural conditions, and another with another set, then, those 2 sets of load cases must be separated by a PERFORM ANALYSIS and CHANGE.

In the original model posted by Himmatlal Chavda, only condition (1) existed - load cases 1 and 2 of his model. So, a PERFORM ANALYSIS and CHANGE commands were needed after each of those two cases, which he did provide.

However, load cases 3 through 19 are all associated with a single structural condition. So, the analysis instruction is needed only after case 19. But he had provided them after each of those cases, so, I removed all instances except the last. But there is no harm in leaving them there as the program is designed to handle them even if they are redundant.

In the snippet that you have attached, I am not able to find an error. Can you post the full text of the error message? Have you ensured that all the load cases which are included in that combination - 1 thru 6, 8, 9, 10 and 13 have all been specified prior to your load case 118? What do you have prior to load case 118?

Re: tension only command

ryanasu — Tue, 17 Jan 2012 19:41:30 GMT

SKLose,

Can you please explain why you removed some of the perform analysis and change commands? I have a model that has several combinations that look like the one below. After 13 combinations, staad tells me the "LOAD" command is not valid on the 14th combination, even though it is identical to the 13th combination. I have repeat load, perform analysis and change after every combo. Should I only have them on certain combos?

LOAD 118 D+0.75(LL+LR/S+W(-Z)+.1W(X))

REPEAT LOAD

1 1.1 2 1.0 3 1.0 4 1.0 5 1.0 6 1.0 8 0.75 9 0.75 10 0.075 13 0.75

PERFORM ANALYSIS

CHANGE

Re: tension only command

SKLose — Mon, 11 Apr 2011 06:55:56 GMT

Attached is your model with the following changes :

1. All combination cases with the LOAD COMBINATION syntax have been replaced with the REPEAT LOAD syntax.

2. Some superfluous instances of the PERFORM ANALYSIS and CHANGE commands have been commented out.

3. Some superfluous instances of the PARAMETER and CODE LRFD statements too have been commented out.

Using a file comparison program, if you compare your original model with this, you can identify the changes easily.

Re: tension only command

SKLose — Mon, 11 Apr 2011 06:23:36 GMT

There is a Q&A at the following link

communities.bentley.com/.../member-tension-and-combination-load-cases-tn.aspx

See if it has the answer to your question.

If not, please state your question more clearly (always a good practice to follow when you need help).