According to application notes by AISC, for Direct Analysis Method the UNREDUCED STIFFNESS shall be used for service checks.
Now, due to limitations by Staad for DAM (for instance, not TENSION MEMBER available), I proceed to:
1. Reduce module of elasticity E to 0.8E
2. Apply Notional Loads.
3. Load combinations are Load cases with repeat load.
4. Pdelta analysis.
But this means that the result shown for deflections are calculated on reduced stiffness. After analysis I can change the module of elasticity back but I have to run the analysis again, and with repeat loads the model takes too long. So I decided to use the CHANGE command after Strength load cases like this:
...LOAD 645 1.2(D) + 1.0(W) + 1.0(RL) + 0.5(RLR) + 1.0(CR4) Z-REFERENCE LOADR1 1.2 R9 1.0 R3 1.0 R5 0.5 R304 1.0 NOTIONAL LOADR1 Z -0.0036 R3 Z -0.003 R5 Z -0.0015 R304 Z -0.003 ********************************************************************************PRIMER ANÁLISIS*******************************************************************************PDELTA ANALYSISCHANGE*CONSTANTSMATERIAL STEEL ALL********************************************************************************COMBINACIONES DE SERVICIO*******************************************************************************LOAD 1000 1.0(D)REFERENCE LOADR1 1.0...
But in analysis, I get a warning indicating that CONSTANTS should not be after an analyisis command. And after analysis or edit, the portion CONSTANTS / MATERIAL input after CHANGE, just disappear.
Is there any syntax missing here?
See the link below. Someone had a similar question.
Change Command and Material Properties - RAM | STAAD Forum - RAM | STAAD | ADINA - Bentley Communities
Thanks, yes, that person had a similar question. But I'm still looking for an answer.
Most likely is just the way it is and I have to constantly input and re-input the constants command just before run analysis, or run analysis two times each one with each type of material, because the constant command keeps disappearing.
I made a simple model using the items you described above. I do get the warning about CONSTANTS appearing after an analysis command, but the CONSTANTS/MATERIAL input doesn't disappear for me after the CHANGE command. What version of STAAD are you using? I'm using v.22.12. Maybe that is why the analysis command disappears for you. The two analyses for me proceed without error (only get the previously mentioned warning) and comparing the results from the lateral only load cases shows the different results for the different material assignments. Maybe Modestas or Abhisek M or Sye can provide an answer about changing CONSTANTS after an analysis to get around the limitations of STAAD with tension only members and the Direct Analysis Method.
MaterialChange.STD
Thank you Jeremy.
My version is the same v22.12. Maybe, if I can ask you to carry out an edition, any, like changing a member size and before run analysis, please check if CONSTANTS command is still there. That is the point when is deleted for me.
Naturally when I'm constantly working in a model, I can "forget" to type it back so my analysis is for previous material and have to run again. For a model that could take up to 20 minutes, is quite time consuming.
If you change the members size through the GUI, the material assignment will mess up, as the program will not understand which material assignment should be used for the new section size. To avoid this, you should be changing section sizes directly in the input file by using the STAAD Editor, I believe.
Thanks Modestas. In fact, I'm talking about edition directly using STAAD Editor when this problem happens.
I can use the Command File and change member sizes without the second analysis being removed.
Thanks. Yes, me too, the second analysis is not removed, the problem is that the command CONSTANTS and the following command MATERIAL input after an analysis, are removed. So I have to type them back again and again.