Auxiliary Cases in Non-Linear Combinations (Seismic - Equivalent Lateral Force Method)

[gary.iwansantoso6FRXD]

Hi,

Just expanding on the discussions from the thread below: 

Solved: Seismic Equivalent Lateral Force / Non-linear uplift released elastic ground - Autodesk Comm...

I want to clarify on how the software works in terms of auxiliary cases and non-linear combinations in the context of ELF seismic analysis.

I am running a similar scenario where I have non-linearity due from releases at the column base to allow uplift under seismic loading. The intent is to make sure that there is no tension reaction at the base nodes so as to avoid taking out said tension through the foundations (compression/bearing only).

I've created a seismic loading case (refer screenshot below) where the blue cases are the lateral components of earthquake and the red cases are the combinations generated from those components. Ignore case numbers, I'm referring to the nature of the cases.

The black case (No. 34) is then the combination of lateral loads and gravity loads. So now I want to clarify that if I turn all cases auxiliary except the final non-linear combination (Case No. 34) - will the individual components be computed as part of the combination? Will it collect every bit of lateral and gravity load from each Cases and solve as if it's in a single load case?

I'm not sure how the automatic generation of loads work (or seismic loads for that matter) in the software back end, because I'm finding that the non-linear combo can converge but either it is empty or it provides non-sensical results (vertical loads appear to be excluded for example).

The best result I found was to let it run and let the individual cases be non-convergent (as suggested by the thread I linked above) but this is quite time inefficient. 

Any ideas on better ways to run this or workarounds?

[gary.iwansantoso6FRXD]

I will simply further - why is it that I get a different answer when I run a non-linear analysis and letting all cases run (with some going non-convergent) vs. only running the load combination with it's individual load cases as auxiliary?

[EduardoMeruvia]

Hi @gary.iwansantoso6FRXD , 

Simple answer to that its cause you most likely have uni-directional support or supports i.e. compression only or tension only supports. Thus, if you run individual cases say wind or seismic which is predominantly a lateral force, when such cases run independently then your building/model is unstable i.e. it can overturn freely as there is no forces (selfweight) to balancing these lateral forces (overturning) which is why you get a non-convergence. In contrast when you run these wind or seismic cases in combination, then they do are balanced which make your overall model stable. 

[gary.iwansantoso6FRXD]

Hi @EduardoMeruvia 

I do indeed have compression-only supports and I do understand the concept. I am indeed seeing the earthquake load cases (lateral force components) go divergent, while the gravity cases and the load combination converging to a result.

However, when I set the load cases within the load combination to be auxiliary - the load combination converges to a different result than if I have no auxiliary cases and let divergent loads (lateral components) be divergent.

I've created the analysis model for earthquake loads to AS1170 (ELF method) as below using the automatic combinations function.

Load Case 1 - Gravity

Load Case 2 - Live Load

Load Case 3 - Earthquake +y direction

Load Case 4 - Earthquake +x direction 

Combination 1 - Case 3 + 0.3*Case 4

Combination 2 - Case 1 + 0.3*Case2 + Combination 1

... (for each direction and eccentricity as per AS1170)

In the above setup - Load Case 3 & 4 and Combination 1 are left to calculate and be divergent, but Combination 2 is still convergent.

But when Load Case 1 to 4 and Combination 1 is set to auxiliary - Combination 2 still converges but gives a different result.

Any ideas?

[EduardoMeruvia]

@gary.iwansantoso6FRXD, I see where you question is coming from. When dealing with ELF method unfortunately you will need to just let Load Case 3 & 4 solve and diverge so you can use those cases in your Combinations as per AS1170. 

When setting Load Case 3 & 4 as auxiliary, because they are "special" cases (i.e. required a series of inputs as per AS1170 for them to convert the masses on lateral equivalent forces) they will not run at all even when within a combination case. That is probably why you are seeing a difference in results between your combinations when you set Load Case 3 & 4 as auxiliary and when you just let them run and diverge. If you look at your reactions forces at the supports for both running with all set as auxiliary and running all divergent, you'll probably notice all your lateral reaction forces are missing on the auxiliary run case. 

[gary.iwansantoso6FRXD]

@EduardoMeruvia 

I had a suspicion it might have something to do with mass conversion, but there was no specific settings for it so I wasn't sure.

Is it not possible to generate only the calculation model to have this mass conversion occur prior to calculation? Would freezing results for the divergent cases will do it?

I've just done another couple runs and confirmed that the "different answer" I was getting was because none of the lateral earthquake loads were included in the load combination. I suppose the simplest workaround is to specify only 5 or so iterations so the analysis for divergent cases can give up quicker.

[EduardoMeruvia]

@gary.iwansantoso6FRXD, freezing the results for the expected divergent case might be the easiest way to go in your case. Once you got the generated forces using the ELF in Robot (i.e. divergent cases) you could use the calculation Restart tool in Robot so every time you need to re-run other load cases or combination you only need to re-run those. 

With the Calculation Restart option, new calculations are run on the selected cases only while the results for the other cases remain unchanged.