topic Re: when to use arc length and incremental method for non linear analysis in Robot Structural Analysis Forum

when to use arc length and incremental method for non linear analysis

me1205377 — Thu, 21 Jun 2018 15:22:56 GMT

Greetings All

can someone explain when it is appropriate to use incremental method and when it is better to use arc length method for solving structures with geometrical non linearity

Re: when to use arc length and incremental method for non linear analysis

Artur.Kosakowski — Fri, 22 Jun 2018 13:37:01 GMT

Hi @me1205377

The arc-length is intended to be used when under the applied load a structure temporarily looses its stability and then 'regains' it with already different shape of deformation or when you want to analyse its collapse (under the load being above the critical level).

If I managed to answer your question(s) press the Accept as Solution button please. This will help other users to find solution(s) much faster. Thank you.

Re: when to use arc length and incremental method for non linear analysis

me1205377 — Sun, 24 Jun 2018 06:01:07 GMT

can i say that if there is a convergence to a problem . i should be able to get the same results by arc length and incremental method

for example a typical up lift case or P small delta scenario should the solution obtained by arc length be more or less the same to a solution obtained be incremental method

Re: when to use arc length and incremental method for non linear analysis

Artur.Kosakowski — Mon, 25 Jun 2018 10:46:20 GMT

Hi @me1205377

No, this is not the idea. They are intended for different objectives rather being used as either or. You use the arc-length to see what will happen on the way towards the (deliberately) excessive load value or displacements which you set as the criteria to stop the analysis. For the typical P-Delta you just use the incremental one (you don't want to 'collapse' your structure).

If one or more of these posts answered your question, please click Accept as Solution on the posts that helped you so others in the community can find them easily.

Re: when to use arc length and incremental method for non linear analysis

AnthonyMcTigue — Thu, 02 Jun 2022 00:59:21 GMT

Is there any documentation out there that explains in detail the difference between the Incremental Method and the Arch Length Method? The Help files on the subject offer little in terms of detail as to which type of situations are suitable for each method and guidance on stop parameters.

The situation I am attempting to model is response of a stiffened RC raft footing of expansive clay soils so I have a situation there the structure temporarily 'loses' and 'regains' stability. I have modeled the expansive clay as follows:

Defined nodes at a depth of 2300 mm below the raft footing (2300 mm determined by AS 2870 code requirements).
Defined a non-linear support at each node consisting of a linear spring with a stiffness parameter determined by multiplying the soils spring stiffness by the area of soil contributing to that spring and defined an upper limit equal to the allowable bearing capacity of each block of soil.
Connected the nodes back to the raft with a dummy member with non-linear releases consisting of a 'lift-off' release at the supported end and a gap at the top end (gap is equal to the amount of contraction of the expansive soil determined from AS 2870).

I believe that the Arc Length method is more appropriate in this instance than the incremental method as it is conceivable that parts or all of the raft will become 'unstable' as the gaps remain open, supports in contact with the raft reach their ultimate bearing capacities and try to redistribute load to supports with spare capacity and some supports lift off.

Thanks,

Anthony