the difference between the dispbeamcolumn and nonlinearbeamc

[vesna]

yes, you can set density to 0 and define mass separately.

[baozai]

vesna wrote:
> yes, you can set density to 0 and define mass separately.

vesna, Thank you.

[wuhaoshrek]

vesna wrote:
> Can you post your code so I can see what you are exactly doing. Then it is
> going to be easier for me to comment on your model.
>
> With -iter option you can specify maximum number of iterations to be
> undertaken to satisfy element compatibility.
> tweak means to fine-tune it. what I meant when I sad "to tweak the
> section properties" is to try to change number of fibers and see if
> you are going to see better results.
>
> I am not sure I understand you last question: "will the decrement of
> the integration points influence the numericial result?" Can you
> explain it better?

Hi, vesna,

I'm curious about the iteration with -iter option in element command, the default number of
that is 1, does it really commonly be enough to iterate only once to get the compatibility for
the element?? AND what's the difference between that iteration with commonly used iteration
in the analysis loop.

Thanks a lot

Hao

[wuhaoshrek]

vesna wrote:
> The best way to figure out what is wrong with your model would be to create
> a very simple model (portal frame - one storey, one bay) and to test your
> elements, sections, materials and analysis objects on that one. Once you
> pass this stage you can extend the model to 3-storey, 3-bay frame.
>
> yes, precision is function of the # of integration points. I would go with
> 5 integration points. 3 points seam two few to me.
>
> I would not use modified Newton as a basic algorithm as it slows down
> analysis due to a big # of iterations to achieve the convergence. Newton is
> much better as it needs up to 10 iterations to converge. If during the
> analysis you have a problem with convergence you can switch to modified
> Newton.
>
> About the number of fibers I would suggest you to look at this report:
> http://peer.berkeley.edu/publications/p ... erhard.pdf.
> It shows how different number and configuration of fibers influence the
> response of an element.

Hi vesna,

I am reading all the post, and here's question again. U mentioned precision is function of the # of integration points,
but in my understanding, what the guy really want to know is: if he decrease the int # from 5 to 3, would the value(like
deformation) he get for the integration point, be less accurate than what he get from using 5 int. point. For instance,
we use Gauss Lobatto method, we out put the deformation value on the integration point 0.5, using 3 and 5 int. points
separately, and we compare to see if there would be any difference.

In my knowledge, since only 3 Gauss points would accurately integrate the formulation(cubic interpolation function for
the element), so we should have the accurate result both with 3 and 5 int. points. Further more, Gaussian integration
theory tells us that the value it calculated on the int. point is accurate.

I am not sure if my understanding is right. Please feel free to comment on me. Thank.

best,

Hao

[vesna]

If the time step of the analysis is small enough very often it is enough to iterate over the element only once. Iterations at the element level are different from iterations at the system level.

[vesna]

Given the forces, deformation at the integration points (curvature and axial strain), whether you have 3 or 5 will be calculated from sectional forces that are exact and are both valid, but they will be calculated at different locations. However, displacements that are calculated by integrating the deformations over element length will differ depending on the number of integration points along the element length. I suggest you to make a simple example that will help you understand the implication of number of integration points on the results.

[Tas]

I would like to add a comment about the default use by OpenSees of the iteration number =1 when using FBE.

The non-iterative force formulation (i.e maxIters=1) is according to what Neuenhofer, Ansgar and FC Filippou are saying in the reference “Neuenhofer, Ansgar, FC Filippou. Evaluation of Nonlinear Frame Finite-Element Models. ASCE Journal of Structural Engineering, Vol. 123, No. 7, July, 1997. ISSN 0733-9445/97/0007-0958-0966. Paper No. 14157. pp. 958-966” about the non-iterative formulation. In this paper it is described that the (element’s) iterative procedure can be replaced by 1 iteration by making a “correction” after that first step.

I think that this is the meaning of the default iteration number =1 for FBE (Vesna correct me if i am wrong).

[vesna]

I looked at the source code to find the reason for default number of iteration and here is what I have found:

1. max number of iteration is 10 (by default) not 1 (I corrected this in the command manual)
2. default tolerance is 1e-12 (I corrected this in the command manual)
3. opensees code for force-based element does not implement non-iterative formulation from the Neuenhofer et al. 1997. It iterates to find a solution considering three different algorithm schemes.

[Tas]

Thank you for your investigation and the explanation to this issue Vesna, i think that it had to be done because the default value of maxIters=1 in the manual was confusing the reader indeed. Persoanally, i had a wrong understanding for so long (i thought that the non-iterative procedure was used by default, this term is also used in the command manual, please correct it: "...which is based on the non iterative (or iterative) force formulation...").

[amir70007]

Hi All

What sholud we do to to accurately capture local response of elements according to:

"To accurately capture local response of elements whose plastic hinges
locations and lengths can be estimated, NIPs of a FBE has to be chosen such
that integration weights at locations of plastic hinges match the plastic hinge
lengths"

[vesna]

Tas, thanks for your comment! I corrected that one as well.

[vesna]

You need to chose number of integration points (NIPs) such that the integration weight at the plastic hinge location equals (approximately) the plastic hinge length.

For example: consider 10 ft tall column whose plastic hinge develops at the top and the bottom of the column. Assume plastic hinge length to be 1 ft.
1) Try 3 integration points => integration weight is: wt(0)/2*L=0.33/2*10=1.66 ft
2) Try 4 integration points => integration weight is: wt(0)/2*L=0.1666/2*10=0.83 ft => closest to 1 ft => select 4 integration points
3) Try 5 integration points => integration weight is: wt(0)/2*L=0.1/2*10=0.5 ft

Note: wt(0) is the integration weight ratio at the column end that can be found here: http://opensees.berkeley.edu/WebSVN/fil ... ration.cpp

[amir70007]

Thank you for your helpful reply...
Is there any source that explains "integration weight plastic hinge" exactly or graphically so I can find out the meaning of that?
How can I determine plastic hinge length in elements?I think a definite plastic hinge occures at a concentrated section...
Does "wt(0)/2*L" change If we consider plastic hinges develop at more more sections of the element?
Do we need to choose NIPs for other elements?

[vesna]

At the bottom of the command manual for the forceBeamColumn element you can find some info on integration schemes and their integration weights.

Plastic hinge length depends on the structural system, component, material. There are empirical formulas that mostly used to estimate the plastic hinge length. Plastic hinge occurs at a concentrated section only in cases of very bad design. The intent of modern building codes is to spread plasticity over a length of the element and this length is called plastic hinge length.

Yes, "wt(0)/2*L" applies only to the case that plastic hinges develop at the element ends.

Remember, this is important only if local response is important for your study.

[wuhaoshrek]

vesna wrote:
> At the bottom of the command manual for the forceBeamColumn element you can
> find some info on integration schemes and their integration weights.
>
> Plastic hinge length depends on the structural system, component, material.
> There are empirical formulas that mostly used to estimate the plastic hinge
> length. Plastic hinge occurs at a concentrated section only in cases of
> very bad design. The intent of modern building codes is to spread
> plasticity over a length of the element and this length is called plastic
> hinge length.
>
> Yes, "wt(0)/2*L" applies only to the case that plastic hinges
> develop at the element ends.
>
> Remember, this is important only if local response is important for your
> study.


Hi vesna,

1. If you let the weight of the bottom int. point to be the same as plastic hinge, for Lobatto int. method
where the int. is on the end of the element, I think this would overestimate the integration result since
deformation at the end would usually be largest. So i suggest use one element with its length equal to
the plastic hinge length, and use more int. points within that element as long as it converges.

2. The plastic hinge concentrates at the bottom critical section occurs when we use Elastic of softening material
in a cantilever column model. Acctually I'm modeling on such a simple model right now. I was wondering
what would be the EXACT solution to such a problem? My personal view is there would be no answer to such
a problem since plastic hinge length would be zero after yield.

3. By the way I am trying to use user default integration method, with 1 int. point in the middle and weights=1.0
on an element with its length equal to the plastic hinge length. The idea is straightforward, just to check if
in this way we can accuarately capture the behavior in plastic hinge. BUT I cannot get converged. OpenSees
failed to converge at the very beginning and prompts that the cannot find compatible force and deformation
for that very element representing the plastic hinge. Vesna, Can you explain why? and give me some suggestions?

Thanks.