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Structure stiffness matrix S

Structure stiffness matrix S

Structure stiffness matrix S

(OP)
Hello,

How can I obtain the structure stiffness matrix [ S ] using STAAD, ETABS, or any other software? ponder

RE: Structure stiffness matrix S

I don't know about ETABS but I'm fairly positive STAAD will not output the stiffness matrix directly.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

RE: Structure stiffness matrix S

I'm just about 100% positive you cannot get that from the program. I couldn't even get STAAD's tech support to give me certain shape functions when I have asked for them. It's a trade secret I guess.

RE: Structure stiffness matrix S

WARose -

Shape functions could only be a trade secret if they developed them internally. I don't think that's the case with STAAD. Some resources:
For beam elements (Matrix Analysis 2nd edition by McGuire, Gallagher, and Ziemian).

For the plate elements you should ask them for a reference for their plate formulation. There is a book by Richard MacNeal (Finite Elements, their design and performance) which criticizes the type of plate element that STAAD uses... suggesting that it is inappropriate (and random) to place left over strain energy in the drilling degree of freedom.

Now, it makes sense for MacNeal to say this because he's talking in theory about proper derivation and construction of Finite Elements. In the case of STAAD's plate, I don't think it's anything to worry about. While the drilling degree of freedom is still likely not captured correctly, their plate element seems to perform approximately as well as the other widely used elements.

RE: Structure stiffness matrix S

Quote:

Shape functions could only be a trade secret if they developed them internally. I don't think that's the case with STAAD. Some resources:
For beam elements (Matrix Analysis 2nd edition by McGuire, Gallagher, and Ziemian).

For the plate elements you should ask them for a reference for their plate formulation.

No big mystery for plates or beams. The d.o.f. I asked for was the shape functions for rotational degrees of freedom for 8 node solid ("brick") elements. When I first started using STAAD (decades ago) they had only translational degrees of freedom at these joints. It made these elements prohibitively stiff. (Especially in dynamic loading problems). I don't know if the rotational degrees of freedom are responsible for the elements getting more flexible.....but I know STAAD guards this one.

RE: Structure stiffness matrix S

For solids, there are some isoparametric element formulations in KJ Bathe's, "Finite Element Procedures". Just translational degrees of freedom I'm pretty sure. But, the element formulation (and shape functions) for solids is supposedly a lot simpler than plates. Probably because of the lack of rotational DOF's. I've never taken the time to go through the full element formulation myself though. So, I'm really just going off of what the RISA lead developer was saying. She's pretty brilliant though, so anytime she says something is "simple" I take it with a grain of salt. :)

RE: Structure stiffness matrix S

Quote:

For solids, there are some isoparametric element formulations in KJ Bathe's, "Finite Element Procedures". Just translational degrees of freedom I'm pretty sure. But, the element formulation (and shape functions) for solids is supposedly a lot simpler than plates.

That's one thing I've never been too sure of: how much of the new "flexibility" in solid elements in STAAD is due to the isoparametric formulation for translation (which I am pretty sure they were not using in some older versions) or the new rotational degrees of freedom. (That's what prompted my question (to them) about those shape functions to begin with.) The isoparametric formulation can address a lot of the shear locking issues.....but I don't recall those brick elements (in STAAD) getting flexible until those rotational d.o.f. were introduced.

I haven't used RISA in years. (Didn't like it when I did.)

RE: Structure stiffness matrix S

(OP)


I think PSA can solve the problem. what do you thik ?

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