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1161 (Materials) (OP)
7 Feb 02 2:04
Greetings

How accurate is Staad finite element analysis.In case of water
tank resting on soil, can we give springs only KFY value, not
releasing MX, MZ.e.g.
supports
1 to 200 KFY 24000
We get very less moments in bottom slab.
Please respond.

Regards.
arniec (Structural)
7 Feb 02 10:15
goto Thread507-7222 - Staad vs competitors for some honest views about STAAD

a
JAE (Structural)
7 Feb 02 12:56
Everybody...do you notice that on Eng-Tips we get a lot of STAAD questions?  But very few of other software vendors...is this because there are many more STAAD users or is it the program?
Ron (Structural)
7 Feb 02 13:09
JAE...I think it is both.  STAAD has been popular despite its numerous shortcomings.  I think a lot of people have made the investment and don't want to throw that away, so they stick with a program that they at least feel comfortable that they know many of its problems.  Kind of "the devil you know" syndrome.  

Having said that, we can see from the numerous posts that many are just finding out how error prone this software can be.  You would think that Research Engineers would take to heart some of the feedback they have gotten over the years.  In my experience, they view the problems to be mostly with the user, not the software.
Qshake (Structural)
7 Feb 02 13:59
JAE, after reading your inquiry I was going to respond until I found RON had taken the words right out of my mouth!

So this will be short...I second what RON has noted and urge the original poster to check out the other staad post for some real feedback.
1161 (Materials) (OP)
8 Feb 02 2:15
Thanks for replies.Please be more spscific that is can I give spring
support on soil as FIXED BUT KFY 12000(variable)

Regards.
nades (Structural)
8 Feb 02 7:52
It has to be
FIXED BUT MX MY KFZ 12000.
because if you do not allow rotation about X and Y axis then
you should have all nodes prevented from rotaions and you may have
only one level of displaced nodes.
1161 (Materials) (OP)
8 Feb 02 11:51
Thanks for response.
Tank is modelled as 3D resting on ground by Staad.
Can spring support be FIXED BUT KFY 12000.

Regards.
bylar (Structural)
8 Feb 02 14:31
I know this does not answer your question but i see from the other responses that they have had similar experiences with Staad that i have had.  They blame  all error or inconsistencies  on the user.  This is the case even after i have shown them programming bugs that arise.

I've invested the money and don't care to spend more for another engineering design system.
nades (Structural)
8 Feb 02 15:53
Dear 1161,
Staad will not prevent you from defining springs as
FIXED BUT KFY 12000
..BUT, it is not realistic and does not represent reality.
in my view, this is wrong. it has to be
FIXED BUT MX MY KFZ 12000
in order to represent plate supported on springs.
kareemyas (Structural)
9 Feb 02 10:48
Dear nades, I think what 1161 says about the soil spring to be KFY is correct, as staad considers y direction as the normal gravity direction,the direction in which all soils support the structures resting on them.
danmccarthy (Civil/Environmental)
12 Feb 02 9:14
STAAD is the most popular structural analysis program because it is much cheaper than the more comprehensive (and more accurate) programs such as STRUDL or ANSYS.
If you are performing an analysis which requires the accuracy of finite elements, it would be best to use a package better than STAAD. STAAD has a very limited library of elements and provides very poor description of its available elements and their strength and limitations. I have known several engineers who have used STAAD for finite element analysis and had disasterous results. Also, the user support services for STAAD is very poor compared to other products if you run into problems.
For basic structural analysis STAAD is probably all you need. It is easy to use and relatively accurate. However, you would be fooling yourself to think it does advanced analysis properly. It will give you an answer but the answer may be wrong.
Helpful Member!  wmccain (Structural)
12 Feb 02 9:38
Gents:

I am a registered user of STAAD.  It does have several shortcomings,
however, if you stick to "standard" problems with finite elements, you
can get good results.  I have, for instance checked the plates against
an analytical solution for vertical walls with varous loadings and found
STAAD to be accurate.  
As for MATS or TANK BOTTOMS on the ground you need to use:
(Joint No)  FIXED BUT FX FZ MX MY MZ KFY XX.XXX.
You must release all the other DOF's or you'll induce bogus membrane
stresses, and it will affect your moment results(this is what you're after,
I presume).  Under no circumstances do you want to fix moments.
 Since you have now left all the other DOF's free in all the
joints, you will induce instabilities in the solution unless you FIX
FX and FZ at one of the joints in your mat and FX OR FZ(not both)
in some other joint in your mat, away from the first.  This will give
you accurate moments(not very sensitive to subgrade modulus, by the way)
and a stable solution.  If you are interested in lateral soil resistance, then
use lateral SPRINGS, do not FIX.
Qshake (Structural)
12 Feb 02 10:39
wmccain, how do you evaluate your results with STAAD?  What is the baseline that enables you to say you've got good results?

In my experience finite elements (true finite elements)must be evaluated for several factors to encompass the behavior or response to the change in those variables.  Not the least of which is the element type itself.  So, in my interpretation, you cannot just run one file and consider the matter complete.  And since the heart of the finite element method is a numerical approximation there is an inherent amount error in the analysis - how does STAAD report this error or even better how does STAAD minimize this error?
JAE (Structural)
12 Feb 02 14:19
wmmcain, I would also like to know how many engineers out there ever typically deal with "standard" designs.  It seems that with so many owners altering or adding onto their existing buildings, there is not a lot of standard problems out there.

JAE (Structural)
12 Feb 02 14:20
And one more thing....how do you know how standard your particular problem is before STAAD starts to give you eroneous results?
Guest (Visitor)
12 Feb 02 15:52
To see how accurate STAAD's plate elements and dynamic analysis are, go to

http://www.reiworld.com/Support/Pro/Verification.pdf

They compare their answers with ANSYS.
wmccain (Structural)
12 Feb 02 19:25
Thanks, born2build.  I've never seen so much whingeing
over one word.  Maybe I should have said "flat plate
related problems."  I do not used STAAD for tubulars,
tubular junctions, or multiple curvature shells.  I do
think, however, and I think the STAAD verification problems
bear me out, that mats on Winkler foundations, rectangular
tank walls, and other "flat" problems with rectangular
elements can be solved accurately assuming the user's
boundary conditions are correct.  On this last point see
nades, above and my response.  I am not defending STAAD.
If anyone has any further problems about the use of the word "standard", then let them scream and yell to their
hearts' content.
1161 (Materials) (OP)
12 Feb 02 22:25
Thanks for responses.How is it if the tank supports are given PINNED as done in Staad manual example no.10.Please comment.
Qshake (Structural)
12 Feb 02 22:53
Gee, if I had a verification problem and I posted/published the results I guess I would make sure the problem was such that any comparison looked favorably on me too.

However, Gentlemen, we are asking you to step out into the unknown, to really apply the product to engineering problems and not textbook examples.  In this area, I ask you, are you completely satisfied with the STAAD product?

I have used STAAD for a number of years and have always felt that the problems I analyzed were appropriate for the software.  In that respect, I got exactly what I needed from STAAD.  But I will not perform complex dynamic analysis or FEA with the program.  Which that program is marketed to do but doesn't live up to those expectations.
wmccain (Structural)
12 Feb 02 23:27
I feel compelled to respond to the
irresponsible remarkes by QSHAKE.  The
"baseline" used to measure the accuracy
of the STAAD solution is stated in my
original post.  The type of element used
is explained in the STAAD manuals.  ERRORS
in a numerical analysis are dependent upon
boundary conditions and assumptions in
the analysis itself, as well as on
the numerical method.  I see in this forum
numerous examples of faulty boundary conditions
associated with the use of the STAAD elements
in foundation mats or tank bottoms.  The post
I responded to as well as previous and following posts
are prime examples of this.


For 1161(visitor)
For example:  A moment's thought will enlighten one
to the fact that to make joints in a soil supported
tank bottom pinned in X and Z would be an error.  Also,
as suggested in another post, to fix moments in Z, X,
and Y, would be in error.
These conditions would produce nonsensical results.   
To evaluate a structural force associated with a
degree of freedom, one must release that degree
of freedom.

The Example 10, which is in the STAADPro 2001 Online
Documentation, is, as far as I am concerned, in error.
I will not work a problem such as this in the manner
suggested by STAAD.  I refer you to the post I placed
earlier, using springs, and limited lateral restraints for
solution stability.

Happy howling,

Guest (Visitor)
13 Feb 02 11:31
I have evaluted STAAD verificacion manula. It seems good. I have question in Problema 11, what method used to got mode shapes?
Guest (Visitor)
13 Feb 02 12:06
In STAAD 2001, they report the method used in eigenvalue extraction. Check the output files. The STAAD engine will say Subspace Iteration method used. The STARDYNE engine has the HQR and LANCZOS methods.
JAE (Structural)
13 Feb 02 14:08
I guess I don't see how remarks in this forum, especially the ones above by Qshake are "irresponsible".  He simply made a point about STAAD relative to his assurance level of the program's accuracy and use.

My single point was that STAAD is marketed aggressively as the best structural program out there and yet over and over again I hear engineers complain about it, seem confused about its application, and doubt its results.

I haven't used the program for some years but when I did, weird results would appear and not correspond to the results from an exact replica model built in STRUDL.  This was about 12 years ago and Eng-Tips forums still have posts appearing with STAAD issues.  

When someone offers me a program to use and then says "Its a great piece of software as long as you keep to standard designs"...would you feel confident in using it?  Not me.
Guest (Visitor)
13 Feb 02 16:15
Thank you carl! I am new user to STAAD in Brasil and I think product is so good. Me company have used Sap 90 in past days. Its is to confusing. Mr. JAE, please don not be mad at me for using the STAAD. I still like to hear your good advise to me. A work friend used the STAAD in US and liked it so much. So we buyed it here. Also our cheif used STAAD 3 in Europe and also liked it much. I look at Exmples Mr. Born2build write and they look good. What are problemas I should look for? Thank you.

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