What Level of Theretical Knowledge do FEA Analysists Need? 16

[tld23]

In thread727-21610 a discussion about the level of theoritical knowledge needed to be an analyst was started. It will obviously depend on the type and complexity of the analysis undertaken. I will play devils advocate by openeing with the statement:

"Provided an analyst is aware of potential problems, can read the software manuals and takes a thorough investigatory approach, he/she need not have any knowledge of FEA theory or coding."

The point is about what is needed rather than what is desirable. Any comments?


TERRY

 

Rapt,

I think that most people do actually realise that the moments from a finite element analysis are not "magically lower moments".
As an example, British codes for flat slabs use an empirical method which gives far higher moments than a finite element analysis. This is why some engineers refer to finite element analysis as giving a lower moment.

I would also like to say that the refernces to 10-15 years of experience required for finite element analysis is complete nonsense. Either these people are still typing in code or they do not realise that not all finite element analysis has to be complicated. A flat slab design using finite element analysis is not terribly taxing on the brain is it?

 

[rapt]

youwhat,

You might, many do not. In supporting my software I have this discussion with many people and you would be amazed how many think that FE will result in less moment than 2D for a regular column layout. I am not comparing to approximate methods such as the BS8110 2-way slab moments, I am talking about properly analysed solutions. The code figures are conservative because they have to allow for an envelope of solutions to allow for span length variations, end connectivity and a lot of other factors. They were actually developed as an envelope of yield line solutions for the variations allowed. They are not elastic moments and cannot even be used for crack control calculations. The people I am hearing are comparing to Frame Analysis of the actual member and its loads.

I have not said that it is not easy to design from a FE output. I am describing 2 areas where designers need experience or supervision in doing this.
1 Getting the model right and the correct analysis results. Experience is needed to know that the results are in the right order/ball park and that modelling errors have not caused erroneous results. An example of this from many years ago with a Frame Analysis is a designer analysing a relatively simple roof frame with sloping members (hipped roof) supported on brick walls. Unfortunately, he put the end supports in as fixed against horizontal movement (the default). Members were very small. On checking just before construction because we thought his members were very light, we found the problem in his analysis. Member size nearly doubled.

2 Looking at how many designers are using results from FE analysis to do concrete design, there are several areas where they are getting it wrong now because of a lack of understanding of both the FE results and design in general. The number of designers who take the FE deflection and multiply it by 2 or 3 and call it a total long term deflection is amazing. They do not understand that cracking has not been considered.
The number of designers who do not realize or ignor the transverse distribution of the moments actross a panel is just as amazing. Deciding that a slab panel is uncracked based on the average stress over the whole width is not logical but it is done continuously in USA design practice.

In my previous posts, I have said that experience is required for ALL design. Not just FE. Otherwise supervision is needed.

 

[DRDearth]

The best way to learn about any FEA code is to create your own custom tutorial problems. Make up some simple problems with know solutions that you can work out for yourself using a pencil and paper, then create a few FEA model idealizations. The bottom line is "if you can't solve a problem with a pencil & paper, you will never be able to solve problems with a computer". As a 2nd approach look around on other FEA web sites and download their tutorial problems. Try to duplicate the results using your code. See this site for starters. David R. Dearth, P.E.
Applied Analysis & Technology
E-mail "AppliedAT@aol.com"

 

[Rufus182]

Rapt this is for you

We've built a large number of flat slab structures based on linear FE model output using a reduced E value based on a cracking moment type relationship. This is in an attempt to determine a realistic long term deflection (& creep) behaviour.

As a moment plot will tell you, not all the domain will crack therefore we would need to have a variable E value in line with moment field across the elements. We need to go iterative here. Compute moment field: any cracking? if yes reduce E for those particular elements, then solve for the deflection field. Of course, we can't do this with our package (ROBOT). Currently, we are applying a reduced E, typically 0.28 of short term E, to the whole domain. Conservative (aren't struct engineers always)?

Surely the Yankee design practice and ours (limeys) is the best compromise between accuracy and ease of solution. After all, we are not working for over-funded F1 design teams here!

Any comments greatly appreciated.
Rufus182
talktorufus@hotmail.com