Designing by Hand vs. FEA 10

[jester86403]

I have just been curious lately about what the upper extent people feel is that can be designed by hand or at what point would designing a structure by hand be no longer close at all to using FEA as far as efficiency goes.

I would like to get responses for buildings, bridges, and other miscellaneous structures if possible.

Obviously almost anything can be designed by hand...I mean high rises were built long before computers, but when is it no longer worth the time to do it by hand?

 

[dougantholz]

The efficiency of computer programs means that I will probably design every two story building (except the most plain regular building) using a computer. I mean when you can knock out a relatively simple 2-story office building in an afternoon why would you do anything else?

 

[jester86403]

Not all engineers work for large companies that provide the software. Many are small firms or sole proprietors that do not necessarily want to spend the money for all the software.

For small jobs that computer efficiency also is not necessarily that efficient if you go through and check all the input and output and setting up the model. I guess depending on how much checking is done to assure the model is accurate would effect the efficiency. Some one who does no checking will find it far faster than those who do.

 

[JStephen]

I've used structural software for two or three applications through the years. One problem is the learning curve to get efficient with the software. Someone that works with it every day is going to be able to knock a problem out in 1/10 the time of someone having to learn as they go. Additionally, you'd expect the more expensive software to be more capable and time saving, if you pay the money and use it a lot.

 

[FSS]

If you can't do by hand, then you will have trouble knowing when you mess up with the FEA. I would be real leery of something that I have no way of checking at least some part by hand.

 

[dougantholz]

I never said I wouldn't do it by hand or couldn't the question really is about production. If I am under pressure, as all consultants are, to design a structure as quickly and efficiently as possible I start with a model. I check the model and work with the model over a period of weeks during the design. I can get the basics down in an afternoon and size some things to start the drafting process. Lots of things still need to be done by hand and will, even on the most complex building, but models are the key now.

 

[youngstructural]

I see value in both Dougan's view and in FSS's post... But I think JAE would probably chim in here with the key one: Allowing ourselves to be rushed or "do the client a favour" is the number one reason for the erossion in the value of our specialist knowledge.

And I NEVER design anything on a computer output without being able to come to within 30% (and preferably much less percent!) of the results with a hand calculation.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[jester86403]

Youngstructural,

I would believe that to be one of the truest statements in the industry. I also believe being put out yourself is the definition of a favor.

I also feel the same way about doing it by hand to check the results. Thats kind of why I started the post. I was told by a few people I know in the industry that they do some small jobs really fast using FEA. Well I got into a discussion and began debating whether or not it is really faster for small jobs since if you are checking the output like you should it will take just as long as doing it by hand.

 

[youngstructural]

You're spot on Jester; Now if we could just make sure that everyone, including our competition, was so responsible about their design work....

*sigh* Food on the table has yet to overcome my ethics and professional duty of care; However I can honestly say it has definately cost me work on occation!

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[GregLocock]

So you double check the FEA (with hand calcs), but you don't analyse your hand calcs any other way?

Hmmmm.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[youngstructural]

Greg: I can't think of many jobs I haven't used a computer on... That said, when I have used only hand calculations, I have checked my hand calcs by both random sampling, as well as by in-house peer review (in the better firms I have been priviledged to work for).

Your Automotive experience probably means that you work with FEA models which are much more dificult to review/double check with hand methods.

Even so, you do raise an interesting point; I would hope everyone double checks all their calcs, irrespective of the way they were produced.

Cheers,

YS

P.S. Love the sarcasm, just can't agree with you this time!

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[haynewp]

I don't know FE (shell and plate elements type of FE), I model little things here and there just to compare results from my hand calcs and intuition. I have been told before that I am behind the times (by a European engineer). But whatever, I have never had a failure of anything either so I hang my hat on that and the fact I don't get complaints of overdesigning often either. Some people seem to refer to matrix models as FE also, but I think of FE as being shell and plate elements.

 

[IFRs]

One benefit from doing small projects with FEA is that you gain confidence and proficiency by doing losts of small problems that you can hand check easily. You get experience and have chances to try the various software options and variations. Then you are more able to be comfortable with your I/O, boundary conditions, etc when it comes to larger jobs.

 

[oneintheeye]

generally i would use fea to check proposals, ideas etc , prelimanary design and sizing then design by hand 'knowing' that the answer should be correct. Partically with foundation design where you could end up with many iterations to find a solution.

 

[oneintheeye]

on a side note using this method I have found serious errors on my 'initial' fea models results during hand checks.
And to agree with youngstructural, any decent company would have all calcs checked by a second engineer after completion.

 

[IFRs]

Of course, by "hand" calcs, I mean MathCAD and Excel documents I have created "by hand", used many times and are trusted in addition to pencil and paper...

 

[MotorVib]

FE's are only as good as the user running them. And I think that we all agree with that.

out side of vary simple problems I make some hand check.

Chris

"In this house, we obey the laws of thermodynamics." Homer Simpson

 

[connect2]

jester86403:
'... buildings, bridges, and other miscellaneous structures ...', that's pretty funny, what else is there?
Haven't used the 'Portal Frame' method in awhile, to many load combinations.

 

[RARWOOD]

douganthotz Are you using FEA or just a regular 3-D program?

 

[jester86403]

connect2,

There are also dams, levees, flyovers, towers, retaining walls, and tunnels