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Designing by Hand vs. FEA
10

Designing by Hand vs. FEA

Designing by Hand vs. FEA

(OP)
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?

RE: Designing by Hand vs. FEA

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?

RE: Designing by Hand vs. FEA

2
(OP)
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.

RE: Designing by Hand vs. FEA

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.

RE: Designing by Hand vs. FEA

5
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.

RE: Designing by Hand vs. FEA

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.

RE: Designing by Hand vs. FEA

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...

RE: Designing by Hand vs. FEA

(OP)
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.   

RE: Designing by Hand vs. FEA

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...

RE: Designing by Hand vs. FEA

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

Hmmmm.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Designing by Hand vs. FEA

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...

RE: Designing by Hand vs. FEA

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.  

RE: Designing by Hand vs. FEA

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.

RE: Designing by Hand vs. FEA

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.  

RE: Designing by Hand vs. FEA

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.  

RE: Designing by Hand vs. FEA

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...

RE: Designing by Hand vs. FEA

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

RE: Designing by Hand vs. FEA

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.
 

RE: Designing by Hand vs. FEA

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

RE: Designing by Hand vs. FEA

(OP)
connect2,

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

RE: Designing by Hand vs. FEA

not to mention, silos, chimneys, bunks, tanks, piers, wharves ..., and other misc. structures.  About covers it.

RE: Designing by Hand vs. FEA

Its amazing how things have changed over the years in structural work.  25 years ago the steel codes and wind codes were very simple and things like portal frames just took a few pages of calculations to obtain member sizes.  Now it probably cant be done by hand because the wind loads vary over the span of the portal.  It takes about 60 to 70 pages of computer printout and eventually the sizes come out the same as 25 years ago. I actually did the exercise not long ago.  Connections are probably a little better designed now and probably reflect more limit states rather than working stress.

Was it all worth the effort of changing from working stress design to Limit state design?  More sophisticated wind codes?  I dont know.  It seems to me that it only came about because of more powerful computers becoming available.  Computers made it happen, not good structural practical design.

Sorry if Ive gone off in a tangent here but these are my thoughts at least.



 

RE: Designing by Hand vs. FEA

I use programs to model stuff on the complicated things.
Obviously simple beam calcs are by hand, but I always do a crude hand calc on the main features of the comples items to double check my modeling.  Many times I have found errors in entering the model by double checking the basics by hand  (wrong fixity, axis of member, braced/unbraced, etc) and I use both methods to double check the other.
I also have my own excel sheets that I use to do iterative tasks "by hand" more quickly.
 

RE: Designing by Hand vs. FEA

Computers can't layout a good layout of the structural system.  Only engineers can.

Quick hand calcs should always precede computer models to lay out a starting point.

RE: Designing by Hand vs. FEA

What a great post for engineers to discuss!

I'm a small firm owner and fall into the category that hand calc's must confirm the computer output. I contend an engineer can't understand a structural system they are designing without doing hand calcs.  Just today I put a $(US)30k structural proposal together for an architectural firm for a commercial design which would be a really nice project, but I won't it because I'm way over priced for this reason.  The computer is a tool not an engineer.  The computer doesn't design structures!

RE: Designing by Hand vs. FEA

Computer can only help engineer, but can not replace engineer.

I just designed a back to back cold rolled form C-channel beam. Using software, it only tells me the yielding failure load is around 19.8 kips uniform load. By hand calcs, I found the failure mode is torsional buckling, the failure load is only 7.7 kips uniform load. I doubt any structural software can replace this kind of engineering judgement.

RE: Designing by Hand vs. FEA

A simple question with a very complicated answer.

As many people have no doubt mentioned above, you should always have a good idea of what to expect before relying on an analysis program.

I would probably divide jobs into simple categories as far as the overall members are concerned.

1. Simple spans and pinned connections - This I would always do by hand because it definitely tends to be quicker. I would even include double spans and more complex loading patterns in this analysis. The only exception for me would be when significant second order moments may be encountered.

2. Framed structures such as portal frames with significant secondary moments - I would tend to do an analysis on the computer and member checks by hand.

3. More complicated fixed structures with significant restraint, indeterminacy and possibly torsion. An example of this would be a complex canopy made in one welded 'lump'. I would either do a quick hand calc to determine sizes and then back it up with a computer analysis or I would do the analysis and refine it to the optimum solution then check the output against hand calcs.

Basically, you should always understand the simplifying assumptions of your hand calculations so that you can make the judgement whether you can live with it. Generally indeterminacy, secondary load paths, restraint, and second order effects are the main reasons why a hand calc would not be accurate.

As far as connections are concerned then it is usually the best approach to us the simplest method, it is easier for someone to check and is also easier to get it right.

Personally I have never used FEM and have only had one or two situations where I thought it would be truly essential.

 

RE: Designing by Hand vs. FEA

Structural engineering is all about determining the loads first, then selecting the material that can resist the loads.  No computer can determine the first on its own.

Computer is useful only when all the load cases have been estimated correctly and all possible load combinations assigned.

I mainly rely on the computer model for "analysis" part.  I look deeper into the capabilities and limitations of each structural software for the "design" part to ensure that computer is performing what I want it to perform (via hand calc etc).

RE: Designing by Hand vs. FEA

RobertEIT - your FEA model wasn't sophisticated enough.  Real (non linear) FEA is perfectly good at handling instability.




 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Designing by Hand vs. FEA

A simple question with a very complicated answer.

As many people have no doubt mentioned above, you should always have a good idea of what to expect before relying on an analysis program.

I would probably divide jobs into simple categories as far as the overall members are concerned.

1. Simple spans and pinned connections - This I would always do by hand because it definitely tends to be quicker. I would even include even double spans and more complex loading patterns in this category. The only exception for me would be when significant second order moments may be encountered.

2. Framed structures such as portal frames with significant secondary moments - I would tend to do an analysis on the computer and member checks by hand.

3. More complicated fixed structures with significant restraint, indeterminacy and possibly torsion. An example of this would be a complex canopy made in one welded 'lump'. I would either do a quick hand calc to determine sizes and then back it up with a computer analysis or I would do the analysis and refine it to the optimum solution then check the output against hand calcs.

Basically, you should always understand the simplifying assumptions of your hand calculations so that you can make the judgement whether you can live with it. Generally indeterminacy, secondary load paths, restraint, and second order effects are the main reasons why a hand calc would not be accurate.

As far as connections are concerned then it is usually the best approach to us the simplest method, it is easier for someone to check and is also easier to get it right.

Personally I have never used FEM and have only had one or two situations where I thought it would be truly essential.
 

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