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?
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
RE: Designing by Hand vs. FEA
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
RE: Designing by Hand vs. FEA
RE: Designing by Hand vs. FEA
RE: Designing by Hand vs. FEA
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
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
*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
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
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
RE: Designing by Hand vs. FEA
RE: Designing by Hand vs. FEA
RE: Designing by Hand vs. FEA
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
RE: Designing by Hand vs. FEA
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
'... 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
RE: Designing by Hand vs. FEA
There are also dams, levees, flyovers, towers, retaining walls, and tunnels
RE: Designing by Hand vs. FEA
RE: Designing by Hand vs. FEA
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
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
Quick hand calcs should always precede computer models to lay out a starting point.
RE: Designing by Hand vs. FEA
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
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
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
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
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
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.