Computer output shows value of real-world judgement
Computer output shows value of real-world judgement
(OP)
So I was looking at a plant deck for an existing building, and in designing some of it using software I have found quite the humorous computer output. Warning: Probably not funny to anyone other than a Structural Engineer, and that might even be a stretch!
Not to restart the "when is overstress okay" debate, but have a look at the screen capture; Funny what a computer considers a "failure"...
I thought it would be useful to mention, as it reinforces yet again the value of engineering judgement.
Cheers,
YS
Not to restart the "when is overstress okay" debate, but have a look at the screen capture; Funny what a computer considers a "failure"...
I thought it would be useful to mention, as it reinforces yet again the value of engineering judgement.
Cheers,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...






RE: Computer output shows value of real-world judgement
RE: Computer output shows value of real-world judgement
The results part doesn't scroll. I imagine that is where the funny bit is located.
RE: Computer output shows value of real-world judgement
Cheers,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: Computer output shows value of real-world judgement
What I'd like to see if a Ramsteel-designed framing plan with insane sizes due to one button being pushed wrong somewhere. I worked on a job like this years ago: the poor guy had pressed the "Shored" button a couple of menu levels deep and it was an unshored composite beams. Those W12x14, 34' long did't stand a chance when they started putting concrete on there. All of the main beams and girders were undersized by at least 2x and the errors got by everybody working on the job -- designer, cad guy, detailer, shop drawing checker, fabricator, erector...didn't get by Mr. Gravity, though, LOL. I need to get a copy of that plan to show my students to make an impression about mindless use of programs.
RE: Computer output shows value of real-world judgement
RE: Computer output shows value of real-world judgement
Don Phillips
http://worthingtonengineering.com
RE: Computer output shows value of real-world judgement
I think it was 1.0000001. Thus NG.
RE: Computer output shows value of real-world judgement
Don Phillips
http://worthingtonengineering.com
RE: Computer output shows value of real-world judgement
RE: Computer output shows value of real-world judgement
Don Phillips
http://worthingtonengineering.com
RE: Computer output shows value of real-world judgement
RE: Computer output shows value of real-world judgement
I am sure this is a case of rounding and display format precision. I am sure if you modify your default output format, you will find that the computed result is something like 0.996 (which is displayed as 1.00 when formatted to two decimal places), but is definitely less than 1 exactly, and so is a "Fail", according to the user-defined test limits.
You have to remember that computers are dumb but obedient servants - if you say you want to test against a pass / fail criterion of greater than or equal to 1.00 exactly is a "Pass", less than 1.00 exactly is a "Fail", then that is what it will do.
If you are prepared to accept a little bit of "overload", you can specify this when you run your Steel Design Check - instead of setting the "Load Factor Limit" equal to 1.00 (which is the default), you could change this to 0.95 say (YOU have to tell the computer what YOU consider to be "a little bit"!), but you would face the same issues then when it reported a Load Factor of 0.951 for one member (which displays as 0.95) as a "Pass", but another member with a load factor of 0.949 (which also displays as 0.95) would be a "Fail". YOU have to draw the line somewhere, and wherever YOU draw the line, the computer will faithfully tell you which side it calculates you are standing on!
Hope this helps!
RE: Computer output shows value of real-world judgement
RE: Computer output shows value of real-world judgement
RE: Computer output shows value of real-world judgement
And of course Julian is right; The screen display is not the value (proper) that the machine is considering. I hope the issue of machine accuracy and the fact that this number is by nature flawed and inaccurate doesn't need to be pointed out!
I just thought it was funny, and something to keep in mind. The blind acceptance of an output as "failed" or even as "okay" is irresponsible from a Professional point of view in any case, however I did think it was worth posting, even just for a chuckle.
Regards,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: Computer output shows value of real-world judgement
Don Phillips
http://worthingtonengineering.com
RE: Computer output shows value of real-world judgement
Our computers and calculators enable us to calculate results to 4, 8, 12 or even more decimal places. However, as engineers, we should never forget that we often only know the input parameters to one decimal place accuracy (or less, sometimes!)
Back in my undergraduate years, I had one lecturer who used to deduct marks for every decimal place of of precision you gave in your final solution greater than the lowest precision of the supplied input data. It was a lesson well-learned.
For example, consider the coefficient of internal pressure in a building due to external wind loading. Your design code may indicate a value of +/- 0.3 say. (Think about what this means - a change of just +/- 0.1 - the implied accuracy of the provided figure - would yield a change in total load effect of +/- 33%!) Your wind loading program may tell you the total uplift load is 773.45 kN, say. By all means, carry this precision through in the subsequent calculations, but if someone asks you, all you REALLY know is that the total uplift is about 800 kN - not 770, not 773, and definitely not 773.5!
In some cases, 0.7 = 1 = 1.3 - to the required engineering accuracy!
RE: Computer output shows value of real-world judgement
That said, it's not just the accuracy of the input, but also the floating-point number error creep. You cannot expect laser accuracy from computers, any more than you can expect engineering judgement. The computer cannot multiply, divide, or do any of the higher mathematical operations on decimal figures without automatically causing errors. The problem is that everything is stored in "pseudo" numbers, as actually represented by a string of base two numbers.
Errors occur in the most routine of computer calculations, and only by the grace of random error cancellation and rounding do the answers come out "okay". There are some very famous examples, including a Patriot missile which failed to stop a Scud attack in the middle east. If memory serves the error was so bad that the missiles were seriously far apart... But the computer thought they were spot-on!
As soon as you get into Matrix Methods, you get into even more expansive realms of errors, and very smart people who try to make them not matter. But they do, and trusting the black box blindly is the fastest way to get people killed that I can think of in our line of work...
Cheers,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...