vierendeel truss analysis 3

[joistg]

I have to analyze warren truss with one diagonal missing somewhere in the centre of the truss.I understand that top and bottom chords will have additional moments. The required opening in the truss should quite large, so there is no room for reinforcing both top and bottom chord of the truss. Also, it is a matter of saving (there is lots of trusses like this). How can I analyze this truss? Also, should I be concerned about deflection and how can I calculate it?

 

[BAretired]

The OP doesn't have an FE program.

BA

 

[BAretired]

joistg,

Your responses are coming too slowly. I asked you what your maximum shear in the middle panel is and also the panel length. You will find that you get a lot more out of these sessions if you respond immediately, not wait until we have all gone to sleep. So, get with it!!!

BA

 

[dik]

I hear you get tired and cranky when you retire... <G>

Dik

 

[csd72]

Has anyone ever looked at doing a picture frame (i.e. 4 sided frame) around these openings.

I would think you could possibly design one based on shear only with a pinned connection to the truss in each corner.

Just a thought.

 

[rb1957]

isn't that what BA proposed back at 24 Feb 11 15:29 ?

 

[dhengr]

BA:
I agree with you, but we have to cool our jets , he gets paid by the week, not by the jobs/day or by the post. Many times I think we are more invested and interested in their problems, and in a deeper way, than they are. They are just churnin it out, doing a job and going anyplace they can for the quickest answer with the least thought and effort or digging on their own part. Whereas some of us older guys love engineering and have too damn much free time on our hands.

Remember the first time we faced many of these types of problems, the trips to the Univ. library because our own ref. books weren’t enough, the head scratching and late evenings because we thought we owed it to our job, the project or our boss, the discussions with older engineers, maybe from other offices, or our old profs., the digging through old plans and calcs. to see how it was done, our friendly fabricator, etc. etc.? Now, ya just click on the internet, some wise guy like you gives good advice, maybe ya take the advice, maybe ya don’t, maybe ya know which is the good advice, or not, and ya don’t know how to check this out either, so ya run it through a FEA program, with bad input assumptions, and call it a day. Ya certainly don’t waste your own free time checking on Eng-Tips for the latest on your questions. Some new structures make me nervous, and I am the nervouser because I’m not sure which ones.

 

[BAretired]

dik,

Tired and cranky...sounds about right.

BA

 

[dhengr]

dik:
Quit being a smart A$$, as I seem to recall, your not too many years away from tired and cranky.

 

[csd72]

rb1957/BA,

missed that one!

 

[BAretired]

dhengr,

Thanks, I think I'll "cool my jets".

BA

 

[dik]

I'm on the 'Freedom 95', kick... so have a bit before I get tired and cranky...

Dik

 

[ToadJones]

I am going to take the soapbox from dhengr for a minute...you can have it right back when I'm finished.

I'll admit I am irritated by the older engineers who reject FEM (seemingly out-of-hand) or make sneeky remarks to cut on it.

I'd venture to say that you are as much irritated with your lacking in ability to use FEM as you are with "younger engineers" using it.

I use FEM almost every day. I also teach guys younger than myself how to properly use FEM and when they are barking up the wrong tree. I have learned a lot about load paths in structures by studying classical methods (on my own time) and then comparing them to my FE models.
You get to the point where you can review the results of your FE models and right away see "hmmm somethings not right".

You guys shouldn't be so quick to assume that "younger engineers" just build pretty models and accept the results as if they were Bible.

I'm also not so proud as to deny I have been shown a thing or two by guys with a lot less experience than I. I tell those guys "thank you, good job". It also goes a long way when the table is turned and I have to tell them that they did something wrong.

Because you fellas learned over the course of generous break-in period on the drafting board how to do things by hand doesn't make using FEM wrong in any way at all. The same guys using FEM might be the guys who should you how to send an email back in the 90's. These guys are expected to learn quickly as they come out of school how to use these software programs. I don't think it is right, but, I know for a fact this is the case. I deal with it everyday. Hell, none of us youngsters ever even had the chance to make our hand writing look exactly like stencils used to make general notes on drawings before CAD, which was probably equally rejected.

Call me what you want, but this isn't as much conjecture as you might think. I am a third generation engineer with 6 siblings working in the engineering community. I have literally heard it all and I have been hearing it since I can even remember.

dhengr....lets hear it!

 

[BAretired]

Toad,

I believe, as you do, that there are many problems which may be solved more quickly and more precisely using FEM than by using hand methods. I started using computers in 1959 when my boss decided that it was time somebody in the office had some idea about their potential. I attended lessons offered in Edmonton, Alberta by a company called Royal McBee. The computer was called LGP-30. I was fascinated by it because it appeared to have lots of potential in solving engineering problems, but I never became very proficient at doing anything very useful on it because it had to be addressed in machine language...even more laborious than Assembly language.

I studied Fortran and wrote a few very simple programs on specially prepared sheets which were subsequently transcribed by a technician on a sort of a typewriter. That output was fed into another machine which punched cards with little rectangular holes. These were read by the computer which converted the punch cards into instructions. One little hole in the wrong place meant your program failed.

I neglected my programming pursuits for about twenty years, but in the early eighties, I decided to revisit the world of computing. I settled on Borland's version of Pascal. I thoroughly enjoyed studying that language and I wrote a Cad system and a 2D Frame Program, both of which I used on real engineering projects, although I must admit, they were not particularly user friendly. I wrote lots of quick and dirty routines to solve specific problems.

Nowadays, there are so many programs available to engineers that it does not pay to attempt to write them from scratch. The problem is that today, the software people don't want to spend the time and money to prepare documentation adequate for the normal person to understand how to use their product. In consequence, young engineers are struggling with how to input data and how to interpret the output.

You said:

I'll admit I am irritated by the older engineers who reject FEM (seemingly out-of-hand) or make sneeky remarks to cut on it.

I do not reject FEM. I believe it is a wonderful tool and eliminates drudgery in calculations. I also believe it should be used only by those who understand the principles of the problem they are trying to solve and who can do a rough check by hand to ensure their answer is in the ballpark. Otherwise, they have no business using FEM or any other computer program.

BA

 

[dhengr]

Amen!

 

[ToadJones]

BA-
Can't disagree with anything you said.
I do disagree with comments like:

"Tomorrow, buy a $5000 computer program, input all the variables that we really can't pin down very well anyway, and run that thing for all the different load conditions, to at least 8 significant decimal places. Impress your boss with the .5% savings in steel if you could only get an angle that size, and assuming all of your guesses on the variables are correct"

It's snide comments like these of the strawman nature that get old.
I know some of it is in good fun, I do it too.

I just think the savings that one might impress his boss with will come in improved productivity once he learns the software.
I think you'll agree that in this day, its a necessity if for no other reason than staying competitive.

 

[dik]

Ditto, BA (and no disrespect intended)... and I've written my own FEM and stiffness programs... they have their place. With the problem at hand, I'd have had a solution before people could have input the data... almost faster than the program could load <G>.

Dik

 

[ToadJones]

Shoot us some geometry and loads and member sizes.....
Let's have a race....and accuracy counts

 

[RFreund]

Thanks for the entertaining thread to read on a Saturday night.

dhangr:
I'm a young engineer and probably can't teach you much about computer programs but I'd pay to have a few of your "courses". You are one smart dude.
Not to discredit any of the other elders on this site.
Makes me feel like theres an infinite amount of information that I don't do.

EIT

 

[paddingtongreen]

@joistg, In the discussion on whether you should, or should not should use that FEA program, you know, the one that you don't have, you could well be left with a great deal of misinformation.

The conservative assumption for manual analysis, is that you have have two, four bay trusses, connected by a beam, bridging the central gap, the beam connected to the first two node points on either side of the gap. In this case, the bottom chord is ignored in the gap, the reactions at the node points distribute through the diagonals. The change of slope of the beam through the gap can be used to estimate the additional deflection.

If this result is not satisfactory, the next thing is to calculate the effect of the bending of the beam on the bottom chord. This is just possible by hand but it is a longish iterative process finding a balance between the effect of the bending of the beam and the force couple of the chords. Unless you have spent good time using and understanding these archaic methods, this really needs an FEA program. (Any reduction in the bending of the beam by the force couple changes the forces many of the other members).

p.s. I would welcome a clear argument as to why and how I'm wrong.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[JAE]

I have ...ahem...over 30 years of engineering experience myself (man I feel old) but I must say that I appreciate ToadJones' post above. Not quite doing engineering as far back as BAretired but I did use a slide rule at one point!

I have to agree with much of what TJ says. I strongly believe that using TODAY's programs - which are much more user friendly than those of the 70's - help the engineer get a much MORE intuitive sense of design than using moment distribution or other energy methods by hand.

I can alter a member size, loading, etc at the drop of a hat and immediately see the effects of any of my input or assumptions. I know this is true because I've experienced it doing both ways (by hand vs. computer). The drudgery of hand calcs doesn't always lead to a good intuitive view of the analysis, rather, I feel that hand calcs can actually interfere with it as you spend more time with the math and less with the engineering design.

The analysis program I use today is very user friendly, has a great manual, I've tested it against hand calcs, I trust it, and it is easy to teach engineering design principles (not just simple bookkeeping analysis methods) to younger engineers. Most of they younger engineer interns "get it" at a vastly quicker pace than my own peers took so many ears ago.

Enjoyed the discussion above - thanks all.