Hand Calculations 2

[mrMikee]

I am curious how many people still do hand calculations for structural analysis and steel design, or know of someone who does? Things like moment distribution, combined stresses, and so on. When are hand calcs appropriate, or not. Does anyone have opinions about this?

Just wondering.

Best Regards,
-Mike

 

[JedClampett]

I only use the computer to check my hand analysis (I prefer slope deflection when I can use it). I have a couple of utilities that I programmed into my calculator, but once again I only use them to for preliminary analysis. Once in a while I'll do an indeterminate analysis on CFRAME. Everything else I do by hand. I can look at my calculations years after I did them and immediately understand what I was doing. Even the mistakes are obvious.
The engineers who review my work (in-house and the occasional plan reviewer)seem to appreciate the hand calculations. Not so much "black box" engineering. Plus with hand calculations, I usually do sketches that explain the design.
I'm afraid I'm the last of a breed. I can get away with it because I'm a senior level employee, but the younger engineers are afraid hand calculations and sketches slow them down.

 

[JStephen]

It just varies according to what you are doing.

For a one-time simple project, hand calcs are fine.
For simple calcs that require references and explanations, etc., I've actually made "word" documents, and it's fairly quick to go through and change the numbers for the case in point.
Spreadsheets work great for lots of things- anytime you're doing the same basic thing several times over. I've done a couple using visual basic (took that as continuing ed for pe licenses), and that can be pretty cool.

 

[NewImhotep]

I replaced my hand caculation by creating several Excel Sheets Which can analysis multi span beams or slabs using moment distribution concept
also design a PT & RC concrete with a lot of functions , and currently it is my basic tools in prelaminary design befor going to a FEA software.

from while to another i trying to improve these sheets and i found it a good replacement for the hand calculation.

 

[WillisV]

For small one time calcs and simple beam designs, hand calcs are great...but for analyzing and designing 20+ story buildings within the schedule expected by today's clients it is next to impossible without using building analysis software to automate the process. Garbage in garbage out is as true as always but these programs do greatly increase the efficiency of design provided they are used correctly.

 

[aggman]

I am a relatively young engineer, 3 1/2 years since my first degree. I generally do quite a bit by hand. All of my silo and bin designs are by hand. It just depends on what I am doing and how exact I need to be. As a minimum I always do a back of the envelope type calculation to check the computer. Many of the more complex structures require computer analysis because of the large indeterminate nature of them. I have always thought that I was the only person with my experience level that did calculations by hand. My supervisor (he's 51) still does a lot by hand, so I was glad to jump on the hand calculation experience since he doesn't jump down my throat when I do it. Another young engineer I know has done very little by hand. When he started with his firm they started teaching him all the little software programs they use (nothing about how to get the outputted results). To me he seems somewhat useless because his analytical skills are weak and he doesn't have a good feel for what is going on.

 

[GBor]

Software programs are great for improving efficiency, but hand calcs are necessary, not to "check the computer" generally, but rather to check the user in my experience. I wouldn't want to hand calc a 20 story building, but I would be interested in hand calcing a simple truss, making certain I'm getting an accurate result and then making the leap to say the 20 story building analysis was correct. Aggman's story of the "useless engineer" is too frequent. Universities aren't teaching applications the way the used to and then we take young engineers, give them a computer, and rely on their results without providing adequate training or support.

OK, off the "bandwagon"...nice to see some people still remember how to calculate a beam.

 

[UcfSE]

Hand calcs are necessary when you work for someone who doesn't buy software. Almost evertything I do is by hand calcs or by a spread sheet I have invested hours in creating. I also will take a look at quick numbers to check computer results. I believe either hand calcs or software are fine as long as you aren't using the software to do something you don't understand or couldn't get yourself in an appropriate amount of time. What's hard about the hand calcs though is trying to produce them for a project budgetted as though I had software to do everything. I am also relatively new in the field with a little over 3 years of experience.

 

[miecz]

I do most of my work in MathCad, which is essentially an electronic hand calc, in that the formulas are right there and can be annotated by text. In most worksheets, I use calculus to develop shear/moment/deflection diagrams for simple and continous beams, and retaining walls. The time invested in developing the worksheets is saved upon reuse. Some structures require black box software, like a 3-span continuous bridge with composite stringers (varying moment of inertia) and moving live load. If anyone has a hand solution for this, I'd love to see it, as I hate using software that I didn't write.

 

[mrMikee]

Thanks for the comments.

I still do a lot of calcs by hand but will use software when I can. I have been writing programs in BASIC and VisualBASIC for about 20 years, and I like VB, but these are very specific in function. About two years ago I bought a calculation program and find that now I am doing less with VB and more with the calc program. These programs (these are several good ones) are more of a free form type of calculation sheet that is something like using MS Word but with the ability to solve equations.

It has occurred to me however, that even though I am using software, my approach and thought process is really the same as hand calcs. And I like this. I get to select unbraced lengths, K factors, column braced lengths, etc. It's all pretty neat, in my opinion.

I am now shopping for another FEA program, but as much as I want to get a program for analysis and design, I can't bring myself to buy into the "black box" approach. But I get the feeling I'm doing things the hard way, again.

-Mike

 

[UcfSE]

When I first started using our software here (RISA 2D and 3D) I would set up several different beams and columns and so on that I could readily solve by hand and check that the software and I came to the same conclusion on allowables and code checks. You also find out what the software does and doesn't consider by doing this. For instance, RISA does check cold-formed steel members but does not check web crippling. It does not specifically point this out on your output sheets. Being able to identify and remember these things helps take away the black box feeling that many programs can have. What's scary is seeing other engineers using software blindly and just trusting the results. I see that all to often.

Personally I wish my company would buy us MathCAD and/or MATLAB. I use these at the University and would love an opportunity to take advantage of what you can do with these programs. They seem to have much more to offer than a spreadsheet with the same hand calc feel to them that mrMikee mentioned.

 

[mrMikee]

UcfSE,

I use CalculationCenter ($595) by Wolfram Research and my son uses MathCAD ($1200). They are similar but different. There's and interesting discussion on thread770-122493, but it's not my intent here to start that one over again. I too like the hand calc feel this software gives me.

One thing I noticed about RISA is that it only checks single angle struts for L/r. Compression in single angles is much more complicated than this, which is one of my pet peeves. I've seen too many engineers not figure this correctly.

jmiec: You mention that you hate software that you didn't write. I too suffer from this affliction.

Thanks,
-Mike

 

[JStephen]

Once upon a time, I was using structural software, and discovered than when you put in a diagonal beam, the plane of the beam is determined by the plane of the end points and the origin (0,0,0) (as best I remember). Not only was it very awkward to rotate the beams to the proper angles, it was very awkward to tell what angle they were at in the first place (webs were in vertical plane in all cases, in reality). That kind of situation just asks for mistakes to be made. But then again, that was a situation that I couldn't do by hand, either.

 

[wds1]

The trouble with hand calculations is that it is hard to apply all the load combinations and draw the max/min envelope. The way the codes are going, they are forcing us to use software. The trouble with software is that you have to hand check all your output which can be more time-consuming.

 

[mrMikee]

I remember my first design job using our new FEA software, and how surprised I was that it took me longer than by doing hand calculations. The reason was way too much output to dig through to do stress calculations. It was a general purpose analysis program that computed stresses but could not code check with AISC.

-Mike

 

[SacreBleu]

The only time I use hand calcs is to occasionally check automated output, and whenever it is not practical to write the calc in Excel or to use canned software.

 

[connect2]

Hand calc's ... we have thousands of pages, but really much beyond 'simple' beams, 'simple' frames, well really anything fairly 'simple' we really only use hand calcs to verify these 'simple' situations and to verify or check our computer generated output. Note to proceding ... one can make even the most 'simple' analysis very 'complex' if one wishes to.

 

[Dinosaur]

jmiec,

I also use Mathcad whenever I can because after the checker looks everything over, all I have to do is change one or two values and the whole thing updates itself. My only regret with Mathcad is that I can't easily place sketches in the work. If I did more projects that were similar geometry, I could smoke some work with all the little worksheets I have now.

Like you, I basically do everything possible by hand. I have been working on a Mathcad worksheet that analyzes a straight multispan girder with varying moment of inertia. It is fairly far along but still has a way to go. We use Mathcad 2000 Pro here in my office, and I have a copy of Mathcad 7 Pro at home.

The secret to solving this problem (say your three span continuous girder) is to realize that it can be formulated using a stiffness model with only two indeterminant degrees of freedom. Once you have the end rotations, you can immediately obtain the end moments for a specific load combination. With the end moments, everything else is analyzing a simply supported beam.

The key to getting the problem down to two DOF is eliminating the nodes where discrete changes occur in the MOI. You can accomplish this using the Hardy Cross Column Analogy. This is a method where a beam vith varying MOI can be reformulated as an equivalent beam with differing rotational stiffness at each end (e.g. the stiffness of the new beam will not be 4EI/L at each end as it is with a prismatic beam). With the rotational stiffness, the remaining translational stiffness and coupling values can be computed directly from the equations for static equilibrium.

Anyway, when you can reduce the problem to so few DOF, you find there are not so many load combinations to worry about. I have conquered the moment design portion and I am working on the fatigue stress and shear portion of the design.

Unfortunately, I have spent many hours on this, much of it my own time and I am not inclined to give the thing away. But if you know what the Hardy Cross Column Analogy is and know how to use Mathcad and the stiffness method, you can make a worksheet to do the same thing.

Good Luck - Ed

 

[miecz]

Dinosaur,

Thanks for the suggestions. It sounds as though the approach will work and I'd love to try it. Clearly though, the solution will take many hours, so it won't happen for a while. Anyone who has spent long hours to develop a solution can appreciate your reluctance to share it. The approach itself is very valuable. I have used stiffness solutions for small frames, but used the 3 moment equation for continuous beams.

To place sketches in a MathCad sheet, you can import AutoCad or Microstation drawings. It you use some other electronic drawing program that doesn't talk to MathCad, there is a handy little utility called "Capture Express" that allows you to cut and paste pretty much anything. I suppose you could even make a hand sketch, scan it, and use the capture utility to paste it into Mathcad.

Jim

 

[Dinosaur]

Jim,

I'm glad you understand about the time factor, but I'll check back from time to time in case you get stuck while you work on it. I usually check this forum twice a week.

I usually draw free body diagrams by hand and scale sketches of connections and the like if I have the time and then I just photocopy them in or scan them and place them as an object. If I used the worksheets more frequently, then I would take more time to get the figures in the file.

Good Luck - Ed

P.S. Do you have the booklet, Moments, Shears and Reactions for Continuous Highway Bridges, published by AISC? That's what I used to check my first run of the procedure. I don't know if it is in print, but you might try contacting AISC if you don't have a copy. All us Hand Calc guys should have a copy if you do bridges. My information says it costs $16 and you can reach AISC @ (312) 670-2400.

And for those of you who do have it (1986 ed.) there is an error in Table 2.0, max dead load moment @ B should read -0.1250 and not -1.1250. In Table A2.0 the value in column 0.6 of span 1, row 0.8 of span 2 should read -0.0288 not -0.2088. I have not done an exhaustive check of the tables, these are just two values I have noticed that were incorrect (typos).