Properties of Combination Sections
Properties of Combination Sections
(OP)
Aside from the Tables in the AISC Manual
where can I find more variations of beam and angle combinations? Is there a way to calculate S1 & S2 & I?
Are there any programs available?
where can I find more variations of beam and angle combinations? Is there a way to calculate S1 & S2 & I?
Are there any programs available?






RE: Properties of Combination Sections
Or you may use RISA-Section (I have not used it, but my coworker had and said that it was good).
RE: Properties of Combination Sections
RE: Properties of Combination Sections
For a stand-alone program, you may look into one of the modules within EnerCalc.
RE: Properties of Combination Sections
RE: Properties of Combination Sections
Why not just go back to basics and find a textbook (or your university notes ?) which shows how to combine section properties?
RE: Properties of Combination Sections
RE: Properties of Combination Sections
My post was aimed specifically at the very simple problem that vintage70 posted.
Essentially this only requires the combination of the section properties of two basic sections (The main I beam and the pair of angles), whose individual properties are known from standard section property tables, and which have a common axis of symmetry (so we know from the start that the principal axes are horizontal and vertical).
I cannot lay my hands on a text that sets out the method of calculation, so what follows is as good as I can offer. Please excuse the limitations that are inherent in these posts (I cannot, for example, set out the neat tabulation of values that I was taught to use back in the 1950's).
Let me start by defining my variables:
Datum line : I usually use a horizontal line through the bottom of the I beam as a datum, from which all vertical offsets (y) are taken. (Some prefer to use the NA of the main beam as the start datum - there are arguments in favour of either choice).
Variables:
Acomb = total area of combined section; Ybar = distance of NA of combined section above the assumed datum line;
AYcomb = 1st moment of area of the combined section about the assumed datum;
Icomb00 = "Moment of Inertia" (or 2nd moment of area if that is what you were taught to call it) I of the combined section about the datum line;
Icombna = I of the combined section about its own Neutral Axis;
ybtm = distance from combined NA to bottom fibre; ytop = distance from combined NA to top fibre.
A1=main beam area; y1=vertical distance to centroid of main beam; I1 = I of main beam about its own Neutral Axis;
A2=area of pair of angles; y2=vertical distance to centroid of angles; I2 = I of pair of angles about their Neutral Axis;
For easy legibility of any formal calculations I tabulate 6 columns for the following properties for every component section involved - Description of component section (with dimensions where appropriate),Area, y, Area*y, Area*y^2, I.
Particularly if you are calculating somewhat more complex sections (eg a PS concrete I beam, with triangular fillets between the web and both flanges) the tabulation keeps everything clear for anyone else looking at the calcs.
Basic equations:
Acomb = A1 + A2 = sum of my table column 2
AYcomb=A1*Y1+A2*Y2 = sum of table column 4
Icomb00 = A1*Y1^2 + I1 + A2*Y2^2 + I2 =sum of table columns 5 and 6
[for the more complex sections, these just become Acomb=A1+A2+A3+A4+..etc]
That is the hard work done
Now we get ybar from ybar=Aycomb/Acomb.
Icombna=Icomb00-Acomb*ybar^2
Here comes the benefit of taking the datum through the bottom of the main beam:
ybtm = ybar, Section modulus to bottom = Icombna/ybtm.
[with any other datum you have to calculate the bottom fibre distance as the distance from the datum to the bottom fibre of the main beam plus ybar]
ytop = Top fibre distance = overall depth - ybtm, Section modulus to top = Icombna/ytop.
All done. In fact it takes much longer to describe than to do on paper. Total calculations involved: 2 mental doublings (single angle area and I to give properties of the pair), 4 multiplications (Ay and Ay^2 ), 4 column additions (A,Ay,Ay^2,I) plus 4 simple expressions.
RE: Properties of Combination Sections
The entire exercise has only two steps, viz. Finding the CG of the combined section and find the MI of the combined section about the axis passing through CG using the formula
Icg = sum(Icg of individual elemnts+ area of element * d^2) where d is the distance between the CG axis of the element and the axis through the CG of the combined section.
As austim has stated this geometrical calculation does not take much time to do by hand.
I am afraid that the computer era is eroding our patience and confidence to do simple hand computations!
RE: Properties of Combination Sections
http://www.structural-engineering.fsnet.co.uk/free.htm
However, I do agree that we should not place too much reliance on computers for such a simple calculations.
RE: Properties of Combination Sections
RE: Properties of Combination Sections
As a matter of fact, you can set up your spreadsheet to calculate your centroidal moments of inertia using the parallel axis theorem, and voila, a single step.
Then, when you find out your stress is still too high, or something else needs nodification, 3 minutes later...finished. Can the hand calc. proponents match that?
You can even document this nicely by incorporating the Acad view of your section an an Excel document.
RE: Properties of Combination Sections
Must we really assume that all engineering design has to be done at a desk with full computer and software support and that it is quite OK for engineers to forget the basics?
RE: Properties of Combination Sections
Really, just a little poke. I agree, the computer is great, but sometimes just setting the problem up takes longer than solving it by hand.
RE: Properties of Combination Sections
The same question has been discussed in different threads under different titles. Should the use of computers and software blunt the mental working capability of an engineer?
RE: Properties of Combination Sections
2. An engineer forgetting the basics is not an engineer. If one becomes useless in situation where there is no access to computers, God shower His mercy on him. (And still there are many such places!)
3. If one 'Knows' how to compute sectional-properties, whether he chooses to use calculator, computer or a sheet of paper is really a trivial issue, as far as he gets the correct values.
4. I regularly use computer programs and also do hand computations. My limited experiencxe tells me, mistakes done on computer are most hard to find, even to experienced eys. Contrary to this a hand calc can be checked by a Grad engineer and he/she may be able to pick up any mistakes there. If I were to bet my hard earned money on some computation, I will most definitely do a hand calc. The satisfaction of doing every mathematical step yourself defeats all self-doubts and fears.
Regards to all,
flame
RE: Properties of Combination Sections
I also understand how to calculate all the section properties that I need by hand, but why waste time over and over when the client is paying a fixed price for my services? It leaves me (and my colleagues) more time for checking and correcting drawings.
Being an engineer in this day and age requires BOTH an understanding of classical methods, and an openness to new technologies to speed up the process.
And this way, when I fly, I don't need to calculate section properties, I can just enjoy the movie and pretzels...
RE: Properties of Combination Sections
Are you the same Austim that suggested a non-linear analysis to analyse tension-only braced frames?
But you calculate section props by hand?
Hmmm...
RE: Properties of Combination Sections
computer is supposed to help us do things faster. there is a tendency of many younger generation engineers to rely fully on computer out-put. cliche'd garbage-in-garbage-out remains for ANYONE who use computer programs. There had been many good articles I've read in the past about structural engineering, computer-use and engineering judgment.
True engineer can solve problems even after you take away the codes, computer and calculator. That is the future generation of engineers I would like to see.
I see many spreadsheets being passed around on the net. I never use any - not because I dont trust the author but I wouldnt have confidence in the spreadsheet unless I took time to know what is going on in EVERY CELL.
Even in-house, I am very careful about distributing spreadsheet tools which I've authored. Peers may not know the assumptions, limitations in the routine.
Bottom line is... Computer is merely a tool. Use it wisely, otherwise it will come back and bite you.
few cents from sunny California.
RE: Properties of Combination Sections
I would like to boil down two concepts - one from austim and one from trainguy.
1. I agree with austim that engineers should know how to do the section calculations by hand. They should know the concepts behind them, and the process to find a section property without a desk/computer. However, in this current economic/engineering culture these days, speed is part and parcel of our services...not just our engineering expertise. As an engineer, I do not have the luxury of doing all my calcs by hand. I must use the technological tools available to me to provide my services to my client.
I'm not advocating engineers becoming blindly dependent upon the black box. I agree that a good engieer should use the CORRECT tool for the job...that may mean a computer program, spreadsheet, hand calc or counting on fingers...whatever will get me the answer, most correctly, in the shortest time.
All this while keeping myself cognizant of quality checking and assurance that I'm doing the right thing.
2. trainguy hits this same point - be open to the tools out there as they become available. BUT, don't use a tool that you know nothing about and have not tested.
RE: Properties of Combination Sections
You may be relieved to know that I am the one and only genuine (perhaps even dinkum) Austim.
I have happily embraced computer technology as it has been developed, and encouraged others to do so, but to use it intelligently and for appropriate purposes. No, my slide rule slides no longer, but my pocket (programmable) electronic calculator still works full time. [Perhaps you were never required to prepare alternative bridge girder designs for tender within an unreasonably brief tender period, before in-flight movies had been invented ?]
My previous lengthy post was directly aimed at Vintage70's initial question - "Is there a way to calculate S1 & S2 & I ?". Rightly or wrongly, I concluded that Vintage70 had either forgotten how to do it, or had never been taught the basic principles. That would not be a good start for setting up a spreadsheet.
1. Re my hand calcs for section properties:
I still stand by my view that rushing to a computer to do the calculation needed by vintage70 is the wrong way to go. In fact, for any beam calculation that I might do, the three properties sought by Vintage70 would only be part of the problem. What about Iyy, J, W (required to determine lateral/torsional buckling properties), and the plastic moduli about each principal axis? ACAD does not appear to offer any help with J and W or the plastic moduli, so I would have to pull out the pencil and paper anyway. And in one quick exercise I have my submission documentation finished.
I have never been one to avoid a challenge, but I cannot (quite) match your nominated 3 minutes. (But have you actually timed yourself)?
Under 'exam conditions' (starting with blank sheet of paper, section table booklet closed and calculator switched on), my best time so far for calculating the elastic moduli to the top and bottom of an Ibeam-plus-angles section is 4min, 34.9 seconds (including time to start and stop the digital stop watch on my laptop). Allowing for improved neatness of presentation for serious work, say about 5 minutes.
But that has already got me further than ACAD can do, since I have written down the two section moduli, which you cannot read from ACAD, and I have got a calculation sheet (including a sketch of the section) to incorporate into my record file.
I never use ACAD for drawing (only for reading drawings that others send to me), so any attempt of mine to confirm your 3 minutes estimate would be pointless. But I seriously question whether you could, within your suggested 3 minutes, find and load the relevant drawing, find a suitable cross-section, verify that it has been drawn accurately, select it, convert it to a region, find its CG, move it so that the relocated CG is at (0,0), record the CG location and M of I, and then calculate and record the two section moduli that Vintage70 wanted.
At the very best, the ACAD process will derive four elastic properties of the gross cross-section (A,ybar,Ix,Iy). Within current Australian practice, that is really not much use as a general procedure, and could mislead inexperienced engineers into some dangerous design decisions, particularly if they were to use it for large plate girders.
The Australian steel design code abandoned ASD methods for all structural work 12 years ago. As a result, the elastic moduli on their own are virtually useless to us. To determine the bending capacity of a section, we have to be able to calculate the plastic modulus and take account of possible reductions in effective section due to the slenderness ratios of the various plate elements involved in the section. Then, of course to calculate the bending capacity of a beam, we have to allow for possible lateral/torsional buckling, which requires us to know the two torsional parameters J and W.
[The best that I can get from ACAD is an upper bound to the design values for A, Ixx, Iyy, which can be reduced significantly by plate slenderness effects.]
Essentially, these factors force us to use standard sections for which all tabulated properties are available, or alternatively to know what we are doing, and calculate for ourselves the design properties for all special sections. In much of my own work (heavily loaded plate girders etc) the latter option is unavoidable.
2. re my non-linear suggestion (which was not entirely tongue-in-cheek. Some day I will try it for myself).
Again, current Australian practice comes into the picture. Since 1990, our LFRD design code has specified quite clearly that if we only use first-order (linear) analyses, then we have to multiply all of our output moments by moment amplification factors which vary according to the critical axial buckling load applied axial load in each member.
Alternatively we may use the output from a non-linear analysis without further modification. (Provided that we keep above a buckling factor of 5, the non-linearity does not have to include the effect of axial load on the member stiffness). The 2nd-order alternative is much 'neater' and simpler in practice. It rarely adds much to the analysis time, and avoids a very messy process once we have our analytical results. So, whenever I can, I adopt 'non-linear' as a standard process.
RE: Properties of Combination Sections
RE: Properties of Combination Sections
I enjoyed everyone's comments very much.
RE: Properties of Combination Sections
The quality of your posts is, as always, highly informative and entertaining. The quantity of some of the above is nothing short of breathtaking. That you have the time to do all those hand calcs, and write posts that long is quite impressive....
OK, maybe 3 minutes was an exaggeration, but if it's 5 or 6, it is that value for any arrangement of angles, channels, symmetric, non-symmetric by my Acad method. You're right on about needing other properties for design (esp. in bridges and buildings fields). You should see the wierd section shapes I encounter in railcar design, some aluminum extrusions are really quite complex. Many of my design criteria refer to permanent deformation and yielding so an ASD method is somewhat common practice here.
Point well made about the ORIGINAL post, it IS an easy hand calc. I'd probably still use a spreadsheet approach (without ACAD) that I'd written myself. At least, the arithmetic is foolproof if the data is entered right. It takes just as much time to enter the data on your hand calculator than it does in Excel, but with the added benefit of a nice print-out.
I can't believe I just took 12 minutes to write this post! I could have done 4 section property calculations...
RE: Properties of Combination Sections
I will let you into my time secret (just between the two of us, mind you).
I am due to retire permanently before the end of this month, and I now have no outstanding work commitments
But I am glad to see that between us all we seem to kept vintage70 happy.
RE: Properties of Combination Sections
You've been a great help. I hope some day I will be just like you----
RETIRED
Stay well
V
RE: Properties of Combination Sections
from another engineer who wants to be in austim's shoes
RE: Properties of Combination Sections
RE: Properties of Combination Sections