properties of combination sections

huangyhg

properties of combination sections
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?
in risa-2d or risa-3d you may specify built-up section and it will calculate properties.
or you may use risa-section (i have not used it, but my coworker had and said that it was good).
try shapebuilder 3.0, but keep in mind, that if the in-service section is loaded there is an initial locked in stress after the section is built up.
shapebuilder is a good program.  once the built-up section is created, it can be imported to the visualanalysis program for analysis and design.
for a stand-alone program, you may look into one of the modules within enercalc.
in autocad, draw your cross section.  convert it to a region.  then do massprop, find the centroid of your shape, then move that to 0,0,0.  then do massprop again, and that will give you some of the values you are looking for.
in 45+ years of practice i have never needed to use software for such a calculation.  if your section will be symmetrical about the plane of the web, then what you want to do should not take more than 5 minutes of hand calculation.
why not just go back to basics and find a textbook (or your university notes ?) which shows how to combine section properties?
i agree with austim on simple shapes. staad makes a product called sectionwizard for complex shapes. i hear sap makes one as well. but since staad is the industry standard, it probably would make sense to go with them. more compatibility.
hi lippie,
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.
austim has explained the process in detail.
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!
i think there is a section property calculator (with some other good free stuff) on this link
thanks to all. especially to you austim. i usually encounter simple beams or symetric combinations. this one was a mc12 with a 10x1/4-in plate and a 2x2x1/4-in angle. so i was curious. thanks for the explanation. i'll try it long way.