built up sections

huangyhg

built up sections
when calculating a built-up section, what is the best way to interpret table b4.1 (13 ed.) b5.1 in the (9th ed)?  i am checking the application of angles as stiffeners and am having difficulty determining my final section modulus.  the angle is a 3x2x1/4 (a-36) and the housing it is welded to is 12 ga. 304 stainless steel.  using the 9th ed. i calc'd the housing contribution to be around 1.5". do i include my weld size also?  is it correct to align the center of mass of the 1.5" with the center of mass of the welded angle?   
can you post a sketch?
no, you don't count the weld.
i'm trying to picture what you are doing and like structuraleit, i'm not getting it.
if it's a complex built up section, draw it in autocad near the origin.  
convert all of the lines to closed polylines.
convert all of the closed polylines to regions.
use the command "massprop" and select all of the regions.
note where it tells you the center of mass is.
move the center of mass of your built-up section to the origin (0,0).
use the massprop command again and select all of the regions.
it should give you everything you need, or a means to calculate it by.

i tried to post a sketch...office protocols limit what i can do on the internet.  basically, what the end result would look like would be a j shape.  however, the major  issue is the interpretation of aisc code.  i assumed that the resulting 1.5" would be centered over the leg of the welded angle, which gives a small section modulus.   
i'm still having a hard time understanding your question.  the section modulus is what it is based on the geometry of the section and the material properties.  table b4.1 is for local buckling of parts in compression.  i guess i'm not sure what the section modulus has to do with table b4.1.
you say you calc'd the housing contribution to be 1.5".  1.5" of what?
i think a light either just came on, or, i blew a fuse.
you're doing a duct or something similar.  the "stiffener" is stiffening up the wall (12 ga ss) of the duct.
if this is the case, it's not really a "stiffener" so to speak.  it's a beam that is supporting the pressure generated inside your duct.  even if it's a rectangle, it's acting more like a tension ring.  think of it as putting a rubber band around a balloon, and blowing it up.
design the angle w/o any consideration of a built up section, for the bending forces coming from the pressure of the duct walls, combined with the tension forces coming from the "vertical" reactions of the angles at each end.  the welds from the angle to the duct, braces the compression portion of your angle.
chip8,
yes, that is what i am doing.  however, i have limits imposed by the owner, height and weight. so, i'd like to take into account as much as possible.   
the direction of the loading in the duct, is perpendicular to your angle.  you're basically asking those forces to make a right turn when they reach the angle, along with distributing them to the angle.
i understand what you are saying, but, it would not be good engineering judgement to take that into account.  use two-way distribution.  you've got a much more rigid section at the top and bottom of the duct (top and bottom duct walls).
thanks for the information.