beam loading help

huangyhg

beam loading help
i haven't done this in years so i am hoping someone will refresh my memory. i need to find the maximum weight that can be handled safely on structural tubing (ansi a500) class b - 2 x 2 x 16 ga wall. the weight will be uniformly distributed along a 48" span.  the supports will be fixed at either end.  i know this seems simple but i just want to make sure i am doing this correctly.  thanks in advance
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if its just strength you are looking for, do the following:
1) determine sx of your section.
2) determine allowable stress (fy) of steel. normally .66 fy therfore you have (0.66 x 50)=33 ksi.
3) determine allowable moment of your section.
mmax= fa (in psi) * sx (in in 3)/ 12
4) mmax= w* l * l/8
where l is your span (4'-0). this will give you in pounds per foot.
hope this helps.
geoff
double check that you have the right tubing spec- i didn't realize it went down to anything that small and light.  also, check to make sure it qualifies as compact, or whatever the criteria is (ie, not too thin-walled relative to width).
possibly also check for crippling of the webs and shear issues at the ends.  if this is a critical   
a500 class b for a square tube: fy=46ksi, not 50 ksi.
the maximum moment for the fix-fix condition (which occurs at the ends) is =wl^2/12 (not wl^2/8).  
i prefer lrfd method instead of asd, so i would take .9(fy)sx for the nominal moment allowed and compare it to factored max moment.  then solve for sx.  this only works if the section compact, and i think in your case, 48" is too long and the walls are too thin, therefore the section is probably non-compact.
no solve for sx, solve for w.  whoops, i'm thinking design, not analysis.