ributary area and concentrated load question

huangyhg

tributary area and concentrated load question
i have question related to tributary area and concentrated loads. if you have a concentrated rooftop load on a metal building, ac unit for example, positioned between two intermediate frames how do you allocate that load to each frame? do you use a ratio of the distance from each frame? or if it falls within the tributary area for one frame do you assign the whole concentrated load to that frame?
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some would go to each frame.
the bigger question is whether the purlins (z-shaped beams between rafters) can take the load.  these can be 8" deep and 20-some feet long--very flimsy.
i agree with comments by 271828.
if it is an existing pre-engineered metal building, there is rarely (most likely never) any additional extra capacity. if the mechanical unit is large, you may also have to include drifted snow around the unit.
good luck!
so if say the distance between frames 20' and the ac unit (3500#) is located 4' from frame a and 16' from frame b would the following be correct:
3500*(16/20) should be applied to frame a
and
3500*(4/16) should be applied to frame b
oops i meant....
3500*(16/20) should be applied to frame a
and
3500*(4/20) should be applied to frame b
that is a pretty accurate assumption.
you've got the mechanics of the situation correct; but if you're asking how to assign the load between frames, i really think you should consult with a structural engineer to get specific advise.
caveat stated, you should certainly heed 271828 and jike's warnings:  it is unlikely that the system is going to take this new load; and i would think that further bracing and blocking are likely required if it does happen to have the reserve capacity....
if you're working with unknown site material, you're going to have a very hard time working out the capacity of the purlins.  you will have to use vernier calipers to determine the material thickness (and know the age of the purlins to be confident that the zinc coating is either 180g/m^2, 275g/m^2 or 350g/m^2; which will impact the mm you need to subtract to determine the material thickness; ie: .1mm, .15mm, .2mm).  the radius of the bends is also critical, as it effects the effective widths calculations.  also cfs purlins can be produced from a wide variety of steel grades, each of which will gain different amounts of strength from cold working (what makes cfs cold formed).  be careful when working with cold formed steel, it is unforgiving.
and do not ignore shear when working with cfs.  if that comment doesn't make sense, contact a structural engineer for help.  in my opinion combined shear and bending action is likely to govern.  in the as/nzs 4600 code this is a permissive (squared sum) combined action check, and i believe it is the same in the american cold formed steel code.
good luck,
let us know how you proceed,
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...
thanks for the help guys. i actually just provided a sample scenario, because i was interested in how the loads would be distributed if you were designing the frames for a new building. i was not exposed to the tributary area concept in school since i didn't take any structural analysis classes in the mechanical discipline and wanted to make sure i understood how it worked. i was confident i understood how that concept applied to point loads and wanted to ask someone who did.
i worked putting up steel buildings for a little while.
mechanical equipment was never over the purlins, always attached to the frame.