Tributary Area and concentrated load question
Tributary Area and concentrated load question
(OP)
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?






RE: Tributary Area and concentrated load question
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.
RE: Tributary Area and concentrated load question
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!
RE: Tributary Area and concentrated load question
3500*(16/20) should be applied to frame A
and
3500*(4/16) should be applied to frame B
RE: Tributary Area and concentrated load question
3500*(16/20) should be applied to frame A
and
3500*(4/20) should be applied to frame B
RE: Tributary Area and concentrated load question
RE: Tributary Area and concentrated load question
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...
RE: Tributary Area and concentrated load question
RE: Tributary Area and concentrated load question
Mechanical equipment was never over the purlins, always attached to the frame.