Steel Joist - Double up, KCS, or Diagrams for RTU unknown locations

[BurgoEng]

I have been given a roof design to check that has 16K2 joists specified, but this design did not account for the placement of HVAC RTU's. I am now being contacted by the fabricator asking if we will be supplying loading diagrams for the joists that may be effected by the RTU's. The problem is that the RTU locations have not been set yet, as they keep changing with the architectural layout of the building. The options I see are as follows...

#1 Have the fabricator provide KCS joists in the "assumed" RTU locations so that the "actual" location does not effect design.
#2 Double-up (side to side) the joists at RTU locations so that the load carrying capacity at this location is doubled and tie the joists directly together via welding plates, etc...
#3 give the joist manufacturer an "assumed" loading diagram with and envelope of possible locations for the RTU's and have them design the joists for worst case within the envelope.

In the past, I have seen my company just have the joists doubled at the RTU locations, but I am not convinced that is the "right" thing to do, and being that I am new to this, I'd like to know the more correct thing to do. Showing KCS joists is the quickest fix but may be overkill? Diagrams are probably more "right" to give, but could be way off when it comes time for construction, especially with actual size and location of RTU not known.

Thoughts, suggestions???? I know this was a long post.

 

[UcfSE]

If you have the maximum operating weights of the RTU's, I would use a KCS joist and locate the RTU for maximum shear and maximum moment and design accordingly. I also usually apply a 1.33 load increase to account for things like uncertainty and dynamic affects. Hopefully the changes occuring for the RTU locations will happen along the joist length so this procedure works, instead of moving perpendicular to the joists. You will need to note the maximum weight of the RTU's on your drawings also.

I wouldn't bother with a load diagram unless you know the exact location of the RTU and it is finished changing. A KCS provides a little more flexibility than the load diagram would in the event the RTU changes again later.

If you don't know the weights or locations of the RTU's there doesn't seem to be much point in proceeding until you do.

 

[archeng59]

agree with using KCS joists.

 

[SperlingPE]

I have specified KCS type joists as stated above. I have also indicated on the drawings that the supplier should provide an x amount of additional joists to be located in the field for RTU's. I have also just indicated an area on the plans where mechanical units are to be placed. This area has joists of an appropriate size or are doubled up.