Light-gage Ceiling Joists

[laff24]

I have 24 ft 800S162-50 ceiling joists interrupted by a 16" high linear diffuser. 4 ft from end of bearing. Which breaks the joist. It would be to expensive to add an additional drop ceiling. Has anyone faced this before. Pretty sure it needs to be a framed section. Please note the crude section will add a diagonal brace. Any other thoughts?

 

[hokie66]

Can't the lighting be coffered between the joists? If not, you may have to hang the cut ends of the joists from the structure above.

 

[KootK]

Can you put the joists up high but then suspend the ceiling 18" lower? You'd need the suspension apparatus but still just the one ceiling. You could also create a moment connection at the offset as your sketch suggests if the numbers could be made to work. I'd find that a pretty scary proposition though. How long does the diffuser run uninterrupted?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dhengr]

Laff24:

It never ceases to amaze me that 10's of Structural Engineering hours, a crappy design solution and thousands of dollars in extra structural work is not big deal to everyone else, when it comes to saving a few bucks on a dropped ceiling or asking the Mech. E. to think a little and come up with a workable solution which allows reasonable structuring. Back charge the Mech. for the whole mess, since they couldn’t be bothered to think a little about a solution which considered the structure. Makes my a$$ tired that everyone else should be able to do their thing the easiest (least thought involved) way, the structure be damned. Then, at whatever cost in time, materials, and forced, inferior, design solution, the Structural Engineer should fix it all, with no extra fee.

Why not the duct work above the joists, with 13-14" wide drops btwn. the joists? Then, put a 1 or 2" deep/high trough, ‘U’ shaped and opening down onto the diffuser. The drops would be attached to flanged openings in the top of the trough.

 

[XR250]

Seems like quite a span for 8", 16ga joists. What is your design live load?
You could make that funky moment connection as you show in your detail, but I would look elsewhere such as suspending things as other have mentioned.
You probably should have known about the diffuser as it was likely super obvious in some crappy BIM section :>
I completely agree with dhengr. I am in a similar situation in the world of light gage and the only reason being is that the HVAC and plumbing contractors got there crap in before the light gage guy did. If he wasn't dragging his feet, it would have been their problem.
Generally speaking, the light gage guy is at the bottom of the pecking order in the eyes of Arch's, EOR's and GC's.

 

[wannabeSE]

How long is the diffuser? 4ft? Can the joists be doubled at each end of the diffuser with headers in between. I too don't like the funky moment splice connections. For similar conditions, I have supported the ends of the joist from above as Hokie66 suggests.

Dhengr - in my experience, it is usually the Architect (not the ME) that is responsible for these conditions.

 

[dhengr]

Alternatively, just go out there with a right angle drill motor, a reciprocating saw and some tin snips; cut 2.5" x 9" holes in the sides of the ducts, and slide the CFM joists through the duct side wall holes and support them as usual, and she’s done. If you want to really be helpful print up some labels which say “pretend these are studs or wooden joists” to stick on the ducts near your holes, and some labels which say “pretend these are pipes, vents or ducts” to stick on the CFM joists near your holes. Then, just as a final note, a label or two which says “do unto others as they so often do unto you. Thanks for the cooperation and attention to detail. Your friendly Structural Engineer.”

 

[laff24]

I designed a "U" joist to the inside of the diffuser. I am using lower soffit framing from the outer face of the plenum to the wall. I am using 7.5 psf for design. The moment connection was not that bad. 6-#10 screws. To answer some other threads. 1. I blame the Architect. This never showed up in one enlarged section. I don't think is shows up in the overall cross sections. 2. The diffusers in some locations are 30 ft long. Thanks everyone for your input.

 

[hokie66]

Your original sketch showed someone walking around on the ceiling. I don't think 7.5 psf is enough live load allowance for an accessible ceiling.

 

[laff24]

Hokie66: That was me climbing up to hang mysekf (joking of course)