Uninhabitable Attic - In an Industrial Building

[phamENG]

Curious about how others here might approach this situation. Working on a project with an existing industrial warehouse. We're going to hang some insulated panels from the roof. Clearance between the panels and the roof will vary from just enough to get a hand tool in to about 7ft. No access unless you get on a lift (panels will be about 20ft above the floor). Thought is to post signs saying no storage, just in case somebody gets the idea to stick something up there.

The panels are theoretically capable of supporting a 20psf live load for "storage areas above ceilings" in ASCE 7-16 Table 4.3-1. The roof structure they're hanging from...not so much. Since this is NOT a storage area...what would you all do? "Scuttles, skylight ribs, and accessible ceilings" only calls for a 200lb point load. If this were new construction, I'd go for the 20psf all day for simplicity, resilience, and options for future use. But for a retrofit...I don't want to make them rebuild a perfectly good roof if it isn't absolutely necessary.

I'm leaning toward a worst case 10psf/200lb point load.

 

[MotorCity]

If you are not going to require rebuilding or reinforcement of the roof, aren't you limited by the capacity of the existing roof? What can the existing roof handle? Since you can prohibit storage, can you also prohibit people from accessing that space? Do the panels need to be "walkable"? I could get behind the 200 lb load if you are going the accessible ceiling route. Not sure how I feel about the 10 psf.....seems like an arbitrary load.

 

[phamENG]

MotorCity - some reinforcement of the roof may be required, and that's fine. But a full 20psf live load and the "reinforcement" would turn into a new roof structure. So it's a pencil sharpening exercise - make sure I'm not being too conservative but also ensuring safety of occupants.

It's not going to a place somebody can wonder into. Like I said - it's 20ft off the floor with no stair or ladder. If you don't go find a lift (or ride a forklift fork to the top and jump?) you're not getting up there. A cage may be doable, but given the difficulty of access I don't think it's necessary. So I'm considering it "accessible" because you can walk on it.

10psf is commonly used in uninhabitable attics without storage in residential design. So in this usage, sure...a little arbitrary. Just trying to bring some context to loading assumptions and my thought process. Applying a uniform load would help give me a little more comfort in the analysis of the existing structural system. If I assume there's a 200lb point load at each hanger, it comes out to about 3.5psf. That's about what the reduced live load is if I use L_o=10psf for the primary structure (ignoring the 0.5L limit). So it seemed like a reasonable number to use in the absence of more direct guidance from the code.

Or am I overthinking this? Just put a 200lb point load at one connection point per member and be done with it? I like that idea, but it feels like I'm missing something...

 

[MotorCity]

Your thought process sounds reasonable. If the roof can take 10 psf and 200 lb point load, I would lean towards that. If you only specify a point load, that may give the layman a false impression thats all its good for. A 10 psf uniform load says "Its good for a couple of people scattered about on a rare occasion, but lets not have the office party up here".

 

[driftLimiter]

You could prepare a special acknowledgement where the owner acknowledges the limits of your proposed construction and agrees to permanently post signage. Certainly would want to cover construction worker live loading if it is walkable at all. I always get a bit spooked when you have stuff hanging overhead, it could be potentially a really bad failure. In the same token tho, if someone tried to walk on a normal T-bar ceiling they would certainly fail it and it wouldn't be the designers fault.

 

[phamENG]

Thank you both. I'll continue down this path. The special acknowledgement is a good idea. The architect has already agreed and I believe he's already working on something similar. The owner will agree to just about anything that reduces up front costs, so the immediate concern is making sure the plan reviewer will approve it. Around here commercial/industrial plan review tends to be just thorough enough to be annoying but not thorough enough to comprehend reasoning (or catch legitimate errors if they're there).

 

[Once20036]

There's a lot of ways to justify this, but generally speaking I`m OK using a single 200# point load in an existing building, if there's no way to access the space and no reason that a person would ever need to make the effort. This sounds a lot like your situation.

I design my roofs for 16psf wind pressure (as required by the C&C chapter). IBC 15 eq`n 16-3 often controls the designs. It's easy to say that the ceiling is rated for 16psf because that load combo considers 16psf wind OR 16psf ceiling load.

In situations where an existing roof hasn't previously considered wind pressure, it may still be eq`n 16-3 that controls.
In this scenario, the ceiling would safely support 20psf as long as the design snow load/roof live load don't occur concurrently. For an area that's inaccessible, its unlikely that these ceiling loads would occur at the same time as a winter blizzard, or the same time as a re-roof project. I`ve even see some people write a letter to the owner stating that a very accessible ceiling wasn't to be used if there's snow on the roof.

Lastly, I like to refer to ASCE 37 to understand the 20psf load. It says that 20psf is a couple people with light hand tools. It wouldn't be reasonable to have a large area loaded at 20psf, while also considering that ASCE description of 20psf. You`d have a couple hundred pounds in the immediate vicinity of a hanger, but its tough to justify across an entire ceiling structure.

It's important that the ceiling system is designed for 20psf, for the safety of person who will inevitably be up there in the future.
Maybe that will be a structural engineer measuring joists for a proposed HVAC unit.

 

[JLNJ]

While you are sharpening your pencil, make sure you take full advantage of live load reduction(s). The odds of having both roofs loaded with significant live load is incredibly small.

 

[phamENG]

Once - thanks for your insight. Are you saying the ceiling system should be designed for 20psf, but the individual hangers should be designed for the 200lbs? So, in essence, the ceiling system will be fine regardless of the location of the worker and the hanger will be okay when the worker is standing at the hanger? (sort of an extreme live load reduction) That's the current course - the manufacturer of the ceiling system uses 20psf LL as a standard. I'm working on figuring out the best (read: realistic and compliant) application of the load to the existing structure.

JLNJ - thanks. LLR was the first thing I did.

 

[Once20036]

Pham,
Close - I do all of the new construction for 20psf LL - the panels, the tees, the hangers, and the attachment of the hanger to the structure (keep a close eye on this last one).
I switch to a 200# criteria only when looking at existing structure that will be expensive to strengthen.
I clearly list the 200# design criteria in the design criteria notes for the code official. Thus far, nobody has ever objected.
I also include the owner and project manager in the decision making process.

To reiterate, I`m considering a limited application of inaccessible new ceilings in an existing structure. These tend to be installed for a sanitary or thermal purpose.

 

[phamENG]

That's what I meant, though I didn't spell it out as succinctly as you did. That condition is exactly what we have here. Thanks again.