Bar Joist Concentrated vs. Distributed Loading 2

[StrucCyclist]

Hello all,

I am aware of the "100 pound rule" regarding concentrated loads applied to joist top or bottom chords. I have a project with proposed solar array loads being applied to a roof framed with 1.5" decking over 16k bar joists spaced 5'-0" on center. The array concentrated loads are approximately 250 pounds spread out over a 14" long by 2.4" wide foot. I'm interested in others' opinion on when a concentrated load becomes distributed enough to be considered a distributed load, particularly when it comes to a bar joist. My thought on this is if the load can be distributed via a pad support that is at least as long as the web spacing (which I don't know yet but can get) then the load could reasonably be considered a distributed load. If this was a single 6" square base I would argue that's a true point load, but a linear foot oriented parallel to the joist is much more like a uniformly distributed load that the joist should have been designed for. I have attached a simple sketch to help clarify.

Am I on the right track here? I have spoken with the joist manufacturer about this with little feedback. There's not much incentive on their part to provide any input.

https://files.engineering.com/getfi...145-4b58-9a6a-8b00826a4707&file=Bar_Joist.jpg

 

[lexpatrie]

Is this a planned structure or something that's already in place with the solar panels being added?

If it's just a potential, it would be fairly common to specify this as an added distributed load (say psf).

SEAOC has some documents on solar loading, I haven't gotten those or read them.

If snow loads govern the design for the roof in general, then something more refined for the solar array might be more common among the various engineers, or if it's one of those "reduced to 12 psf" roof live load projects.

If the solar equipment zone is in the "wrong" spot, you might encounter stress reversal at mid-span (i.e. pattern live load) on the joists and that would be a concern, whether psf or plf.

This psf/plf is fairly similar to sprinkler loads, the loads themselves are more realistically plf, but for smaller pipes they tend to be treated as psf. Larger pipes, provided you are aware of their location that "early" would put them on plan, probably. Mechanical loads like ductwork and electrical conduit runs are similar.

 

[StrucCyclist]

lexpatrie,

Thanks for the response. This is an existing structure, about 25 years old. The joist design is a TL/LL of 275/150. That's what the joists are designed for as I have access to the structure record drawings and the joist submittal drawings. Patterned live load is a good point. The load being "distributed" along a 12-18" length every 42" or so would be a discontinuous live load, but would also occur repetitively along the entire 20-foot length of the joist.

 

[BAretired]

http://www.vulcraft.com

7/31/2012

CONCENTRATED LOADS AT JOIST CHORDS
100 Pound Rule:
One of the most frequently asked questions Vulcraft receives from customers and erectors concerns joist reinforcement at concentrated loads and if field installed members are really necessary. Because of the myriad of questions, SJI conducted research and determined that loads up to 100 pounds placed between panel points have a negligible effect on the overall performance of the joist.

I am aware of the "100 pound rule" regarding concentrated loads applied to joist top or bottom chords. I have a project with proposed solar array loads being applied to a roof framed with 1.5" decking over 16k bar joists spaced 5'-0" on center. The array concentrated loads are approximately 250 pounds spread out over a 14" long by 2.4" wide foot.

Typical panel length is usually 24". I will assume that is the case here.

According to SJI 100# placed between panel points would have a negligible effect on the joist. That would be a shear of 50# and a simple span moment of 100*24/4 = 600"#.

250# spread over 14" would produce a shear of 125# and a simple span moment of 125(12-14/4)=1062"#. Your values are higher than SJI values by a factor of 2.5 and 1.77 for shear and moment respectively.

Am I on the right track here?

I don't think so!

 

[cliff234]

I have a question about the "100 pound rule". What if I weigh 250 pounds and am walking on a joist-framed roof? Will I be overloading every joist that I walk on?

 

[StrucCyclist]

BAretired: I agree the proposed 14" long foot is not going to work. The shear and moments within the panel don't compute, as you noted. My intent at this point is to tell the solar array folks that the feet supporting these arrays need to be lengthened such that the original joist design load isn't exceed. Which for this configuration is going to be about 24"+/-.

 

[StrucCyclist]

@cliff234: I asked Vulcraft this question and their response is that a load like you describe is accounted for in the design live load. Not the greatest answer.

 

[lexpatrie]

Well, I'm expecting that Cliff234 is a temporary point load. Maybe you should wear snowshoes next time Cliff, and take it easy at the buffet!

 

[BAretired]

I have a question about the "100 pound rule". What if I weigh 250 pounds and am walking on a joist-framed roof? Will I be overloading every joist that I walk on?

Not necessarily, but you could not use the SJI "100 pound rule" to justify placing a 250# point load between panel points.