OWSJ CISC Provisions

[EngDM]

Hey all,

There is a portion of clause 16.5.6.3 for top chords that is a bit unclear to me. There is a sentence that reads "For top chords with panel lengths not exceeding 610mm, Mf resulting from any uniformly distributed loading may be neglected". However clause 16.5.6.1 b) states that the top chord has to be designed for moment when a concentrated load is applied between panel points. I guess my question is, when you are analyzing a roof truss that was originally designed under the assumption that moment can be neglected for a new RTU or other point load, do you only consider the Mf resulting from the point load and ignore the UDL as clause 16.5.6.3 suggests? If the joist was originally spec'd to 95%-99% as is typical for joist manufacturers while neglecting moment, then any load I apply instantly fails the OWSJ since now I have to consider moment, even if the moment changes by <1%. I'm having a hard time justifying the need for the amount of reinforcing this analysis requires; I'm adding a 400lb unit located 3" from a panel point so it induces next to no moment.

 

[jayrod12]

Can't you just relocate the point load? Or provide an angle strut from directly below the point load, down to the closest bottom chord panel point? That's what I always do.

 

[EngDM]

Can't you just relocate the point load? Or provide an angle strut from directly below the point load, down to the closest bottom chord panel point? That's what I always do.

Owner doesn't want to open up suites to provide a single angle strut. The unit fits between two panel points leaving 3" to each if the unit is centered. If I move the unit then the strut is for sure how we would do it.

I've read some other forums that say if the load is under X and withing 4-6" (sources vary) then you don't need a strut, but I can't find an actual source to how those numbers were obtained.

From my OP, the way the code reads is unclear of if you can ignore the UDL Mf as long as panel points are 610mm, even in the presence of concentrated loads.

 

[jayrod12]

I'd be hesitant to ignore the udl while accounting for the pt load.

What about just building your sleeper curb on the roof large enough and stiff enough to span to the panel points directly?

 

[EngDM]

I'd be hesitant to ignore the udl while accounting for the pt load.

I agree. I'll see what I can find in SJI, there appears to be some guidance but to what extent is is applicable I have yet to discover. Sleeper curbs might be the way to go.

 

[Once20036]

SJI previously had a similar provision that allowed you to ignore flexure from the roof load where your top chord panel points were 2' apart or less.
That provision has since been removed from their code and a flexural check is required for all joists.
I agree with JRod. Once you`re checking for flexure, I`d check for all of the flexure and not limit it to flexure from the point load.

The small eccentricity allowed (often 4" for K joists, 6" for LH joists) isn't codified anywhere. I`ve seen it in typical details from each of the major joist manufacturers and I don't hesitate to use it on my jobs.

 

[EngDM]

SJI previously had a similar provision that allowed you to ignore flexure from the roof load where your top chord panel points were 2' apart or less.
That provision has since been removed from their code and a flexural check is required for all joists.
I agree with JRod. Once you`re checking for flexure, I`d check for all of the flexure and not limit it to flexure from the point load.

Wonder if CISC will adopt that flexure check revision any time soon.

Anyways, thanks for the input.