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According to the IRC, a 1200S162-68 cold-formed steel C-section can hold 40psf total load at an almost 22' span when used as a floor joist, but can only hold 25psf total load at a 21' span when used as a ceiling joist.

Why is this? Black magic? If a structural component is used as a ceiling joist it is magically some % weaker than if an identical one was used as a floor joist? I'm scratching my brain at this one.

Furthermore, why would a joist supporting an attic floor not be considered a floor joist (especially if it were a habitable attic)? A piece of metal should have the same mechanical properties whether it is on the ground floor or 8' above. enter image description here

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    $\begingroup$ It looks like you have two accounts. You should read this to get it fixed: engineering.stackexchange.com/help/merging-accounts $\endgroup$
    – hazzey
    Nov 10, 2021 at 22:59
  • $\begingroup$ While the physical material does not change, factor of safety (or any other term you wish to call the approximation fudge to help account for wear, usage, etc) upon it does. $\endgroup$
    – Abel
    Nov 11, 2021 at 13:14
  • $\begingroup$ attic spaces may not be considered to have a floor. As a result you have a difference in unsupported length. Notice your ceiling joist table states unsupported, mid span and thirds. Meanwhile your floor table give no such bracing condition and may be considered continuously supported due to the flooring system that is eventually put in place? $\endgroup$
    – Forward Ed
    Apr 10 at 17:38

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I think the reason is that the attic allows for additional storage, for which the limit is to be calculated but not stipulated in the code. Let's check to see what is the implied allowance for storage load.

Since the permissible total load deflection is the same for both cases, let's set and equate the deflection equations:

$\Delta = \dfrac{5w_{flr}L_{flr}^4}{384EI} = \dfrac{5w_{cel}L_{cel}^4}{384EI}$

For 24" joist spacing:

$w_{flr}$ = (30 + 10)*2 = 80 plf; $L_{flr}$ = 21.67'

$w_{cel}$ = ? ; $L{cel}$ = 21'

After canceling the identical parameters on both sides, the equation of deflection become:

$w_{flr}L_{flr}^4 = w_{cel}L{cel}^4$

$w_{cel} = \dfrac{w_{flr}L_{flr}^4}{L{cel}^4} = \dfrac{80*21.67^4}{21^4} = 90 plf$

The total allowable uniform load of the ceiling joist $W = w/spacing = 90/2 = 45 psf$

The maximum permissible $storage load = 45 - 20(L) -5(D) = 20 psf.$

Conclusion: The reason for the code to recommend a shorter span for the ceiling joist, which has a lower specified live load and dead load compared to the floor joist, is to account for the anticipated weight of the "attic storage". After all, the advantage is tilting the ceiling joist for the clear bracing requirement in the table, and probably the stationary nature of the storage load. Nevertheless, the joist at both applications must satisfy the stress check and the same set of deflection criteria in the end.

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  • $\begingroup$ Your storage load would be a live load and in this case the storage is 20 as listed in the table header is it not? If I read the final line it appears you are saying the live load and the storage load are two separate loads which are both 20. I was wondering if it was more of a case of lateral bracing. Drywall is not considered structural support, so your ceiling joist are only braced at the transverse tie locations. In the presented table this is 1/3 points. A floor on the other hand "might" be considered continuously braced. $\endgroup$
    – Forward Ed
    Apr 10 at 17:23
  • $\begingroup$ The simple thing here is to back-check the allowable load for the span in question, which turns out to be 45 psf. From this, less the code specified 20 PSF live load and 5 psf dead load, here we found the 20 psf extra capacity than answers the OP's question why the same size ceiling joist and floor joist have different allowable span length as shown in the table. $\endgroup$
    – r13
    Apr 11 at 1:35
  • $\begingroup$ Thanks for letting me know about the editing error. I've corrected it this time, but in the future, you can too, The OP will be informed of your "edit", then decide to accept or reject the edit. $\endgroup$
    – r13
    Apr 11 at 1:41
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    $\begingroup$ Actually until you have edit permissions, you need to suggest edits and those edits have a minimum 6 character requirement. Only author and those with enough rep for edit permissions can do smaller than 6 character edits. $\endgroup$
    – Forward Ed
    Apr 11 at 1:47

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