Floor Joists Code vs Engineering

Question

Floor Joists Code vs Engineering

woodman1967 (Structural)

(OP)

10 Jun 24 18:53

I have a question about the 2015 IRC on joist spans.

Table R502.3.1(2) (residential living areas LL-40psf, DL-10psf, L/delta = 360) states that for a 2"x10" @ 16" SPF #2 the max span is 15'-5". But I have an engineered calc sheet (Woodworks Sizer 11 USA) that shows the max span is 15'. The calc sheet shows a clear span of 15'. When the IRC states 15'-5" span does it mean clear span?

Just curious why the IRC allows for a slightly longer span than engineering allows. I've added a copy of the calc sheet in case there was something missed in the calculation

Thanks all

https://files.engineering.com/getfile.aspx?folder=6c2fa576-067a-42a0-82a4-d

phamENG (Structural) · Answer 1 · 2024-06-10 19:19:10

Quote (2015 NDS 3.2.1 Span of Bending Members)
For simple, continuous and cantilevered bending members, the span shall be taken as the distance from face to face of supports, plus 1/2 the required bearing length at each end.

So that's 15' 3-1/2" clear for the IRC table. What kind of span do you get with no self weight? A very basic floor assembly usually has the joist included in the 10psf. Carpet, 1/2 plywood, joists, and 1/2 gyp ceiling. Doesn't add up to much. You might squeeze out an extra couple inches.

woodman1967 (Structural) · Answer 2 · 2024-06-10 20:18:46

That's exactly what it is. Soon as the self-weight was removed the max span worked out to be 15'-3 1/2".

I agree with you on what dead load should be. Usually spec a dead load of 15psf or higher depending on the floor finish.

Thanks for spotting the issue.

lexpatrie (Structural) · Answer 3 · 2024-06-11 20:43:16

You may as well join the fun in the main forum under limits of prescriptive design...

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Floor Joists Code vs Engineering