floor joist spacing and double joists

[jfmann77]

Design capacity of (2) joists connected together to form unified joist assembly...... and spaced at 24 inches....... is same as single joists at 12 inches.
However, technically.......since doubled sawn-lumber floor joists are not listed in prescriptive tables..........such design is not "prescriptive"....... so that engineered design is necessary and should be obtained. This issue is of greater importance when multiple I-joists are grouped together to form unified joist assembly.
Key issues to be considered.......which must be addressed by proper design & construction;
(1) Whether load will always be applied to top of both joists such that joists will act together........irrespective of how joists are connected together. If not, then method of connection becomes more important.
(2) Support requirements at each end of double-joist. Any hangers used must have adequate nailing for design shear force at end of joist.
(3) Although relatively minor; Effects of greater load........from ends of doubled-joists..........that might occur for other elements, such as shallow header beam.
(4) Design capacity of floor sheathing for greater spacing of floor joists.

 

[Inspector Gadget]

Would you accept doubled joists on 24" centers to be equal to the same joists on 12" centers? Obviously decking would have to be rated for the span. My education and experience says the doubled is actually better, based on the improbability of both joists having a weak spot in same place.

Thank you!

Is it in the book as a prescriptive solution?

Simple answer. If no, then no. The issue isn't load, the issue is deflection.

 

[Yankee Chronicler]

Is it in the book as a prescriptive solution?

Simple answer. If no, then no. The issue isn't load, the issue is deflection.

Deflection of the joists is exactly the same as the deflection of joists spaced 12 inches on center. Any deflection issue is in the subfloor/floor assembly itself, which is spanning 24 inches (actually 20.5 inches) rather than 13.5 or 8.5 inches.

 

[bill1952]

Probably just not finding it but where in IRC does it permit spacers - like the commonly used 1/2 ply - between multiple member headers? The nailing tables and the figures all suggest no spacer to me. And if the spacer is allowed, where does it say maximum thickness and how does that affect nailing? Any reason I can't use a 4 1/4" spacer for a prescriptive double 2x10 header in a staggered stud wall on 2x8 plates? Is it even important that a double 2x10 header be fastened together? It's in nailing schedule so if you fasten them together, it say how, but no requirement they be fastened.

 

[bill1952]

Here is an example. Is it prescriptive compliant with IRC if the double two-by is from the table for the given span?

 

[jfmann77]

As long as double joist-members act together......with each member having thickness of 1-1/2 inches...... double joists then act as unified joist-assembly having thickness of 3-inches. Joists separated by gap, with or without fillers, will act as such "assembly" only if load is applied to top of both joists via flat board or other element having adequate stiffness to spread load to both single-joists evenly.
Design capacity of any structural element, including beam, consists of (1) Strength, which governs stress, and (2) Stiffness, which governs deflection.

Other issue to consider for increased spacing of joists is support for interior walls;
(1) Wall running across joists. If only single wall base plate.........and if in-wall column is about midway between floor joists........then there is potential for problem due to excessive bending of wall base plate spanning between floor joists spaced at 24 inches. This could be much more severe if base plate has joint between floor joists.
(2) Wall running parallel to joists, on floor sheathing, about midway between floor joists.......especially if such wall might be supporting upper floor framing members.

 

[steveray]

Is it even important that a double 2x10 header be fastened together? It's in nailing schedule so if you fasten them together, it say how, but no requirement they be fastened.

If it is top loaded...no....Not really...

 

[Inspector Gadget]

Deflection of the joists is exactly the same as the deflection of joists spaced 12 inches on center. Any deflection issue is in the subfloor/floor assembly itself, which is spanning 24 inches (actually 20.5 inches) rather than 13.5 or 8.5 inches.

That's what I meant. Shoulda been clearer.