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floor joist spacing and double joists

I'm under the impression that IRC lets the code users do simple calculations without requiring an RDP.

For example R507.3.1 on the minimum footing size for decks requires us to calculate the tributary area for sizing a footing.

If we can be trusted to do that without an RDP, we can be trusted to recognize that the floor joist span tables are based on load per member.

Cheers, Wayne
 
wwhitney said:
I'm under the impression that IRC lets the code users do simple calculations without requiring an RDP.

For example R507.3.1 on the minimum footing size for decks requires us to calculate the tributary area for sizing a footing.

If we can be trusted to do that without an RDP, we can be trusted to recognize that the floor joist span tables are based on load per member.

Cheers, Wayne
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RDP requirements are largely a matter of state law. See IRC R106.1.

Note that the final sentence (in orange) that precedes the exception is the basis by which many BO and Plans Examiners will require the use of an RDP for non-prescriptive construction for structures that are typically exempt under statutes governing RDP. (For Mark)

2021 IRC - R106.1 Submittal Documents​

Submittal documents consisting of construction documents, and other data shall be submitted in two or more sets, or in a digital format where allowed by the building official, with each application for a permit. The construction documents shall be prepared by a registered design professional where required by the statutes of the jurisdiction in which the project is to be constructed. Where special conditions exist, the building official is authorized to require additional construction documents to be prepared by a registered design professional.
Exception: The building official is authorized to waive the submission of construction documents and other data not required to be prepared by a registered design professional if it is found that the nature of the work applied for is such that reviewing of construction documents is not necessary to obtain compliance with this code.
 
tmurray said:
You've just summoned Mark K to come and say that the codes do not dictate when an RDP is required, state legislation does.
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Oh, I know I did... haha. But I (and many others) live in a State where the Building Codes are adopted by law, so in that case, the codes do dictate that the BO has the authority to require a RDP.
 
bill1952 said:
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!
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What is provided should be what the designer specified.

With respect to strength they are the same but there may be other considerations.

The comment on weak spots is an attempt to rationalize a position. If the members were properly graded experience suggests that we need not be concerned about potential weak spots.
 
classicT said:
Oh, I know I did... haha. But I (and many others) live in a State where the Building Codes are adopted by law, so in that case, the codes do dictate that the BO has the authority to require a RDP.
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I live in a state that regulates the practice of engineering and that also adopts a state building code. This does not give the body empowered to adopt building codes the power to regulate the practice of engineering. Consult with an attorney.
 
Mark K said:
I live in a state that regulates the practice of engineering and that also adopts a state building code. This does not give the body empowered to adopt building codes the power to regulate the practice of engineering. Consult with an attorney.
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Called it! ;) I knew you'd show up. Haha!

I have had the legal guidance, and because both are adopted via legislative statute, both apply. AHJ (BO) has the ability to require the use of an RDP via code. Basically, the provisions are not exclusive of each other. It is an either or type situation. The requirement for an RDP can be due to regulations pertaining to the practice of engineering/architecture or as required under the adopted code or both.
 
classicT said:
Oh, I know I did... haha. But I (and many others) live in a State where the Building Codes are adopted by law, so in that case, the codes do dictate that the BO has the authority to require a RDP.
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But the code says construction documents shall be prepared by a registered design professional " where required by the statutes of the jurisdiction in which the project is to be constructed." In my state, there is an exemption to the practice of architecture for most buildings under 5,000 square feet of total floor area and for one- and two-family dwellings. There is NO comparable exemption for the practice of engineering. If something isn't spelled out [prescribed] in the code, it requires engineering, and that means it MUST be done by a licensed P.E.
 
Yankee Chronicler said:
But the code says construction documents shall be prepared by a registered design professional " where required by the statutes of the jurisdiction in which the project is to be constructed." In my state, there is an exemption to the practice of architecture for most buildings under 5,000 square feet of total floor area and for one- and two-family dwellings. There is NO comparable exemption for the practice of engineering. If something isn't spelled out [prescribed] in the code, it requires engineering, and that means it MUST be done by a licensed P.E.
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Agreed, mostly anyways. That was the point I was making. The code official has the authority to require design by a RDP for non-prescriptive designs.
classicT said:
Where special conditions exist, the building official is authorized to require additional construction documents to be prepared by a registered design professional.
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That said, the code official is not required to do such. They are authorized to.

Basically, in the context of this thread, I as a code official could require the submission of a beam calculation that demonstrates doubled-2x10 at 24"oc are structurally acceptable. Given that the member is non-prescriptive, I would be within the confines of R106.1 to require an RDP.

However, I as a code official, I could also choose to accept the beam calculation output (based upon review of the calculation and approval of the program utilized) without said calculation being prepared by a RDP. For example, if prepared by the local lumber supply house via a recognized calculator, I could accept the non-prescriptive design. The design being justified and those who performed the calculation doing so under an exemption that does not require professional registration as a RDP.
 
It would seem if you don't allow the doubled joist/spacing I proposed without an RDP, you could not allow an LVL without an RDP. At least I don't think the exact size and spacing of an LVL is prescribed in the IRC.
 
bill1952 said:
It would seem if you don't allow the doubled joist/spacing I proposed without an RDP, you could not allow an LVL without an RDP. At least I don't think the exact size and spacing of an LVL is prescribed in the IRC.
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I'd accept a beam calculation or a manufacturer's span tables.
 
I don't see the difference of calculating the tributary area to come up with a plf for the lvl and using a span table versus using sawn lumber span tables the same way.

The IRC allows you to use 1 1/2" t&g decking on "joists or beams" up to 5' o.c. prescriptively but not span tables for that.
 
bill1952 said:
The IRC allows you to use 1 1/2" t&g decking on "joists or beams" up to 5' o.c. prescriptively but not span tables for that.
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You're referring to Table R503.1? That IS a span table. The spans are 16", 24", 48", 54", and 60". That table is about as prescriptive as you can get. It tells you ("prescribes") the maximum span of lumber floor sheathing. How can you say there's no span table for that?

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OK, here's what I've got for the naysayers:

2021 IRC R502.3 second sentence says "For other grades and species and for other loading conditions, refer to the AWC STJR." That's the American Wood Council's 2021 Span Table for Joists and Rafters, available here: https://awc.org/codes-and-standards/span-tables/

Appendix A of that document explains the methodology of the span tables and indicates that the allowable span of a member is based on its uniform load w, which depends on, for deflection, "live load and joist spacing," and for strength, "dead plus live load and joist spacing." It is clear from this that the uniform load w is calculated based on the load per joist.

Further, Example 3 of the Appendix discusses how to use the joist span tables to deal with a corridor with 80 psf live load requirement, by finding a solution based on 40 psf live load, and then using twice as many joists. This again shows that what matters is the loading per joist.

Based on the above examples, when using double joists, applying the span table based on half the actual spacing of the joist pairs is within the purview of what the IRC allows.

Cheers, Wayne
 
Basically, it should go like this:

Applicant: These joists are sized based on 12" o.c. spacing, but for ease of installation and to make room for utilities in the joist bays, they will be installed in pairs, with each pair 24" o.c.

Plans examiner: I don't see a span table for double joists in R503.

Applicant: OK, but members are sized based on the load per member. For both single joists at 12" o.c., and double joists at 24" o.c. per pair, each individual joist carries the load of the same amount of the floor area, a 12" wide strip. So the necessary joist size is identical for the two cases. "Spacing" in the span tables just means "tributary width."

Plans examiner: OK, that makes sense.

Cheers, Wayne
 
Yankee Chronicler said:
You're referring to Table R503.1? That IS a span table. The spans are 16", 24", 48", 54", and 60". That table is about as prescriptive as you can get. It tells you ("prescribes") the maximum span of lumber floor sheathing. How can you say there's no span table for that?

View attachment 12568
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Span tables for joists or beams on up to 60" centers is what isn't there, except as Wayne points out. I know 3 joists from the 16" o.c. table will work fine when sistered at 48" o.c. Actually a 6 by would be better.

Mark K said:
The comment on weak spots is an attempt to rationalize a position. If the members were properly graded experience suggests that we need not be concerned about potential weak spots.
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I've read engineers make this argument, though for ganging roof trusses. I'd admire your faith in the grading system. In NYS the IRC is amended to permit ungraded lumber. Hurrah for the wood lot owners lobby. And also in NYS, RDPs at or above 1500 SF. Hurrah for the RFP lobby. Too bad you can't find one for small or limited work.
 
bill1952 said:
Span tables for joists or beams on up to 60" centers is what isn't there, except as Wayne points out. I know 3 joists from the 16" o.c. table will work fine when sistered at 48" o.c. Actually a 6 by would be better.
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Table R503.1 has nothing to do with the joists or the joist spans. It only addresses the floor sheathing. It doesn't address how joists spaced 60 inches apart get engineered (or not engineered).
 
Yankee Chronicler said:
Table R503.1 has nothing to do with the joists or the joist spans. It only addresses the floor sheathing. It doesn't address how joists spaced 60 inches apart get engineered (or not engineered).
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That was my question. They show joists on 60" centers. How is that done prescriptively?
 
bill1952 said:
That was my question. They show joists on 60" centers. How is that done prescriptively?
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Who says it has to be done prescriptively? The table shows that 1-1/2" lumber can span up to 60 inches when used as floor sheathing. The fact that 2x6's can be used as floor sheathing prescriptively doesn't say or even imply that joists can be spaced 60 inches on center prescriptively.
 
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