• Welcome to the new and improved Building Code Forum. We appreciate you being here and hope that you are getting the information that you need concerning all codes of the building trades. This is a free forum to the public due to the generosity of the Sawhorses, Corporate Supporters and Supporters who have upgraded their accounts. If you would like to have improved access to the forum please upgrade to Sawhorse by first logging in then clicking here: Upgrades

floor joist spacing and double joists

B

bill1952

SAWHORSE
Joined
Aug 12, 2021
Messages
2,116
Location
Clayton NY
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!
 
classicT said:
I'd accept with the supporting design calculations provided for review (i.e. beam calc.).
Click to expand...
The only calculation required is that each 24" wide strip of the floor has 2 joists, so that makes 24" / 2 = 12" o.c. on average for the joist framing. That means the 12" o.c. column of the joist span tables applies directly.

There's also the question of connecting the two joists of each pair, but that is covered by Table R602.3(1) line 28 "Built-up girders and beams, 2-inch lumber layers".

Cheers, Wayne
 
wwhitney said:
The only calculation required is that each 24" wide strip of the floor has 2 joists, so that makes 24" / 2 = 12" o.c. on average for the joist framing. That means the 12" o.c. column of the joist span tables applies directly.

There's also the question of connecting the two joists of each pair, but that is covered by Table R602.3(1) line 28 "Built-up girders and beams, 2-inch lumber layers".

Cheers, Wayne
Click to expand...
By rough approximation only is your calculation correct. I understand that the load applied to each joist is equivalent; however, how a joist performs when sistered to another is not the same.

If you feel comfortable making that determination, go ahead. I would not accept it though, as it is not prescriptive IRC.
 
classicT said:
I understand that the load applied to each joist is equivalent; however, how a joist performs when sistered to another is not the same.
Click to expand...
Yes, the load applied to each joist is the same.

Care to expand on that last part about sistered joists behaving differently than single joists, even when the load per joist is the same? I'm not aware of any way sistered joists would perform worse, at least for new construction where both sistered joists are installed at the same time.

Saying the OP's proposed arrangement is outside the prescriptive IRC is a very narrow reading of the term "joist spacing" in the IRC joist span tables.

Cheers, Wayne
 
Basically what I am saying is that it is reasonable to read "Spacing" in the IRC span tables to mean "maximum tributary width per member". And as far as I know there's zero engineering downside to that interpretation.

But as I'm not an engineer I may be mistaken on the second point, so I would welcome any information to the contrary.

Cheers, Wayne
 
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!
Click to expand...
The numbers, when you run them are exactly the same, whether single joists at 12" o.c. or two joists @ 24" o.c.

The percentages for Bending Stress Y, Bearing Stress, Shear Stress Y, and Deflection Y do not change whatsoever. There may be some changes at the granular level but they are minuscule and don't change the result of normal calculations.
 
jar546 said:
The numbers, when you run them are exactly the same, whether single joists at 12" o.c. or two joists @ 24" o.c.

The percentages for Bending Stress Y, Bearing Stress, Shear Stress Y, and Deflection Y do not change whatsoever. There may be some changes at the granular level but they are minuscule and don't change the result of normal calculations.
Click to expand...
Yes. But would you accept it?
 
wwhitney said:
Given your first statement, why require anything other than the reference to the 12" o.c. table column?

Cheers, Wayne
Click to expand...
Because it is not a prescriptive design. My knowledge base and my legal authority are two different things. There is no 24" o.c. table that matches, nor can be interpolated; therefore, it is not prescriptive.
 
jar546 said:
Because it is not a prescriptive design. My knowledge base and my legal authority are two different things. There is no 24" o.c. table that matches, nor can be interpolated; therefore, it is not prescriptive.
Click to expand...
I'm really having trouble wrapping my head around this point of view that it is not prescriptive. What is wrong with reading "spacing" as "tributary width per member"? Spacing is not a defined term, as it's obvious that for this application, the way spacing matters is only in terms of load per joist.

This feels like a dumbing down of the building trade. If "A" is a clear prescriptive solution, and if with only minimal knowledge you can see that "B" is equivalent, then "B" should be allowed as prescriptive. And I guess I'm expecting plans examiners to have that minimal engineering knowledge.

If you really feel that you would need the table column heading to read "tributary width per member" before you can approve it without any thought, then you still have the option to allow the same result under R104.11, which only requires a small amount of thought (assuming that minimal engineering knowledge).

Cheers, Wayne
 
Last edited:
e hilton said:
What about the subfloor? It may be strong enough to not fail, but will it deflect too much?
Click to expand...
The subfloor is covered by R503, and for this purpose it is appropriate to consider the joist spacing 24". Makes no difference if that's a single 2x12 or double 2x10s, as far as the floor sheathing goes. (Well, double 2x10s would only give a 21" unsupported span, so it's slightly better.)

Cheers, Wayne
 
wwhitney said:
I'm really having trouble wrapping my head around this point of view that it is not prescriptive. What is wrong with reading "spacing" as "tributary width per member"? Spacing is not a defined term, as it's obvious that for this application, the way spacing matters is only in terms of load per joist.
Click to expand...

What's wrong with it is that it doesn't say that. Prescriptive means that it's plainly written, not that "In my opinion it's functionally the same as."

I happen to agree that, as long as the subfloor is rated for 24-inch spans, the performance of the joists will be the same. But it's NOT prescriptive -- it's an engineering judgment.
 
wwhitney said:
This feels like a dumbing down of the building trade. If "A" is a clear prescriptive solution, and if with only minimal knowledge you can see that "B" is equivalent, then "B" should be allowed as prescriptive. And I guess I'm expecting plans examiners to have that minimal engineering knowledge.
Click to expand...
Wayne, while your academic perspective offers valuable insights into building construction methods, it's crucial to understand the distinct legal and procedural framework within which Building Officials, Plans Examiners, and Inspectors operate. These professionals are legally bound to follow the codes, which explicitly require that non-prescriptive methods be supported by engineering approval. This isn't merely a matter of logical equivalence or theoretical understanding; it's about adhering to a structured process that ensures public safety and compliance with state statutes. State-specific training and certifications underscore this, affirming a commitment to the intent and integrity of building regulations. Building codes are complex, and what appears logically sound must also navigate the intricacies of legal and procedural mandates to ensure that all construction adheres to the highest standards of safety and regulatory compliance.
 
Building officials largely have some measure of discretion in interpreting the codes. The question is whether allowing the use of tables beyond their stated range falls within this discretion (unless code specifically prohibits extending the use). Unfortunately, these situations rarely result in the hard and fast rules we see to like most. Discretion is different for each official and applied differently for each individual situation depending on the variables at play by a single official.

Someone may have the education and experience to look at the table and understand that extending its application in this specific instance will not have a negative effect, but not all officials can do this. Nor can the official with the experience and understanding come to the conclusion that it will not have a negative effect in any circumstance.
 
So will you allow joists that meet the span for 24" o.c. to be installed at 18" o.c.? Code doesn't say 24" maximum. Or does inspection fail if one joist is 2" off (assume decking span rating is ok)?

I can't believe an RDP is required for this kind of interpretation.
 
bill1952 said:
So will you allow joists that meet the span for 24" o.c. to be installed at 18" o.c.? Code doesn't say 24" maximum. Or does inspection fail if one joist is 2" off (assume decking span rating is ok)?

I can't believe an RDP is required for this kind of interpretation.
Click to expand...
There is no prescriptive method for doubling of joists in order to increase the on center rating. Not prescriptive is not prescriptive.
 
Mr. Inspector said:
I might consider approving what bill1953 proposes if the tables would let 12" spacing of a joist be twice as long as 24" spacing does, but it doesn't.
Click to expand...

That's not an even comparison.

Reducing the joist spacing by half reduces the uniform load by half. It's tempting to think that then doubling the span just brings the load back to the same load factor. BUT ... if memory serves, the formula for bending in a beam (which is all a joist is) under uniform loading is WL^2/8, which means the bending moment increases linearly (there's that word again) with the load, but exponentially as the span increases. So, double the span, and the bending moment increases by a factor of four, not double.
 
"What would a judge say?"

This is the filter I run most decisions through. However, the codes do not speak to every circumstance and it is up to the inspector to determine what is acceptable if it varies from the prescriptive methods.

As I get further into this career I less and less frequently require an engineer to become involved in matters like these. This is not to say that I let more things slide, I more often tell the client to build it prescriptively or pay for the professional, this usually convinces them to build it prescriptively. I do not get paid enough to assume risk for somebody else's project.
 
Yankee Chronicler said:
WL^2/8, which means the bending moment increases linearly (there's that word again) with the load, but exponentially as the span increases.
Click to expand...
Terminological note: exponential means the variable is in the exponent. L^2 is a power law, not an exponential law. 2^L is an exponential law.

Cheers, Wayne
 
You must log in or register to reply here.
Top