4x6 floor joists??

Started by HighlandLanding, September 27, 2023, 10:46:54 AM

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HighlandLanding

Hi New Member Here! Though I've read through other threads and enjoy the content!

I am building a second floor to a barn that I have and I'd like to span 16' with 4x6 joists.  I am wondering if this is doable. They will be rough sawn true 4x6.  

The space is a 30'x16' room and I'm trying to avoid having to use center posts.  The 4x6 joists will be tied into and sitting upon 2x6 top plates.  The 30' will be split equally into two separate rooms and I'm going to use a large LVL in the wall space between the rooms (as in the LVL will span the 16' halfway through the 30').  I also thought that I could incorporate 3x5 steel I beams into the joists for added rigidity (and box them in with rough sawn lumber).  

What I'm wondering is if this is safe, will they sag, springy floor?  I am going to put 2x6 tongue and groove flooring on top.  The reason for the short 6" heigh is a space shortage above.  

I appreciate any and all insight.
Thanks,
Adam 

MountainDon

I doubt that 4x6 beams can be safely used to span 16 feet without some sag and bounce. I have not run any figures to verify that feeling, but here are some quick numbers to add to the "thinking pot".

The AWC joist calculator indicates that a Doug-Fir, #2, 2x6, 12" OC, can only span 10'9" when using the normal 40 PSF floor load and a standard L/360 deflection.  If you had perfectly clear select structural Doug-Fir the span would increase to 11'4". That would also be rather pricey, and maybe hard to obtain.

Note that a single 4x6 usually is not rated for as high a load capacity as a double 2x6 sandwich or two normally spaced 2x6 joists. Part of the reason is that a single 4x6 timber will have knots that pass clear through. OTOH, a two 2x6 pair will have the knots scattered along the length.

Perhaps 4x6 every 12" might be workable.

Steel beams with wood cosmetic cladding could work but you would need some engineering advice on that.

There is a beam calculator available at timbertoolbox.com.  That may take some learning to use but is useful. Sorry, I cannot access and use it easily on my small phone screen.
Just because something has been done and has not failed, doesn't mean it is good design.


gmcdermott

Quote from: HighlandLanding on September 27, 2023, 10:46:54 AMHi New Member Here! Though I've read through other threads and enjoy the content!

I am building a second floor to a barn that I have and I'd like to span 16' with 4x6 joists.  I am wondering if this is doable. They will be rough sawn true 4x6. 

The space is a 30'x16' room and I'm trying to avoid having to use center posts.  The 4x6 joists will be tied into and sitting upon 2x6 top plates.  The 30' will be split equally into two separate rooms and I'm going to use a large LVL in the wall space between the rooms (as in the LVL will span the 16' halfway through the 30').  I also thought that I could incorporate 3x5 steel I beams into the joists for added rigidity (and box them in with rough sawn lumber). 

What I'm wondering is if this is safe, will they sag, springy floor?  I am going to put 2x6 tongue and groove flooring on top.  The reason for the short 6" heigh is a space shortage above. 

I appreciate any and all insight.
Thanks,
Adam
I think incorporating 3x5 steel I-beams into the joists would provide additional rigidity and help to reduce deflection, but it's important to ensure that the I-beams are properly integrated into the framing system and that the loads are transferred effectively.

billy boy

I'm no expert, but I've seen similar setups before. Using an LVL and steel I beams sounds like a solid plan for support. The 4x6 joists might work, especially if they're true 4x6 and tied into the 2x6 top plates well. Just make sure everything's level and properly secured. Maybe get a local builder's opinion to be safe.

OlJarhead

I'm curious, why 4x6's?  I know a 2x8 can span 12' (I did that calc back in '09 when building my cabin and they've worked fine since) but going beyond that requires 2x12's (A DF#2 2x12 can span just over 17' with standard loading).


peacesun

Costs can vary a lot based on location, materials, and your specific plans. My advice would be to get a few local quotes from contractors—they can give you a better ballpark based on your area.

ulnaoperating

Quote from: HighlandLanding on September 27, 2023, 10:46:54 AMHi New Member Here! Though I've read through other threads and enjoy the content!
I am building a second floor to a barn that I have and I'd like to span 16' with 4x6 joists.  I am wondering if this is doable. They will be rough sawn true 4x6. 
The space is a 30'x16' room and I'm trying to avoid having to use center posts.  The 4x6 joists will be tied into and sitting upon 2x6 top plates.  The 30' will be split equally into two separate rooms and I'm going to use a large LVL in the wall space between the rooms (as in the LVL will span the 16' halfway through the 30').  I also thought that I could incorporate 3x5 steel I beams into the joists for added rigidity (and box them in with rough sawn lumber). 
What I'm wondering is if this is safe, will they sag, springy floor?  I am going to put 2x6 tongue and groove flooring on top.  The reason for the short 6" heigh is a space shortage above. 
I appreciate any and all insight.
Thanks,
Adam
Consult a structural engineer or a competent contractor to assess whether this setup is technically sound and will prevent drooping or bouncy floors. They will be able to evaluate the load-bearing capacity of the 4x6 joists, their connection to the 2x6 top plates, the use of LVL beams, and the inclusion of steel I beams for increased rigidity.