Wood Floor Trusses @ 12" OC?

[jandlo]

Does anyone here have experience using wood floor trusses at 12" OC? I have 26'-0" spans and the only way to get that to work with my depth restriction is 12" OC. I'm hesitant because I feel like it will make using the space for utilities very challenging - especially duct work. This is for townhouse construction.

 

[XR250]

I would not do that. Even with 12" o.c., the floor will be bouncy AF and will sag (seen this many times)
Find a way to add a midspan beam. For 26 ft, I would not use anything less than an 18" truss.

 

[Celt83]

What is your structure depth limitation, if its less than 16-18” then your likely not going to be getting through the truss panels anyway once you factor insulation thickness into the duct sizes. You may want to consider a higher stress tier I-Joist.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[jandlo]

I plan to use 18 inch deep trusses so they should be reasonable for the span. I'm just concerned about the difficulty getting utilities into the space. The owner would like to use trusses specifically for that purpose... To avoid the need for soffits.

 

[KootK]

It's rare but there is such a thing as 2 ply floor trusses. That would get you 17" clear between trusses for services.

 

[XR250]

ugh, sorry, I thought you were trying to use 12" trusses.

 

[jandlo]

I don't think I need to go to extremes to maximize the spacing between joists. I would have been comfortable with a more common 16" on Center spacing. I see plenty of 16 in deep trusses at 16 inches on Center with ductwork running in it. I'm sure 18 inch deep trusses have plenty of space but I worry that it will be difficult to get it into place or have to replace a section in the future with 12 inch on Center spacing. Maybe I'm overthinking it. I don't see much conversation about that condition online.

 

[Celt83]

See plenty of 18” trusses at around the 26’ mark at 24” or 19.2” spacing. Reach out to a local supplier they are usually pretty helpful in running a quick truss calc or two.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[XR250]

I have also seen double top and bottom chord trusses (stacked).

 

[HouseBoy]

On a recent project, the floor truss designer used flat "roof" trusses (doubled) to achieve the load capacity. Maybe triples would work at 16" spacing. IDK where the "Achilles heal" is for this design but... I aways thought floor trusses had the advantage os keeping the chords as far apart as possible but when I spoke with the designer, he said is could work with a shallower center to center and just needed to get the web connections to do more. 7" wide trusses seems awfully wide...

 

[jandlo]

I'll reach out to a manufacturer. I suspect 18" deep trusses at 16" OC for residential floor loading (40PSF) is doable, but I worry about deflection/vibration.

I'm still also very curious about 12" truss spacing and the practicality of installing ductwork. I'd be curious to know if anyone has seen this done successfully, or unsuccessfully.

 

[phamENG]

MiTek's roof and floor truss manual is a helpful resource: Link

Page 21 of the PDF has some reasonably guidelines for spans, spacings, and joist depths for various loading arrangements (TC Live/TC Dead/BC Live/BC Dead). Be warned that they are for code minimum deflection using SP #1 chords. The truss manufacturer can swap out for better/engineered lumber and make other modifications to improve performance, but they come at a cost of course.

Are you specifying strong backs? If not, you should. They help significantly in reducing vibrations.

A question for anyone who might know - is there a good method for approximating an EI for MPC trusses that includes shear deflection for vibration calculations? I haven't come across one yet.

 

[KootK]

I'd assumed that there was some exceptional loading at play here. If not, this should be fine at 16" oc.

A question for anyone who might know - is there a good method for approximating an EI for MPC trusses that includes shear deflection for vibration calculations? I haven't come across one yet.

I've been using 85% of the moment of inertia calculated based on the chords. That's not particularly scientific though and it's not sanctioned by the truss world or anything.

I'm often surprised by the reticence of EOR's to ballpark this stuff on their own, particularly when it's floor trusses which are geometrically simple. If I feel as though I might be pushing the envelope with a floor truss, I'll just calculate Sx & 0.85I based on double MSR chords on the flat and run my strength and serviceability numbers off of that. If I care about being cost conscious, it's SPF and single ply chords. I feel that's plenty adequate for preliminary design. Most any chord local bending can be resolved by adjusting the panel point spacing. And pretty much any joint can be plated, even if it takes an actual human to get that done. Or, at the least, this is the attitude of most truss suppliers.

I suppose that, given my extensive background with pre-eng wood trusses, it's difficult for me to put myself in the shoes of someone less comfortable with those products. As a child, I never would have guessed that metal plate connected truss engineering would be my lame superpower.