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.

If you have rebar embedded in the concrete and they extend from one pour area to the next adjacent it can only be better than separate piers. It possibly may not be as good as a continuous pour, but it would never be worse than piers.

Version 3.3.1 - 09.27.2023
- Fixed a bug with the labeling algorithm for the molding module.

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 

Version 1.9.1 - 09.26.2022
- Added a negative X and negative Y slope direction for slab depressions.

Very useful information thanks.  Based on what I'm reading I'm inclined to totally eliminate the extra below-grade CMU and run the pipes below the footing.  This makes excavation very simple.  

The last foundation challenge is the switch from piers to perimeter footing creates much bigger challenge in volume of concrete.  I imagine most people pick piers in the first place in part because it is suitable for hand mixing in remote locations.  I'm thinking of doing sections of footing tied together with rebar (not in seismic zone D) to relieve the stress of coming up with enough volume in a short period of time.  Does anyone have any reason to believe this could be worse than doing piers, or does this sound like a dumb idea?

Our concrete footings are 18" below grade. The fresh water and waste lines are about 3 to 4 feet below grade. Power lines at 40" below grade. We use 18" as the frost depth. No unusual protections around the service lines.

Quote from: MountainDon on September 25, 2023, 08:39:03 PMAll of our plumbing lines pass below the perimeter foundation of our high desert home. Placement of the lines before any concrete is poured is key to having things in the right place after the concrete is done.

Interesting, thanks for your response.  How far underneath the footing did your lines pass?  What kind of precautions were taken around the lines?

All of our plumbing lines pass below the perimeter foundation of our high desert home. Placement of the lines before any concrete is poured is key to having things in the right place after the concrete is done.

As a followup... the masonry between footing and grade is basically only there to allow passing plumbing through without interrupting the solid cement footing.  Would be interested if anyone has experience passing plumbing BELOW the footing, as this would allow to shorten the height of masonry considerably.