floor joists

[dug]

I have purchased countryplans 20 by 30 1 1/2 story cottage plans and intend to use the concrete pier foundation. My concern is that the 20 ft. floor joist span seems like a long stretch. I am considering adding a 3rd beam under the joists and building an interior, load bearing wall on top of the 3rd. beam which would also support the 2nd. story loft joists. The floor plan I have designed would put the center beam 2 ft. off center leaving a span of 8 and 12 ft for the joists. I am no engineer and have little building experience but it would seem that I could use lighter lumber for the floor and loft joists (2x12 are specified) and still have a stiffer floor.
Any thoughts on this would be greatly appreciated.  

[n74tg]

I haven't seen the plans for the cottage, but is there some reason why you need an 8' and 12' span rather than two equal 10' spans.  I ask this because the house I'm building is 30' wide and has two beams giving me 10' spans exactly.  I'm using 2x10 floor joists on 24" centers and from a website that calculates floor deflections it shows my floor (L/540) to be stiffer than most codes require (L/360). 

I have the Span Tables (free, pdf files) from southernpine.com and they show that even for #2 visually graded lumber that a 2x10 floor joist on 24" spacing is good for up to a 13'-1" span (using 40 psf live load, 10 psf dead load, L/360 deflection).

A 2x8 on 24" spacing is good for up to a 11' span and a 2x6 is good for up to a 8'-6" span. 

Please do not just automatically take these numbers as being good for your application.  Take the time to download the tables yourself and read all the fine print at the bottom of each table.  Likewise, take into consideration that the building code in your area may have more stringent requirements.  Finally, these numbers are for southern pine lumber and not applicable to any other wood species. 

Good luck.   

[John_C]

Sure you could go smaller, down to 2x8 #2SYP or similar.  But how much insulation do you want to put there?  That may factor in as well.

[dug]

N74tg-

Thanks for reply. I wanted the beam off center because I would like to put an interior wall above it that would also support the 2nd. floor loft. Because of my (interior) design layout I do not want this wall centered.

John-

I agree on the insulation, would probably go with 2 by 10.

[glenn kangiser]

Span tables here to calculate for you.

http://countryplans.com/smf/index.php?topic=2729.0

[rwanders]

I vote for the third beam---uneven spans are not a problem and a stiffer floor will be sweet long after the pleasure of a few dollars less wears off. If you use appropriate sized wood I-beams a 20 foot span is also easily obtained. As one of our fearless senior contributors says: " When in doubt build it stout with something you know about."

[OldDog]

I do everything the "Country Way"

I have 3 beams for 16' span 2x8's on 16" center.

[glenn kangiser]

I have purchased countryplans 20 by 30 1 1/2 story cottage plans and intend to use the concrete pier foundation. My concern is that the 20 ft. floor joist span seems like a long stretch. I am considering adding a 3rd beam under the joists and building an interior, load bearing wall on top of the 3rd. beam which would also support the 2nd. story loft joists. The floor plan I have designed would put the center beam 2 ft. off center leaving a span of 8 and 12 ft for the joists. I am no engineer and have little building experience but it would seem that I could use lighter lumber for the floor and loft joists (2x12 are specified) and still have a stiffer floor.
Any thoughts on this would be greatly appreciated.  

Without a basement I see no problem with the first floor, and If you don't mind to post then no problem w/ the second either.