Stiffness of Floor Joists.

[rwalter]

Ok this might sound stupid but here goes. I designed a cabin that is 20x32 feet that I plan on building. I am planning on using 2"x12"x20' SPF lumber for the floor joists on 12" centers.  I know that that will meet the 40 Live 10 Dead load with 360L deflection requirements of the code here. My question is how stiff will that floor be? My father-in-law keeps telling me I should run a beam down the center of the cabin. Well of course I could do that but then why use 2x12's. The span table for SPF lumber on the American Wood Council's web site has got the 2x12's rated at 20 feet 7 inches and using southern SPF at 19 feet 4 inches. Now the actual span I will have will be about 19 feet since I am planning on using 6 inch concrete block for the crawl space.  So has anyone else out there done this and how stiff is your floor?

[Amanda_931]

Back in the early days of this version of the forum there was a series of Engineering....for beginners topics.  Here's part one:

http://countryplans.com/cgi-bin/yabb/YaBB.cgi?board=01;action=display;num=1105069884

If nobody comments on it it it will stay on page 12 of the archives, some more there, and a couple more, on page 11 and maybe 10.

[John Raabe]

Over that span a 1/360 deflection could allow almost 2/3 of an inch of movement. That's more than enough to rattle the china cabinet when Big Bertha is dancing.

I actually expect your design would do considerably better than 1/360, maybe 1/480.

Still, the question is what deflection is tolerable? For some people (builders who never want a call-back from fussy clients) the answer is almost none.

I live with floor joists that were built to the deflection limit of the span tables. We live right on the 3/4 subfloor (adding hardwood would stiffen the floor). There is some slight bounce when we have a party. I have noticed it three times in 24 years. No one has ever complained.

PS - in the 20' wide cottages I have designed, I have two types of floor systems: One full span with either the highest quality 2x12s or 11" I-joists (for 1/480 deflection). The other with a beam at the middle and standard 2x joists.

Several customers who used the I-joists were tentative about the choice but where happy with the outcome and feel the floor is very solid. One person also reported that they were happy with the 2x12 structure as well. I've had nobody say the floor felt too bouncy.

[rwalter]

Amanda,

I know the 2x12's will meet code and span it with out a problem and that they won't fail. What I am wondering is how much bounce if any will I feel when walking on the floors and over the years will I experience any sagging. I realize I could always put in a beam at a later date. Just want to see what other people are doing and experiencing with similar floor framing.

[rwalter]

John, thanks for the information. I had thought about using the engineered I beams but I had concerns regarding the burn through times in case of a fire. I think I will probably go with the 2x12's and just make sure they are not southern SPF. According to the span calculator that I am using at the  American Wood Council's web site the SPF grade #2 joists should be able to span the distance with the 40/10 load at L/480, (they state the span for this lumber at 19 feet 1 inch).

http://www.awc.org/technical/spantables/index.html

[JRR]

If other space factors allow it, you could beef-up the bottom edge of the joist .... say a couple of 2x4s, one on either side.  You would need "select" 2x4s .. no knots, etc.  The 2x4s don't have to be full length to be effective, as the joists aren't subjected to bending moments at their ends ... only shear.

This can be done as a corrective action "after the fact" , by using floor jacks etc.  But it would be better to glue-and-screw reinforcements in-place before the subflooring is installed.

Instead of 2x4s, you could center rip a 2x6.  Roll the cutting blade over as close to 45 deg as you can get it.   Install with pointy edge up.

[John Raabe]

Nice tip JRR.

Another combined structural/visual option would be to integrate this into the ceiling system. Use the 2x4s on each side to hold up a ceiling panel and thus make the beefed up joists into an exposed beam ceiling. Perhaps trim the bottoms with a 1x with a 3/4" reveal.