Floor Joist Beam Spans

[RicodJour]

Code is the minimum acceptable construction. Why would anyone want to build something that failed to meet such a low standard?

R

[HerHusband]

James,

A few years back we built a 24'x28' garage. I wanted a completely open lower level, and a fully usable attic (i.e. No truss webs). I never expected to put any real weight in the attic.

I used regular #2 Douglas Fir, 2x12 joists at 16" OC to span the 24' width of the garage. I added solid 2x12 blocking every 8 feet along the span. I used 2x6 rafters at an 8/12 pitch and ended up with about 7 feet of headroom running down the center of the attic.

Yes, with a 24' span the attic floor is a bit bouncy, but not nearly as much as I expected. If you stand in one place and bounce up and down you can feel it, but it's not really detectable in normal walking. Still, I wouldn't want to store an engine block or a library of books up there.

Mostly, we just use the attic to store items like long PVC pipes, empty stereo/TV boxes, holiday decorations, etc. But, I do have a few heavier items up there too.

I could have easily strengthened the floor by doubling up joists or going to 12" spacing.

We used 2x10 douglas fir joists at 16" OC in some parts of our house. That's close to the maximum span for doug fir, but the floors still feel quite solid.

You could use wood I-joists and easily span 24' or more. They're light, straight, and about the same price as 2x12's around here.

Alternatively, you may want to check with a truss company for "attic trusses". These can easily span 24' and still have a usable room in the middle of the attic. I decided against these because of cost, and because I was mostly working alone. It would have been very difficult to install trusses by myself.

Take care,

Anthony

[James]

Thanks for the input Anthony, and your experience is interesting and on point.

From all the input received here, I now have no doubt that I can span 20 feet easily, using #2 southern pine 2 X 12 beams , on 16 inch centers.

I remain curious if anyone can still answer these remaining questions that were in my original post:

--------------------------------------------------

Is #2 visually graded, the most common lumber found in lumber yards ?

Which would be better, 2 x 10 on 12 inch centers or 2 x 12 on

16 inch centers ?? It seems that the latter may be a bit cheaper, but price isn't the primary concern.

What happens if you use a 2 X 12 beam, but put them on 24 inch centers instead of 16 inch?? Do you simply get more deflection, and less load capacity ?

[RicodJour]

I'd have doubts.

US species and the loads you indicated - you can't span 20' easily or even barely. Your house, your call.

R 222 15069 body James wrote:

I'd have doubts.

US species and the loads you indicated - you can't span 20' easily or even barely. Your house, your call.

R

[crhras]

I agree for the most part. Howerver, here in Chicago ocasionally it seems that the code was influenced by politics and requires things that don't make sense and only serve to put more money in someone's pocket.

[Michael Daly]

So, because of a few odd bits in the code, you reject it all?

Mike

[James]

R:

Well, I must respectfully disagree. In addition to assurances from folks here WHO HAVE DONE IT, as stated in my OP, I have walked on a floor made with 2 X 10's spanning 16 feet. There was no bounce. Surely I can go to 2 X 12's and span 4 more feet.

The chart you pointed to is neat !! Thanks !! It does indicate I would have to use 12 inch centers.

--James--

I'd have doubts.

US species and the loads you indicated - you can't span 20' easily or even barely. Your house, your call.

R

[crhras]

Interesting... where did I say that?

[RicodJour]

Respectfully disagree with what or whom? The Canadian Wood Council? Let me know what they think of your research - walking on an entirely different floor system and asking people on usenet.

If you want to revise your criteria to justify what you seem set on doing, please feel free. Instead of having 40 PSF as your live load, go with 30 and there won't be a problem. Just be aware that storage can frequently exceed live loads commonly found in a house, and rather quickly.

Right. Or go with a higher grade of wood. Before you count your chickens though, find out if you can get SYP framing lumber where you are for a reasonable price. In my neck of the woods framing lumber is either Hem Fir or Doug Fir. You can get 2x12 in pressure treated lumber but you do NOT want to do that. Besides the greatly increased cost, for no reason, you would have to use different connectors and fasteners and there would be substantially increased shrinkage.

Instead of going through all of that extra effort, why not rethink your objective, realize that indeed it has all been done before, and select a commonly available wood such as Hem Fir Select Structural on 16" centers. Most likely that will be your best alternative.

You should also check out some of the other design tools online at CWC. For example, they have DimensionCalc which will tell you that your

2x12 framing lumber will shrink about 1/4" as it dries. That is area of construction commonly ignored by all too may professional designers and all amateurs. R

[RicodJour]

That doesn't read correctly. Make that you can get 2x12 SYP in pressure treated...

R

[Michael Daly]

Ok, what are you saying? Some parts of the codes are crap. Big deal - those parts are not relevant.

Mike

[Michael Daly]

You seem to rely more on beliefs than code data. You find 2x10 ok for 16 feet - how does that translate into 2x12s ok at 20 feet? He has pointed you to one set of code limits and it shows your ideas are not satisfactory. What you need to do now is check your local code and find out what the requirements are.

Where's Hammurabi when you need him?

Mike

[Mulan]

The longest allowable span I could find on line was 18' 5" -18' 11" using 2 x 12 size of #2 douglas fir 12 nches apart.

Maybe you could > James wrote:

[Mulan]

The longest allowable span I could find on line was 18' 5" -18' 11" using 2 x 12 size of #2 douglas fir 12 nches apart.

Maybe you could > James wrote:

[terry]

When I built my large modified-post-and-beam barn/garage I went thru the same gyrations of tables and codes, all of which came up with different numbers.

All approaches use a 'safety factor' between the point of probable failure (read 'broken') and 'working loads'. The factor varies from 4 to 7, and other 'basic' numbers like Modulus Of Rupture (MOR) has different 'opinions' in different publications. So the results vary...

I got the Wood Handbook from the US Forest Service: Huge file at:

and spent hours understanding the formulas etc. It may be worth it to you to do this... as you seem to want to dig in and do your own work based on your own understanding, rather than all the conflicting 'experts'...

I don't have any neat design package, but I'll send you my .123 spreadsheet if you want. I think Excel will open it... Basically you just need the values for: MOR (Modulus of rupture) you want to use, with some safety factor, and the acceptable PSI for the end bearing area (end of beams on supports)...

Here's a look at the raw numbers for hemlock. Get any other wood out of the handbook.

--------------------- Hemlock Density: 50.00 Lb/ft3 Grn 28.00 Lb/ft3 dry Hemlock Weight (Lb. Per Board Foot) 4.20 green 2.30 dry

Hemlock Modulus Of Rupture (psi) 6400.00 green 8900.00 dry Safety factor 4.00 green 6.00 dry HEMLOCK: Safe Working Loads: GREEN DRY Hemlock Working Stress-Beam 1600.00 psi 1483.33 psi

Hemlock Parallel (Post end) 950.00 psi 1050.00 psi Hemlock Perpendicular (Beam Bearing) 360.00 psi 650.00 psi

---------------------------------------------------------------------------------------------------------- (Need a fixed font to see these)

As you can see I used a safety factor of 4 for the green hemlock I was using, and that naturally increases to a factor of 7 when dry. So I was OK with my 6400 pound Yukon driving on the first floor, and it'll have a better safety factor now, 2 years later. And I haven't put much load on the second floor yet....

The nice thing about a spreadsheet is you can try out different size beams and see the resulting MOR numbers and decide on your safety factor.

I just designed and built a 12by12 deck on our Cabin that is about 14 feet above ground. Most codes require 100 Pounds Per Square Foot for a 'Party Deck Load"... California requires 150 PSF in a few towns, as I understand it, after several people were killed in a deck collapse. I used 150 PSF, and had to go to an 8by8 beam on the outer edge because of a 12 foot span. That beam costs $50 here in Vermont. I was able to make tradeoffs on joist size, span and so on. I added a center support beam to keep the joist span to 6 feet, which made a huge difference and allowed 2 by 6 PT joists.

ANYWAY, you might decide to do all this, or not.... and use some tables,

I checked all my barn calculations against tables and they were close....

Have fun!!

Regards, Terry King ...On The Mediterranean in Carthage (Back ...In The Woods In Vermont for the Summer) [e-mail]

a few barn photos at:

[Bob Morrison]

Use engineered lumber I-joists. 14" TJI 210 @ 16" o/c should be about right.

[Glenn]

[HerHusband]

Typically, yes. I request kiln dried lumber when available, but it's rare to find kiln dried around here for anything larger than a 2x8 or so.

Taller joists usually result in a stronger floor.

A much shorter span rating, and you have to use thicker plywood to span the 24" joist spacing.

A 20' wide building with a 2x6 supporting wall at each end would result in a 19'-1" free span. That leaves several options:

Assuming a 30 Live/10 Dead (sleeping areas/attics)

2x10 at 12" OC = 19'-10" span 2x12 at 16" OC = 21'-1" span

Bump the load rating to 40 Live/10 Dead (all rooms)

2x12 at 12" OC = 21'-9" span

On the other hand, if the attic isn't tall enough to walk through and we're talking limited light storage, you could size the "floor" as CEILING joists.

Assuming a 20 Live/10 Dead (Drywall, No rooms, Limited storage)

2x10 at 16" OC = 20'-9" span 2x10 at 12" OC = 23'-11" span 2x12 at 16" OC = 24'-4" span

Obviously, it depends on what James plans to store up there, but I'd probably opt for 2x12's at 16" OC.

Anthony

[crhras]

Right, that's exactly what I was saying.

[jusair]

crowned spanning the 20 ft in the center of the 30 ft span then run joists perpendicular to the beam giving you a span of only abot 14'9 or 14' 7.5