View Full Version : Beam Spans?

01-13-2006, 06:13 AM
I've seen and used the span table link many many times. But I have a little different of a question.

One of the rooms I'll be adding on will be 28' long by 15'deep (outside deminisions). This is going to be a great room partitioned at the end may or may not be another room.

To achieve the minimum clearance of 18" to my joists i'm going to need to use 2x6 joists. I'll probably be doing a 2x6 or 2x8 ( I like overkill) wood stem wall on a continuous concrete footer. At least this is how i'll have it drawn up.

Could I use 4x6 or 6x6 beams to span that 15' distance at 6' apart and then use joist hangers to the hang my 2x6's? The 2x6 joist span meets code at 6'.

But i'm not finding much about beam spans.

If nessecary I may cut that 15' deep distance in half with a second stem wall. But if the 4x6's or 6x6's will easily span that then why bother.

That or I could use spot peirs.

01-13-2006, 06:51 AM
So I think I answered my own question. I'm gonna have to have the center stem wall or spot peirs. I was kinda figuring on it anyhow.

01-17-2006, 12:07 PM
I think I'll end up using 16' double 2x6 joists spannning the entire 15'. The spans will be single pieces of wood. BUT the length will be enterupted halfway by a center stem wall. This way I use no beams. Just double 2x6" joists. The maximum span for a 2x6" joist is 9'. I'll have double 2x6" joists going 7.5' on 16's. Over kill is good.

I like thinking out loud.

01-30-2006, 07:09 AM
I'm bumping this back up just to see if anyone has any comments on it.


01-30-2006, 02:14 PM
I would probably try and opt for 2x10 DFir #2 @ 16" o.c. which would span the whole distance. Might compare the cost difference and see if there is a way to use 2x10 in lieu of 2x6.

A 6x6 spanning 15' with a tributary area of 6' (3' either side) is insufficient. Ends up with over 3" of deflection under total load and just under 3" for live load. This is L/59.. it should be L/360 minimum.

01-31-2006, 02:03 AM
Awesome. I doubt i'll be able to run 2x10's. Thats where I run into the whole 18" above grade thing and matching up to the level of the current structure i'm dealing with. I should have explained that.

So would the double 2x6's on 16"o.c. work then? My spans would be less than 7.5'.

01-31-2006, 03:03 AM
Missed the cutting the span in half part....sorry. 2x6 @ 24" o.c. gives a L/662 which is pretty good :) Should mention that the joist must be continuous along the 15' length.

02-01-2006, 04:26 AM
That's the plan. It will be a continuous joist that is supported in the center. So the unsupported spans will only be 7' to 7.5'. I'll probably set the boards on 16" o.c. just out of my own paranoia.

Thanks for your help!

02-22-2006, 05:52 AM
What is deflection limit?

Is a higher (L/662) deflection limit better?

If so what would my deflection limit be with PT 2x6 Southern Pine with 7'6" spans on 16"o.c. centers?

Also, how do live load and dead loads change when you double up on a joist? Would a double 2x6 double the live and dead loads? Sounds to easy to me. I've heard it doubles deflection limits.

I apologize for my ignorance on this. I do know what live and dead loads are but the whole deflection limits thing kind throws me for a loop.

02-22-2006, 10:55 AM
I think deflection is the amount of resistance to sag the board will have when weight is placed on it. I don't know what the numbers represent, but I'd like to! I have a guess, but I'll keep it to myself and let someone else answer for sure.

I believe L/360 is considered good for total loads, while the L/662 would be considered great!

It would seem (to me, at least) that doubled 2x4's at 16" oc would be less advantageous than single 2x4's at 8" oc (same amount of wood).

Just my thoughts, now lets hear from someone who actually knows. :lol:

02-23-2006, 09:25 AM
Rich? Cole? You guys are smart. Any comments?

02-24-2006, 09:28 AM
L = Length of unsupported span.
Live loads and dead loads are pounds per square foot so it will remain constant over an area. Load and deflection are not directly proportional - a double 2x6 will not give you twice the span with the same deflection amount. I shouldn't say won't - in some cases it may give you twice the span. What will often happen is the span will increase but deflection will not become an issue - it will start to affect total allowable load.
For instance - 2x6 @ 16" o.c. 8'6" span - 40psf live load, 12psf dead load. The member performs at L/373 for total load.. now double the span and double the joist - you only get L/86.
Now bringing the span down to 10'6" with a double 2x6 the deflection is L/377 but fails due to total load.
These are just quick numbers to show some relationships and there are many more variables to consider than just deflection or loads.. etc.

02-24-2006, 10:23 AM
I think I understand. So will my idea of 2x6's on 16o.c. with 7.5' spans have good strength? From what you said it sounds like it will.

02-24-2006, 10:25 AM
So if I did keep my spans the same but doubled the joist my deflection and loads would increase?

For instance still a 7.5' span on 16o.c. but with double 2x6's?

02-24-2006, 05:46 PM
Yes.. but why?
The deflection right now is plenty good (2x6 @ 16" o.c. hem fir # 2).. with allowable loads of 40psf live and 12psf dead. Total deflection under total loading conditions is about 3/32" (L/844).
I guess I don't understand why you want to overkill it. The difference in deflection that you gain is only .033" (1/32").

02-24-2006, 06:38 PM
Rich? Cole? You guys are smart. Any comments?

Sorry guys, I have been super busy.

02-25-2006, 12:03 PM
Its really for my own knowledge. But I think I understand now. Its kind of like hitting a certain point and then your gains are minimal.

So i'm guessing L/844 is a really good deflection limit. The higher the number the better. I'll stick with the single 2x6's on 16 o.c. with 7.5" spans. I'll run a single conitinuous board the full 15 feet but supported in the middle. At least this is how i'll have him draw it.

Thanks for the help Rich!!

02-25-2006, 04:46 PM
Yes - if the span and loads are within limits L/844 is excellent. If you want a stiffer floor. I personally like a little give - not so hard on the knees of the elderly. Also remember - deflection is fine and all but you can have a high deflection rating and still have a floor that fails and is bouncy.

02-26-2006, 06:17 AM
I guess in the end its all up to the draftsman anyhow. At least now I kinda know something.