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Jeremy Hillary Boob, PhD 06-06-2013 12:11 PM

deck joist span question
 
1 Attachment(s)
We've got a deck that is need of a complete rebuild and the attached image shows what I'd ideally like to do. The posts would be 6x6, the joists 2x12 (at 12" oc), and the beams would be triple 2x12s. The issue I have is that the deck beam span tables all say "framing from one side only" but, if I understand this correctly, for the middle beam, the framing is from 2 sides. I'd like to only have just the 3 beams as located in the picture, but I can easily add more posts if that's an issue (note that the posts shown in the drawing are 4' apart).

I'm still just in the thinking stage, so I was hoping to get a better understanding of any issues here. Thanks.

GBrackins 06-06-2013 05:18 PM

when attempting to size a beam to support joists coming from both sizes add together the lengths of the two joists and divide by 2 (10' + 18')/2 = 14' then go find the 14' joist span in Table 3 on page 2 of the Addendum that came out the end of May. Changes to the design values of southern pine caused these changes.

hope this helps, good luck!

Jeremy Hillary Boob, PhD 06-07-2013 07:04 PM

Quote:

Originally Posted by GBrackins (Post 1196436)
when attempting to size a beam to support joists coming from both sizes add together the lengths of the two joists and divide by 2 (10' + 18')/2 = 14' then go find the 14' joist span in Table 3 on page 2 of the Addendum that came out the end of May. Changes to the design values of southern pine caused these changes.

hope this helps, good luck!

Thanks, I really appreciate the response. However, I'm still confused. I believe this is the latest version of the deck design document that you refer to:

http://www.awc.org/publications/dca/dca6/dca6-09.pdf

The info on southern pine doesn't affect me, since I'll use PT douglas fir. In any case, that document contains the following on page C4:

"With appropriate assumptions, Table 3 could be used to size beams with joists spanning from both sides. Since tabulated values for beams assume 1⁄2 of the joist span to calculate tributary area, using 2x the joist span for cases where joists span symmetrically (equal joist spans) from opposite sides is acceptable. For example, assume there are 8'-0" joists spanning from opposite sides of the same beam. The column in Table 3 labeled for 16'-0" joist spans can be used to size a beam in this case."

It seems to me that this is saying that wrt Table, 3, I should use 10' + 18' = 28' (which is too big for Table 3, since that only goes up to 18' joist span), instead of (10' + 18')/2 = 14'. That's why I was thinking that I'd need more posts to support the beam than is shown in Table 3. Let me know if I've got this wrong.

TarheelTerp 06-07-2013 07:19 PM

Quote:

Originally Posted by Jeremy Hillary Boob, PhD (Post 1197138)
It seems to me that this is saying that wrt Table, 3, I should use 10' + 18' = 28' ...

So... how tall a joist will that lead you to use?

GBrackins 06-07-2013 07:32 PM

Quote:

Originally Posted by Jeremy Hillary Boob, PhD (Post 1197138)
document contains the following on page C4:

"With appropriate assumptions, Table 3 could be used to size beams with joists spanning from both sides. Since tabulated values for beams assume 1⁄2 of the joist span to calculate tributary area

you are absolutely correct, I forgot the Table is set up to apply 1/2 the joist span to determine the beam, so you would have to add both together (10' + 18' = 28') and it would not have your span in the table so you would be unable to determine the intermediate beam by prescriptive means, unless you installed another beam to get your combined joist spans down to 18'

GBrackins 06-07-2013 07:47 PM

I apologize for earlier incorrect statement. :(

Jeremy Hillary Boob, PhD 06-08-2013 09:23 AM

Quote:

Originally Posted by TarheelTerp (Post 1197145)
So... how tall a joist will that lead you to use?

I don't believe the joist depth is an issue here (according to Table 2, douglas fir, 2x12 at 12" oc can span up to 18', and my max actual joist span is 17.5'). The issue is whether the middle beam can support the added weight of having joists on both sides. From Table 3, if we use a beam of three 2x12s, and a max "beam span" (i.e., post spacing) of 8' 1", then we're allowed an effective 18' joist span. If the beam span is less (like, say, 4' in my drawing) then the beam could support more weight. However, the standard tables don't go any further. I guess that means I need an engineer, but I'd like to make sense of this myself before going that route. So, thanks for your help (and patience)...

tony.g 06-08-2013 11:00 AM

The central beam will be supporting one-half the load from each side.
If those square bits are posts, then your central beam is spanning only 4ft?.
If so, that triple beam will be more than adequate to support the live and dead load from each side.
So what's the problem, or am I misreading/misunderstanding?

Jeremy Hillary Boob, PhD 06-09-2013 08:37 AM

Quote:

Originally Posted by tony.g (Post 1197449)
The central beam will be supporting one-half the load from each side.
If those square bits are posts, then your central beam is spanning only 4ft?.
If so, that triple beam will be more than adequate to support the live and dead load from each side.
So what's the problem, or am I misreading/misunderstanding?

Yes, I think that is correct. It's just that this configuration is not show up on the standard tables and that's why I was asking.

jagans 06-09-2013 09:23 AM

Is there some reason you do not want to run your girder parallel to and at 14 feet from the house, and run your joists perpendicular, cantileveing the joists over the girder out to 16 feet? Your decking would then run parallel to the house, instead of perpendicular.

CarpenterSFO 06-09-2013 01:50 PM

Quote:

Originally Posted by jagans (Post 1197908)
Is there some reason you do not want to run your girder parallel to and at 14 feet from the house, and run your joists perpendicular, cantileveing the joists over the girder out to 16 feet? Your decking would then run parallel to the house, instead of perpendicular.

First thing I wondered, too.

CarpenterSFO 06-09-2013 02:07 PM

Quote:

Originally Posted by jagans (Post 1197908)
Is there some reason you do not want to run your girder parallel to and at 14 feet from the house, and run your joists perpendicular, cantileveing the joists over the girder out to 16 feet? Your decking would then run parallel to the house, instead of perpendicular.

And the general question is: why did you pick this particular approach? How high is the deck? Why the 10' and 17.5' spans specifically? Pics? Even 2x12s may seem a little bouncy at 17.5'. If it's a lower deck, and you don't need the extra space for an open view below, then put in some more posts and reduce that span.

MTN REMODEL LLC 06-09-2013 03:15 PM

Quote:

Originally Posted by CarpenterSFO (Post 1198089)
And the general question is: why did you pick this particular approach? How high is the deck? Why the 10' and 17.5' spans specifically? Pics? Even 2x12s may seem a little bouncy at 17.5'. If it's a lower deck, and you don't need the extra space for an open view below, then put in some more posts and reduce that span.

Hey Bob...CarpSFO.... Welcome over here... you will/would be a great addition..:thumbsup:

Got kinda quiet over on CT for the most part....

Peter

Jeremy Hillary Boob, PhD 06-10-2013 01:14 AM

Quote:

Originally Posted by jagans (Post 1197908)
Is there some reason you do not want to run your girder parallel to and at 14 feet from the house, and run your joists perpendicular, cantileveing the joists over the girder out to 16 feet? Your decking would then run parallel to the house, instead of perpendicular.

Yes, there is actually a concrete pad that I'd like to use underneath, and that's why the 17.5' and 10' spans are ideal. So, I could put in another set of posts/beam, but would prefer not to.

As to running the beams parallel to the house, I did consider that. But triple 2x12 PT douglas fir beams won't span 17.5', at least not at the spacings given in the standard tables. You can get that distance with yellow pine, but I don't have easy access to that.

The deck is about 10' off the ground, and another advantage of making it freestanding and running the beams perpendicular to the house is some additional headroom.

jagans 06-10-2013 09:22 AM

Quote:

Originally Posted by Jeremy Hillary Boob, PhD (Post 1198382)
Yes, there is actually a concrete pad that I'd like to use underneath, and that's why the 17.5' and 10' spans are ideal. So, I could put in another set of posts/beam, but would prefer not to.

As to running the beams parallel to the house, I did consider that. But triple 2x12 PT douglas fir beams won't span 17.5', at least not at the spacings given in the standard tables. You can get that distance with yellow pine, but I don't have easy access to that.

The deck is about 10' off the ground, and another advantage of making it freestanding and running the beams perpendicular to the house is some additional headroom.

You have strange logic. Im sure I dont understand it, especially the thing about headroom, unless you mean the headroom between the joists, which is ludicrous.

I think you are hung up on the concrete pad, when it is fairly easy to go through it. You cannot span 17.5 feet with dimensional lumber, I really dont care what the span tables say. Especially with the crap lumber they sell today.


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